Ventilation/perfusion distributions revisited

A major cause of hypoxemia in anesthesia is ventilation-perfusion (VA/Q) mismatch. With more advanced surgery and an aging population, monitoring of VA/Q is of increasing importance.

The classic multiple inert gas elimination technique has been simplified with a new approach based on mass spectrometry. VA/Q distributions can also be measured, at the bedside, by varying inspired oxygen concentration. MRI, 3-dimensional single photon emission computed tomography, positron emission tomography, and electrical impedance tomography enable imaging of perfusion and ventilation, and in some of the techniques also the distribution of inflammation. One-lung ventilation with thoracoscopy and capnothorax require careful monitoring of VA/Q, made possible bedside by electrical impedance tomography. Carbon dioxide, but not air, for pneumoperitoneum enhances shift of perfusion to ventilated regions. Ventilatory support during cardiopulmonary resuscitation causes less VA/Q mismatch when inspired oxygen concentrations are lower. Mechanisms of redistribution of lung blood flow by inhaled nitric oxide include endothelin-mediated vasoconstriction in collapsed lung regions.

Methods are continuously developing to simplify measurement of VA/Q and also to relate VA/Q to inflammation. The recording of VA/Q has helped to explain important aspects of gas exchange in thoracic anesthesiology and in intensive care medicine.