Pulmonary Vascular Resistance (PVR) Calculator

How does this pulmonary vascular resistance calculator work?

This health tool determines vascular resistance in the pulmonary circulation based on arterial and atrial pressure as well as pulmonary flow. This measure is hemodynamically useful in the management of several pulmonary and cardiovascular conditions.

The three parameters to be input within this PVR calculator are:

■ Mean Pulmonary Arterial Pressure – measured in mmHg, cmH₂O, kPa, atm or psi. Represents input to the pulmonary blood circuit. Normal values are between 10 and 20 mmHg;

■ Left Atrial Pressure – measured in mmHg, cmH₂O, kPa, atm or psi. Represents the output of the pulmonary circuit at the place where blood is transferred to the left atrium. Normal range is between 6 and 12 mmHg;

■ Pulmonary flow –  measured in L/min with possibility to be input in mL/min as well in the pulmonary vascular resistance calculator. In most cases pulmonary flow is equivalent to cardiac output which is normally between 4 and 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 possible formulas are:

■ PVR measured in dynes-sec/cm⁵ = 80 x (Mean Pulmonary Arterial Pressure in mmHg – Left Atrial Pressure in mmHg) / Pulmonary Flow in L/min;

■ The simplification for PVR measured in mmHg-min/L is (Mean Pulmonary Arterial Pressure in mmHg – Left Atrial Pressure in mmHg) / Pulmonary Flow in L/min.

Example of a PVR calculation

Taking into account the following determinations:

■ Mean Pulmonary Arterial Pressure = 15 mmHg;

■ Left Atrial Pressure = 12 mmHg;

■ Pulmonary flow, cardiac output = 6.5 L/min.

The pulmonary vascular resistance = 80 x (15 – 12) / 6.5 = 36.92 dynes-sec/cm⁵.

Pulmonary vascular resistance (PVR)

This is the vascular resistance of the pulmonary circulation, meaning the pressure difference across the pulmonary circuit divided by the rate of blood flow through it.

The three of the properties of blood vessels, that are determinants of resistance to blood flow, are:

■ Vessel length (L);

■ Vessel radius or circumference (r);

■ Blood viscosity (η).

Out of these, the main determinant of changes in pressure is vessel radius, with vasoconstriction (decrease in blood vessel diameter) that increases PVR and vasodilation (increase in diameter) that decreases PVR.

PVR depends on lung volume and it is lowest at functional residual capacity (FRC). PVR is influenced by small arteriolar tone from resistance arterioles which are vessels with diameters between 100 and 450 µm and by resistance in the pre-capillary arterioles which have even smaller diameters.

The former type of arterioles is known to be autoregulatory and can dynamically change to increase or decrease blood flow to meet physical demand.

Increased PVR or pulmonary hypertension is caused by:

■ Pulmonary vascular disease;

■ Pulmonary embolism;

■ Pulmonary vasculitis;

■ Hypoxia.

Decreased PVR is mainly caused by calcium channel blockers, delivery of O₂, isoproterenol or aminophylline.

References

1) Barratt-Boyes BG, Wood EH. (1958) 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; 51(1):72-90.

2) Baim D. (2006) ed. Grossman's Cardiac Catheterization, Angiography, and Intervention. Lipincott Williams & Wilkins 7th ed.

3) Skimming JW, Cassin S, Nichols WW. (1997) Special Article: Calculating Vascular Resistances. Clin. Cardiol. 20, 805-808.

4) Davidson WR, Fee EC (1990) Influence of aging on pulmonary hemo- dynamics in a population free of coronary artery disease. Am J Cardiol; 65: 1454-1458.

5) Krovetz LJ, Goldbloom S. (1972) Normal standards for cardiovascular data. II Pressure and vascular resistances. Johns Hopkins Med J; 130:174 - 186.