Cardiac Output Measurement Techniques in Anesthesia
Physiological Factors Affecting Cardiac Output During Anesthesia
Anesthetic agents reduce cardiac output through multiple mechanisms that must be anticipated and managed proactively. 1
- Age-related pharmacodynamic changes decrease anesthetic requirements progressively, with elderly patients requiring substantially lower doses of both inhalational and intravenous agents to achieve the same depth of anesthesia 1
- Relative overdosing occurs commonly when doses are not adjusted for age, resulting in prolonged significant hypotension and reduced cardiac output 1
- The "triple low" phenomenon (low BIS, hypotension despite low inspired agent concentration) is associated with higher mortality and prolonged hospital stay, reflecting excessive myocardial depression 1
- Poorly compliant ventricles and vasculature in elderly patients make traditional preload assessment unreliable, as central venous pressure correlates poorly with blood volume and fluid responsiveness 1
- Vasodilation from anesthetic agents reduces systemic vascular resistance, requiring careful titration of vasopressors once optimal intravascular volume is achieved 1
Invasive Cardiac Output Measurement Methods
Pulmonary Artery Catheter (PAC) - Thermodilution
Right heart catheterization with thermodilution remains the most accurate and best validated technique for cardiac output measurement, particularly in cardiac surgery patients. 1, 2
Indications:
- Cardiac surgery with complex hemodynamics 1
- Patients requiring simultaneous assessment of pulmonary artery pressures, mixed venous oxygen saturation, and cardiac output 2
- Cardiac intensive care unit patients with structural heart disease 2
Contraindications:
- Routine use in low-risk patients undergoing off-pump CABG (no mortality benefit demonstrated) 1
- Octogenarian patients (propensity-matched analysis showed OR 1.24 for mortality, 95% CI 1.03-1.50) 1
- High-risk patients in general cardiac surgery (OR 1.30 for mortality, 95% CI 1.14-1.48) 1
Complications:
- Arrhythmias during insertion 1
- Pulmonary embolism or hemorrhage 1
- Technical errors including unreliable data or false interpretation 1
Measurement Technique:
- Cold saline bolus injection into right atrium with temperature change detection at pulmonary artery thermistor 2, 3
- Provides continuous mixed venous oxygen saturation monitoring 1
- Allows calculation of derived hemodynamic parameters 3
Arterial Waveform Analysis (Pulse Contour Analysis)
Minimally invasive cardiac output devices using pulse contour analysis are frequently inaccurate at the extremes of measurement and at the limits of physiology. 1, 4
Indications:
- Intraoperative monitoring when PAC is not indicated 1
- Patients requiring frequent arterial blood gas analysis 1
- Early detection of hypotension in high-risk patients 1
Limitations:
- Poor agreement with PAC (mean percentage error 41%, exceeding the accepted 30% threshold) 1
- Particularly inaccurate during CPB weaning with hemodynamic instability, temperature changes, and alterations in vascular tone 1
- No clinical outcome studies validating their use 1
- Requires arterial line calibration and may drift over time 5
Measurement Technique:
- Algorithm-based calculation from arterial waveform characteristics 1
- Provides stroke volume, stroke volume variation (SVV), pulse pressure variation (PPV), and cardiac index 1
- SVV calculation: [(SVmax - SVmin) / SVmean] × 100 during mechanical ventilation 6
Transpulmonary Thermodilution
This technique provides accurate cardiac output measurement with less invasiveness than PAC. 3, 7
Measurement Technique:
- Cold indicator injection via central venous catheter with detection at femoral or axillary arterial thermistor 3
- Based on simple physical principles with minimal assumptions 7
- More reliable than mathematical modeling-based devices 7
Non-Invasive Cardiac Output Measurement Methods
Transthoracic/Transesophageal Echocardiography
Echocardiography is recommended as the initial diagnostic study in the setting of shock and provides both cardiac output measurement and differential diagnosis. 1
Indications for Intraoperative TEE:
- All open-heart (valvular) and thoracic aortic procedures 1
- Selected CABG operations, particularly with acute life-threatening hemodynamic instability 1
- Assessment of ventricular contractility, structural abnormalities, and surgical results 1
Contraindications:
- Esophageal pathology (stricture, varices, tumor, recent surgery) 1
- Routine use requires reconsideration given national audit data showing major complications in 0.08% and mortality in 0.03% of cases 1
Measurement Technique:
- Stroke Volume = LVOT Cross-Sectional Area × LVOT VTI 6
- LVOT diameter measured mid-systole from parasternal long-axis view, inner-edge to inner-edge 6
- CSA = π × (LVOT diameter)² / 4 6
- VTI obtained by tracing Doppler flow signal with sample volume 0.5 cm proximal to valve 6
- Cardiac Output = Stroke Volume × Heart Rate 3
Limitations:
- High operator dependency 5
- Requires specific training and competence 1
- Spontaneous breathing or inadequate ventilator synchronization invalidates SVV measurement 6
- Improper Doppler beam alignment underestimates VTI and stroke volume 6
Esophageal Doppler Monitoring
Doppler directed at the aorta may be less accurate in elderly patients due to poor aortic compliance, potentially overestimating cardiac output and resulting in insufficient fluid resuscitation. 1
Indications:
- NICE guidelines recommend consideration for major or high-risk surgery 1
- Limited evidence in elderly and emergency surgery populations 1
Measurement Technique:
- Continuous wave Doppler measurement of descending aortic blood flow 3
- Provides stroke volume and cardiac output estimates 5
CO₂ Rebreathing Methods
Foreign gas rebreathing using acetylene or nitrous oxide has been shown to be reliable and safe for noninvasive cardiac output assessment during exercise, but requires patient cooperation. 1
Limitations:
- Requires subject cooperation, difficult in some patients 1
- High CO₂ concentrations may cause lightheadedness or feelings of suffocation 1
- Accuracy compromised in advanced pulmonary disease 1
- Not routinely used in anesthesia practice 1
Bioimpedance and Bioreactance
These techniques provide non-invasive continuous monitoring but are primarily used in investigational settings. 3, 5
Measurement Technique:
- Electrical impedance changes across thorax correlate with stroke volume 3
- Bioreactance measures phase shifts in electrical currents 3
Limitations:
Clinical Significance of Derived Hemodynamic Values
Cardiac Index (CI)
Maintain Cardiac Index ≥ 2.2 L/min/m² individualized to the patient during surgery using appropriate vasopressors and inotropes. 1
- Normalizes cardiac output to body surface area 3
- More meaningful than absolute cardiac output for comparing between patients 3
- Goal-directed hemodynamic therapy (GDHT) targets CI optimization in high-risk patients 1
Stroke Volume (SV) and Stroke Volume Variation (SVV)
Using stroke volume as a guide to resuscitation and vasopressor use is likely to reduce unnecessary fluid overload and improve outcomes. 1
- SV = Cardiac Output / Heart Rate 3
- SVV predicts fluid responsiveness in mechanically ventilated patients 6
- SVV >12-15% typically indicates fluid responsiveness 6
- Tidal volume dependency: SVV increases proportionally with tidal volume depth 6
- Maximizing stroke volume may not always be correct; avoiding hypovolemia and hypotension while ensuring adequate perfusion is key 1
Systemic Vascular Resistance (SVR) and SVR Index (SVRI)
SVR = (MAP - CVP) / CO × 80 (in dynes·sec·cm⁻⁵) 3
- Normal range: 800-1200 dynes·sec·cm⁻⁵ 3
- SVRI normalizes to body surface area 3
- Guides vasopressor vs. inotrope selection 1
- Low SVR with adequate CO suggests need for vasopressors (norepinephrine first-line) 1
- High SVR with low CO suggests need for afterload reduction 3
Left Ventricular Stroke Work (LVSW) and LVSW Index (LVSWI)
LVSW = SV × (MAP - PCWP) × 0.0136 (in gram-meters) 3
- Reflects myocardial contractility and work performed per beat 3
- LVSWI normalizes to body surface area 3
- Decreased LVSW indicates impaired contractility requiring inotropic support 3
- Useful for differentiating cardiogenic from distributive shock 3
Pulmonary Vascular Resistance (PVR) and PVR Index (PVRI)
PVR = (MPAP - PCWP) / CO × 80 (in dynes·sec·cm⁻⁵) 3
- Normal range: 20-120 dynes·sec·cm⁻⁵ 3
- PVRI normalizes to body surface area 3
- Elevated PVR indicates pulmonary hypertension requiring specific management 3
- Guides use of pulmonary vasodilators (inhaled nitric oxide, prostacyclin) 3
- Important in right ventricular failure assessment 3
Practical Recommendations for Anesthesia Practice
Arterial lines and/or central venous pressure catheters should be considered at an early stage in elderly unwell patients to aid physiological assessment and deliver vasopressors. 1
- Maintain MAP 60-65 mmHg during surgery to reduce end-organ injury 1
- Recent UK data in patients >65 years showed no mortality difference between MAP targets of 60 vs. 65 mmHg 1
- Optimize flow before commencing vasopressors 1
- Administer fluid therapy with great care in divided boluses to assess response 1
- Norepinephrine is first-line vasopressor for sepsis or vasodilatory shock, safe via large peripheral vein until central access established 1
- Consider bedside transthoracic echocardiography for hemodynamically unstable patients to assess contractility and guide inotrope/vasopressor use 1
- GDHT should be considered during surgery in high-risk patients to optimize cardiac index 1
Critical Pitfalls to Avoid
- Do not rely on CVP alone for volume assessment in elderly patients with poorly compliant ventricles 1
- Verify critical values with direct measurement rather than unvalidated digital displays 4
- Avoid routine PAC use in low-risk or elderly patients given mortality data 1
- Do not assume pulse contour analysis accuracy during hemodynamic instability or CPB weaning 1
- Ensure adequate training before using any cardiac output monitoring device 1
- Adjust anesthetic doses for age to prevent the "triple low" phenomenon 1