Lung reexpansion--for better or worse?

struck with the complexity of the relationship between the frequency, type and duration of apnea, the degree of arterial desaturation, and the type and incidence of cardiac dysrhythmias. Recent investigations have demonstrated quite different relationships between arterial desaturation associated with apnea, and concomitant ventricular and supraventricular arrhythmias. Guilleminault et aP found no consistent relationship between the degree of hypoxemia and ventricular arrhythmias, while Zwillich et al2 demonstrated a strong positive correlation between the level of arterial oxygen desaturation during an apneic event and the degree of bradycardia. It is axiomatic that longer apneic episodes will produce a greater degree of oxygen desaturation, and according to the study by Zwillich et al,2 a predictable decline in heart rate. In fact, they state that virtually every apnea is associated with some degree of bradycardia. The mechanism for this appears to be enhanced vagal tone, probably due to a decrease in reflex respiratory inhibition of vagal tone.2'3 Ventricular ectopy is not so manifestly predictable. In their study of hypoxemia and ventricular ar-rhythmias in OSA, Shepard and colleagues4 have provided excellent data which substantially enhance our understanding of this complex relationship. In their study, as well as the extensive study by Guilleminault et al,' no correlation was noted between episodes of ventricular dysfunction and arterial oxygen saturation. The data of Shepard et all suggest that there is a threshold of oxygen saturation below which the risk of encountering ventricular ectopy is markedly increased. They noted a twofold to threefold increase in PVC frequency in a subgroup of 16 patients whose arterial oxygen desaturation reached 60 percent or less. This threshold effect explains quite nicely why the correlation is low when the full range of arterial oxygen saturation is evaluated. In effect, there is minimal correlation until the oxygen saturation goes below 60 percent. Should we become too encouraged that an important and useful parameter has been unmasked by these findings? The authors themselves point out that the increase in ventricular ectopy in patients with oxygen saturations below 60 percent was due primarily to seven patients who had a marked increase in PVCs. Two of the other patients showed no increase in ventricular ectopic activity with comparable oxygen desaturation. However, over half of the 31 patients studied (17) exhibited complex ventricular arrhyth-mias. It is evident from this result that complex ar rhythmias are present with oxygen saturation above 60 percent. Thus, oxygen saturation does not …