SVR (systemic vascular resistance) describes how much resistance the body’s blood vessels provide to blood flow. This SVR Calculator computes SVR from mean arterial pressure (MAP) and cardiac output (CO) and shows the value in both dyn·s/cm⁵ and Wood units.
What SVR Means (and Why It Matters)
SVR estimates the overall resistance of the systemic circulation. Clinicians use it to understand how blood vessels are constricting or relaxing and how that affects blood pressure and perfusion.
SVR rises when resistance increases (vasoconstriction) and falls when resistance decreases (vasodilation). Because SVR depends on both pressure and flow, it is more informative than blood pressure alone.
Core Formula for SVR
Most SVR calculations use MAP and CO. The key relationship is:
- MAP is the mean arterial pressure in mmHg.
- CO is cardiac output in L/min.
- SVR is commonly reported as dyn·s/cm⁵.
SVR in dyn·s/cm⁵
The standard bedside formula is:
SVR (dyn·s/cm⁵) = (MAP − CVP) × 80 / CO
Where:
- CVP is central venous pressure in mmHg. If you don’t have CVP, some settings use 0 mmHg, but that changes accuracy.
- 80 is a unit conversion constant that yields dyn·s/cm⁵ when MAP and CVP are in mmHg and CO is in L/min.
SVR in Wood units
Wood units are another common form:
SVR (Wood) = SVR (dyn·s/cm⁵) / 80
Wood units are numerically smaller and often used in cardiology and anesthesia contexts.
How to Get the Inputs (MAP, CO, and CVP)
Mean Arterial Pressure (MAP)
MAP is the average pressure driving blood through the systemic circulation. If you have systolic (SBP) and diastolic (DBP) blood pressure, a common approximation is:
MAP ≈ DBP + (SBP − DBP) / 3
Some clinical systems also provide MAP directly.
Cardiac Output (CO)
CO is the volume of blood pumped by the heart per minute. It is typically measured or estimated by methods such as:
- Thermodilution or indicator dilution
- Echocardiography-based calculations
- Noninvasive monitors that estimate stroke volume and heart rate
CO must be in L/min for the SVR formulas used in this calculator.
Central Venous Pressure (CVP)
CVP reflects right-sided filling pressures. In the SVR formula, CVP is subtracted from MAP because the effective driving pressure for systemic flow is closer to MAP − CVP.
If your scenario doesn’t include CVP, you can set CVP to 0 mmHg in the calculator, but understand that this assumption can overestimate SVR.
Step-by-Step: Using the SVR Calculator
- Enter MAP in mmHg (or compute it elsewhere and paste the value).
- Enter CO in L/min.
- Enter CVP in mmHg if available (otherwise use 0).
- Choose the output unit you want to emphasize (the calculator provides both).
- Review the results for dyn·s/cm⁵ and Wood units.
Validation matters: CO must be greater than 0, and values should be physiologically plausible to avoid misleading results.
Practical Examples
Example 1: High SVR from Vasoconstriction
Suppose a patient has MAP = 90 mmHg, CVP = 5 mmHg, and CO = 4.0 L/min. The driving pressure is MAP − CVP = 85 mmHg.
SVR (dyn·s/cm⁵) = (85 × 80) / 4.0 = 1700 dyn·s/cm⁵. Wood units = 1700 / 80 = 21.25 Wood.
This pattern suggests increased systemic resistance, which can occur with vasopressor therapy or severe vasoconstriction.
Example 2: Low SVR from Vasodilation
Consider MAP = 65 mmHg, CVP = 2 mmHg, and CO = 6.0 L/min. The driving pressure is 63 mmHg.
SVR (dyn·s/cm⁵) = (63 × 80) / 6.0 = 840 dyn·s/cm⁵. Wood units = 840 / 80 = 10.5 Wood.
This indicates lower systemic resistance, which can happen in vasodilatory shock or when vasodilation dominates.
Common Mistakes (and How to Avoid Them)
- Using the wrong units for CO: CO must be in L/min for the 80 conversion constant.
- Forgetting CVP: If CVP is known, subtracting it improves accuracy of MAP − CVP.
- Entering CO as 0: The formula divides by CO, so the calculator will flag invalid input.
- Mixing MAP definitions: Use MAP in mmHg, not pulse pressure or systolic-only values.
Interpretation Tips
SVR is a calculated hemodynamic index, not a direct measurement. Trends over time are often more useful than a single number.
Higher SVR generally means systemic vasoconstriction; lower SVR generally means vasodilation. Interpretation should always consider heart rate, oxygen delivery, lactate, and clinical context.
SVR Calculator Quick Reference Table
| Variable | Symbol | Units | Used in Formula |
|---|---|---|---|
| Mean arterial pressure | MAP | mmHg | (MAP − CVP) |
| Central venous pressure | CVP | mmHg | subtracted from MAP |
| Cardiac output | CO | L/min | divides the result |
| Systemic vascular resistance | SVR | dyn·s/cm⁵ and Wood | reported outputs |
Frequently Asked Questions
What does SVR stand for, and what does it measure?
SVR stands for systemic vascular resistance. It estimates the resistance the systemic blood vessels create against blood flow. SVR helps clinicians understand whether blood vessel tone is driving changes in blood pressure and perfusion, especially when combined with cardiac output and central venous pressure.
How do you calculate SVR from MAP and cardiac output?
Use SVR (dyn·s/cm⁵) = (MAP − CVP) × 80 / CO. MAP and CVP are in mmHg, and CO is in L/min. If CVP is unknown, some workflows assume 0 mmHg, but that can overestimate SVR.
Why do we subtract CVP from MAP in the SVR formula?
Subtracting CVP accounts for the pressure in the right-sided venous system that must be overcome for systemic circulation to receive effective driving pressure. MAP alone reflects systemic arterial pressure, but the actual pressure gradient driving flow is closer to MAP − CVP.
What are Wood units, and how are they related to dyn·s/cm⁵?
Wood units are an alternative SVR scale. The relationship is Wood = dyn·s/cm⁵ ÷ 80. Because the conversion uses the same constant as the SVR dyn·s/cm⁵ formula, the numbers differ but represent the same hemodynamic concept.
What SVR values are considered high or low?
“High” and “low” SVR depend on patient context, measurement method, and clinical goals. In general, higher SVR indicates vasoconstriction and lower SVR indicates vasodilation. Always interpret SVR alongside MAP, CO, oxygenation, lactate, and the patient’s overall status.
Bottom Line
The SVR Calculator computes systemic vascular resistance using MAP, CO, and (optionally) CVP. It outputs SVR in both dyn·s/cm⁵ and Wood units, making it easier to compare results with common clinical references.