Cardiac autonomic control in obstructive sleep apnea: effects of long-term CPAP therapy

Obstructive sleep apnea in epilepsy

Obstructive sleep apnea can affect an individual with epilepsy profoundly. These relatively common disorders can coexist and potentially exacerbate each other. The identification and appropriate treatment of OSA may have far-reaching consequences in improving a patient's quality of life and recurrence of seizures. Clinicians must be aware of the relationship of these disorders and keenly question epilepsy patients, regardless of their body habitus, regarding potential symptoms of sleep apnea. Although the underlying pathogenic mechanisms are unclear, we can model the information gained from the observations to further the understanding of the relationship between sleep and epilepsy.

Autonomic modulation of heart rate during obstructive versus central apneas in patients with sleep-disordered breathing

Sleep-disordered breathing is associated with an altered sympathovagal balance determined by the nocturnal cyclic alternating of apneas and hyperventilation. The aim of this study was to determine whether the autonomic modulation of heart rate during obstructive apneas (OA) and central apneas (CA) in patients with sleep-disordered breathing is different. Therefore, by using the time-varying Wigner-Ville transform spectral analysis we described, in 17 patients, the time course of the low-frequency (LF) and the high-frequency (HF) components of the interbeat interval (R-R interval) reflecting, at large, respectively, the sympathetic and the parasympathetic modulation, during OA (n = 185) and CA (n = 51) and during the postapneic hyperventilation. In both types of apneas we found cyclic lengthening/shortening in R-R interval, during apneas/postapneic hyperventilation, respectively, with more marked bradycardia during OA (R-R: 1,011 +/- 23 versus 893 +/- 30 ms2, p < 0.01). In OA the HF oscillations decreased from the apnea to the postapneic hyperventilation (from 1,964 +/- 244 to 387 +/- 98 ms2, p < 0.0001), whereas the LF oscillations increased (from 2,649 +/- 230 to 9,820 +/- 716 ms2, p < 0.0001). Conversely, in CA the HF oscillations increased from the apnea to the postapneic hyperventilation (from 452 +/- 177 to 1,485 +/- 406 ms2, p < 0.0001), whereas the LF component remained unchanged. These results show markedly different autonomic alterations during and after OA versus CA, suggesting a surge in sympathetic modulation after the obstructive episodes.

Model-based assessment of autonomic control in obstructive sleep apnea syndrome during sleep

Respiration, R-R interval, blood pressure, and other polysomnographic variables were recorded in eight normal subjects and nine patients with untreated obstructive sleep apnea syndrome in wakefulness and sleep. To increase respiratory and cardiovascular variability, a computer-controlled ventilator delivered randomly modulated inspiratory pressures that were superimposed on a baseline continuous positive airway pressure. A mathematical model allowed heart rate variability to be partitioned into a component mediated by respiratory-cardiac coupling and one mediated by the baroreflexes. Respiratory-cardiac coupling gain was lower in patients versus normal subjects (36.9 +/- 3.3 versus 66.1 +/- 5.6 milliseconds L-1, p < 0.03). Baroreflex gain in patients was also depressed relative to normal subjects (2.3 +/- 0.4 versus 4.9 +/- 0.7 milliseconds mm Hg-1; p < 0.02). Baroreflex gain increased two- to threefold from wakefulness to sleep in normal subjects, but was relatively unaffected by state change in patients. Along with results derived from spectral analysis of cardiovascular variability, these findings confirm previous reports that obstructive sleep apnea syndrome is associated with reduced parasympathetic and elevated sympathetic activity. The model-based approach provides a more precise characterization of heart rate variability that can be employed in conjunction with spectral analysis for the noninvasive detection and assessment of autonomic cardiovascular abnormality in obstructive sleep apnea syndrome.

Sleep disordered breathing and risk factors for cardiovascular disease

Patients with sleep disordered breathing (SDB) are at increased risk for cardiovascular disease including hypertension, angina, myocardial infarction, and stroke. Neurohumoral and hemodynamic responses to untreated sleep apnea are likely mechanisms that produce functional and structural changes within the cardiovascular system. Obesity, higher blood pressure, and advancing age, which are common characteristics of patients with SDB, contribute to the overall risk for cardiovascular disease. Recent studies indicate that OSA is associated with or aggravates other risk markers for cardiovascular disease. These factors include leptin, C-reactive protein, homocysteine, and insulin resistance syndrome. Elevations in C-reactive protein and glucose intolerance may be correlated with the severity of SDB. The impact of alleviating SDB on these cardiovascular risk factors has not been fully elucidated. Regardless, assessment of overall cardiovascular risk in patients with sleep apnea is warranted to identify those individuals that are high-risk who require immediate attention and intervention or in those that should be treated more aggressively.

Continuous positive airway pressure treatment improves baroreflex control of heart rate during sleep in severe obstructive sleep apnea syndrome

The role of the arterial baroreflex in the cardiovascular changes associated with the obstructive sleep apnea syndrome (OSAS), and the effect of nasal continuous positive airway pressure (CPAP) treatment on baroreflex function during sleep are unknown. Baroreflex control of heart rate was studied in 29 normotensive patients with OSAS under no treatment, in 11 age-matched control subjects, and in 10 patients at CPAP withdrawal after 5.5 +/- 3.7 (range 3-14) months of treatment. Baroreflex control of heart rate was assessed by "sequence method" analysis of continuous blood pressure recordings (Finapres) obtained during nocturnal polysomnography. In untreated OSAS, baroreflex sensitivity (BRS) was low during wakefulness and non-rapid eye movement (REM) stage 2 sleep compared with control subjects, and correlated inversely with mean lowest Sa(O(2)) and the blood pressure increase after apneas. After CPAP treatment, the apnea-hypopnea index was lower, and mean lowest Sa(O(2)) higher than before treatment. After CPAP, patients were more bradycardic, blood pressure and its standard deviation decreased as Sa(O(2)) improved in non-REM stage 2 sleep, and BRS increased (nocturnal wakefulness: +59%; non-REM stage 2 sleep: +68% over pretreatment values). Our data suggest that baroreflex dysfunction in OSAS may be at least partly accounted for by nocturnal intermittent hypoxemia, and can be reversed by long-term CPAP treatment.

Sleep apnoea and hypertension

This review article provides an update on two major issues. First, the most recent evidence supporting the occurrence of an association between obstructive sleep apnoea syndrome, or more generally sleep-disordered breathing, and arterial hypertension in humans is summarized and discussed. This includes an evaluation of both cross-sectional and longitudinal studies. Second, new insights into the mechanisms responsible for the appearance of chronic hypertension in individuals suffering from recurrent nocturnal apnoeic episodes are provided, based both on experimental studies in animals and on clinical studies in humans. The relevance of these data for the clinical management of hypertensive patients with sleep-disordered breathing, and the possibility of obtaining a reduction in blood pressure through the application of nasal continuous positive air pressure, is also addressed.