Immediate effects of arousal from sleep on cardiac autonomic outflow in the absence of breathing in dogs

To determine the immediate effects of arousal from non-rapid-eye-movement (non-REM) sleep on cardiac sympathetic and parasympathetic activities, six dogs were studied breathing through an endotracheal tube inserted into a chronic tracheostomy. Mean heart rates (HRs) during non-REM sleep were compared with 1) awake periods immediately after spontaneous arousals (ARs) and 2) later periods of stable relaxed wakefulness (RW). During spontaneous breathing, HR increased after AR (mean = 31.0%; P < 0.001) and in RW (mean = 7.6%; P < 0.001). To avoid the confounding influence of changes in breathing pattern, lung volume, and blood gases accompanying AR on HR, further studies were performed during constant mechanical hyperventilation that eliminated spontaneous breathing. In this condition, HR still increased after AR (mean = 29.9%; P < 0.001) and in RW (mean = 5.7%; P < 0.001), suggesting that the HR increases could be mediated by an effect of the state change per se on autonomic activity. This interpretation was confirmed when the HR increases were essentially abolished by combined cardiac sympathetic and parasympathetic block. In contrast, parasympathetic block alone did not prevent the HR increases after AR (mean = 12.2%; P < 0.001) or in RW (mean = 12.3%; P < 0.001), whereas sympathetic block alone almost abolished the HR increases in RW (mean = 3.6%) but did not prevent the HR increases during AR (mean = 30.2%; P < 0.001). The results show that, compared with non-REM sleep, AR is associated with acute cardiac sympathetic activation and parasympathetic withdrawal, whereas stable RW is associated mainly with sympathetic activation. These effects may have clinical relevance to the cardiovascular sequelae of breathing disorders that cause repetitive arousals from sleep.