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Invasive and noninvasive cardiovascular monitoring options for cardiac surgery

Open AccessPublished:April 10, 2022DOI:https://doi.org/10.1016/j.xjon.2022.02.028

      Key Words

      Figure thumbnail fx1
      Cardiovascular monitoring options for the cardiac ICU.
      Pulmonary artery catheters can be used selectively. Novel technology providing minimally or noninvasive monitoring offers alternatives and may enhance recovery after cardiac surgery.
      Heart disease is the most common cause of death in the United States, with more than 655,000 deaths in the 2018 Centers for Disease Control and Prevention report. Surgical treatment of heart disease is common, expensive, and greater risk than many other surgical endeavors. Cardiovascular monitoring is an important process within this complex clinical setting that requires careful consideration of the patient and the clinical team, as well as the institutional resources to optimize a precise and personalized approach. A comprehensive evaluation of monitoring must consider patient anatomy and cardiopulmonary physiology, hemodynamic and physiologic goals, the phases of care, direct and indirect costs, and operational considerations such as duration of monitoring, therapeutic protocols, and team expertise. Invasive monitoring is a universally accepted component of cardiac surgical perioperative care, but there remain unresolved controversies as to the optimal monitoring strategies to optimize efficacy and efficiency. One of the long-standing controversies is the use of a pulmonary artery catheter (PAC) versus other monitoring alternatives (Figure 1).
      Figure thumbnail gr1
      Figure 1Common invasive and noninvasive hemodynamic monitoring options for management in the cardiac intensive care unit. Footnote: Arterial lactic acid is shown in mmol/L. Right panel, Invasive and noninvasive pulse contour analysis technology. Left upper panel, Swan-Ganz Catheter (Edwards LifeSciences). Left lower panel, Handheld point-of-care ultrasound. SvO2, Central/mixed venous oxygen saturation from centrally placed upper extremity venous access or Swan-Ganz.

      Pulmonary Arterial Catheters

      PACs, introduced in 1970 by Swan and Ganz,
      • Swan H.J.
      • Ganz W.
      • Forrester J.
      • Marcus H.
      • Diamond G.
      • Chonette D.
      Catheterization of the heart in man with use of a flow-directed balloon-tipped catheter.
      transformed the bedside assessment of hemodynamic monitoring in critically ill and postoperative patients by providing real-time estimation of cardiac output (CO). This is performed by thermodilution or with continuous measurement of mixed venous oxygen saturation. The PAC also enables accurate measurement of the central venous, right ventricular (RV), pulmonary artery, and pulmonary arterial wedge pressures, as well as indirect derivation of several other parameters such as systemic and pulmonary vascular resistance.
      PAC use remains common in cardiac surgery, with 68% of respondents in a 2015 survey reporting its use in greater than 75% of cases.
      • Judge O.
      • Ji F.
      • Fleming N.
      • Liu H.
      Current use of the pulmonary artery catheter in cardiac surgery: a survey study.
      However, several studies have failed to demonstrate a survival benefit. In a meta-analysis of 13 randomized clinical trials including 5051 subjects comparing use of a PAC versus nonuse in critically ill patients that included cardiac patients, the use of a PAC did not reduce overall mortality or days in hospital.
      • Shah M.R.
      • Vic Hasselblad V.
      • Stevenson L.W.
      • Binanay C.
      • O'Connor C.M.
      • Sopko G.
      • et al.
      Impact of the pulmonary artery catheter in critically ill patients: meta-analysis of randomized clinical trials.
      Other studies of patients undergoing cardiac surgery showed no reduction in morbidity, as well as an increase in mortality, duration of mechanical ventilation, and length of stay (LOS) in the intensive care unit (ICU).
      • Joseph C.
      • Garrubba M.
      • Smith J.A.
      • Melder A.
      Does the use of a pulmonary artery catheter make a difference during or after cardiac surgery?.
      ,
      • Chiang Y.
      • Hosseinian L.
      • Rhee A.
      • Itagaki S.
      • Cavallaro P.
      • Chikwe J.
      Questionable benefit of the pulmonary artery catheter after cardiac surgery in high-risk patients.
      Hospitalization costs were greater. However, patients receiving PAC were older and more likely to have pulmonary hypertension, chronic obstructive pulmonary disease, obesity, and chronic renal failure.
      • Chiang Y.
      • Hosseinian L.
      • Rhee A.
      • Itagaki S.
      • Cavallaro P.
      • Chikwe J.
      Questionable benefit of the pulmonary artery catheter after cardiac surgery in high-risk patients.
      Two recent large observational studies compared patients undergoing cardiac surgery with PAC, matched by propensity score, also showed no impact on mortality or outcomes. A recent publication by Brown and colleagues
      • Brown J.A.
      • Aranda-Michel E.
      • Kilic A.
      • Serna-Gallegos D.
      • Bianco V.
      • Thoma F.W.
      • et al.
      The impact of pulmonary artery catheter use in cardiac surgery.
      matched 3519 balanced pairs in a total of 11,820 patients undergoing coronary artery bypass grafting (CABG), aortic valve replacement, mitral valve replacement or repair, and a combination of CABG and aortic or mitral surgery. The cohort included urgent and emergent cases, and the study failed to demonstrate a survival benefit or improved outcomes. In addition, patients had prolonged ICU stay (48 vs 39 hours; P < .001) and increased transfusions (40.4% vs 35.5%; P < .001). A second prospective observational study composed of 5065 patients undergoing CABG in 1273 propensity-matched pairs demonstrated increased risk of all-cause mortality (adjusted odds ratio [AOR], 2.08), severe end-organ complications (cardiac AOR, 1.58; renal AOR, 2.47; cerebral AOR, 2.02), prolonged ICU stay (AOR, 1.55), longer time to extubation (P < .00001), and larger positive fluid balance (P = .003) with the use of a PAC.
      • Schwann N.M.
      • Hillel Z.
      • Hoeft A.
      • Barash P.
      • Möhnle P.
      • Miao Y.
      • et al.
      Lack of effectiveness of the pulmonary artery catheter in cardiac surgery.
      Conversely, Shaw and colleagues
      • Shaw A.D.
      • Mythen M.G.
      • Shook D.
      • Hayashida D.K.
      • Zhang X.
      • Skaar J.R.
      • et al.
      Pulmonary artery catheter use in adult patients undergoing cardiac surgery: a retrospective, cohort study.
      showed decreased LOS (9.39 days vs 8.56 days; P < .001) and cardiopulmonary morbidity (P < .001) with a PAC, but no difference in 30-day mortality (P = .516).
      The current opinion is to use PACs more selectively (Table 1), such as in the evaluation and management of patients in shock; in those undergoing high-risk surgery; or in patients with pulmonary artery hypertension. PAC has been recommended in patients presenting with refractory cardiogenic shock or in patients undergoing acute mechanical circulatory support to monitor effectiveness of therapy, optimize device settings, assess the need for escalation, and guide timing and rate of weaning.
      • Saxena A.
      • Garan A.R.
      • Kapur N.K.
      • O'Neill W.W.
      • Lindenfeld J.
      • Pinney S.P.
      • et al.
      Value of hemodynamic monitoring in patients with cardiogenic shock undergoing mechanical circulatory support.
      Other considerations include patients with a history of RV failure and those undergoing advanced heart failure surgery.
      • Ranucci M.
      Which cardiac surgical patients can benefit from placement of a pulmonary artery catheter?.
      Table 1Clinical situations associated with usefulness of PAC in the cardiac ICU
      ConditionCVPRVPPAPPVR(I)PCWPSVR(I)CO/CI
      Acute cardiogenic shock↔/↑
      Acute vasoplegic (distributive) shock↔/↓↔/↓↔/↓↔/↑
      Acute tamponade (obstructive shock)↔/↑↔/↑
      Acute hemorrhage (hypovolemic shock)↔/↑
      Acute pulmonary hypertension
      Acute ventricular septal defect (VSD)
      Right ventricular failure↔/↓↔/↑
      CVP, Central venous pressure; RVP, right ventricular pressure; PAP, pulmonary artery pressure; PVR, pulmonary vascular resistance; I, indexed; PCWP, pulmonary capillary wedge pressure; SVR, systemic vascular resistance; CO, cardiac output; CI, cardiac index.
      PAC measurement requires familiarity with the device, as frequent inaccuracies can be observed. When compared with the direct Fick method, the PAC may be associated with a percentage error of >50%.
      • Baylor P.
      Lack of agreement between thermodilution and Fick methods in the measurement of cardiac output.
      In addition, complications include arrhythmia, valvular damage, pulmonary infarction, infection, and thromboembolism. Pulmonary artery perforation is rare but carries a 70% risk of mortality.
      Following the 1996 publication by the SUPPORT (Study to Understand Prognosis on Preferences for Outcomes and Risk of Treatments) investigators that showed an increased 30-day mortality and ICU stay with PAC,∗ there has been a surge of alternative hemodynamic monitoring devices to replace the PAC with less-invasive methods while capturing the same important physiologic parameters and eliminating PAC-related complications. Pulse contour analysis, ultrasonography, partial carbon dioxide rebreathing, bioimpedance, and pulse-wave transit time (PWTT) are some of the methods that estimate CO (Table 2).
      • Thiele R.H.
      • Bartels K.
      • Gan T.J.
      Cardiac output monitoring: a contemporary assessment and review.
      ,
      • Kenaan M.
      • Gajera M.
      • Goonewardena S.N.
      Hemodynamic assessment in the contemporary intensive care unit: a review of circulatory monitoring devices.
      When evaluating these innovative devices, one must consider that they provide mathematical estimations and extrapolations of hemodynamic values, given their minimally invasive nature. The percentage error (ie, accuracy), derived by the Bland–Altman analysis, is the difference in the measured value (usually CO) from the reference method. A value up to 30% is considered clinically acceptable.
      • Critchley L.A.
      • Critchley J.A.
      A meta-analysis of studies using bias and precision statistics to compare cardiac output measurement techniques.
      The concordance rate (ie, precision) is a surrogate for a device's trending ability with changes in a patient's hemodynamic status compared with the reference method.
      Table 2Summary of cardiovascular monitoring options
      TechnologyCO determinationPE and CR, %AdvantagesLimitationsEstimated pricing (USD)
      Pulse contour analysis

      Minimally invasive: pressure recording analytical method (PRAM)
      Examples of devices include FloTrac/Vigileo (Edwards Lifesciences); LiDCOrapid and PulseCO (LiDCO Group Plc); MostCare (Vygon Health); ProAQT/Pulsioflex (Getinge).
      CO derived from arterial waveform pressure signal.PE: 23-74 (41)

      CR: 84-93
      • Minimally or noninvasive
      • Estimates several hemodynamic variables: CO, SV, SVV, SVR
      • Ease of use
      • Unreliable in certain clinical situations
      • Requires recalibration after acute hemodynamic changes.
      Device: $18,000-$20,000

      Set up: $290

      Service: daily cost $25

        Noninvasive pulse contour analysis

      • Continuous noninvasive cardiac output (CNCO
        Examples of devices include CNAP system (CNSystems Medizintechnik GmbH); ClearSight (Edwards Lifesciences).
        )
      • Multibeat analysis (MBA
        An example of a device includes Argos (Retia Medical).
        )
      • Radial artery applanation tonometry (RAAT
        An example of a device includes T-line 200 pro (Tensys Medical Inc).
        )
      PE: 23-58 (41)

      CR: 84-100
      Esophageal Doppler
      Examples of device include CardioQ (Deltex Medical); HemoSonic (Arrow International).
      CO estimated from shift in Doppler frequency, blood flow velocity in aorta, and nomogram-based patient cross-sectional area.PE: 43

      CR: not reported
      • Provide corrected flow time and SVV
      • Continuous monitoring
      • Intubation of the esophagus
      • Positioning error
      • Learning curve
      • Inaccurate in aortic diseases
      • Population variation (nomogram)
      Device: $20,000-$27,000

      Individual set up: $190

      Service: daily cost $5
      Transthoracic Doppler
      An example of device includes: USCOM (USCOM Ltd).
      CO derived from SV from blood velocity through the aortic or pulmonary valve, and nomogram-based patient cross-sectional area.PE: 56-62

      CR: not reported
      • Noninvasive
      • Rapid
      • Cost-effective
      • Short learning curve
      • Operator dependent
      • Difficult to achieve continuous monitoring
      • Population variation (nomogram)
      Device: $27,000

      Setup and service contract not disclosed
      TEE
      TEE, Transesophageal echocardiography. An example of a miniaturized device includes the hTEE probe (ImaCor Inc).
      CO calculated using LVOT diameter and velocityPE: not reported

      CR: not reported
      • Provides information about preload, afterload, and contractility
      • Diagnosis of acute conditions
      • Miniaturized probes for continuous monitoring
      • Invasive
      • Intubation of the esophagus
      • Expertise (operator)
      Device: $70,000

      Setup $1250

      Service: daily cost not disclosed
      Point-of-care TTE
      TTE, Transthoracic echocardiography. Examples of devices include the Butterfly iQ (Butterfly Network Inc); Lumify (Philips); and Vscan (GE Healthcare).
      PE: 25-40

      CR: 94
      • Training opportunities
      • Diagnosis
      • Availability of portable devices.
      • Limited data on TTE-derived CO in cardiac surgical patient
      • Expertise (operator)
      Device: $2000-$7000;

      Set-up $1000

      Service: daily cost $0 - $5
      Partial CO2 Rebreathing
      An example of a device includes NICO monitor (Novametrix Medical Systems).
      Uses Fick equation to calculate CO from the change in ratio of CO2 production and ETCO2 in response to intermittent partial rebreathing.PE: 45

      CR: not reported
      • Noninvasive
      • No calibration
      • Utility in mechanically ventilated patients
      • Underestimation of CO under certain mechanical ventilatory settings
      • Inaccurate for patients with primary pulmonary pathophysiology
      • Inability to assess volume and fluid responsiveness
      • Expertise
      Device: $18,500

      Setup $215

      Service contract—daily cost $5
      Bioimpedance
      An example of a device includes ECOM (ECOM Medical Inc).
      Impedance changes from variations in blood resistance to induced current over volume fluctuations of the cardiac cycle. Sensors on endotracheal tube. CO determined by SV and HRPE: 40-53

      CR: 87-99
      Requires intubation, invasive
      Bioimpedance/bioreactance
      An example of a device includes NICOM (Cheetah Medical).
      Phase shift between applied current and measure of returning voltage between 4 sensors. Correlated with blood flow. CO determined by SV and HRPE: 43

      CR: not reported
      • Noninvasive
      • Averages signal over 1 minute (arrhythmias)
      • Predictive of fluid responsiveness
      • Ease of use
      • Versatile in different settings
      • Influenced by mode of ventilation, fluid, cardiothoracic procedures and conditions, low CO, and electrocautery.
      • Sensitive to location of electrodes, body size, temperature, and humidity
      Device: $20,000

      Setup $500; Service: daily cost not disclosed
      PWTT
      PWTT, pulse-wave transit time. An example of a device includes esCCO (Nihon Kohden).
      Time for pulse pressure waveform to propagate between two arterial sites. CO derived from inverse correlation between PWTT and SV.PE: 41-64

      CR: 76
      • Uses basic instruments in the ICU: ECG and pulse oximetry
      • Intuitive
      • Inexpensive
      Decreased concordance with mechanical ventilation
      CO, Cardiac output; PE, percentage error expressed as range and (mean) (%); CR, concordance rate, expressed as range (%); USD, US dollars; SV, stroke volume; SVV, stroke volume variation; SVR, systemic vascular resistance; LVOT, left ventricular outflow tract; CO2, carbon dioxide; ETCO2, end-tidal carbon dioxide; HR, heart rate; ICU, intensive care unit; ECG, electrocardiogram.
      Examples of devices include FloTrac/Vigileo (Edwards Lifesciences); LiDCOrapid and PulseCO (LiDCO Group Plc); MostCare (Vygon Health); ProAQT/Pulsioflex (Getinge).
      Examples of devices include CNAP system (CNSystems Medizintechnik GmbH); ClearSight (Edwards Lifesciences).
      An example of a device includes Argos (Retia Medical).
      § An example of a device includes T-line 200 pro (Tensys Medical Inc).
      Examples of device include CardioQ (Deltex Medical); HemoSonic (Arrow International).
      An example of device includes: USCOM (USCOM Ltd).
      # TEE, Transesophageal echocardiography. An example of a miniaturized device includes the hTEE probe (ImaCor Inc).
      ∗∗ TTE, Transthoracic echocardiography. Examples of devices include the Butterfly iQ (Butterfly Network Inc); Lumify (Philips); and Vscan (GE Healthcare).
      †† An example of a device includes NICO monitor (Novametrix Medical Systems).
      ‡‡ An example of a device includes ECOM (ECOM Medical Inc).
      §§ An example of a device includes NICOM (Cheetah Medical).
      ‖‖ PWTT, pulse-wave transit time. An example of a device includes esCCO (Nihon Kohden).

      Alternative Technologies

      Pulse Contour Analysis

      Pulse contour analysis is by far the most-used and -studied technology for minimally invasive estimation of CO and is derived from the arterial waveform pressure signal using proprietary algorithms to each device. The CO may be derived by pulse contour analysis alone or in conjunction with transpulmonary thermodilution or indicator dilution. Most of these technologies are uncalibrated such that they do not require intermittent alignment, and many are noninvasive. Volume clamp methods typically involving finger cuffs and radial artery applanation tonometry and allow for real-time hemodynamic monitoring without the use of arterial lines. The noninvasive nature of these devices is appealing for monitoring in the era of enhanced recovery after cardiac surgery protocols (Enhanced Recovery After Surgery Cardiac). Numerous small, randomized clinical trials have been published comparing CO measured by bolus thermodilution with commercially available technologies. Invasive pulse contour technologies that use an arterial line have demonstrated percentage errors ranging from 23% to 74% and concordance rates from 84% to 93% compared with thermodilution.
      • Asamoto M.
      • Orii R.
      • Otsuji M.
      • Bougaki M.
      • Imai Y.
      • Yamada Y.
      Reliability of cardiac output measurements using LiDCOrapid™ and FloTrac/Vigileo™ across broad ranges of cardiac output values.
      • Barile L.
      • Landoni G.
      • Pieri M.
      • Ruggeri L.
      • Maj G.
      • Nigro Neto C.
      • et al.
      Cardiac index assessment by the pressure recording analytic method in critically ill unstable patients after cardiac surgery.
      • Romagnoli S.
      • Romano S.M.
      • Bevilacqua S.
      • Ciappi F.
      • Lazzeri C.
      • Peris A.
      • et al.
      Cardiac output by arterial pulse contour: reliability under hemodynamic derangements.
      • Maeda T.
      • Hamaguchi E.
      • Kubo N.
      • Shimokawa A.
      • Kanazawa H.
      • Ohnishi Y.
      The accuracy and trending ability of cardiac index measured by the fourth-generation FloTrac/Vigileo system™ and the Fick method in cardiac surgery patients.
      • Missant C.
      • Rex S.
      • Wouters P.F.
      Accuracy of cardiac output measurements with pulse contour analysis (PulseCO) and Doppler echocardiography during off-pump coronary artery bypass grafting.
      • Greiwe G.
      • Peters V.
      • Hapfelmeier A.
      • Romagnoli S.
      • Kubik M.
      • Saugel B.
      Cardiac output estimation by multi-beat analysis of the radial arterial blood pressure waveform versus intermittent pulmonary artery thermodilution: a method comparison study in patients treated in the intensive care unit after off-pump coronary artery bypass surgery.
      • Saugel B.
      • Heeschen J.
      • Hapfelmeier A.
      • Romagnoli S.
      • Greiwe G.
      Cardiac output estimation using multi-beat analysis of the radial arterial blood pressure waveform: a method comparison study in patients having off-pump coronary artery bypass surgery using intermittent pulmonary artery thermodilution as the reference method.
      Noninvasive pulse contour technologies using radial artery applanation tonometry have demonstrated percentage errors ranging from 23% to 58% and concordance rates from 84% to 100% compared with thermodilution.
      • Wagner J.Y.
      • Körner A.
      • Schulte-Uentrop L.
      • Kubik M.
      • Reichenspurner H.
      • Kluge S.
      • et al.
      A comparison of volume clamp method-based continuous noninvasive cardiac output (CNCO) measurement versus intermittent pulmonary artery thermodilution in postoperative cardiothoracic surgery patients.
      • Maass S.W.
      • Roekaerts P.M.
      • Lancé M.D.
      Cardiac output measurement by bioimpedance and noninvasive pulse contour analysis compared with the continuous pulmonary artery thermodilution technique.
      • Wagner J.Y.
      • Sarwari H.
      • Schön G.
      • Kubik M.
      • Kluge S.
      • Reichenspurner H.
      • et al.
      Radial artery applanation tonometry for continuous noninvasive cardiac output measurement: a comparison with intermittent pulmonary artery thermodilution in patients after cardiothoracic surgery.
      • Broch O.
      • Renner J.
      • Gruenewald M.
      • Meybohm P.
      • Schöttler J.
      • Caliebe A.
      • et al.
      A comparison of the Nexfin® and transcardiopulmonary thermodilution to estimate cardiac output during coronary artery surgery.
      In a meta-analysis, not exclusively in cardiac surgery patients, 24 studies compared pulse contour analysis with a reference method and showed a mean percentage error of 41% and precision of 1.22 L/min.
      • Peyton P.J.
      • Chong S.W.
      Minimally invasive measurement of cardiac output during surgery and critical care: a meta-analysis of accuracy and precision.
      The overall accuracy of pulse contour analysis is clinically significant and devices that integrate this technology may play a role in routine postcardiac surgery care, especially when compared with other minimally invasive hemodynamic monitoring modalities. Their high concordance rate suggests accurate trending ability and can be used at the bedside for titrating medications and measuring acute changes in hemodynamic status. In addition, the recent development of algorithms founded on machine-learning from large intraoperative datasets can assist in detection of sub-optimal hemodynamic parameters prior to the development of hypotension.
      • Hatib F.
      • Jian Z.
      • Buddi S.
      • Lee C.
      • Settels J.
      • Sibert K.
      • et al.
      Machine-learning algorithm to predict hypotension based on high-fidelity arterial pressure waveform analysis.
      ,
      • Shin B.
      • Maler S.A.
      • Reddy K.
      • Fleming N.W.
      Use of the hypotension prediction index during cardiac surgery.
      Limitations of arterial pulse contour analysis include aortic regurgitation, arrhythmias, intra-aortic balloon counter-pulsation, hemodynamic instability, and extracorporeal circulation.
      • Barile L.
      • Landoni G.
      • Pieri M.
      • Ruggeri L.
      • Maj G.
      • Nigro Neto C.
      • et al.
      Cardiac index assessment by the pressure recording analytic method in critically ill unstable patients after cardiac surgery.
      ,
      • Romagnoli S.
      • Romano S.M.
      • Bevilacqua S.
      • Ciappi F.
      • Lazzeri C.
      • Peris A.
      • et al.
      Cardiac output by arterial pulse contour: reliability under hemodynamic derangements.
      However, newer algorithms have begun to address some of these limitations.

      Ultrasonography

      Ultrasonography has evolved since the 1970s including 2-dimensional, 3-dimensional/4-dimensional echocardiography, esophageal Doppler, and the ultrasonic cardiac output monitor (USCOM). Esophageal Doppler ultrasonography can estimate CO but can also provide estimates of left ventricular (LV) function, preload, and contractility. A few small observational studies and clinical trials have shown a percentage error of 43%, a high degree of bias, and poor correlation when compared with PAC thermodilution in CABG and/or valve surgery patients.
      • Missant C.
      • Rex S.
      • Wouters P.F.
      Accuracy of cardiac output measurements with pulse contour analysis (PulseCO) and Doppler echocardiography during off-pump coronary artery bypass grafting.
      ,
      • Collins S.
      • Girard F.
      • Boudreault D.
      • Chouinard P.
      • Normandin L.
      • Couture P.
      • et al.
      Esophageal Doppler and thermodilution are not interchangeable for determination of cardiac output.
      • Sharma J.
      • Bhise M.
      • Singh A.
      • Mehta Y.
      • Trehan N.
      Hemodynamic measurements after cardiac surgery: transesophageal Doppler versus pulmonary artery catheter.
      • Jaeggi P.
      • Hofer C.K.
      • Klaghofer R.
      • Fodor P.
      • Genoni M.
      • Zollinger A.
      Measurement of cardiac output after cardiac surgery by a new transesophageal Doppler device.
      Given this, and the invasive nature of intubating the esophagus in the ICU, the utility of esophageal Doppler to assess hemodynamic parameters in the postoperative period is limited.
      Transthoracic Doppler ultrasonography uses similar technology in a noninvasive manner with probe placement at the supraclavicular, suprasternal or parasternal positions. A meta-analysis of 6 studies found a mean percentage error of 43% compared with thermodilution
      • Chong S.W.
      • Peyton P.J.
      A meta-analysis of the accuracy and precision of the ultrasonic cardiac output monitor (USCOM).
      ; however, not all publications included cardiac surgical patients. The USCOM monitor is safe, noninvasive, rapid, and cost-effective. However, operator dependency and poorly estimated valve area are sources of error. Among cardiac surgical patients, USCOM estimations of height-derived aortic and pulmonary valve area showed poor correlation with echocardiographic measurements, and failure to obtain Doppler readings occurred in nearly 25% of patients.
      • Van den Oever H.L.
      • Murphy E.J.
      • Christie-Taylor G.A.
      USCOM (Ultrasonic Cardiac Output Monitors) lacks agreement with thermodilution cardiac output and transoesophageal echocardiography valve measurements.
      Transesophageal echocardiography (TEE) is the standard of care for intraoperative imaging of valvular repairs and for evaluation of ventricular function, and is often used to evaluate the unstable patient in the ICU who has limited acoustic windows. Continuous monitoring has been made possible with miniature TEE probes (ie, transoral miniaturized hemodynamic TEE) and intensive care providers can be trained to interpret TEE, allowing continuous direct visualization of biventricular function and preload. Anatomic detail, however, requires a formal evaluation by a certified echocardiographer and this mode of monitoring is invasive, and is better suited for complex patients requiring mechanical ventilation postoperatively.
      • Sarosiek K.
      • Kang C.Y.
      • Johnson C.M.
      • Pitcher H.
      • Hirose H.
      • Cavarocchi N.C.
      Perioperative use of the IMACOR hemodynamic transesophageal echocardiography probe in cardiac surgery patients: initial experience.
      In medical ICUs, transthoracic echocardiography (TTE) has shown great utility in the diagnosis of structural heart defects, evaluation of postoperative hypotension, or in the investigation of strokes. TTE estimation of CO in surgical patients, but not after cardiac surgery, was found to have a percentage error of 40% compared with PAC.
      • Olivieri P.P.
      • Patel R.
      • Kolb S.
      • Fatima S.
      • Galvagno S.M.
      • J Haase D.J.
      • et al.
      Echo is a good, not perfect, measure of cardiac output in critically ill surgical patients.
      Accuracy is limited by the inability to measure the LV outflow tract diameter. In mechanically ventilated patients in the ICU, TTE had a lower percentage error: 25%, and a concordance rate of 94% when compared with thermodilution-estimated CO,
      • Mercado P.
      • Maizel J.
      • Beyls C.
      • Titeca-Beauport D.
      • Joris M.
      • Kontar L.
      • et al.
      Transthoracic echocardiography: an accurate and precise method for estimating cardiac output in the critically ill patient.
      but serial examinations are labor intensive and impractical. However, with the advent point of care or handheld ultrasound, clinicians can evaluate an acute problem in a time sensitive fashion at the bedside. This is particularly helpful in the context of heart failure, shock, cardiac arrest, and tamponade and to evaluate biventricular function and volume status in the context of respiratory failure and sepsis. Newer technology uses mobile application-based technology. Furthermore, artificial intelligence and telerobotic addition to point-of-care technology may optimize acquisition and interpretation of data. Further studies are needed to evaluate the benefits and accuracy of handheld ultrasound in monitoring of cardiac surgical patients.

      Less-Common Alternatives

      Partial carbon dioxide rebreathing technique studies were mostly performed in the early 2000s. Noninvasive cardiac output monitor was used for determination of CO in sedated, mechanically ventilated subjects following elective cardiac surgery with excellent accuracy (bias of 0.050 L/min) when compared with other studies involving simulation of pulmonary pathophysiology.
      • Binder J.C.
      • Parkin W.G.
      Non-invasive cardiac output determination: comparison of a new partial-rebreathing technique with thermodilution.
      The mean percentage error from thermodilution-estimated CO of the method is 45%.
      • Peyton P.J.
      • Chong S.W.
      Minimally invasive measurement of cardiac output during surgery and critical care: a meta-analysis of accuracy and precision.
      However, noninvasive cardiac output monitor requires specialized equipment and familiarity with the system, and low tidal volumes, low minute ventilation, and spontaneous breathing are limiting factors.
      • Tachibana K.
      • Imanaka H.
      • Miyano H.
      • Takeuchi M.
      • Kumon K.
      • Nishimura M.
      Effect of ventilatory settings on accuracy of cardiac output measurement using partial CO(2) rebreathing.
      ,
      • Tachibana K.
      • Imanaka H.
      • Takeuchi M.
      • Takauchi Y.
      • Miyano H.
      • Nishimura M.
      Noninvasive cardiac output measurement using partial carbon dioxide rebreathing is less accurate at settings of reduced minute ventilation and when spontaneous breathing is present.
      Bioimpedance systems are based on the principle that the electrical resistance of blood changes with movement and fluctuations in volume. In patients undergoing elective cardiac surgery, the endotracheal cardiac output monitor system has shown percentage errors of 40% to 53% and concordance rates of 87% to 100% when compared with thermodilution reference methods.
      • Maass S.W.
      • Roekaerts P.M.
      • Lancé M.D.
      Cardiac output measurement by bioimpedance and noninvasive pulse contour analysis compared with the continuous pulmonary artery thermodilution technique.
      ,
      • Maus T.M.
      • Reber B.
      • Banks D.A.
      • Berry A.
      • Guerrero E.
      • Manecke G.R.
      Cardiac output determination from endotracheally measured impedance cardiography: clinical evaluation of endotracheal cardiac output monitor.
      ,
      • Ball T.R.
      • Culp B.C.
      • Patel V.
      • Gloyna D.F.
      • Ciceri D.P.
      • Culp Jr., W.C.
      Comparison of the endotracheal cardiac output monitor to thermodilution in cardiac surgery patients.
      In a meta-analysis, 13 studies conducted with postsurgical and critically ill subjects resulted in a mean percentage error of 43% for transthoracic electrical bioimpedance devices.
      • Peyton P.J.
      • Chong S.W.
      Minimally invasive measurement of cardiac output during surgery and critical care: a meta-analysis of accuracy and precision.
      This system is influenced by mode of ventilation, thoracic fluid content, movement, arrhythmias, low flow states, and electrocautery.
      PWTT integrates data from basic instruments (eg, electrocardiogram and pulse oximetry) to estimate continuous cardiac output with percentage error ranges between 41% and 64% and concordance rates of 76% when compared with thermodilution measured intra- and postoperatively after cardiac surgery.
      • Smetkin A.A.
      • Hussain A.
      • Fot E.V.
      • Zakharov V.I.
      • Izotova N.N.
      • Yudina A.S.
      • et al.
      Estimated continuous cardiac output based on pulse wave transit time in off-pump coronary artery bypass grafting: a comparison with transpulmonary thermodilution.
      ,
      • Ball T.R.
      • Tricinella A.P.
      • Kimbrough B.A.
      • Luna S.
      • Gloyna D.F.
      • Villamaria F.J.
      • et al.
      Accuracy of noninvasive estimated continuous cardiac output (esCCO) compared to thermodilution cardiac output: a pilot study in cardiac patients.
      PWTT systems are noninvasive, easily interpreted, and relatively inexpensive. However, different ventilatory settings and maneuvers tend to decrease concordance rates.
      • Thonnerieux M.
      • Alexander B.
      • Binet C.
      • Obadia J.F.
      • Bastien O.
      • Desebbe O.
      The ability of esCCO and ECOM monitors to measure trends in cardiac output during alveolar recruitment maneuver after cardiac surgery: a comparison with the pulmonary thermodilution method.

      Surrogates of CO

      Assuring adequate end organ perfusion after cardiac surgery through maintenance of appropriate hemodynamic goals is essential for optimizing postoperative outcomes, and perfusion markers such as blood arterial lactate and central venous oxygen saturation may serve as important tools in the guidance of cardiothoracic critical care management. In a prospective randomized clinic trial of 502 patients,
      • Hajjar L.A.
      • Almeida J.P.
      • Fukushima J.T.
      • Rhodes A.
      • Vincent J.L.
      • Osawa E.A.
      • et al.
      High lactate levels are predictors of major complications after cardiac surgery.
      arterial lactate levels >3 mmol/L at 6 hours after ICU admission were an independent risk factor for major complications including acute kidney injury, cardiogenic shock, acute respiratory lung disease, and mortality in adult patients after cardiac surgery. Lactate is also a strong predictor of mortality during extracorporeal life support after failure to wean from cardiopulmonary bypass following surgery
      • Park S.J.
      • Kim S.P.
      • Kim J.B.
      • Jung S.H.
      • Choo S.J.
      • Chung C.H.
      • et al.
      Blood lactate level during extracorporeal life support as a surrogate marker for survival.
      but is nonspecific: tissue hypoxia (eg, sepsis, compartment syndrome, hepatic insufficiency, mesenteric ischemia) as well as nonhypoxic causes (eg, hypothermia, drug therapy) can lead to lactate elevation. Central venous oxygen saturation is another commonly used marker of adequate cardiocirculatory function and both low (<60%) and “supranormal” (>80%) results are associated with increased in-hospital mortality, 3-year mortality, postoperative hemodialysis, and prolonged hospital LOS in the context of cardiac surgery.
      • Balzer F.
      • Sander M.
      • Simon M.
      • Spies C.
      • Habicher M.
      • Treskatsch S.
      • et al.
      High central venous saturation after cardiac surgery is associated with increased organ failure and long-term mortality: an observational cross-sectional study.
      ,
      • Perz S.
      • Uhlig T.
      • Kohl M.
      • Bredle D.L.
      • Reinhart K.
      • Bauer M.
      • et al.
      Low and “supranormal” central venous oxygen saturation and markers of tissue hypoxia in cardiac surgery patients: a prospective observational study.
      Such variables to monitor hemodynamic parameters are best used in addition to other monitoring strategies and adjustments must be weighed against the confidence in which the clinician places in their understanding of the patient's pathophysiologic presentation.

      Summary

      Until recently, PACs have been used routinely in cardiovascular critical care in a large proportion of centers surveyed
      • Judge O.
      • Ji F.
      • Fleming N.
      • Liu H.
      Current use of the pulmonary artery catheter in cardiac surgery: a survey study.
      despite evidence demonstrating a lack of clinical benefit in routine cardiac surgery. This may be attributed to ICU models and staffing, and the perceived high reliability of the data. Overtreatment has been demonstrated in many studies using PACs, with negative impact on mechanical ventilation time, fluid balance, postoperative transfusions, ICU LOS, complications, and mortality. A majority of elective cardiac surgical procedures can be performed without a PAC, however; complex multivalve operations, heart failure surgery, heart transplantation, and emergent presentation with shock may warrant invasive monitoring with a PAC.
      Introducing new technologies or a change in practice requires a clinical champion and a commitment to staff education. Alternative noninvasive monitoring strategies described herein, in conjunction with biomarkers, provide excellent surrogates to clinicians. Recent interest in enhanced recovery after cardiac surgery and value-based care may shift the care paradigm. Selective PAC and reducing duration of invasive lines while maintaining monitoring capacity, may accelerate mobilization, prevent excessive fluid administration, transfusions, and accrued ICU postoperative morbidity. A shift to noninvasive miniaturized novel technology could allow patients to be monitored in more adapted ward setting to continue optimizing their recovery. In addition, predictive analytics, technological advancements in robotics, as well as and machine learning algorithms have improved image acquisition, and may further aid clinicians in the interpretation of data, as they get more sophisticated. Finally, improved communication systems and critical care staffing models with virtual support from experts evaluated during the coronavirus disease 2019 pandemic,
      • Igra A.
      • McGuire H.
      • Naldrett I.
      • Cervera-Jackson R.
      • Lewis R.
      • Morgan C.
      • et al.
      Rapid deployment of virtual ICU support during the COVID-19 pandemic.
      ,
      • Dhala A.
      • Sasangohar F.
      • Kash B.
      • Ahmadi N.
      • Masud F.
      Rapid implementation and innovative applications of a virtual intensive care unit during the COVID-19 pandemic: case study.
      are examples of innovation in perioperative care systems and give us an opportunity to reconsider our relationship with technology.

      Conflict of Interest Statement

      The authors reported no conflicts of interest.
      The Journal policy requires editors and reviewers to disclose conflicts of interest and to decline handling or reviewing manuscripts for which they may have a conflict of interest. The editors and reviewers of this article have no conflicts of interest.

      References

        • Swan H.J.
        • Ganz W.
        • Forrester J.
        • Marcus H.
        • Diamond G.
        • Chonette D.
        Catheterization of the heart in man with use of a flow-directed balloon-tipped catheter.
        N Engl J Med. 1970; 283: 447-451
        • Judge O.
        • Ji F.
        • Fleming N.
        • Liu H.
        Current use of the pulmonary artery catheter in cardiac surgery: a survey study.
        J Cardiothorac Vasc Anesth. 2015; 29: 69-75
        • Shah M.R.
        • Vic Hasselblad V.
        • Stevenson L.W.
        • Binanay C.
        • O'Connor C.M.
        • Sopko G.
        • et al.
        Impact of the pulmonary artery catheter in critically ill patients: meta-analysis of randomized clinical trials.
        JAMA. 2005; 294: 1664-1670
        • Joseph C.
        • Garrubba M.
        • Smith J.A.
        • Melder A.
        Does the use of a pulmonary artery catheter make a difference during or after cardiac surgery?.
        Heart Lung Circ. 2018; 27: 952-960
        • Chiang Y.
        • Hosseinian L.
        • Rhee A.
        • Itagaki S.
        • Cavallaro P.
        • Chikwe J.
        Questionable benefit of the pulmonary artery catheter after cardiac surgery in high-risk patients.
        J Cardiothorac Vasc Anesth. 2015; 29: 76-81
        • Brown J.A.
        • Aranda-Michel E.
        • Kilic A.
        • Serna-Gallegos D.
        • Bianco V.
        • Thoma F.W.
        • et al.
        The impact of pulmonary artery catheter use in cardiac surgery.
        J Thorac Cardiovasc Surg. February 2, 2021; ([Epub ahead of print])
        • Schwann N.M.
        • Hillel Z.
        • Hoeft A.
        • Barash P.
        • Möhnle P.
        • Miao Y.
        • et al.
        Lack of effectiveness of the pulmonary artery catheter in cardiac surgery.
        Anesth Analg. 2011; 113: 994-1002
        • Shaw A.D.
        • Mythen M.G.
        • Shook D.
        • Hayashida D.K.
        • Zhang X.
        • Skaar J.R.
        • et al.
        Pulmonary artery catheter use in adult patients undergoing cardiac surgery: a retrospective, cohort study.
        Perioper Med (Lond). 2018; 7: 24
        • Saxena A.
        • Garan A.R.
        • Kapur N.K.
        • O'Neill W.W.
        • Lindenfeld J.
        • Pinney S.P.
        • et al.
        Value of hemodynamic monitoring in patients with cardiogenic shock undergoing mechanical circulatory support.
        Circulation. 2020; 141: 1184-1197
        • Ranucci M.
        Which cardiac surgical patients can benefit from placement of a pulmonary artery catheter?.
        Crit Care. 2006; 10: S6
        • Baylor P.
        Lack of agreement between thermodilution and Fick methods in the measurement of cardiac output.
        J Intensive Care Med. 2006; 21: 93-98
        • Thiele R.H.
        • Bartels K.
        • Gan T.J.
        Cardiac output monitoring: a contemporary assessment and review.
        Crit Care Med. 2015; 43: 177-185
        • Kenaan M.
        • Gajera M.
        • Goonewardena S.N.
        Hemodynamic assessment in the contemporary intensive care unit: a review of circulatory monitoring devices.
        Crit Care Clin. 2014; 30: 413-445
        • Critchley L.A.
        • Critchley J.A.
        A meta-analysis of studies using bias and precision statistics to compare cardiac output measurement techniques.
        J Clin Monit Comput. 1999; 15: 85-91
        • Asamoto M.
        • Orii R.
        • Otsuji M.
        • Bougaki M.
        • Imai Y.
        • Yamada Y.
        Reliability of cardiac output measurements using LiDCOrapid™ and FloTrac/Vigileo™ across broad ranges of cardiac output values.
        J Clin Monit Comput. 2017; 31: 709-716
        • Barile L.
        • Landoni G.
        • Pieri M.
        • Ruggeri L.
        • Maj G.
        • Nigro Neto C.
        • et al.
        Cardiac index assessment by the pressure recording analytic method in critically ill unstable patients after cardiac surgery.
        J Cardiothorac Vasc Anesth. 2013; 27: 1108-1113
        • Romagnoli S.
        • Romano S.M.
        • Bevilacqua S.
        • Ciappi F.
        • Lazzeri C.
        • Peris A.
        • et al.
        Cardiac output by arterial pulse contour: reliability under hemodynamic derangements.
        Interact Cardiovasc Thorac Surg. 2009; 8: 642-646
        • Maeda T.
        • Hamaguchi E.
        • Kubo N.
        • Shimokawa A.
        • Kanazawa H.
        • Ohnishi Y.
        The accuracy and trending ability of cardiac index measured by the fourth-generation FloTrac/Vigileo system™ and the Fick method in cardiac surgery patients.
        J Clin Monit Comput. 2019; 33: 767-776
        • Missant C.
        • Rex S.
        • Wouters P.F.
        Accuracy of cardiac output measurements with pulse contour analysis (PulseCO) and Doppler echocardiography during off-pump coronary artery bypass grafting.
        Eur J Anaesthesiol. 2008; 25: 243-248
        • Greiwe G.
        • Peters V.
        • Hapfelmeier A.
        • Romagnoli S.
        • Kubik M.
        • Saugel B.
        Cardiac output estimation by multi-beat analysis of the radial arterial blood pressure waveform versus intermittent pulmonary artery thermodilution: a method comparison study in patients treated in the intensive care unit after off-pump coronary artery bypass surgery.
        J Clin Monit Comput. 2020; 34: 643-648
        • Saugel B.
        • Heeschen J.
        • Hapfelmeier A.
        • Romagnoli S.
        • Greiwe G.
        Cardiac output estimation using multi-beat analysis of the radial arterial blood pressure waveform: a method comparison study in patients having off-pump coronary artery bypass surgery using intermittent pulmonary artery thermodilution as the reference method.
        J Clin Monit Comput. 2020; 34: 649-654
        • Wagner J.Y.
        • Körner A.
        • Schulte-Uentrop L.
        • Kubik M.
        • Reichenspurner H.
        • Kluge S.
        • et al.
        A comparison of volume clamp method-based continuous noninvasive cardiac output (CNCO) measurement versus intermittent pulmonary artery thermodilution in postoperative cardiothoracic surgery patients.
        J Clin Monit Comput. 2018; 32: 235-244
        • Maass S.W.
        • Roekaerts P.M.
        • Lancé M.D.
        Cardiac output measurement by bioimpedance and noninvasive pulse contour analysis compared with the continuous pulmonary artery thermodilution technique.
        J Cardiothorac Vasc Anesth. 2014; 28: 534-539
        • Wagner J.Y.
        • Sarwari H.
        • Schön G.
        • Kubik M.
        • Kluge S.
        • Reichenspurner H.
        • et al.
        Radial artery applanation tonometry for continuous noninvasive cardiac output measurement: a comparison with intermittent pulmonary artery thermodilution in patients after cardiothoracic surgery.
        Crit Care Med. 2015; 43: 1423-1428
        • Broch O.
        • Renner J.
        • Gruenewald M.
        • Meybohm P.
        • Schöttler J.
        • Caliebe A.
        • et al.
        A comparison of the Nexfin® and transcardiopulmonary thermodilution to estimate cardiac output during coronary artery surgery.
        Anaesthesia. 2012; 67: 377-383
        • Peyton P.J.
        • Chong S.W.
        Minimally invasive measurement of cardiac output during surgery and critical care: a meta-analysis of accuracy and precision.
        Anesthesiology. 2010; 113: 1220-1235
        • Hatib F.
        • Jian Z.
        • Buddi S.
        • Lee C.
        • Settels J.
        • Sibert K.
        • et al.
        Machine-learning algorithm to predict hypotension based on high-fidelity arterial pressure waveform analysis.
        Anesthesiology. 2018; 129: 663-674
        • Shin B.
        • Maler S.A.
        • Reddy K.
        • Fleming N.W.
        Use of the hypotension prediction index during cardiac surgery.
        J Cardiothorac Vasc Anesth. 2021; 35: 1769-1775
        • Collins S.
        • Girard F.
        • Boudreault D.
        • Chouinard P.
        • Normandin L.
        • Couture P.
        • et al.
        Esophageal Doppler and thermodilution are not interchangeable for determination of cardiac output.
        Can J Anaesth. 2005; 52: 978-985
        • Sharma J.
        • Bhise M.
        • Singh A.
        • Mehta Y.
        • Trehan N.
        Hemodynamic measurements after cardiac surgery: transesophageal Doppler versus pulmonary artery catheter.
        J Cardiothorac Vasc Anesth. 2005; 19: 746-750
        • Jaeggi P.
        • Hofer C.K.
        • Klaghofer R.
        • Fodor P.
        • Genoni M.
        • Zollinger A.
        Measurement of cardiac output after cardiac surgery by a new transesophageal Doppler device.
        J Cardiothorac Vasc Anesth. 2003; 17: 217-220
        • Chong S.W.
        • Peyton P.J.
        A meta-analysis of the accuracy and precision of the ultrasonic cardiac output monitor (USCOM).
        Anaesthesia. 2012; 67: 1266-1271
        • Van den Oever H.L.
        • Murphy E.J.
        • Christie-Taylor G.A.
        USCOM (Ultrasonic Cardiac Output Monitors) lacks agreement with thermodilution cardiac output and transoesophageal echocardiography valve measurements.
        Anaesth Intensive Care. 2007; 35: 903-910
        • Sarosiek K.
        • Kang C.Y.
        • Johnson C.M.
        • Pitcher H.
        • Hirose H.
        • Cavarocchi N.C.
        Perioperative use of the IMACOR hemodynamic transesophageal echocardiography probe in cardiac surgery patients: initial experience.
        ASAIO J. 2014; 60: 553-558
        • Olivieri P.P.
        • Patel R.
        • Kolb S.
        • Fatima S.
        • Galvagno S.M.
        • J Haase D.J.
        • et al.
        Echo is a good, not perfect, measure of cardiac output in critically ill surgical patients.
        J Trauma Acute Care Surg. 2019; 87: 379-385
        • Mercado P.
        • Maizel J.
        • Beyls C.
        • Titeca-Beauport D.
        • Joris M.
        • Kontar L.
        • et al.
        Transthoracic echocardiography: an accurate and precise method for estimating cardiac output in the critically ill patient.
        Crit Care. 2017; 21: 136
        • Binder J.C.
        • Parkin W.G.
        Non-invasive cardiac output determination: comparison of a new partial-rebreathing technique with thermodilution.
        Anaesth Intensive Care. 2001; 29: 19-23
        • Tachibana K.
        • Imanaka H.
        • Miyano H.
        • Takeuchi M.
        • Kumon K.
        • Nishimura M.
        Effect of ventilatory settings on accuracy of cardiac output measurement using partial CO(2) rebreathing.
        Anesthesiology. 2002; 96: 96-102
        • Tachibana K.
        • Imanaka H.
        • Takeuchi M.
        • Takauchi Y.
        • Miyano H.
        • Nishimura M.
        Noninvasive cardiac output measurement using partial carbon dioxide rebreathing is less accurate at settings of reduced minute ventilation and when spontaneous breathing is present.
        Anesthesiology. 2003; 98: 830-837
        • Maus T.M.
        • Reber B.
        • Banks D.A.
        • Berry A.
        • Guerrero E.
        • Manecke G.R.
        Cardiac output determination from endotracheally measured impedance cardiography: clinical evaluation of endotracheal cardiac output monitor.
        J Cardiothorac Vasc Anesth. 2011; 25: 770-775
        • Ball T.R.
        • Culp B.C.
        • Patel V.
        • Gloyna D.F.
        • Ciceri D.P.
        • Culp Jr., W.C.
        Comparison of the endotracheal cardiac output monitor to thermodilution in cardiac surgery patients.
        J Cardiothorac Vasc Anesth. 2010; 24: 762-766
        • Smetkin A.A.
        • Hussain A.
        • Fot E.V.
        • Zakharov V.I.
        • Izotova N.N.
        • Yudina A.S.
        • et al.
        Estimated continuous cardiac output based on pulse wave transit time in off-pump coronary artery bypass grafting: a comparison with transpulmonary thermodilution.
        J Clin Monit Comput. 2017; 31: 361-370
        • Ball T.R.
        • Tricinella A.P.
        • Kimbrough B.A.
        • Luna S.
        • Gloyna D.F.
        • Villamaria F.J.
        • et al.
        Accuracy of noninvasive estimated continuous cardiac output (esCCO) compared to thermodilution cardiac output: a pilot study in cardiac patients.
        J Cardiothorac Vasc Anesth. 2013; 27: 1128-1132
        • Thonnerieux M.
        • Alexander B.
        • Binet C.
        • Obadia J.F.
        • Bastien O.
        • Desebbe O.
        The ability of esCCO and ECOM monitors to measure trends in cardiac output during alveolar recruitment maneuver after cardiac surgery: a comparison with the pulmonary thermodilution method.
        Anesth Analg. 2015; 121: 383-391
        • Hajjar L.A.
        • Almeida J.P.
        • Fukushima J.T.
        • Rhodes A.
        • Vincent J.L.
        • Osawa E.A.
        • et al.
        High lactate levels are predictors of major complications after cardiac surgery.
        J Thorac Cardiovasc Surg. 2013; 146: 455-460
        • Park S.J.
        • Kim S.P.
        • Kim J.B.
        • Jung S.H.
        • Choo S.J.
        • Chung C.H.
        • et al.
        Blood lactate level during extracorporeal life support as a surrogate marker for survival.
        J Thorac Cardiovasc Surg. 2014; 148: 714-720
        • Balzer F.
        • Sander M.
        • Simon M.
        • Spies C.
        • Habicher M.
        • Treskatsch S.
        • et al.
        High central venous saturation after cardiac surgery is associated with increased organ failure and long-term mortality: an observational cross-sectional study.
        Crit Care. 2015; 19: 168
        • Perz S.
        • Uhlig T.
        • Kohl M.
        • Bredle D.L.
        • Reinhart K.
        • Bauer M.
        • et al.
        Low and “supranormal” central venous oxygen saturation and markers of tissue hypoxia in cardiac surgery patients: a prospective observational study.
        Intensive Care Med. 2011; 37: 52-59
        • Igra A.
        • McGuire H.
        • Naldrett I.
        • Cervera-Jackson R.
        • Lewis R.
        • Morgan C.
        • et al.
        Rapid deployment of virtual ICU support during the COVID-19 pandemic.
        Future Healthc J. 2020; 7: 181-184https://doi.org/10.7861/fhj.2020-0157
        • Dhala A.
        • Sasangohar F.
        • Kash B.
        • Ahmadi N.
        • Masud F.
        Rapid implementation and innovative applications of a virtual intensive care unit during the COVID-19 pandemic: case study.
        J Med Internet Res. 2020; 22: e20143https://doi.org/10.2196/20143