How to Measure Systemic Vascular Resistance
SVR is calculated using the formula: SVR = (MAP - RAP) / CO, where MAP is mean arterial pressure, RAP is right atrial pressure (or central venous pressure), and CO is cardiac output, with the result multiplied by 80 to express it in standard units of dynes·s·cm⁻⁵. 1, 2
Gold Standard Measurement Method
Direct invasive measurement via pulmonary artery catheter provides the most accurate assessment of SVR. 1 This requires:
- Mean arterial pressure (MAP) measured from a 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 3
- Cardiac output (CO) measured in L/min, typically via thermodilution or Fick method 3
The normal range for SVR is 800-1200 dynes·s·cm⁻⁵ (or 10-15 Wood units). 1
Simplified Estimation Without CVP Measurement
For practical clinical purposes, SVR can be estimated using a fixed CVP value of 10 mmHg without causing significant deviation or loss of trending ability. 4 This approach is particularly useful when central venous access is not available, as the clinical range of CVP (5-15 mmHg) has minimal impact compared to the much larger range of MAP (60-120 mmHg). 4
- Using a fixed CVP of 10 mmHg showed excellent accuracy with mean absolute percentage error of only 6.3% in neurosurgery and 5.7% in cardiac surgery patients 4
- All fixed CVP values (0,5,10,15, or 20 mmHg) maintained excellent trending ability with concordance rates >0.99 4
- Caution is needed when actual CVP is expected to be outside the typical clinical range (5-15 mmHg) 4
Non-Invasive Estimation Methods
Echocardiographic Doppler Method
An SVR analog (eSVR) can be estimated non-invasively as the ratio of systolic blood pressure to left ventricular outflow tract velocity time integral. 5 This method:
- Correlates closely with directly measured SVR 5
- Provides prognostic information for cardiovascular outcomes 5
- Requires only standard echocardiography and blood pressure measurement 5
Photoplethysmography-Based Estimation
Spectral and morphological features from finger photoplethysmography combined with heart rate and MAP can estimate SVR using multivariate regression models, though this remains primarily a research tool. 6
Clinical Assessment of SVR Status
Clinical signs suggesting markedly elevated SVR include: 1, 2
- Absent or weak distal pulses
- Cold extremities
- Prolonged capillary refill time (>2 seconds)
- Narrow pulse pressure with relatively increased diastolic blood pressure
Critical Considerations for Accurate Measurement
In the cardiovascular ICU, SVR must be maintained greater than pulmonary vascular resistance (PVR) to ensure adequate right ventricular coronary perfusion. 1, 3 If systolic pulmonary arterial pressure exceeds systolic systemic arterial pressure, right ventricular ischemia results. 3
Common Pitfalls to Avoid
- Measurement timing matters: Record pressures at end-expiration during spontaneous breathing, or end-inspiration during mechanical ventilation 3
- Zero the pressure transducer at the mid-thoracic line for accurate readings 3
- In patients with tricuspid regurgitation and right ventricular dilatation, thermodilution cardiac output measurements can be erroneous, affecting all derived resistance calculations 3
- Volume status assessment in pulmonary hypertension patients is notoriously difficult, and non-invasive estimates of central venous pressures may be misleading 1, 3
Continuous Monitoring Approach
For hemodynamically unstable patients, continuous on-line monitoring of SVR can be achieved through computer analysis of the peripheral arterial waveform, utilizing the peak dP/dt and pressure at peak dP/dt with appropriate calibration constants. 7 This method showed excellent correlation (r = 0.98) with thermodilution-derived SVR across a wide range of values (450-4400 dynes·s·cm⁻⁵). 7
Continuous monitoring of physiologic parameters is essential to achieve correct dosing of vasoactive medications when managing abnormal SVR. 1