Sleep Apnea in Cardiology
Disclosure: Dr. Malhotra has significant research grants from Respironics and Restore Medical; he also has significant Consultant/Advisory Board relationships with Pfizer, Apnex, JMI, Restore Medical, Cephalon, Itamar Medical, and NMT Medical. Dr. Rahangdale and Dr. Yim Yeh have no conflicts.
Pub Date: Monday, July 28, 2008
Author: Shilpa Rahangdale, MD, Atul Malhotra, MD and Susie Yim Yeh, MD
Obstructive and central sleep apnea (OSA and CSA) have overlapping but different etiologies and pathophysiologies, but both diseases are prevalent in cardiovascular patients. Both OSA and CSA lead to blood gas derangements and catecholamine surges, which can have deleterious effects on cardiovascular health. OSA is associated with upper airway collapse during sleep , generally requiring progressively increasing respiratory effort and subsequent arousal from sleep to resume breathing. Patients with CSA typically have unstable ventilatory control, which can cause wide oscillations in breathing with fluctuating CO2 levels. OSA is more common than CSA, with prevalence estimates suggesting that mild OSA occurs in roughly one of every five adults and moderate OSA in one of every 15 (depending on criteria). These rates, however, are likely an underestimate of today's prevalence, given the pandemic of obesity. Although CSA is less common than OSA, the prevalence of CSA in well-treated congestive heart failure (CHF) populations is high , and Cheyne Stokes respiration (CSR, a form of CSA) has been associated with poor outcome. The "Scientific Statement: Sleep Apnea and Cardiovascular Disease" by Somers et al. thoroughly summarizes the available data on the relationship between OSA, CSA, and cardiovascular disease.
OSA causes acute hemodynamic alterations, due to the sustained efforts to breathe during pharyngeal collapse, yielding markedly negative intrathoracic pressure, hypoxemia, and arousal from sleep. Negative intrathoracic pressure increases transmural cardiac pressures, effectively increasing ventricular wall tension and afterload. Additionally, augmented venous return and increased pulmonary arterial pressures, due to hypoxia, may elevate right ventricular pressures resulting in a leftward shift in the intraventricular septum. Concurrently, hypoxemia and arousals cause sympathoexcitation, which increases blood pressure and heart rate. Thus, each obstructive event may be associated with impaired ventricular filling, increased ventricular afterload, and increased myocardial oxygen demand in the hypoxemic patient. Institution of continuous positive airway pressure (CPAP) therapy for sleep apnea decreases transmural cardiac pressures and venous return , eliminates hypoxemia and respiratory events, and suppresses catecholamines.
In addition to altered hemodynamics in OSA, both OSA and CSA are associated with intermittent hypoxemia. Repetitive episodes of desaturation, followed by reoxygenation, occur every night in sleep-apnea patients (up to 80-120/hour in severe cases). These repetitive apneas resemble ischemia-reperfusion events, and the reoxygenation phase yields reactive oxygen species. Endothelial cells are especially vulnerable to oxidative stress, due to the central role of nitric oxide (NO) in mediating endothelial cell health. Reactive oxygen species decrease NO production and inactivate what NO may be present, thus reducing the protective effect of endothelium-derived NO. In addition, intermittent hypoxia generates a proinflammatory vascular environment by activating nuclear factor (NF)-kB in endothelial cells, promoting atherosclerosis. The severity of hypoxemia predicts the degree of endothelial dysfunction found in patients with OSA. Though data do not prove causality, OSA patients with marked desaturations apparently have an increased risk of cardiovascular disease.
Somers et al. review mechanisms by which OSA and CSA may cause cardiovascular abnormalities. These mechanisms include intermittent hypoxia, sympathoexcitation, and hemodynamic alterations (see "Scientific Statement: Sleep Apnea and Cardiovascular Disease," Fig. 1). The authors summarize evolving evidence, which strongly suggests that OSA itself contributes to cardiovascular risk. Growing evidence exists that CSA may identify CHF patients with a poor prognosis and, therefore, cannot be considered an epiphenomenon in CHF. However, the authors also note that despite growing numbers of OSA and CSA patients, concomitant with the epidemics of obesity and CHF, respectively, sleep-disordered breathing remains largely undiagnosed and untreated. Given the high prevalence of OSA and CSA, and their probable impact on cardiovascular health, one wonders why so many patients are undiagnosed and if there is justification to pursue these conditions aggressively.
A lack of awareness regarding sleep apnea may contribute to the burden of undetected disease. The syndrome of OSA is classically defined by both sleep-disordered breathing on polysomnography and symptoms of daytime sleepiness and snoring. Those patients with cardiovascular disease, in particular, CHF and stroke, appear to experience minimal daytime sleepiness with OSA.[17,18] Thus, patients with severe sleep apnea defined by polysomnography may go unrecognized without a high index of suspicion.
However, even the sleepy patient may not seek diagnosis and treatment of OSA for several reasons. First, patients tend to underestimate symptoms or have become accustomed to feeling poorly by altering behaviors such as intake of caffeine. Thus, they may not appreciate their degree of symptomatology. Second, other sequelae of sleep-disordered breathing such as increased cardiovascular risk and neurocognitive dysfunction remain largely unrecognized in the general population. Third, the treatment of OSA (PAP) may be inconvenient and burdensome, discouraging patients from seeking a diagnosis (e.g., the cure may be worse than the disease in their estimation). However, the realization that some patients experience life-changing improvements in symptoms can provide motivation.
If even sleepy patients are reluctant to seek diagnosis and treatment, then how do we convince nonsleepy patients to come forward? After all, compliance with CPAP improves with perceived benefit from therapy (e.g., amelioration in symptoms). Until the cardiovascular risk of asymptomatic OSA patients is better studied and treatment of asymptomatic OSA is proven to improve cardiovascular health, these patients may remain reluctant to seek evaluation. Currently, investigators  are conducting the Randomized Intervention with CPAP in CAD patients with Sleep Apnea (RICCADSA) study of CPAP in nonsleepy OSA patients from cardiology clinics. The results of this RICCADSA study may have major clinical implications. Meanwhile, patient education is critical to the management of many chronic diseases, including sleep apnea.
Patient reluctance to undergo overnight diagnostic testing in sleep centers may also contribute to the underdiagnosis of sleep apnea. New changes in Medicare coverage, since the compilation of the "Scientific Statement," now offer the alternative of home testing for some patients. Although home testing for sleep apnea may have limitations based on the type of test ordered, it does offer more information than nocturnal oximetry, which many patients find acceptable. However, patients with coronary artery disease, CHF, and cerebrovascular disease may have central apneas, which may be misdiagnosed using portable equipment. Moreover, the validation of home testing is still emerging for complicated patients, yielding the suggestion that home testing should be reserved for patients without cardiovascular disease. Notably, most patients tolerate in-laboratory polysomnography quite well with only very rare issues of discomfort or inability to sleep.
As stated, patient acceptance is limited by the cumbersome nature of PAP devices. Indeed, early reports suggested that many patients fail to tolerate PAP for >4 hours/night. Newer technologies, including a wider array of better fitting, comfortable masks, heated humidification of the delivered air, and more sophisticated airflow delivery approaches, may increase comfort.[23-25] Patient acceptance may also improve with education about the potential cardiovascular consequences of untreated sleep apnea [26-29], alternatives to PAP therapy, and intensive support during the provision of PAP therapy. In fact, adherence rates to CPAP are comparable to other chronic medical therapies, such as inhalers in asthma or antiepileptic drugs. Non-PAP alternatives, including weight loss, oral appliances, and surgery, should be considered if PAP is untenable, although the data regarding cardiovascular benefits using these alternative strategies for OSA are limited.[31,32]
Some cardiologists may question the importance of sleep apnea because the association of cardiovascular risk with OSA may be confounded by comorbidities. For instance, OSA may be a marker of obesity and its associated complications. Indeed, early observational trials that demonstrated abnormalities in cardiovascular parameters (blood pressure and other surrogate markers of atherosclerotic risk) in OSA patients did not adequately control for known cardiovascular risk factors such as obesity, hypertension, and smoking status. Somers et al. succinctly review newer, more compelling data that continue to support a link between OSA and hypertension, CHF, stroke, arrhythmias, and myocardial ischemia/infarction after rigorously controlling for known confounding variables. In addition, animal models, cell-culture experiments, and most recently, interventional studies, support the notion that OSA in and of itself has important effects on cardiovascular markers.[13,33-35] Indeed, a causal link between OSA and systemic hypertension is now established.
Large, multicenter, randomized, controlled trials demonstrating improved cardiovascular outcomes with CPAP therapy are lacking. The primary limitations in designing such trials have been the uncertain cardiovascular benefits of treating OSA and ethical concerns regarding appropriate controls. An ethical dilemma exists in exposing untreated OSA patients to the risk of daytime symptoms and injury. Previous studies have reported a sevenfold increase in risk of car accidents in OSA patients , suggesting that the danger of untreated OSA is not trivial. One option would be to study asymptomatic individuals, but the biology of these patients may be different from clinical OSA syndrome, and asymptomatic patients may be poorly adherent to CPAP therapy. Additionally, an appropriate control for CPAP has been much debated, and even the potential solution of sham-CPAP has possible issues of disrupting sleep and/or being partially effective. Despite these limitations, there is a need for more definitive studies in this field. In addition to the available evidence reviewed in the "Scientific Statement," there exist important biologic reasons, both at the hemodynamic and cellular levels, to believe that intermittent hypoxia and sympathoexcitation associated with OSA can cause independent abnormalities in the cardiovascular system, beyond the risk associated with obesity alone. Valiant efforts are currently underway to randomize large OSA populations (e.g., in China and Australia) to CPAP, assessing hard cardiovascular outcomes. Although the results of this research are years away, these data will be eagerly anticipated.
Until then, there are efforts to target specific populations such as CHF patients. Although randomized controlled studies of CPAP effects on mortality in CHF patients with OSA and CSR are ongoing, studies have shown that CPAP in CHF patients with OSA may improve ejection fraction.[40,41] Somers et al. have also shown that treatment of OSA in patients with atrial fibrillation may prevent recurrence after cardioversion. Although CPAP in stroke patients with OSA remains challenging, due to the increased age and neurocognitive impairment in those afflicted, the recent 10-year study of stroke patients with and without OSA suggests that OSA does increase mortality in this population. Indeed, further research is required, but the observation that certain cardiovascular diseases may benefit if concomitant OSA is treated has potential clinical implications.
In contrast, although CSA is likely predictive of increased mortality in CHF, the only multicenter, randomized, controlled trial (CANPAP) performed was negative. This finding may reflect inadequate power, since improvements in CHF outcome reduced the expected number of events, or because CPAP is not the ideal therapy for CSA. In fact, an early increase in mortality seen in the CPAP-treated group may have been secondary to the decrease in venous return and worsened cardiac output associated with CPAP in hypovolemic patients. Further analyses of the CANPAP data, however, showed that patients treated with CPAP who achieved an apnea-hypopnea index (AHI) <15 events/hour ("responders") had a mortality benefit. Whether response to CPAP therapy is a marker of a more favorable CHF phenotype, or effective treatment of CSA itself improves mortality, is unclear. In aggregate, there are currently insufficient data to advocate for widespread treatment of CSA in CHF with currently available therapies. However, newer PAP therapies, which stabilize breathing more effectively in CSR, are being studied.[46,47]
In summary, Somers et al. provide an exhaustive review of a topic of burgeoning relevance. The time has come to define the consequences of sleep apnea to include not just symptoms but, also, the effects of accelerated cardiovascular risk. Although further work is required prior to recommending widespread treatment of all sleep apnea patients, the provocative epidemiological, physiological, and clinical data available thus far should presently prompt one to treat symptomatic patients at risk for cardiovascular events. Patient acceptance of the diagnostic and therapeutic process should improve with the adoption of newer technologies, increased patient options in PAP therapy, and careful patient education and follow up. In addition, the increased mortality associated with CSA in CHF may be modifiable if additional novel therapeutic devices and/or drugs are identified. Our hope is that the "Scientific Statement" will energize a collaborative endeavor to improve the care of cardiovascular patients and patients at risk for these conditions.
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-- The opinions expressed in this commentary are not necessarily those of the editors or of the American Heart Association