Lessons from clinical practice and the evidence base
Accurate cardiac output (CO) monitoring is a cornerstone of early post‑operative management following coronary artery bypass grafting (CABG). In this group of patients, small differences in haemodynamic interpretation can lead to markedly different therapeutic pathways, with important consequences for myocardial protection, fluid balance, and overall recovery.
A recent clinical experience at a London based hospital offers a valuable opportunity to reflect on why precision in CO measurement matters so much in the post‑CABG setting, and how clinicians can use monitoring data thoughtfully in complex physiological conditions.
A Clinical Scenario That Raises an Important Question
During routine post‑operative monitoring of a CABG patient, two minimally invasive cardiac output monitoring systems were connected simultaneously. Despite being derived from the same arterial waveform, the CO readings differed substantially:
- One system reported a cardiac output of 2.7 L/min
- The second system reported 4.8 L/min
These values were not just numerically different; they implied two very different physiological states. Given the potential treatment implications, the clinical team appropriately requested a consultant‑led echocardiographic assessment to clarify the patient’s true haemodynamic status.
Echocardiography estimated the cardiac output at 4.8 L/min, aligning with the second system.
This single case does not stand alone as proof, but it does highlight a broader and well‑recognised challenge: when cardiac output measurements diverge, clinical decision‑making becomes uncertain.
Why Cardiac Output is Central After CABG
In the immediate post‑CABG period, cardiac output is one of the key parameters guiding therapy. Reduced output may reflect myocardial stunning, graft‑related issues, hypovolaemia, or inappropriate afterload, while preserved output supports a more conservative, protective strategy.
A reported CO of 2.7 L/min in a post‑CABG patient would typically prompt concern for low cardiac output syndrome (LCOS). This may lead clinicians to consider:
- Initiation or escalation of inotropic support
- Fluid administration
- Increased vasopressor therapy
- Escalation of monitoring or invasive investigation
- Early concern regarding graft performance
Each of these interventions carries potential risk, particularly in a myocardium recovering from cardiopulmonary bypass.
By contrast, a CO of 4.8 L/min suggests adequate global perfusion, allowing clinicians to:
- Avoid unnecessary inotropes
- Prevent fluid overload
- Focus on myocardial protection
- Continue standard post‑operative care
In short, one number may push clinicians into resuscitation mode, while another supports a conservative approach. This is why the accuracy of cardiac output monitoring is not an academic concern but a practical clinical necessity.
Understanding Discrepancies in Pulse Contour‑Derived CO
Minimally invasive pulse contour systems offer continuous CO monitoring without the risks associated with pulmonary artery catheterisation. However, their accuracy depends on how arterial pressure waveforms are interpreted.
Different technologies use different assumptions. Some systems estimate cardiac output using population‑based or demographic corrections to account for vascular compliance and resistance. Others rely on beat‑to‑beat analysis of the arterial waveform without preloaded patient data.
This distinction becomes particularly important in post‑cardiac surgery patients, where vascular tone, compliance, and ventriculo‑arterial coupling are frequently altered by cardiopulmonary bypass, vasoactive drugs, inflammation, and surgical stress.
In these settings, reliance on assumed or averaged vascular characteristics may introduce error, particularly when systemic vascular resistance is changing rapidly.
What the Evidence Tells Us
The observations from this clinical case are consistent with findings in the published literature. A prospective observational study comparing pulse contour methods with transthoracic echocardiography in surgical patients demonstrated significant differences in agreement between technologies¹.
In that study:
- One system showed poor agreement with echocardiography, with percentage errors approaching 50%, exceeding the threshold generally considered acceptable for clinical decision‑making.
- Another system demonstrated strong correlation with echocardiography, with percentage errors between 17% and 22%, falling within accepted limits for cardiac output monitoring.
Importantly, these discrepancies were most evident in patients with altered vascular impedance, a physiology frequently encountered after cardiac and vascular surgery.
While echocardiography is not continuous and has its own limitations, it remains a valuable reference modality when there is doubt about haemodynamic measurements, particularly when treatment decisions carry significant risk.
From Numbers to Decisions: Why Precision Matters
Cardiac output monitoring does not exist in isolation. Clinicians interpret CO values alongside blood pressure, lactate, urine output, echocardiographic findings, and clinical examination. However, when a displayed CO value is significantly inaccurate, it can bias interpretation of all other parameters.
The risk in post‑CABG care is twofold:
- Over‑treatment, exposing patients to unnecessary inotropes, fluids, or vasopressors
- Under‑treatment, where true myocardial dysfunction may be masked by inaccurate reassurance
Both carry implications for myocardial oxygen demand, graft patency, fluid balance, and ICU length of stay.
Accurate monitoring supports targeted, proportionate therapy – key principles in enhanced recovery and myocardial protection strategies.
Clinical Collaboration and Ongoing Evaluation
One encouraging aspect of this experience is the collaborative response. Rather than acting immediately on a single number, the team involved senior clinical review, confirmatory imaging, and multidisciplinary discussion.
Ongoing collaboration between clinicians, clinical technologists, and physiologists has since allowed repeated comparison of cardiac output values across different patient scenarios. Such real‑world evaluation complements published data and helps teams develop confidence in the tools they use daily.
Importantly, this approach reinforces that monitoring technologies should support clinical judgement, not replace it.
Reflection: Accuracy as a Clinical, Not Technical, Issue
For clinicians caring for post‑CABG patients, cardiac output is not merely a physiological metric. It is a decision‑shaping parameter that influences therapy from the earliest hours of recovery.
Experiences such as this serve as a reminder that accuracy matters because treatment follows numbers, and numbers guide actions. Careful interpretation, awareness of technology limitations, and confirmation when values do not align with clinical context are essential components of safe haemodynamic management.
In post‑cardiac surgery care, precision is not about chasing perfection. It is about ensuring that the information guiding therapy is reliable enough to protect patients when they are most vulnerable.
References (Vancouver)
- Romagnoli S, Ricci Z, Romano SM, et al. FloTrac/Vigileo™ and MostCare®/PRAM versus echocardiography for cardiac output estimation in vascular surgery. J Cardiothorac Vasc Anesth. 2013;27(6):1113–1120. [6- FloTrac…echocardio | PDF]
