Ventilation/perfusion relationships.

Over the last 10 years the investigation of VA/Q relationships has been significantly advanced as a result of the application of computer-assisted engineering techniques to the traditional concepts and equations of gas exchange. The price paid for this increase in ability to measure VA/Q relationships has been an increase in the complexity of technique, especially as seen by clinicians without a special interest in mathematics. Advances have taken place in the understanding of mechanisms of altered gas exchange in disease, but just as importantly in this author's mind, advances have taken place in our understanding of the problem of indirectly obtaining information from an inaccessible organ. This has been illustrated at some length in this review, and has been applied not only to the newer, more complex approaches to gas exchange, but also to the more traditional methods for assessing gas exchange such as the measurement of venous admixture. Thus, one must have a good understanding of the basic information content of any method before it can be used appropriately.