Normal Systemic Vascular Resistance (SVR) Index
The normal range for systemic vascular resistance (SVR) is 800-1200 dynes·sec·cm⁻⁵, representing optimal vascular tone for adequate tissue perfusion. 1
Understanding SVR
Systemic vascular resistance is a critical hemodynamic parameter that represents the resistance against which the left ventricle must pump to eject blood into the arterial system. It is primarily determined by:
- Arteriolar tone
- Blood viscosity
- Vascular capacitance
Calculation of SVR
SVR is calculated using the following formula:
- SVR = (Mean Arterial Pressure - Right Atrial Pressure) / Cardiac Output
- Units: dynes·sec·cm⁻⁵
Clinical Interpretation
SVR values can be categorized as:
- Low SVR: <800 dynes·sec·cm⁻⁵
- Associated with distributive shock
- Values <450 dynes·sec·cm⁻⁵ correlate with increased mortality regardless of etiology 1
- Normal SVR: 800-1200 dynes·sec·cm⁻⁵
- High SVR: >1200 dynes·sec·cm⁻⁵
- Indicates increased afterload
- May require vasodilator therapy
Clinical Applications of SVR Monitoring
SVR measurement is valuable in several clinical scenarios:
Shock differentiation:
- Low SVR: Distributive shock (septic, anaphylactic)
- High SVR: Cardiogenic shock, hypertensive crisis 2
Heart failure management:
- Guides vasodilator therapy
- Elevated estimated SVR has been associated with increased risk of heart failure, major cardiovascular events, and mortality 3
Post-cardiopulmonary bypass:
- Low SVR state is common (44% of patients)
- These patients may respond better to vasopressors than to volume loading 4
Critical care:
- Maintaining SVR between 800-1200 dynes·sec·cm⁻⁵ is crucial for optimizing cardiac function in critically ill patients 1
Relationship to Other Hemodynamic Parameters
SVR has important relationships with other hemodynamic parameters:
- Cardiac Output: Inversely related when other factors remain constant
- Blood Pressure: Directly related (MAP = CO × SVR + RAP)
- Ventriculo-arterial coupling: Optimal Ea/Ees ratio of 0.7-0.8 indicates efficient coupling between the heart and vascular system 5
Non-invasive Assessment
While traditionally measured via invasive methods, non-invasive techniques have been developed:
Doppler echocardiography:
- The ratio of peak mitral regurgitant velocity to left ventricular outflow time-velocity integral (MRV/TVI₍ᴸⱽᴼᵀ₎) correlates well with SVR (r = 0.842) 6
- MRV/TVI₍ᴸⱽᴼᵀ₎ > 0.27 has 70% sensitivity and 77% specificity for SVR > 14 Wood units
- MRV/TVI₍ᴸⱽᴼᵀ₎ < 0.2 has 92% sensitivity and 88% specificity for SVR < 10 Wood units
Photoplethysmography (PPG):
- Features derived from finger PPG waveform combined with heart rate and mean arterial pressure can classify SVR with reasonable accuracy 7
Clinical Pitfalls and Caveats
- SVR should be interpreted in the context of other hemodynamic parameters and the patient's condition
- Single measurements may not reflect the dynamic nature of cardiovascular physiology
- Persistently elevated SVR during follow-up is associated with worse outcomes than transient elevation 3
- Measurements obtained under general anesthesia may not reflect the patient's true hemodynamic state 5
Remember that SVR represents the "steady" component of left ventricular afterload, and maintaining it within the normal range of 800-1200 dynes·sec·cm⁻⁵ is crucial for optimizing cardiac function and tissue perfusion.