THE RESPIRATORY RESPONSE TO EXERCISE

Abstract

THE pulmonary ventilation during mild or moderate muscular exercise in normal subjects is linearly related to the rate of doing work. In simple exercise involving large groups of muscles, as in walking on a treadmill or cycling on an ergometer, the efficiency of muscular exercise is constant. By efficiency is meant the relationship between the increase in oxygen consumption due to the work and the external work done. It may seem strange that all subjects, regardless of age, sex or disease, use almost the same amount of extra oxygen to do the same simple muscular task, but nevertheless it is true. This constant mechanical efficiency makes it possible to substitute external work rate for a measurement of oxygen consumption in exercise tests. Before discussing exercise testing from a ventilatory point of view, the difference between the ventilatory response to mild or moderate exercise and the response to severe exercise will be considered. By severe exercise is meant exercise at a rate within 30 per cent or so of the maximum load that the subject can maintain for 6 minutes. The difference in normal subjects during steady-state exercise on a bicycle ergometer or on a treadmill is shown in Figure 1. This is an idealized graph based on a similar one from a review by Dejours and incorporating some data from our laboratory on directly measured arterial blood gas tensions corrected for changes in arterial or esophageal temperature. The graph shows ventilation, blood lactic acid, arterial oxygen and carbon dioxide tensions and pH, together with respiratory exchange ratio[R] plotted against oxygen consumption, rate of doing work in kg-m per minute, and pulse rate. The first part of the graph, marked A, shows the relationships existing during mild and moderate exercise, and the second part, B, shows what happens during severe exercise.