Discriminating between the effect of shunt and reduced VA/Q on arterial oxygen saturation is particularly useful in clinical practice

There is an extensive literature on methods for discriminating between an i ncreased shunt and a reduced ratio of ventilation to perfusion. In this rev iew we prefer the terms "VA/Q" and "reduced or low VA/Q" rather than "V/Q i nequality" to refer to the effects on arterial oxygenation of reducing V/Q below 0.8 to about 0.1. Almost without exception the conventional methods f or measuring shunt and reduced VA/Q are invasive as well as technically com plex. For most clinicians who are dealing with a hypoxemic patient the rele vance of these entities is not so obvious as to justify the time and diffic ulty in either understanding or measuring them. However this review shows t hat, while an increased shunt and a decreased VA/Q both reduce arterial oxy gen saturation (SaO(2)) at a particular inspired oxygen concentration (PIO2 ), the effect of shunt and reduced VA/Q have important clinical differences on the relationship between PIO2 and SaO(2). The review also outlines a si mple non-invasive method for measuring shunt and reduced VA/Q which illustr ates the value of discriminating between them in clinical practice.