How is systemic vascular resistance (SVR) measured?

How to Measure Systemic Vascular Resistance (SVR)

SVR is calculated using the formula: SVR = (Mean Arterial Pressure - Right Atrial Pressure) / Cardiac Output, with the result multiplied by 80 to express it in standard units of dynes·s·cm⁻⁵. 1, 2

Direct Invasive Measurement (Gold Standard)

Right heart catheterization with pulmonary artery catheter provides the most accurate SVR assessment. 1, 2

Required Measurements

• Mean Arterial Pressure (MAP): Obtained from systemic arterial line, or calculated as (Systolic BP + 2 × Diastolic BP) / 3 3
• Right Atrial Pressure (RAP) or Central Venous Pressure (CVP): Measured via central venous catheter 4, 3
• Cardiac Output (CO): Measured in L/min, typically via thermodilution technique through pulmonary artery catheter 4, 3

Calculation Steps

1. Measure MAP from arterial line 3
2. Measure RAP/CVP from central venous catheter 3
3. Measure CO via thermodilution (inject 10-15 mL cold saline through proximal port, measure temperature change at distal thermistor) 5
4. Apply formula: SVR = (MAP - RAP) / CO 1, 2
5. Multiply result by 80 to convert to dynes·s·cm⁻⁵ 1

Normal SVR range is 800-1200 dynes·s·cm⁻⁵. 1

Non-Invasive and Continuous Monitoring Methods

Arterial Pressure Waveform Analysis

Pulse contour analysis of the peripheral arterial waveform enables continuous on-line SVR monitoring without repeated thermodilution measurements. 6, 7

• The method analyzes peak dP/dt (rate of pressure change) and pressure at peak dP/dt from the arterial waveform 6
• Requires initial calibration with thermodilution cardiac output measurement 6, 7
• Correlation with thermodilution-derived SVR is r = 0.92-0.98 6, 7
• Maintains accuracy for 24 hours without recalibration 7
• SVR values measured ranged from 450 to 4400 dynes·s·cm⁻⁵ with excellent correlation 6

Finger Arterial Pressure Waveform Analysis

Non-invasive finger arterial pressure monitoring devices can estimate SVR with acceptable concordance to invasive measurements in heart failure patients. 8

• Variability coefficient of 18% compared to Swan-Ganz catheterization 8
• Useful for continuous monitoring when invasive access is not available or desired 8

Clinical Assessment Without Direct Measurement

When invasive monitoring is unavailable, clinical signs suggest markedly elevated SVR: 1, 2

• Absent or weak distal pulses 1
• Cold extremities 1, 2
• Prolonged capillary refill time (>2 seconds) 1, 2
• Narrow pulse pressure with relatively increased diastolic blood pressure 1

Critical Technical Considerations

Measurement Timing and Conditions

• Zero the external pressure transducer at the mid-thoracic line before measurements 4, 3
• Record pressures at end-expiration during spontaneous breathing 3
• Maintain standardized conditions, as general anesthesia can lower systemic arterial blood pressure and affect calculations 3

Common Pitfalls to Avoid

• Tricuspid regurgitation with right ventricular dilatation causes erroneous thermodilution cardiac output measurements, affecting all derived SVR calculations 3
• Acute changes in SVR from vasoactive medications (e.g., increasing norepinephrine) can transiently alter measured central blood volumes, though SVR calculations remain valid 5
• Non-invasive estimates of central venous pressure may be misleading in pulmonary hypertension patients; direct central line measurement is required 3

Clinical Context for SVR Interpretation

SVR must be interpreted alongside cardiac output and blood pressure to guide therapy: 2

• Normal ventricular function + elevated SVR = hypertension with maintained cardiac output 2
• Reduced ventricular function + normal BP + high SVR = reduced cardiac output 2
• In septic shock, maintain cardiac index 3.3-6.0 L/min/m² for best outcomes (versus >2.0 L/min/m² for non-septic patients) 1, 2

In cardiovascular ICU settings, SVR must be maintained greater than pulmonary vascular resistance (PVR) to ensure adequate right ventricular coronary perfusion and prevent right ventricular ischemia. 1, 3