The resistance offered by the vasculature of the lungs is known as the pulmonary vascular resistance (PVR). By entering the Mean Pulmonary Arterial Pressure, Capillary Wedge Pressure and cardiac output, PVR due to the vasculature of lungs can be calculated.
Formula
Calculation of Pulmonary vascular resistance (PVR) due to the vasculature of lungs is made easier. Free online medical calculator.
PVR explained
This pulmonary vascular resistance (PVR) calculator estimates vascular resistance in the pulmonary circulation based on arterial and atrial pressure difference and pulmonary blood flow.
There are three variables to be specified, as described in the table below:
| Variable | Measurement units | Description | Normal values |
|---|---|---|---|
| Mean Pulmonary Arterial Pressure | mmHg, cmH2O, kPa, atm or psi | Input to the pulmonary blood circuit. | 10 – 20 mmHg |
| Left Atrial Pressure | mmHg, cmH2O, kPa, atm or psi | Output of the pulmonary circuit at the place where blood is transferred to the left atrium. | 6 – 12 mmHg |
| Pulmonary flow | L/min, mL/min | In most cases, pulmonary flow is equivalent to cardiac output. | 4 – 8 L/min |
Vascular resistance is measured in dynes-sec/cm5, but can also be expressed in mmHg-min/L, measurement known as hybrid reference units or Wood units.
Normal PVR is between 20 and 130 dynes-sec/cm5.
The PVR formula is:
PVR measured in dynes-sec/cm5 = 80 x (Mean Pulmonary Arterial Pressure in mmHg – Left Atrial Pressure in mmHg) / Pulmonary Flow in L/min
There is also a simplified formula that provides PVR in mmHg-min/L:
PVR measured in mmHg-min/L = (Mean Pulmonary Arterial Pressure in mmHg – Left Atrial Pressure in mmHg) / Pulmonary Flow in L/min.
About pulmonary vascular resistance
PVR is a characteristic of the pulmonary circulation and represents the difference of pressure across the pulmonary circuit, divided by the rate of blood flow going through it. This definition and the usual calculation have been explored above.
The main physical determinants of resistance to blood flow in a blood vessel are:
- Vessel length (L);
- Vessel radius or circumference (r);
- Blood viscosity (η).
Vessel radius controls:
- Vasoconstriction (decrease in blood vessel diameter and increase in PVR);
- Vasodilation (increase in blood vessel diameter and decrease in PVR).
PVR is influenced by the tone in the small resistance arterioles (vessels with diameters between 100 and 450 µm) and by the resistance in pre-capillary arterioles (even smaller diameters).
PVR also depends on lung volume and was determined to be at its lowest at functional residual capacity (FRC).
Causes of high PVR or pulmonary hypertension include:
- Pulmonary vascular disease;
- Pulmonary embolism;
- Pulmonary vasculitis;
- Hypoxia.
Lower than normal PVR is most commonly caused by dysfunction in one of the following: calcium channel blockers, delivery of O2, isoproterenol or aminophylline.
Example of a PVR calculation
When:
- Mean Pulmonary Arterial Pressure = 16 mmHg;
- Left Atrial Pressure = 13 mmHg;
- Pulmonary flow, cardiac output = 5.5 L/min.
The pulmonary vascular resistance = 80 x (16 – 13) / 5.5 = 43.64 dynes-sec/cm5.
References
- Barratt-Boyes BG, Wood EH. Cardiac output and related measurements and pressure values in the right heart and associated vessels, together with an analysis of the hemo-dynamic response to the inhalation of high oxygen mixtures in healthy subjects. J Lab Clin Med. 1958; 51(1):72-90.
- Baim D. (2006) Grossman's Cardiac Catheterization, Angiography, and Intervention. Lipincott Williams & Wilkins 7th ed.
- Skimming JW, Cassin S, Nichols WW. Special Article: Calculating Vascular Resistances. Clin. Cardiol. 1997; 20, 805-808.