PVR Calculator - Pulmonary Vascular Resistance

Pulmonary vascular resistance (known as PVR in medical abbreviations) determines if there has been a change in the arteries that deliver the blood to the lungs.

A quick reminder - this type of circulation also exists under the name pulmonary circulation, and includes the unoxygenated blood that the right ventricle expels into the pulmonary arteries. Then perfusion/ventilation happens, and oxygenated blood from the lungs comes back into the left atrium via the pulmonary veins, to be sent around the body.

If the pressure in the pulmonary arteries and veins is too high, we have pulmonary hypertension. The right ventricle has to pump more forcefully to overcome that pressure and successfully push the blood into the pulmonary vessels. This may lead to right ventricle dilation - where the ventricle becomes too big, and therefore more power has to be put into pushing the blood. As a result, we face right ventricular dysfunction and failure, strongly related to decreased patient survival.

A similar concept to PVR is peripheral resistance, calculated from the systemic circulation.

Normal PVR ranges vary depending on the source and the association which defines the cut-off limits. However, it tends to be less than 250 dynes * sec * cm^-5.

What may cause low PVR?

• Drugs, vasodilators, which lower both pulmonary and peripheral resistance;
• High blood pH, known as alkalemia;
• Low PaCO2, hypocapnia; and
• A lot of exercises.

On the other hand, what are the reasons for high PVR, and thus pulmonary hypertension?

• Drugs, vasoconstrictors;
• Low PaO2, known under the term hypoxemia;
• Low blood pH, acidemia;
• High PaCO2, hypercapnia;
• Atelectasis - the collapse of a lung due to a lack of oxygen exchange, also determined by AA gradient changes; and
• Pulmonary embolism (you can rule it out with PERC calculator).

Once you know the PVR medical abbreviation use, how do you use it in practice? To calculate PVR you need three items: left atrial pressure (LAP), mean pulmonary artery pressure (MPAP), calculated similarly to mean arterial pressure for peripheral resistance, and cardiac output (CO):

PVR = 80 * (MPAP - LAP) / CO

If we focus on units:

• MPAP is in [mmHg];
• LAP is in [mmHg]; and
• CO is in [L/min].

Therefore, for PVR it's dynes * sec * cm^-5.

The normal values for each variable in the formula are as follows:

Let's put what we've learned to work with an example. We have a patient with suspected right heart failure. We want to check whether he has a normal pulmonary vascular resistance (again: known as PVR in medical abbreviation) or any pulmonary hypertension.

His results are:

• Mean Pulmonary Arterial Pressure: 23 mmHg;
• Left Atrial Pressure - 4 mmHg; and
• Cardiac Output - 6 L/min.

PVR = 80 * (23 mmHg - 4 mmHg) / 6 L/min

PVR = 253.3 dynes * sec * cm^-5

The patient is above the normal PVR range.

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