How this instrument works
Pulmonary vascular resistance is Ohm's law borrowed for the lungs: resistance equals the pressure drop across a circuit divided by the flow through it. Here the circuit is the pulmonary vasculature, the drop is mean pulmonary artery pressure minus the wedge pressure (a stand-in for left-atrial pressure), and the flow is cardiac output. Divide the first by the second and you get how much the right ventricle has to strain to push the same blood through narrowed, stiffened, or simply constricted lung vessels.
The unit named after physiologist Paul Wood is a plain mmHg-per-litre-per-minute ratio, kept small and readable at the bedside. Multiply by eighty and the same resistance appears in dynes·s·cm⁻⁵, the older convention some catheterization labs and older charts still print. Neither unit changes what happened in the vessels; they are the same figure dressed two ways, and this instrument reports both from one set of inputs.
A resting value under roughly two to three Wood units, or under about two hundred dynes·s·cm⁻⁵, is generally considered within the normal working range for the pulmonary circuit. Numbers well above that threshold point toward elevated resistance in the lung vasculature itself, a finding that current pulmonary hypertension guidelines from the European Society of Cardiology and European Respiratory Society treat as a key diagnostic threshold alongside the pressure reading, not a replacement for it.
- Enter Mean pulmonary artery pressure (mmHg), the average PA reading from the catheter.
- Enter Pulmonary capillary wedge pressure (mmHg), the catheter's stand-in for left-atrial pressure.
- Enter Cardiac output (L/min), measured by thermodilution or the Fick method during the same study.
- Read PVR (Wood units) and PVR (dynes·s·cm⁻⁵) — the same resistance in both conventions.
Worked example — mPAP 25, wedge 10, output 5 L/min
A catheterization shows a mean pulmonary artery pressure of 25 mmHg, a wedge pressure of 10 mmHg, and a cardiac output of 5 L/min. The pressure drop across the lung circuit is 25 minus 10, or 15 mmHg. Divide that by the flow of 5 L/min and the resistance comes to exactly 3.0 Wood units — right at the upper edge of what most labs call normal.
Multiply 3.0 by 80 and the same measurement reads 240 dynes·s·cm⁻⁵ on an older chart. Change only the cardiac output to 4 L/min with everything else held the same and the drop of 15 divides by a smaller flow, pushing the resistance up to 3.75 Wood units — a reminder that a falling output alone, with pressures unchanged, will still raise the calculated resistance.
Questions
What counts as a normal PVR?
Roughly under 2 to 3 Wood units, or under about 160-240 dynes·s·cm⁻⁵, at rest. Current European pulmonary hypertension guidelines use a resistance above 2 Wood units, combined with an elevated mean pulmonary artery pressure, as part of the hemodynamic definition of pulmonary vascular disease — resistance alone, without the pressure reading, is not a diagnosis.
Why does the formula subtract wedge pressure instead of using PAP alone?
Because raw pulmonary artery pressure can climb for reasons that have nothing to do with the lung vessels themselves, most commonly left-heart disease pushing pressure backward into the lungs. Subtracting wedge pressure, which approximates left-atrial pressure, isolates the pressure drop actually caused by the pulmonary vasculature, so the resistance figure reflects the lung circuit rather than a downstream heart problem.
Why do some reports use Wood units and others dynes?
Wood units are named for physiologist Paul Wood and stay close to the raw catheter numbers — mmHg divided by litres per minute — which is convenient at the bedside. Dynes·s·cm⁻⁵ is the older CGS convention some labs never dropped. The two describe the identical resistance; this calculator multiplies by 80 to move between them so neither format needs its own separate calculation.
Can a falling cardiac output raise PVR even if pressures don't change?
Yes, and it is a common source of confusion reading a catheterization trend. Because output sits in the denominator, a heart that is simply pumping less blood will produce a higher calculated resistance even if the pulmonary vessels themselves are unchanged. Trending PVR alongside cardiac output, not in isolation, avoids mistaking a flow problem for a vascular one.
Does this number diagnose pulmonary hypertension by itself?
No. PVR is one hemodynamic figure from a right-heart catheterization, interpreted together with mean pulmonary artery pressure, wedge pressure, and the clinical picture by the physician who performed the study. This calculator only carries out the arithmetic from the three numbers you already have; it does not read the tracing or diagnose the underlying cause.
References
- 2022 ESC/ERS Guidelines for pulmonary hypertension, Eur Heart J
- NHLBI — What Is Pulmonary Hypertension?
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.