SVR Calculator – Systemic Vascular Resistance

SVR Estimator Using MAP, CVP, and CO

Systemic Vascular Resistance Formula

Formula

\[ SVR = 80 \cdot \frac{MAP - CVP}{CO} \]

Where:

\(SVR\) — systemic vascular resistance (dyn·s/cm⁵)
\(MAP\) — mean arterial pressure (mmHg)
\(CVP\) — central venous pressure (mmHg)
\(CO\) — cardiac output (L/min)
\(80\) — conversion factor from mmHg·min/L (Wood units) to dyn·s/cm⁵

The formula follows from the hemodynamic analogy of Ohm's law: the pressure drop across the systemic circulation (\(MAP - CVP\)) divided by the flow through it (\(CO\)) gives the resistance.

Systemic Vascular Resistance – Calculation Example

Example: a critically ill patient has a mean arterial pressure of 90 mmHg, a central venous pressure of 5 mmHg, and a cardiac output of 5 L/min.

\[ SVR = 80 \cdot \frac{90 - 5}{5} = 80 \cdot \frac{85}{5} = 80 \cdot 17 = 1360 \text{ dyn·s/cm}^5 \]

Result: SVR = 1360 dyn·s/cm⁵, which is above the typical reference range and suggests increased vascular tone (vasoconstriction).

How to Use the SVR Calculator

Enter the Mean Arterial Pressure (MAP) in mmHg. If you only have systolic and diastolic pressures, use the Mean Arterial Pressure calculator first.
Enter the Central Venous Pressure (CVP) in mmHg, measured with a central venous catheter. If no value is available, a typical value of 5–8 mmHg is often assumed.
Enter the Cardiac Output (CO) in L/min. You can obtain this from thermodilution or estimate it with the Cardiac Output (Fick Method) calculator.
Read the Systemic Vascular Resistance result in dyn·s/cm⁵.

What Is Systemic Vascular Resistance?

Systemic vascular resistance is the resistance that the systemic blood vessels oppose to the blood flow generated by the left ventricle. In clinical terms, it is the quantitative expression of afterload: the load the left heart must overcome with every contraction. SVR is determined mainly by the diameter of the arterioles — the small resistance vessels — which constrict or dilate under sympathetic, hormonal, and local metabolic control.

The concept comes directly from Ohm's law applied to the circulation: pressure equals flow times resistance. Rearranging gives resistance as the pressure drop across the vascular bed divided by the flow through it. Here the pressure drop is the difference between the mean arterial pressure (the pressure at the origin of the systemic circuit) and the central venous pressure (the pressure at its return), and the flow is the cardiac output.

Because resistance cannot be measured directly, it is always a derived value. That means its accuracy depends entirely on the accuracy of the three inputs — particularly the cardiac output, which appears in the denominator, so small measurement errors in CO produce proportionally larger errors in SVR.

Interpreting SVR Values

In adults, a typical reference range is roughly 800–1200 dyn·s/cm⁵ (about 10–15 Wood units). Ranges vary between laboratories and patient populations, so always interpret SVR in the context of the clinical picture and the reference values used in your institution.

Elevated SVR (above roughly 1200 dyn·s/cm⁵) indicates vasoconstriction. Common causes include hypovolemic or cardiogenic shock (where the body compensates by clamping down the vessels), late septic shock, hypertension, and vasopressor therapy. A high SVR with a low cardiac output is the classic pattern of cardiogenic shock.

Low SVR (below roughly 800 dyn·s/cm⁵) indicates vasodilation, most characteristically in early septic shock, but also in anaphylaxis, neurogenic shock, liver failure, and after vasodilator drugs. The typical septic picture is a low SVR with a high cardiac output — the heart pumps strongly, but the vessels offer too little resistance to maintain pressure.

SVR is most useful when read together with cardiac output and filling pressures rather than in isolation. For example, the same elevated SVR can reflect compensatory vasoconstriction in a hypovolemic patient or pathological vasoconstriction in heart failure. For a related measure of cardiac performance, see the Ejection Fraction calculator.

Units, Assumptions and Limitations

Units. The factor 80 converts the result from mmHg·min/L (Wood units) to dyn·s/cm⁵, the traditional clinical unit. In SI units, resistance is expressed in MPa·s/m³ (1 mmHg·min/L ≈ 8 MPa·s/m³).
Assumed inputs. The formula assumes steady-state flow. In rapidly changing hemodynamic states (arrhythmias, ongoing resuscitation), a single set of values may not represent the patient's average condition.
Estimating missing values. If CVP is not measured, clinicians often assume a value of 5 mmHg; this introduces error, especially when the true CVP is far from that assumption. MAP derived from a non-invasive cuff pressure is less accurate than an arterial line, particularly in shock.
Not a standalone diagnosis. SVR is a derived index intended to support, not replace, clinical assessment. Treatment decisions should rest on the full hemodynamic profile and the patient's overall condition.
Special populations. Reference ranges differ in children, during pregnancy, under anesthesia, and in patients on mechanical ventilation or vasoactive drugs.

For broader cardiovascular risk context, you may also find the Framingham Heart Disease Risk calculator useful.

Frequently Asked Questions

How do I calculate SVR without a CVP measurement?

If no central line is available, you can substitute an assumed CVP. In clinical practice 5 mmHg is the most common estimate, though some use up to 8 mmHg. For example, with MAP 90 mmHg, assumed CVP 5 mmHg, and CO 4 L/min: \(SVR = 80 \cdot (90-5)/4 = 1700\) dyn·s/cm⁵. Keep in mind that the further the true CVP is from your assumption, the larger the error — a 5 mmHg CVP error changes SVR by 80·5/CO.

How do I convert SVR between dyn·s/cm⁵, Wood units, and SI units?

To convert dyn·s/cm⁵ to Wood units (mmHg·min/L), divide by 80; to go from Wood units back to dyn·s/cm⁵, multiply by 80. So 1200 dyn·s/cm⁵ is 15 Wood units. For SI units (MPa·s/m³), multiply Wood units by 8, or divide dyn·s/cm⁵ by 10: 1200 dyn·s/cm⁵ ≈ 120 MPa·s/m³. Wood units are still preferred at the bedside because the numbers are small and intuitive.

What is the difference between SVR and SVRI?

SVRI is systemic vascular resistance indexed to body surface area: \(SVRI = SVR \times BSA\), typically reported in dyn·s/cm⁵·m². Indexing matters because larger patients normally have higher cardiac output, so their raw SVR looks artificially low. Roughly, an SVRI above ~2000 dyn·s/cm⁵·m² suggests vasoconstriction and below ~1600–1800 suggests vasodilation, but cutoffs vary by institution. Use SVRI when comparing patients of very different sizes, especially in pediatric or obese patients.

Why is my SVR negative or extremely low?

A negative SVR appears whenever CVP exceeds MAP, which should not occur physiologically in a stable patient — it almost always signals a data error such as a transducer at the wrong height, a mislabeled value, or swapped MAP and CVP entries. An extremely low positive SVR usually means the cardiac output entry is too high (e.g., entered in mL/min instead of L/min) or reflects genuine profound vasodilation. Re-check all three inputs before interpreting the result.

What are common mistakes when calculating SVR?

The most frequent errors are: using systolic or pulse pressure instead of mean arterial pressure; forgetting to subtract CVP at all (which inflates SVR by roughly 400–600 dyn·s/cm⁵); entering cardiac output in mL/min rather than L/min (a 1000-fold error); and calculating MAP as a simple average of systolic and diastolic instead of the weighted formula \(MAP \approx DBP + (SBP-DBP)/3\). Also make sure all pressures come from the same time point.

How is SVR different from PVR?

PVR (pulmonary vascular resistance) is the same Ohm's-law calculation applied to the right side of the heart: \(PVR = 80 \cdot (mPAP - PAWP)/CO\), using mean pulmonary artery pressure and pulmonary artery wedge pressure instead of MAP and CVP. Typical PVR is much lower, about 100–200 dyn·s/cm⁵ (1–2.5 Wood units), because the pulmonary circulation is a low-pressure system. Together, SVR and PVR describe the afterload of the left and right ventricles respectively.

Can I use this calculator for children?

The formula itself is valid at any age, but adult reference ranges do not apply. In children, SVR depends heavily on age, weight, and body surface area, and indexed values (SVRI) are generally used instead. Normal neonatal SVRI, for example, differs substantially from that of a teenager. If you are evaluating a pediatric patient, use age-specific reference ranges from a pediatric textbook or your institution and interpret the result with a specialist.