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Cheyne-Stokes respiration with central sleep apnea (CSR-CSA) is a form of periodic breathing, commonly observed in patients with heart failure (HF), in which central apneas alternate with hyperpneas that have a waxing-waning pattern of tidal volume. Uniform criteria by which to diagnose a clinically significant degree of CSR-CSA have yet to be established. CSR-CSA is caused by respiratory control system instability characterized by a tendency to hyperventilate. Central apnea occurs when Pa(CO(2)) falls below the threshold for apnea during sleep due to ventilatory overshoot. Patients with CSR-CSA are generally hypocapnic, with a Pa(CO(2)) closer than normal to the apneic threshold such that even slight augmentation in ventilation drives Pa(CO(2)) below threshold and triggers apnea. Factors contributing to hyperventilation in HF include stimulation of pulmonary irritant receptors by pulmonary congestion, increased chemoreceptor sensitivity, reduced cerebrovascular blood flow, and recurrent arousals from sleep. Controversy remains as to whether CSR-CSA is simply a reflection of HF severity, or whether it exerts unique adverse effects on prognosis. The main adverse influence of CSR-CSA on cardiovascular function appears to be excessive sympathetic nervous system activity due to apnea-related hypoxia and arousals from sleep. A number of studies have examined the potential relationship between CSR-CSA and mortality in HF. Most reported that CSR-CSA was associated with an increased risk for mortality, but these studies were small. Further research is therefore needed to elucidate mechanisms which contribute to the pathogenesis of CSR-CSA, and to determine whether its treatment can reduce morbidity and mortality in patients with HF.

Obstructive sleep apnea may contribute to the development of pulmonary hypertension and RVF primarily through pulmonary vasoconstriction secondary to hypoxia. Several recent studies indicate, however, that intermittent apnea-related hypoxia is not sufficient to cause sustained pulmonary hypertension. These studies have been consistent in showing that pulmonary hypertension and RVF are almost invariably seen in the presence of diurnal hypoxia. Sustained pulmonary hypertension, therefore, appears to be associated with sustained hypoxia as is the case in COPD. Patients with OSA who have hypoxia while awake are, as a rule, obese and have mild-to-moderate diffuse obstructive airways disease. Thus, most cases of pulmonary hypertension in association with OSA result from a combination of OSA, obesity, and diffuse obstructive airways disease, a so-called overlap syndrome. However, from the therapeutic viewpoint, it is apparent that treatment of OSA by NCPAP or tracheostomy, in such cases, is usually sufficient to reverse pulmonary hypertension and RVF. More recent work has provided strong evidence that OSA can play a role in the pathogenesis of LV heart failure in patients with CHF of otherwise unknown etiology. It is likely that this occurs through a combination of increased LV afterload related to exaggerated negative Pit swings during obstructive apneas, to intermittent hypoxia, and to chronically elevated sympathoadrenal activity. Reversal of OSA by NCPAP in these patients may relieve LV heart failure. These findings add a new dimension to our understanding of the pathophysiologic effects of OSA on the cardiovascular system by demonstrating that the LV is a structure that may suffer functional impairment secondary to the stresses imposed by OSA. Finally, it has now become apparent that CSR in patients with CHF can cause symptoms of a sleep apnea syndrome when associated with intermittent hypoxia and arousals from sleep. Reversal of CSR during sleep by NCPAP can lead to alleviation of these symptoms and possibly to reduced cardiac dyspnea and LV systolic function as well. Taken together, this suggests that much more extensive use of polysomnography may be warranted in the investigation of cardiovascular disease. The reasons are compelling: sleep apnea disorders are common and eminently treatable conditions whose reversal can result in improved right and left heart function and symptomatic improvement in patients with impaired myocardial function.

SUMMARY The critical issue in considering the diagnosis and management of CSA is to determine the physiologic process underlying the disorder. CSA includes a pathophysiologically and clinically heterogeneous group of disorders that can be divided into two main groups on the basis of the awake Paco2: a hypercapnic group, in whom the disorder is related to central alveolar hypoventilation or neuromuscular disease, and a nonhypercapnic group, in whom there is no identifiable underlying disorder. The common feature of these two groups is recurrent episodes of central apnea during sleep related to withdrawal of the wakefulness drive to breathing. In the hypercapnic group the clinical history is dominated by recurrent episodes of respiratory failure and its complications, with the sleep disturbance being a secondary feature. CSA in these patients is simply an exaggeration, by sleep, of their hypoventilation disorder. Treatment in most cases involves mechanical assisted ventilation during sleep, which can be very effective in reversing CSA and respiratory failure. In contrast, idiopathic CSA is characterized by a tendency to hyperventilation. This tendency is reinforced during sleep by recurrent arousals, which tend to propagate the CSA. Unlike hypercapnic CSA, idiopathic CSA is a relatively benign condition in which cardiorespiratory failure is not a feature. Treatment of this disorder is problematic, but the use of nocturnal nasal CPAP appears to be quite effective.

A number of novel and important observations have recently arisen that emphasize the interaction between sleep apnea and cardiovascular function. New evidence of a role for obstructive sleep apnea as an independent factor in the genesis of hypertension and nocturnal myocardial ischemia has been described. Advances have been made in the understanding of the acute impact of sleep-disordered breathing on hemodynamic function, and a better understanding of the interaction between sleep-disordered breathing and congestive heart failure is now emerging. There is now strong evidence that reversal of sleep-related breathing disorders by nasal continuous positive airway pressure leads to improvements in markers of cardiovascular outcome in selected patients with congestive heart failure. These findings augur well for the development of new diagnostic approaches and treatment strategies for patients with sleep apnea and coexisting cardiovascular disease.

It has long been recognised that patients with respiratory and cardiac disease suffer from symptoms during the night when they would normally be seeking respite. These disturbances include nocturnal dyspnea, cough, wheezing and angina. Until the advent of polysomnographic monitoring about 25 yr ago, however, the pathophysiology of these nocturnal disturbances remained elusive. Since that time, investigators have made significant advances in the understanding of the pathogenesis of many of these disturbances which will be briefly reviewed below. As the subject of this article is disturbances of sleep in patients who suffer from respiratory and cardiac disease, the sleep apnea syndromes which are unique to sleep, will not be discussed except as they may contribute to symptoms of respiratory and cardiovascular disease.