What are the techniques for measuring cardiac output in patients undergoing anesthesia, particularly those with pre-existing cardiovascular disease?

Cardiac Output Measurement Techniques in Anesthesia

Physiological Factors Affecting Cardiac Output During Anesthesia

Anesthetic agents systematically reduce cardiac output through myocardial depression and vasodilation, requiring proactive dose adjustment particularly in elderly patients and those with cardiovascular disease. 1

Age-Related Considerations

• Elderly patients require substantially lower doses of both inhalational and intravenous agents due to age-related pharmacodynamic changes, with failure to adjust doses resulting in prolonged hypotension and reduced cardiac output 1
• The "triple low" phenomenon (low blood pressure, low cardiac output, low bispectral index) 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

Hemodynamic Effects of Anesthesia

• Vasodilation from anesthetic agents reduces systemic vascular resistance, requiring careful titration of vasopressors once optimal intravascular volume is achieved 1
• Both general and regional anesthesia have pronounced effects on perioperative sympathetic nervous tone, with neuraxial techniques significantly decreasing sympathetic nervous influx 2
• In diabetic patients with cardiac autonomic neuropathy, interactions between anesthesia and dysautonomia lead to increased risk of perioperative haemodynamic instability 2

Invasive Cardiac Output Measurement Methods

Pulmonary Artery Catheterization (Thermodilution)

Right heart catheterization with thermodilution remains the most accurate and best validated technique for cardiac output measurement, particularly at the extremes of measurement and limits of physiology. 2, 1

Indications

• Cardiac surgery patients with complex hemodynamics requiring simultaneous assessment of pulmonary artery pressures, mixed venous oxygen saturation, and cardiac output 2, 1
• Selected cases to reduce hospital length of stay (Class IIa recommendation) 2
• Patients with severe valvular disease undergoing elevated-risk surgery requiring invasive hemodynamic monitoring 2

Contraindications

• Standard contraindications for central venous access (coagulopathy, infection at insertion site)
• Severe tricuspid regurgitation (relative contraindication)
• Left bundle branch block (risk of complete heart block)

Complications

• Arrhythmias during insertion
• Pulmonary artery rupture
• Catheter knotting
• Infection and thrombosis

Measurement Technique

• Cold saline bolus injection into right atrium with temperature change detection at pulmonary artery thermistor 3
• Requires accurate measurements of pressure and flow, particularly in patients with low cardiac output or low transvalvular pressure gradients 2

Pulse Contour Analysis

Pulse contour analysis may be considered in selected patients but is frequently inaccurate at the extremes of measurement and at the limits of physiology. 2, 4

Indications

• Cardiac surgery patients requiring continuous cardiac output monitoring (Class IIb recommendation) 2
• Patients requiring less invasive monitoring than pulmonary artery catheterization

Measurement Technique

• Uses transpulmonary bolus thermodilution measurements to calibrate the system 5
• Provides beat-to-beat cardiac output measurement with acceptable bias and precision (mean difference 0.31 ± 1.25 L/min compared to conventional thermodilution) 5

Limitations

• Requires arterial line access
• Accuracy decreases with significant arrhythmias, severe aortic regurgitation, or intra-aortic balloon pump use
• Device-specific training required as cardiac output monitoring can be technically demanding 2

Non-Invasive Cardiac Output Measurement Methods

Echocardiography

Echocardiography is recommended as the initial diagnostic study in the setting of shock because it provides both cardiac output measurement and differential diagnosis of the underlying etiology. 2, 1

Transthoracic Echocardiography (TTE)

• Provides dynamic and functional assessment of cardiac output 6
• Cardiac output calculated at left ventricular outflow tract (LVOT) using velocity-time integral and cross-sectional area 6
• Less invasive alternative to thermodilution but operator-dependent 2

Transesophageal Echocardiography (TEE)

• Recommended in cardiac surgery procedures unless there is an absolute contraindication (Class I recommendation) 2
• Measurements can be obtained at mitral annulus or LVOT 6
• Critical caveat: TEE measurements at mitral annulus show poor concordance with thermodilution (Lin concordance = 0.071), while LVOT measurements show better but still limited concordance (Lin concordance = 0.232) 6

Contraindications for TEE

• Esophageal pathology (stricture, varices, tumor, recent surgery)
• Severe cervical spine instability
• Active upper gastrointestinal bleeding

Esophageal Doppler Monitoring

• May be less accurate in elderly patients due to poor aortic compliance, potentially overestimating cardiac output and resulting in insufficient fluid resuscitation 1
• Requires proper probe positioning and operator experience

Other Non-Invasive Methods

• Transthoracic impedance and combined Doppler ultrasound techniques are available but have limited validation in anesthesia practice 7
• Paucity of data exists around cardiac output monitoring in specialist populations, particularly pediatrics and obstetrics 2

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 to optimize cardiac output and reduce morbidity and mortality. 1

Stroke Volume (SV) and Stroke Volume Variation (SVV)

• Use stroke volume as a guide to resuscitation and vasopressor use to reduce unnecessary fluid overload and improve outcomes 1
• Stroke volume variation predicts fluid responsiveness in mechanically ventilated patients 1
• Optimize flow before commencing vasopressors 1

Systemic Vascular Resistance (SVR) and Systemic Vascular Resistance Index (SVRI)

• SVR guides vasopressor versus inotrope selection 1
• Low SVR with adequate cardiac output suggests need for vasopressors 1
• High SVR with low cardiac output suggests need for afterload reduction 1
• Normal SVR = 800-1200 dynes·sec·cm⁻⁵; SVRI adjusts for body surface area

Left Ventricular Stroke Work (LVSW)

• Reflects myocardial contractility and afterload
• Useful in assessing ventricular performance independent of preload
• Calculated as: LVSW = SV × (MAP - PCWP) × 0.0136

Pulmonary Vascular Resistance (PVR) and Pulmonary Vascular Resistance Index (PVRI)

• Essential in patients with pulmonary hypertension or right ventricular dysfunction
• PVR = (Mean PAP - PCWP) / Cardiac Output
• Normal PVR = 20-120 dynes·sec·cm⁻⁵
• Elevated PVR guides pulmonary vasodilator therapy

Practical Management Algorithm for Patients with Cardiovascular Disease

Preoperative Assessment

• Consider arterial lines and/or central venous pressure catheters at an early stage in elderly unwell patients to aid physiological assessment and deliver vasopressors 1
• Patients with severe valvular stenosis require invasive hemodynamic monitoring for elevated-risk surgery 2

Intraoperative Management

• Maintain mean arterial pressure 60-65 mmHg during surgery to reduce end-organ injury 1
• Administer fluid therapy with great care in divided boluses to assess response 1
• Norepinephrine is first-line vasopressor for sepsis or vasodilatory shock 1

Monitoring Strategy Based on Risk

• Low-risk surgery: Standard ASA monitoring sufficient
• Intermediate-risk surgery with cardiovascular comorbidity: Consider arterial line and advanced hemodynamic monitoring 2
• High-risk surgery or severe cardiovascular disease: Pulmonary artery catheterization or pulse contour analysis with TEE 2

Common Pitfalls to Avoid

• Do not rely solely on central venous pressure for volume assessment in elderly patients with poorly compliant ventricles 1
• Avoid excessive fluid administration (>3 L in first 24 hours after lung resection increases acute lung injury risk with mortality up to 50%) 4
• Ensure device-specific training as cardiac output monitoring is technically demanding 2
• Verify critical measurements manually rather than relying on unvalidated digital displays at extremes of measurement 4