The incidence of obstructive sleep apnea (OSA) has reached epidemic proportions, and it is an often unrecognized cause of perioperative morbidity and mortality. Profound hypoxic injury from apnea during the postoperative period is often misdiagnosed as cardiac arrest due to other causes. Almost a quarter of patients entering a hospital for elective surgery have OSA, and >80% of these cases are undiagnosed at the time of surgery. The perioperative period puts patients at high risk of apneic episodes because of drug effects from sedatives, narcotics, and general anesthesia, as well as from the effects of postoperative rapid eye movement sleep changes and postoperative positioning in the hospital bed.

The T-tube model of systemic arterial circulation was examined with respect to the physiological relevance of model parameters. root aortic pressure [Pao(t)] and flow [Qao(t)] and descending aortic flow [Qb(t)] were measured in anesthetized, open-chest dogs under control conditions, during inflation of a balloon positioned in the left external iliac artery (n = 5), and during infusion of vasoactive drugs nitroprusside (NTP, n = 4) and phenylephrine (PHL, n = 5). With Pao(t) as the input, the model accurately predicted both Qao(t) and Qb(t) under all conditions (r2 > 0.96). The balloon inflation data established the ability of the model to discriminate between proximal and distal arterial mechanical properties. Furthermore, proximal properties (i.e., tube characteristic impedances and transit times) were independent of distal properties such as terminal compliances and resistances (or equivalently, wave reflections). The effects of NTP and PHL were pharmacologically consistent and served to further validate this model. NTP primarily affected distal (load) properties, whereas PHL altered both load and tube parameters. Physiological interpretation of model parameters, particularly compliance, is also discussed. The ability of the model to correctly discriminate between proximal and distal arterial properties is relevant because these properties may affect cardiovascular function differently.