Vascular Resistance Calculator: SVR and PVR Formulas, Wood Units, and Clinical Interpretation

Vascular resistance is a hemodynamic concept derived from the Ohm's law analogy applied to fluid dynamics, where resistance equals the pressure gradient driving flow divided by the flow rate: R = ΔP / Q. In the cardiovascular system, the pressure gradient across the systemic circulation is the difference between mean arterial pressure (MAP) and central venous pressure (CVP), while the gradient across the pulmonary circulation is the difference between mean pulmonary artery pressure (mPAP) and pulmonary capillary wedge pressure (PCWP) or left atrial pressure. Cardiac output (CO) represents the flow rate through both circuits.

Systemic vascular resistance (SVR) reflects the total opposition to left ventricular ejection imposed by the arterial system, predominantly determined by arteriolar smooth muscle tone. SVR is a key determinant of left ventricular afterload and directly influences both blood pressure and cardiac performance. Elevated SVR, as seen in hypertensive crises, cardiogenic shock (compensatory vasoconstriction), and the vasoconstricted phase of sepsis, increases myocardial work and may reduce stroke volume. Reduced SVR occurs in distributive shock (sepsis, anaphylaxis), cirrhosis (portal hypertension with splanchnic vasodilation), and during vasodilator therapy.

Pulmonary vascular resistance (PVR) quantifies the opposition to right ventricular outflow through the pulmonary vasculature and is normally much lower than SVR because the pulmonary circulation is a low-pressure, high-compliance system. The ratio of PVR to SVR is approximately 1:10 in health. PVR is elevated in pulmonary arterial hypertension, chronic thromboembolic disease, hypoxic vasoconstriction, and acute respiratory distress syndrome (ARDS). Elevated PVR increases right ventricular afterload, which can lead to RV dilation, dysfunction, and ultimately right heart failure if the RV cannot compensate. Accurate PVR calculation using the Fick method is essential for diagnosing and managing pulmonary vascular disease.

SVR is calculated using the formula: SVR = (MAP − CVP) / CO × 80, where MAP is mean arterial pressure in mmHg, CVP is central venous pressure in mmHg, and CO is cardiac output in L/min. The multiplication factor of 80 converts the result from hybrid units (mmHg/L/min, also known as Wood units) to metric CGS resistance units (dyn·s·cm⁻⁵). Normal SVR ranges from 800 to 1200 dyn·s·cm⁻⁵ (10–15 Wood units). The SVR index (SVRI) is calculated by substituting cardiac index (CI) for CO: SVRI = (MAP − CVP) / CI × 80, with a normal range of 1970 to 2390 dyn·s·cm⁻⁵·m².

PVR is calculated as: PVR = (mPAP − PCWP) / CO × 80, where mPAP is mean pulmonary artery pressure and PCWP is pulmonary capillary wedge pressure, both in mmHg. Normal PVR is less than 160 dyn·s·cm⁻⁵ (< 2 Wood units). In pulmonary hypertension evaluation, PVR is often expressed in Wood units without the multiplication factor: PVR (Wood units) = (mPAP − PCWP) / CO. The PVRI normalizes to body surface area: PVRI = (mPAP − PCWP) / CI, expressed in Wood units·m². All pressure measurements should be obtained at end-expiration and averaged over several respiratory cycles to minimize artifact.

The accuracy of vascular resistance calculations is directly dependent on the reliability of the cardiac output measurement. An underestimated CO produces a falsely elevated resistance, while an overestimated CO yields a falsely low resistance. In conditions where thermodilution is known to be inaccurate — severe tricuspid regurgitation, very low cardiac output, intracardiac shunts, or atrial fibrillation with marked beat-to-beat variability — the Fick method provides a more reliable CO for resistance calculations. Simultaneous arterial and PA catheter pressure measurements with Fick CO yield the most internally consistent hemodynamic dataset for vascular resistance determination.

The conversion between Wood units and dyn·s·cm⁻⁵ is straightforward: multiply Wood units by 80 to obtain dyn·s·cm⁻⁵, or divide dyn·s·cm⁻⁵ by 80 to obtain Wood units. The factor of 80 derives from the unit conversion: 1 mmHg = 1333.22 dyn/cm², and 1 L/min = 1/60 × 1000 cm³/s, so the conversion factor is 1333.22 / (1000/60) = 79.99 ≈ 80. Wood units are preferred in most clinical practice and cardiology literature because they are simpler and more intuitive: a PVR of 3 Wood units means that for each liter per minute of cardiac output, there is a 3 mmHg pressure drop across the pulmonary vasculature per unit of flow.

The choice of units varies by clinical context and regional practice. North American catheterization laboratories predominantly use Wood units for PVR reporting, while some European centers and research publications use dyn·s·cm⁻⁵. For SVR, dyn·s·cm⁻⁵ is more commonly used because the absolute values are larger and more familiar to ICU clinicians (e.g., SVR of 1000 dyn·s·cm⁻⁵ is more intuitive than 12.5 Wood units for systemic resistance). Regardless of the unit system, it is critical that clinicians and laboratory staff consistently use the same units when trending serial measurements and comparing values across institutions.

When documenting hemodynamic data, standard practice is to report both the raw pressure measurements and calculated indices with units clearly specified. A comprehensive hemodynamic report includes: CO and CI (with method: Fick vs thermodilution), MAP, CVP, mPAP (systolic/diastolic/mean), PCWP, SVR (dyn·s·cm⁻⁵), PVR (Wood units), and derived parameters such as stroke volume, LVSWI, and RVSWI. Including the cardiac output method is essential because Fick and thermodilution may produce different values in the same patient, and the resistance calculations are only as accurate as the CO measurement used in their derivation.

SVR provides critical information for classifying shock etiology and guiding vasoactive medication therapy. In cardiogenic shock, SVR is typically elevated (> 1200 dyn·s·cm⁻⁵) due to compensatory sympathetic activation and neurohormonal vasoconstriction aimed at maintaining blood pressure despite reduced CO. In distributive shock (sepsis, anaphylaxis), SVR is characteristically low (< 800 dyn·s·cm⁻⁵) with a high or normal CO, reflecting inappropriate vasodilation. Differentiating these shock types guides treatment: cardiogenic shock requires inotropes and potentially afterload reduction, while distributive shock requires vasopressors to restore SVR and mean arterial pressure.

PVR interpretation drives management of pulmonary vascular disease. A PVR greater than 2 Wood units in the setting of mPAP > 20 mmHg and PCWP ≤ 15 mmHg confirms pre-capillary pulmonary hypertension. PVR greater than 5 Wood units generally indicates severe pulmonary vascular disease. In patients being evaluated for cardiac transplantation, a PVR greater than 5 Wood units (or PVRI > 6 Wood units·m²) that does not decrease below 2.5 Wood units with vasodilator challenge is a relative contraindication to orthotopic heart transplantation due to the risk of donor right ventricular failure. Serial PVR measurements during PAH-specific therapy track treatment response: a decreasing PVR correlates with improved exercise capacity and survival.

Vasoactive medications predictably alter vascular resistance parameters, enabling targeted hemodynamic optimization. Nitroprusside and nicardipine reduce both SVR and PVR through direct arterial vasodilation. Norepinephrine increases SVR through alpha-1 receptor-mediated vasoconstriction while having variable effects on PVR. Inhaled nitric oxide and inhaled prostacyclin selectively reduce PVR without significant SVR reduction, making them ideal for isolated right ventricular failure with elevated PVR. Milrinone reduces both SVR and PVR through phosphodiesterase-3 inhibition while increasing inotropy, positioning it as a particularly useful agent in biventricular failure with pulmonary hypertension. Real-time vascular resistance monitoring via the PA catheter guides the selection and dose titration of these agents to achieve hemodynamic targets.