81 Time of pulmonary circulation

Pulmonary circulation time: transit of blood through the lungs, how it is measured and what prolongs or shortens it.

The pulmonary circulation time is the time blood takes to travel from the right ventricle through the pulmonary artery, the capillaries of the lungs and the pulmonary veins to the left atrium. Here oxygen-poor blood releases carbon dioxide and takes up oxygen. The lungs contain only about 0.5 litres of blood, but receive the whole cardiac output, so the mean pulmonary transit time at rest is only a few seconds (roughly 5–8 s), and a red cell spends less than a second in a lung capillary. The transit time shortens when cardiac output rises and lengthens when the heart pumps less or pulmonary vessels are congested.

Pulmonary circulationGas exchangeTransit time

Pulmonary blood volume≈ 0.5 L

Mean transit at rest≈ 5–8 s

Capillary transit≈ 0.75 s

Measured byindicator dilution, contrast imaging

Figure 1. Blood passes alternately through the pulmonary and the systemic circulation. Times are approximate averages for an adult at rest with a cardiac output of about 5 L/min; individual paths through short or long vascular beds are faster or slower.

How it is measured

The pulmonary transit time is the difference between the arrival times of an indicator in the pulmonary artery (or right ventricle) and in the left atrium (or left ventricle). It is measured with indicator-dilution techniques, contrast echocardiography, or first-pass imaging with CT, MRI or radionuclides. Multiplied by cardiac output, it gives the pulmonary blood volume.

Clinical significance

Prolonged: left heart failure and mitral valve disease (congested pulmonary vessels and low output), pulmonary hypertension, large pulmonary embolism, severe lung disease
Shortened: high cardiac output (exercise, fever, anaemia, hyperthyroidism); intrapulmonary shunts
Inflammation, raised pressure in lung capillaries and changes in blood rheology also alter pulmonary transit

When to see a doctor

Breathlessness on exertion or when lying flat, waking at night breathless, coughing up blood or sudden chest pain need medical assessment.

References

Meier P, Zierler KL. On the theory of the indicator-dilution method for measurement of blood flow and volume. J Appl Physiol. 1954;6(12):731–744. doi:10.1152/jappl.1954.6.12.731
Vincent JL. Understanding cardiac output. Crit Care. 2008;12(4):174. doi:10.1186/cc6975

This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

Published on 1 May 2024