Pathophysiologic and therapeutic implications of sleep apnea in congestive heart failure

International Consensus Statement on Obstructive Sleep Apnea

BACKGROUND

Evaluation and interpretation of the literature on obstructive sleep apnea (OSA) allows for consolidation and determination of the key factors important for clinical management of the adult OSA patient. Toward this goal, an international collaborative of multidisciplinary experts in sleep apnea evaluation and treatment have produced the International Consensus statement on Obstructive Sleep Apnea (ICS:OSA).

METHODS

Using previously defined methodology, focal topics in OSA were assigned as literature review (LR), evidence-based review (EBR), or evidence-based review with recommendations (EBR-R) formats. Each topic incorporated the available and relevant evidence which was summarized and graded on study quality. Each topic and section underwent iterative review and the ICS:OSA was created and reviewed by all authors for consensus.

RESULTS

The ICS:OSA addresses OSA syndrome definitions, pathophysiology, epidemiology, risk factors for disease, screening methods, diagnostic testing types, multiple treatment modalities, and effects of OSA treatment on multiple OSA-associated comorbidities. Specific focus on outcomes with positive airway pressure (PAP) and surgical treatments were evaluated.

CONCLUSION

This review of the literature consolidates the available knowledge and identifies the limitations of the current evidence on OSA. This effort aims to create a resource for OSA evidence-based practice and identify future research needs. Knowledge gaps and research opportunities include improving the metrics of OSA disease, determining the optimal OSA screening paradigms, developing strategies for PAP adherence and longitudinal care, enhancing selection of PAP alternatives and surgery, understanding health risk outcomes, and translating evidence into individualized approaches to therapy.

Ventrikuläre Arrhythmien bei obstruktiver und zentraler Schlafapnoe

Hintergrund

Ventrikuläre Arrhythmien treten mit einer hohen Prävalenz auf und sind mit einer hohen Morbidität und Mortalität assoziiert. Sowohl die obstruktive (OSA) als auch die zentrale (ZSA) Schlafapnoe können auf Grund ihrer Pathophysiologie zu vermehrten ventrikulären Arrhythmien beitragen.

Ziel

Dieser Artikel soll die komplexen Zusammenhänge und Erkenntnisse jüngster Forschungen bezüglich schlafbezogenen Atmungsstörungen (SBAS) und ventrikulärer Arrhythmien und deren Therapiemöglichkeiten beleuchten.

Material und Methoden

Es erfolgte eine Literaturrecherche basierend auf prospektiven, retrospektiven, klinischen und experimentellen Studien sowie Reviews, Metaanalysen und aktuellen Leitlinien, die seit 2014 in der Medline-Datenbank gelistet wurden.

Ergebnisse

Es besteht ein bidirektionaler Zusammenhang zwischen der SBAS und ventrikulären Arrhythmien. Intermittierende Hypoxie, oxidativer Stress, wiederkehrende Arousals, intrathorakale Druckschwankungen und kardiales Remodeling tragen im Rahmen der SBAS zu einer erhöhten ventrikulären Arrhythmieneigung bei. Der Schweregrad der OSA, gemessen mittels Apnoe-Hypopnoe-Index, ist mit der Prävalenz ventrikulärer Arrhythmien assoziiert. Ähnliche Ergebnisse liegen für Patienten mit ZSA und Herzinsuffizienz vor. Studien zu ventrikulären Arrhythmien bei ZSA-Patienten ohne Herzinsuffizienz fehlen. Eine Positivdrucktherapie (PAP) bei OSA- oder ZSA-Patienten führte in verschiedenen Studien zu einer reduzierten Anzahl an ventrikulären Arrhythmien. Dieser Zusammenhang konnte jedoch nicht in allen Studien bestätigt werden. Ventrikuläre Arrhythmien treten bei der OSA gehäuft nachts auf, bei der ZSA gleichmäßig über den Tag verteilt.

Diskussion

Bisherige Studien weisen einen Zusammenhang zwischen der OSA bzw. der ZSA und ventrikulären Arrhythmien trotz unterschiedlicher Pathophysiologie nach. Hinsichtlich des Effektes der PAP auf ventrikuläre Arrhythmien bei Patienten mit OSA und ZSA sind weitere Studien erforderlich.

Contact-free unconstraint respiratory measurements with load cells under the bed in awake healthy volunteers: breath-by-breath comparison with pneumotachography

Rate of respiration is a fundamental vital sign. Accuracy and precision of respiratory rate measurements with contact-free load cell sensors under the bed legs were assessed by breath-by-breath comparison with the pneumotachography technique during two different dynamic breathing tasks in 16 awake human adults resting on the bed. The subject voluntarily increased and decreased the respiratory rate between 4 and 16 breaths/min ( n = 8) and 10 and 40 breaths/min ( n = 8) at every 2 breaths in 6 different lying postures such as supine, left lateral, right lateral, and 30, 45, and 60° sitting postures. Reciprocal phase changes of the upper and lower load cell signals accorded with the respiratory phases indicating respiratory-related shifts of the centroid along the long axis of the bed. Bland-Altman analyses revealed 0.66 and 1.59 breaths/min standard deviation differences between the techniques (limits of agreement: −1.22 to 1.36 and −2.96 to 3.30) and 0.07 and 0.17 breaths/min fixed bias differences (accuracy) (confidence interval: 0.04 to 0.10 and 0.12 to 0.22) for the mean respiratory rates of 10.5 ± 3.7 and 24.6 ± 8.9 breaths/min, respectively, regardless of the body postures on the bed. Proportional underestimation by this technique was evident for respiratory rates >40 breaths/min. Sample breath increase up to 10 breaths improved the precision from 1.59 to 0.26 breaths/min. Abnormally faster and slower respirations were accurately detected. We conclude that contact-free unconstraint respiratory rate measurements with load cells under the bed legs are accurate and may serve as a new clinical and investigational tool.

NEW & NOTEWORTHY Four load cells placed under the bed legs successfully captured a centroid shift during respiration in human subjects lying on a bed. Breath-by-breath comparison of the breaths covering a wide respiratory rate range by pneumotachography confirmed reliability of the contact-free unconstraint respiratory rate measurements by small standard deviations and biases regardless of body postures. Abnormally faster and slower respirations were accurately detected. This technique should be an asset as a new clinical and investigational tool.

The Effects of Continuous Positive Airway Pressure on Premature Ventricular Contractions and Ventricular Wall Stress in Patients with Heart Failure and Sleep Apnea

Background

We aimed to investigate the effects of continuous positive airway pressure (CPAP) treatment on electrocardiography (ECG), premature ventricular contraction load on 24-hour Holter recordings, and implantable cardioverter defibrillator (ICD) shocks in patients with obstructive sleep apnea syndrome (OSAS) and heart failure.

Methods

Patients with heart failure and ICD and patients with newly diagnosed OSAS were divided into two groups according to CPAP treatment. To compare the impact of CPAP on ECG parameters, both baseline and 6-month ECG, 24-hour Holter ECG, ambulatory blood pressure monitoring, echocardiography, polysomnography, and laboratory parameters were collected.

Results

CPAP treatment significantly reduced the frequency of premature ventricular contractions, T-peak to T-end, corrected QT, corrected QT dispersion, and T-peak to T-end/corrected QT ratio in the study group (p < 0.001 for all). Although the baseline NT-pro-BNP levels were similar between study and control groups, after six months, the NT-pro-BNP levels of the study group were significantly lower than that of the control group (39.18 ± 7.57 versus 46.11 ± 7.65; p < 0.001).

Conclusions

CPAP treatment in patients with heart failure and ICD and in patients with newly diagnosed OSAS may have beneficial effects on premature ventricular contractions and electrocardiographic arrhythmia indices and NT-pro-BNP levels. However, these results are needed to be clarified with further studies.

Identifying Predictors of Central Sleep Apnea/Cheyne-Stokes Breathing in Chronic Heart Failure: a Pathophysiological Approach

Chronic heart failure (CHF) is a major health problem associated with increased mortality, despite modern treatment options. Central sleep apnea (CSA)/Cheyne-Stokes breathing (CSB) is a common and yet largely under-diagnosed co-morbidity, adding significantly to the poor prognosis in CHF because of a number of acute and chronic effects, including intermittent hypoxia, sympathetic overactivation, disturbed sleep architecture and impaired physical tolerance. It is characterized by repetitive periods of crescendo-decrescendo ventilatory pattern, alternating with central apneas and hypopneas. The pathogenesis of CSA/CSB is based on the concept of loop gain, comprising three major components: controller gain, plant gain and feedback gain. Laboratory polysomnography, being the golden standard for diagnosing sleep-disordered breathing (SDB) at present, is a costly and highly specialized procedure unable to meet the vast diagnostic demand. Unlike obstructive sleep apnea, CSA/CSB has a low clinical profile. Therefore, a reliable predictive system is needed for identifying CHF patients who are most likely to suffer from CSA/CSB, optimizing polysomnography use. The candidate predictors should be standardized, easily accessible and low-priced in order to be applied in daily medical routine. The present review focuses on a pathophysiological approach to the selection of some predictors based on parameters reflecting the etiology, the pathogenesis and the consequences of CSA/CSB in CHF.

Ventricular assist devices and sleep-disordered breathing

Congestive heart failure is one of the leading causes of morbidity and mortality in the United States, and left ventricular assist devices have revolutionized treatment of end-stage heart failure. Given that sleep apnea results in significant morbidity in these patients with advanced heart failure, practicing sleep physicians need to have an understanding of left ventricular assist devices. In this review, we summarize what is known about ventricular assist devices as they relate to sleep medicine.

Sleep Disturbances in Patients with Chronic Heart Failure and Their Holistic Consequences—What Different Care Actions can be Implemented?

Background: Sleep disturbances are prevalent among elderly, especially among those with chronic heart failure (CHF) and can affect all dimensions of quality of life (QOL) negatively.

Aim: To describe the most common causes leading to sleep disturbances in patients with CHF, their consequences from a holistic perspective and different care actions that can be implemented.

Methods: MEDLINE and CINAHL databases were searched from 1989 to July 2004.

Findings: Sleep disordered breathing (SDB), and insomnia were the most common causes for sleep disturbances and occurs in 45–82% (SDB) and one-third (insomnia) of all patients with CHF. SDB cause a disturbed sleep structure with frequent awakenings, as well as several adverse effects on the cardiovascular system causing increased morbidity and mortality. Insomnia, caused by anxiety, an unknown life situation in relation to the debut of CHF, or symptoms/deteriorations of CHF can lead to negative effects on all aspects of QOL, as well as daytime sleepiness.

Conclusion: The high prevalence of sleep disturbances and their holistic consequences should be taken into account when nurses asses and plan the care for patients with CHF. Randomized studies with large sample sizes evaluating non-pharmacological nursing interventions that improve sleep are needed.

Contribution of peripheral and central chemoreceptors to sympatho-excitation in heart failure

Chronic heart failure (CHF) is a major public health problem. Tonic hyper-activation of sympathetic neural outflow is commonly observed in patients with CHF. Importantly, sympatho-excitation in CHF exacerbates its progression and is strongly related to poor prognosis and high mortality risk. Increases in both peripheral and central chemoreflex drive are considered markers of the severity of CHF. The principal peripheral chemoreceptors are the carotid bodies (CBs) and alteration in their function has been described in CHF. Mainly, during CHF the CB chemosensitivity is enhanced leading to increases in ventilation and sympathetic outflow. In addition to peripheral control of breathing, central chemoreceptors (CCs) are considered a dominant mechanism in ventilatory regulation. Potentiation of the ventilatory and sympathetic drive in response to CC activation has been shown in patients with CHF as well as in animal models. Therefore, improving understanding of the contribution of the peripheral and central chemoreflexes to augmented sympathetic discharge in CHF could help in developing new therapeutic approaches intended to attenuate the progression of CHF. Accordingly, the main focus of this review is to discuss recent evidence that peripheral and central chemoreflex function are altered in CHF and that they contribute to autonomic imbalance and progression of CHF.

Impact of Obstructive Sleep Apnea on Outcomes of Catheter Ablation of Atrial Fibrillation

Obstructive sleep apnea (OSA) is a growing epidemic in the United States and significantly contributes to the increasing prevalence of atrial fibrillation (AF) in the U.S.

POPULATION

Although a strong correlation between OSA and AF has been demonstrated, a causal relationship between these two conditions has not been definitively established. Evidence of OSA is an important consideration of AF management and impacts the success rate of catheter ablation. The presence of OSA tends to predict a lower success rate and higher complication rate for catheter ablation of AF. However, recent studies evaluating OSA as an independent risk predictor of AF recurrence following an ablation procedure have yielded conflicting results. A greater understanding of these conditions would allow for a more specific therapy targeting the type of AF associated with OSA. The following review provides a brief summary of obstructive sleep apnea etiology, focuses on the relationship between OSA and AF, and discusses the impact of OSA on the outcomes of catheter ablation of AF.

The intuitive case for β-blockers in patients with ESRD

Sudden cardiac death (SCD) is common in dialysis patients accounting for up to 25% of all-cause mortality. Unlike in the general population, occlusive coronary artery disease is implicated in a minority of these deaths. Activation of the sympathetic nervous system is prevalent in the dialysis population and may underlie this high rate of SCD. β-blockers reduce SCD in the general population and, given their mode of action, β-blockers would seem to be an ideal class of agents to prevent SCD in dialysis patients. In this review, we will explore the etiology of SCD in dialysis patients and discuss the evidence supporting the use of β-blockers in patients with ESRD. We will also examine potential impediments to the use β-blocker in the dialysis population and outline directions for future trials in this area.

Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation

Cerebral blood flow (CBF) and its distribution are highly sensitive to changes in the partial pressure of arterial CO(2) (Pa(CO(2))). This physiological response, termed cerebrovascular CO(2) reactivity, is a vital homeostatic function that helps regulate and maintain central pH and, therefore, affects the respiratory central chemoreceptor stimulus. CBF increases with hypercapnia to wash out CO(2) from brain tissue, thereby attenuating the rise in central Pco(2), whereas hypocapnia causes cerebral vasoconstriction, which reduces CBF and attenuates the fall of brain tissue Pco(2). Cerebrovascular reactivity and ventilatory response to Pa(CO(2)) are therefore tightly linked, so that the regulation of CBF has an important role in stabilizing breathing during fluctuating levels of chemical stimuli. Indeed, recent reports indicate that cerebrovascular responsiveness to CO(2), primarily via its effects at the level of the central chemoreceptors, is an important determinant of eupneic and hypercapnic ventilatory responsiveness in otherwise healthy humans during wakefulness, sleep, and exercise and at high altitude. In particular, reductions in cerebrovascular responsiveness to CO(2) that provoke an increase in the gain of the chemoreflex control of breathing may underpin breathing instability during central sleep apnea in patients with congestive heart failure and on ascent to high altitude. In this review, we summarize the major factors that regulate CBF to emphasize the integrated mechanisms, in addition to Pa(CO(2)), that control CBF. We discuss in detail the assessment and interpretation of cerebrovascular reactivity to CO(2). Next, we provide a detailed update on the integration of the role of cerebrovascular CO(2) reactivity and CBF in regulation of chemoreflex control of breathing in health and disease. Finally, we describe the use of a newly developed steady-state modeling approach to examine the effects of changes in CBF on the chemoreflex control of breathing and suggest avenues for future research.

The prevalence and significance of periodic leg movements during sleep in patients with congestive heart failure

PURPOSE

The aim of this study was to evaluate (1) the prevalence of periodic leg movements during sleep (PLMs) in a consecutive sample of congestive heart failure (CHF) outpatients; (2) the presence of correlation between PLMs, subjective daytime sleepiness, and sleep architecture; and (3) the heart rate response to PLMs in CHF.

MATERIALS AND METHODS

Seventy-nine [50 men, age 59 +/- 11 years, body mass index (BMI) 26 +/- 5 kg/m(2)] consecutive adult stable outpatients with CHF [left ventricular ejection fraction (LVEF) 36 +/- 6%] were prospectively evaluated. The patients underwent assessment of echocardiography, sleepiness (Epworth Scale), and overnight in-lab polysomnography.

RESULTS

Fifteen patients (19%) had PLM index >5. These subjects were similar in sex distribution, BMI, subjective somnolence, LVEF, and apnea-hypopnea index (AHI), but were significantly older than subjects without PLMs. Sleep architecture was similar in subjects with and without PLMs. There was a small but significant elevation of heart rate after PLMs (80.1 +/- 9.4 vs. 81.5 +/- 9.2; p < 0.001). The cardiac acceleration was also present in absence of electroencephalogram activation.

CONCLUSION

The prevalence of PLMs in consecutive sample of adult CHF outpatients was 19%. There were no differences in subjective daytime sleepiness, sleep architecture, AHI, and severity of CHF in subjects with and without PLMs. PLMs caused a small but statistically significant cardiac acceleration.

Effects of CPAP on left ventricular structure and myocardial performance index in male patients with obstructive sleep apnoea

BACKGROUND

Obstructive sleep apnoea (OSA) has the potential to cause heart failure. We aimed to determine the effects of nasal continuous positive airway pressure (CPAP) therapy on left ventricular structure and myocardial performance index (MPI) in severe OSA patients.

METHODS

Sixty-seven subjects without any cardiac or pulmonary disease had overnight polysomnography and echocardiography. In 33 males with severe OSA, thickness of interventricular septum (IVS) and posterior wall (LVPW) were measured by M-mode. Left ventricular MPI was calculated as (isovolumic contraction time+isovolumic relaxation time)/aortic ejection time by Doppler.

RESULTS

Eight males were non-compliant with CPAP. Mean age was 47.9+/-8.2 years, and 20 of 25 patients (80.0%) were hypertensive. Patients had high body mass index (BMI: 31.0+/-3.9 kg/m(2)), but there was no change in BMI from baseline after 6 months. Thickness of IVS (11.0+/-1.1mm) and LVPW (11.0+/-1.0mm) at baseline were significantly decreased after 6 months of CPAP therapy (10.5+/-0.9 mm, P<0.001 and 10.4+/-0.7 mm, P<0.0001, respectively). Left ventricular MPI (60.1+/-13.8%) significantly decreased (53.0+/-10.7%, P<0.0001) after CPAP usage.

CONCLUSIONS

In male patients with severe OSA, CPAP therapy significantly decreases left ventricular wall thickness and improves global function even with 6 months of usage.

Validation and clinical utility of a simple in-home testing tool for sleep-disordered breathing and arrhythmias in heart failure: results of the Sleep Events, Arrhythmias, and Respiratory Analysis in Congestive Heart Failure (SEARCH) study

Fifty patients with New York Heart Association class III systolic heart failure were enrolled in this prospective multicenter study that compared the diagnostic accuracy of a home-based cardiorespiratory testing system with standard attended polysomnography. Patients underwent at least 2 nights of evaluation and were scored by blinded observers. At diagnostic cutoff points of > or =5, > or =10, and > or =15 events per hour for respiratory disturbance severity, polysomnography demonstrated a sleep-disordered breathing prevalence of 69%, 59%, and 49%, respectively. Compared with polysomnography, the cardiorespiratory testing system demonstrated predictive accuracies of 73%, 73%, and 75%, which improved to 87%, 87%, and 83%, respectively, when analysis of covariance suggested reanalysis omitting one site's data. The system accurately identified both suspected and unsuspected arrhythmias. The device was judged by 80% of patients to be easy or very easy to use, and 74% of patients expressed a preference for the in-home system. Therefore, this system represents a reasonable home testing device in these patients.

Treatment of sleep apnea in congestive heart failure with a dental device

The aim of the present study was to investigate the effect of a mandibular advancement device (MAD) for the treatment of sleep apnea (SA) on plasma brain natriuretic peptide (BNP), left ventricular ejection fraction (LVEF), and health-related qualify of life (HRQL) in patients with mild to moderate stable congestive heart failure (CHF). Seventeen male patients aged 68.4+/-5.5 with an apnea-hypopnea index (AHI) >or=10 were equipped with an individually fitted MAD. SA was evaluated using a portable respiratory multirecording system before and after the initiation of treatment. Eleven patients completed follow-up and were evaluated after 6 months of treatment. The AHI reduced from 25.4+/-10.3 to 16.5+/-10.0 (p=0.033) compared to baseline and mean plasma BNP levels decreased from 195.8+/-180.5 pg/ml to 148.1+/-139.9pg/ml (p=0.035). SA-related symptoms, e.g., excessive daytime sleepiness, were also reduced (p=0.003). LVEF and HRQL were unchanged. We conclude that SA treatment with a MAD on patients with mild to moderate stable CHF appears to result in the reduction of plasma BNP levels. Further studies to investigate if the observed reduction in BNP concentrations also result in improved prognosis are warranted.

Effects of CPAP on right ventricular myocardial performance index in obstructive sleep apnea patients without hypertension

OBJECTIVES

Obstructive sleep apnoea (OSA) might cause right ventricular dysfunction and pulmonary hypertension. We aimed to determine the effects of nasal continuous positive airway pressure (CPAP) therapy on right ventricular myocardial performance index (MPI) in OSA patients without hypertension.

METHODS

49 subjects without hypertension, diabetes mellitus, any cardiac and pulmonary disease had overnight polysomnography and echocardiography. In 18 moderate-severe OSA (apnea-hypopnea index > or = 15) patients, right ventricular free wall diameter (RVFWD) was measured by M-mode, and right ventricular MPI was calculated as (isovolumic contraction time+ isovolumic relaxation time) / pulmonary ejection time using Doppler at baseline and after 6 months CPAP therapy.

RESULTS

Mean age was 46.5 +/- 4.9 year. Patients had high body mass index (BMI: 30.6 +/- 4,0 kg/m2), but there was no change in either BMI or blood pressures after 6 months. Right ventricular end-diastolic and end-systolic diameters were in normal limits at baseline, and did not change after CPAP usage. Baseline RVFWD (7.1 +/- 2.1 mm) significantly decreased after CPAP therapy (6.2 +/- 1.7 mm, p < 0.001). 15 of patients (83%) had right ventricular diastolic dysfunction at baseline, and it was completely improved in 11 of them (73%) by CPAP usage. Right ventricular global dysfunction was shown in 11 patients (61%) with a high MPI (62.2 +/- 9.3%) at baseline; and MPI was significantly decreased after CPAP therapy (47.3 +/- 8.4%, p < 0.0001), and it was completely corrected in 4 of them (36%).

CONCLUSION

CPAP therapy significantly decreases RVFWD and improves right ventricular diastolic and global functions (MPI) in OSA patients without hypertension.

Oxygen Therapy Prevents Ventricular Arrhythmias in Patients With Congestive Heart Failure and Sleep Apnea

BACKGROUND

Although ventricular arrhythmia is critical for the prognosis of patients with severe congestive heart failure (CHF), it is difficult to control the arrhythmia using conservative therapies. However, many CHF patients also have sleep apnea syndrome (SAS) and oxygen supply improves their prognosis. The beneficial effects of oxygen treatment for ventricular arrhythmia have not yet been clarified, so the present study was designed to evaluate the effects of oxygen treatment for premature ventricular contraction (PVC).

METHODS AND RESULTS

Patients with CHF and SAS were divided into 3 groups: (1) the "PVC declined" group that included patients who had frequent PVCs and oxygen treatment that suppressed the number of PVC; (2) the "PVC not affected" group that included patients who had frequent PVCs and oxygen treatment did not affect the number of PVC; and (3) the "few PVC" group that included patients who had no or few PVCs. The group 1 patients showed higher apnea-hypopnea index, standard deviation of all R-R intervals, left ventricular ejection fraction, and brain natriuretic peptide levels than the patients in group 2. Oxygen treatment in group 3 did not affect the PVC frequency.

CONCLUSIONS

Oxygen treatment may be useful for preventing ventricular arrhythmia in selected patients with CHF and SAS.

Cheyne-Stokes respiration in patients with congestive heart failure: causes and consequences

Cheyne-Stokes respiration is a form of periodic breathing in which central apneas and hypopneas alternate with periods of hyperventilation, producing a waxing and waning pattern of tidal volume. This review focuses on the causes and consequences of Cheyne-Stokes respiration in patients with congestive heart failure, in whom the prevalence is strikingly high and ranges from 30% to 50%. Several factors have been implicated in the genesis of Cheyne-Stokes respiration, including low cardiac output and recurrent hypoxia. The key pathophysiological mechanism triggering Cheyne-Stokes respiration is hyperventilation and low arterial CO2 (PaCO2) that when below the apneic threshold triggers a central apnea. Hyperventilation is associated with pulmonary congestion, and Cheyne-Stokes respiration is more prone to occur during sleep, when the respiratory system is mainly dependent on chemical control. It is associated with recurrent dips in oxygen saturation and arousals from sleep, with oscillations in blood pressure and heart rate, sympathetic activation and increased risk of ventricular tachycardia. Cheyne-Stokes respiration is an independent marker of poor prognosis and may participate in a vicious cycle, further stressing the failing heart.

Positive pressure ventilation in the management of acute and chronic cardiac failure: a systematic review and meta-analysis

Chronic heart failure (CHF) is a common condition and is associated with excess morbidity and mortality, in spite of the many advances in its treatment. Chronic stable heart failure is also associated with an increased incidence of sleep-related breathing disorders, such as central sleep apnoea (CSA) and Cheyne Stokes respiration (CSR). Continuous positive airways pressure (CPAP) has been shown to alleviate the symptoms of CHF, improve left ventricular function and oxygenation. To a certain extent, CPAP also abolishes sleep-related breathing disorders in patients with chronic heart failure. In patients with acute pulmonary oedema, the use of positive pressure ventilation improves cardiac haemodynamic indices, as well as symptoms and oxygenation, and is associated with a lower need for intubation. However, some studies have cast doubts about its safety and suggest a higher rate of myocardial infarction associated with its use. In our opinion, non-invasive positive pressure ventilation and CPAP offers an adjunctive mode of therapy in patients with acute pulmonary oedema and chronic heart failure, who may not be suitable for intubation and in those not responsive to conventional therapies. Non-invasive ventilation also helps to improve oxygenation in those patients with exhaustion and respiratory acidosis. Many trials are still ongoing and the results of these studies would throw more light on the present role of non-invasive ventilation in the management of CHF.

Impact of sleep-related breathing disorders on health-related quality of life in patients with chronic heart failure

BACKGROUND

Quality of life in patients with chronic heart failure (HF) is often severely compromised. Sleep-related breathing disorders (SRBD) like Cheyne-Stokes Respiration (CSR) or obstructive sleep apnea (OSAS) are often observed in patients with severe HF resulting in fragmentation of sleep, excessive daytime sleepiness and an increased mortality. While an apnea/hypopnea-index (AHI) >30/h represents an independent predictor of poor prognosis, clinical relevance of even minor SRBD with an AHI <30/h remains unclear with respect to quality of life, exercise capacity or depression rate.

METHODS

Sixty-nine consecutive ambulatory patients with stable HF (NYHA II-III, EF 25%) underwent two night polygraphies with a six-channel ambulatory recording. Spiroergometry was performed, and patients were examined for sleep quality (PSQI), depressed mood (BDI) and health-related quality of life (SF-36). The data were compared to 10 age-matched healthy controls and 11 patients with OSAS (AHI 14-29/h) not suffering from HF.

RESULTS

Fifty-one patients completed follow up. 52% were positively diagnosed for SRBD (AHI 16-30/h: 12 patients CSR, 5 patients OSAS, 9 patients mixed); 25 patients (48%) showed no relevant SRBD. Patients with HF and SRBD had lower quality of life than patients without SRBD and HF. The severity of SRBD as indicated by the AHI significantly correlated with quality of life measures: Bodily pain, physical functioning and social functioning showed largest impairment in patients with HF and SRBD. Furthermore, elevated depression rates in correlation to the AHI were only observed in patients with SRBD similar to patients with OSAS without HF.

CONCLUSION

Even minor SRBD in patients with HF independently influence quality of life and correlate with estimation of depression and sleep disturbances.

Sleep.7: positive airway pressure therapy for obstructive sleep apnoea/hypopnoea syndrome

The use of continuous positive airway pressure (CPAP) in treating symptoms associated with OSAHS is reviewed. Although it is an imperfect intervention, it continues to evolve and improve in such a way that patients who would not have been able to use this treatment even in the recent past can benefit from it today.

Central Sympathetic Inhibition Augments Sleep-Related Ultradian Rhythm of Parasympathetic Tone in Patients With Chronic Heart Failure

BACKGROUND

Abnormal sleep dynamics in patients with heart failure is one of the mechanisms for the relative predominance of central sympathetic outflow over parasympathetic tone. This study was designed to examine whether central sympathoinhibition could improve the sympathovagal imbalance related to rapid-eye-movement (REM)/non-REM ultradian sleep rhythm in these patients.

METHODS AND RESULTS

Beat-by-beat RR intervals of overnight electrocardiogram were serially subject to power spectral analysis in 14 patients with chronic heart failure and 13 age-matched subjects with normal cardiac function. To assess autonomic sleep dynamics, the ultradian rhythm was extracted from all-night consecutive high-frequency (HF) components of heart rate variability (HRV) before and after administration of an (alpha2)-adrenergic agonist, guanfacine. Night-time HRV in heart failure was characterized by an attenuated ultradian rhythm of HF-components with a concomitant reduction in averaged HF power. Guanfacine reduced blood pressure, heart rate, and plasma norepinephrine concentrations by 7%, 8%, and 34% (p < 0.01), respectively. After guanfacine, HF power rose by 154% (p < 0.01) with a prominent augmentation of the all-night ultradian rhythm (+361%, p < 0.01).

CONCLUSIONS

Central sympathoinhibition augments a sleep-related ultradian rhythm of parasympathetic tone, suggesting a potential benefit to autonomic balancing and sleep quality in patients with chronic heart failure.

Role of central sympathoexcitation in enhanced hypercapnic chemosensitivity in patients with heart failure

BACKGROUND

Enhanced central hypercapnic chemosensitivity is known to mediate excessive exercise ventilation and to indicate a poor prognosis in patients with chronic heart failure. The present study was designed to elucidate the role of central sympathetic activity in the enhancement of hypercapnic chemosensitivity.

METHODS

Central hypercapnic chemosensitivity and plasma norepinephrine were measured in 99 patients with chronic heart failure. In 40 patients, the alpha index was derived from simultaneous analysis of R-R interval and systolic blood pressure variability. The effects of a central sympatholytic agent, guanfacine (0.25 mg/day), on hypercapnic chemosensitivity and exercise ventilatory response were studied in 20 of these patients.

RESULTS

Hypercapnic chemosensitivity was enhanced in 76% of the patients and correlated significantly with plasma norepinephrine levels (r = 0.49, P < .01) at rest. There was a significant inverse relationship between central chemosensitivity and the alpha index (r = -0.41, P < .01). Guanfacine significantly reduced plasma norepinephrine levels by 29% (P < .01) and chemosensitivity by 31% (P < .01). The beneficial effect of central sympathoinhibition with guanfacine was observed specifically in patients who had enhanced chemosensitivity prior to drug administration. Similarly, the patients with excessive exercise ventilation showed a greater reduction in exercise ventilation with this agent.

CONCLUSIONS

The present findings suggest that central sympathoexcitation could play an important role in the pathogenesis of enhanced hypercapnic chemosensitivity and a resultant increase in exercise ventilation in chronic heart failure.

Smoking and sleep disorders

Cigarette smoking is the most important cause of preventable disease, disability, and premature death in the United States. In addition to adverse effects on respiratory, cardiovascular, cerebrovascular, and other systems, accumulating evidence indicates that cigarette smoking may also increase morbidity by adversely affecting sleep. This article focuses on the effects of cigarette smoking, nicotine, and pharmacologic agents used for smoking cessation on neuronal systems regulating sleep and clinically apparent sleep disorders.

Dilated cardiomyopathy in Kugelberg-Welander disease: coexisting sleep disordered breathing and its treatment with continuous positive airway pressure

A rare complication of possible secondary dilated cardiomyopathy to Kugelberg-Welander disease was described in a 53-year-old patient with this inherent motor neuron disease, whom we diagnosed after a genetic analysis of the defective survival motor neuron gene. An association of sleep disordered breathing of Cheyne-Stokes respiration was diagnosed, which was virtually eliminated with continuous positive airway pressure via nasal mask. Considering the paucity of therapeutic options in most degenerative neuromuscular disorders, ameliorations in not only sleep quality but also cardiac function with continuous positive airway pressure have clinical implications.

Prevalence of Sleep Disordered Breathing in a Heart Failure Program

Recent data show that a high percentage of patients with systolic left ventricular dysfunction have sleep-disordered breathing (SDB), contributing to the incidence of morbidity and mortality in heart failure. This study examines the prevalence of sleep disorders in stable heart failure patients regardless of ejection fraction. On three consecutive days in a heart failure clinic, all patients were asked to participate in a screening for SDB. This screening involved the placement of an outpatient device (ClearPath, Nexan, Inc., Alpharetta, GA), which collects thoracic impedance, oxyhemoglobin saturation, and 2-lead electrocardiogram data. Sixteen patients (42%) had moderate or severe SDB, and 22 patients (55%) had mild or no significant SDB. Fourteen of the 16 patients with moderate or severe SDB subsequently received treatment by confirming SDB and the continuous positive airway pressure in a sleep lab. Forty-two percent of patients with stable heart failure presenting to a heart failure clinic screened positive for SDB, despite receiving optimal standard of care.

Use of a mandibular advancement device in patients with congestive heart failure and sleep apnoea

OBJECTIVE

To evaluate the practical use of the mandibular advancement device (MAD) for treatment of sleep apnoea (SA) in patients with congestive heart failure (CHF) over 1 year.

SUBJECTS

Twenty-five patients aged 66 +/- 8 years (mean +/- SD) met the inclusion criteria and were included in the study.

DESIGN

In a prospective, clinical trial, the apnoea-hypopnoea index (AHI), a measure of SA, was determined with a portable device. Failure to enter treatment and compliance, adverse events and signs and symptoms of temporomandibular disorders (TMD) were examined before intervention and 4-6 weeks, 6 months, and 1 year after intervention.

RESULTS

Six patients had removable dentures of whom four had complete dentures. Before treatment, eight patients reported minor symptoms of TMD. The AHI fell from 19.3 +/- 12.1 to 11.8 +/- 9.5 (p = 0.004) with use of the device. In most patients, use of the MAD had no severe effects on the signs and symptoms of TMD. Adverse events such as pain in the temporomandibular joints, soreness in the teeth, and tiredness in the jaws were reported by 10 patients. Dental complications were observed in two patients. Sixty-four per cent of the patients were still using the MAD at the 1-year follow-up.

CONCLUSIONS

Both the general and oral health of CHF patients were important in treatment with a MAD. The MAD therapy had no severe effect on the masticatory system and edentulous patients could be treated.

Heart failure therapy at a crossroad: are there limits to the neurohormonal model?

The advent of neurohormonal blockade in heart failure (HF) has been an overwhelming success, but current evidence points to a ceiling effect as newer neurohormonal targets are exploited in an incremental manner. This has lead us to question whether the neurohormonal model of HF can be sustained by simply stacking multiple neurohormonal or cytokine blockers together as treatment. A unifying theme in some of these disparate trials relates to either a lack of efficacy or, more importantly, adversity resulting in regression of already achieved benefits. It is our contention that the available evidence has uncovered the remarkable complexity of interaction within the context of the neurohormonal construct. As we stand at a crossroad in HF and begin to fervently pursue non-neurohormonal therapeutic targets, we must also direct attention at navigating the multifaceted labyrinth of the neurohormonal model that has led to the current imbroglio.

Interpreting heart rate variability sleep/wake patterns in cardiac patients

Heart rate variability (HRV) measurement is an important tool in cardiac care that can provide clinicians and researchers with a 24-hour noninvasive measure of autonomic nervous system activity. Sleep and wake have profoundly different effects on HRV patterns and therefore significant implications for HRV interpretation. This article provides a brief overview of the processes underlying HRV, the standard measures of HRV, a basic overview of wake and sleep, the HRV patterns associated with different sleep and wake states, and the patterns of HRV exhibited in common cardiac conditions. The article concludes with an overview of some general health history factors that are important to consider when interpreting HRV patterns in the clinical and research setting.

Effect of ventilatory drive on carbon dioxide sensitivity below eupnea during sleep

We determined the effects of changing ventilatory stimuli on the hypocapnia-induced apneic and hypopneic thresholds in sleeping dogs. End-tidal carbon dioxide pressure (PET(CO2)) was gradually reduced during non-rapid eye movement sleep by increasing tidal volume with pressure support mechanical ventilation, causing a reduction in diaphragm electromyogram amplitude until apnea/periodic breathing occurred. We used the reduction in PET(CO2) below spontaneous breathing required to produce apnea (DeltaPET(CO2)) as an index of the susceptibility to apnea. DeltaPET(CO2) was -5 mm Hg in control animals and changed in proportion to background ventilatory drive, increasing with metabolic acidosis (-6.7 mm Hg) and nonhypoxic peripheral chemoreceptor stimulation (almitrine; -5.9 mm Hg) and decreasing with metabolic alkalosis (-3.7 mm Hg). Hypoxia was the exception; DeltaPET(CO2) narrowed (-4.1 mm Hg) despite the accompanying hyperventilation. Thus, hyperventilation and hypocapnia, per se, widened the DeltaPET(CO2) thereby protecting against apnea and hypopnea, whereas reduced ventilatory drive and hypoventilation narrowed the DeltaPET(CO2) and increased the susceptibility to apnea. Hypoxia sensitized the ventilatory responsiveness to CO2 below eupnea and narrowed the DeltaPET(CO2); this effect of hypoxia was not attributable to an imbalance between peripheral and central chemoreceptor stimulation, per se. We conclude that the DeltaPET(CO2) and the ventilatory sensitivity to CO2 between eupnea and the apneic threshold are changeable in the face of variations in the magnitude, direction, and/or type of ventilatory stimulus, thereby altering the susceptibility for apnea, hypopnea, and periodic breathing in sleep.

Sleep-Breathing Disorders and Heart Failure

Cheyne-Stokes respiration is known to be associated with severe left heart failure. Because of severe desaturation, sleep fragmentation, arousals, and an increase in sympathetic activity, Cheyne-Stokes respiration may lead to a further impairment of cardiac function and to a worsening of quality of life. Although the pathology of Cheyne-Stokes respiration is not fully understood, enhanced chemoreceptor sensitivity, prolonged circulation time, as well as decreased pulmonary gas stores and increased ventilatory drive may be contributing factors. Therapeutic options include the improvement of cardiac failure; medical treatment, such as using theophylline; continous positive airway pressure ventilation; and low-flow oxygen supply. Because of severe cardiac insufficiency, change of endothoracic pressure may worsen the hemodynamic situation in some patients. Therefore, this form of treatment has to be used carefully. Another possible treatment is a low-flow oxygen supply, which will prevent severe desaturations. This therapeutic approach might be a good alternative to noninvasive ventilation. However, it is controversial whether oxygen supply will improve quality of sleep of the patients, even in long-term treatment.

Hemodynamic effects of simulated obstructive apneas in humans with and without heart failure

STUDY OBJECTIVES

To determine whether generation of negative intrathoracic pressure during apnea would cause more pronounced and sustained reductions in cardiac output in patients with congestive heart failure (CHF) than in healthy subjects.

DESIGN

Physiologic intervention study.

SETTING

Cardiorespiratory physiology laboratory.

PARTICIPANTS

Nine patients with CHF and nine healthy control subjects matched for age and sex.

INTERVENTIONS

Patients with CHF and healthy subjects generated - 30 cm H(2)O of intrathoracic pressure during 15-s Mueller maneuvers (MMs) to simulate the acute hemodynamic effects and aftereffects of obstructive apneas.

RESULTS

In both groups, MMs caused an immediate rise in left ventricular transmural pressure during systole (LVPtmsys) [p < 0.05], but in CHF patients, this immediate increase was followed by a significant drop in LVPtmsys (p < 0.05), associated with significantly greater reductions in systolic BP and cardiac index than in healthy subjects (- 25 +/- 3 mm Hg vs - 11 +/- 2 mm Hg [p < 0.05] and - 0.53 +/- 0.11 L/min/m(2) vs - 0.15 +/- 0.11 L/min/m(2) [p < 0.05], respectively). Healthy subjects recovered promptly, but in CHF patients, these adverse hemodynamic effects were sustained following release of the MM.

CONCLUSIONS

CHF patients experience more pronounced and sustained reductions in BP and cardiac output both during and following the MM than do healthy subjects. These findings suggest the potential for adverse hemodynamic effects and aftereffects of negative intrathoracic pressure generation during obstructive sleep apnea in patients with CHF.

Arterial baroreceptor and cardiopulmonary reflex control of sympathetic outflow in human heart failure

Several observations indicate that the arterial baroreflex control of sympathetic nerve activity is preserved, even in advanced heart failure. These include: (1) augmentation of muscle sympathetic nerve activity burst amplitude and duration following a premature beat; (2) rapid recognition of changes in blood pressure induced by ventricular arrhythmias; (3) muscle sympathetic alternans and a steep inverse relationship between changes in diastolic pressure and the subsequent sympathetic burst amplitude during pulsus alternans; (4) similar inhibition of muscle sympathetic nerve activity in subjects with normal and impaired left ventricular systolic function by increases in intrathoracic aortic transmural pressure; (5) documentation, by cross-spectral analysis, of similar gain in the transfer function between blood pressure and muscle sympathetic nerve activity in these two groups; and (6) during sodium nitroprusside infusion, similar reflex increases in total body norepinephrine spillover in normal and heart-failure subjects. When nonhypotensive lower-body negative pressure was applied to test the hypothesis that selective reduction of atrial and pulmonary pressures would exert a cardiac sympathoinhibitory response in heart failure, there was no effect in control subjects, but cardiac norepinephrine spillover fell by 25% (P < .05) in those with systolic dysfunction. In summary, human heart failure is characterized by a rapidly responsive and sensitive arterial baroreflex, and by activation of a cardiac sympathoexcitatory reflex related to increased cardiopulmonary filling pressures.

Noninvasive pressure preset ventilation for the treatment of Cheyne-Stokes respiration during sleep

Cheyne-Stokes respiration (CSR) during sleep is common in patients with congestive heart failure (CHF). This pattern of breathing fragments sleep, leading to daytime symptoms of sleepiness and fatigue. It was hypothesized that by controlling CSR with noninvasive pressure preset ventilation (NPPV), there would be a decrease in sleep fragmentation and an improvement in sleep quality. Nine patients (eight males, one female; mean +/- SD 65 +/- 11 yrs) with symptomatic CSR diagnosed on overnight polysomnography (apnoea/hypopnoea index (AHI) 49 +/- 10 x h(-1), minimum arterial oxygen saturation (Sa,O2, 77 +/- 7%) and CHF (left ventricular ejection fraction 25 +/- 8%) were studied. After a period of acclimatization to NPPV (variable positive airway pressure (VPAP) II ST, Sydney, NSW, Australia and bilevel positive airway pressure (BiPAP), Murraysville, PA, USA), sleep studies were repeated on therapy. NPPV almost completely abolished CSR in all patients with a reduction in AHI from 49 +/- 10 to 6 +/- 5 x h(-1) (p<0.001). Residual respiratory events were primarily due to upper airway obstruction at sleep on-set. Arousal index was markedly decreased from 42 +/- 6 to 17 +/- 7 x h(-1) (p <0.001). Sleep architecture showed a trend toward improvement with a reduction in stage 1 and 2 (79 +/- 7% during the diagnostic night versus 72 +/- 10% during NPPV, (p=0.057)), whilst sleep efficiency, slow-wave sleep (SWS), and rapid eye movement (REM) were not altered. Controlling Cheyne-Stokes respiration with noninvasive pressure preset ventilation resulted in reduced arousal and improved sleep quality in the patients with congestive heart failure. Noninvasive pressure preset ventilation should be considered a potential therapy for Cheyne-Stokes respiration in congestive heart failure in those patients who do not respond or fail to tolerate nasal continuous positive airway pressure therapy.

Left ventricular dysfunction, pulmonary hypertension, obesity, and sleep apnea

The purpose of this study was to determine the frequency of central and obstructive sleep apnea in adult patients who have echocardiographic evidence of left ventricular dysfunction and pulmonary hypertension. Subjects with left ventricular dysfunction, pulmonary hypertension (pulmonary artery systolic pressure >30 mm Hg) and no lung disease were evaluated for risk factors associated with pulmonary hypertension. Of eight eligible adults, six completed the study. Subjects were from suburban and inner city family practices. Spirometric assessment, pulse oximetry on room air, rheumatologic evaluation, polysomnography, and additional history were taken. All six subjects had sleep apnea (apnea-plus-hypopnea index, or AHI, > or = 20): obstructive, central, or mixed. All were obese, and almost all the subjects had a restrictive pattern on spirometry, which is consistent with obesity. All had a pulmonary artery systolic blood pressure of 35 mm Hg or greater. None had daytime hypoxemia or collagen vascular disease, and none had ever used appetite suppressants. This study found a strong association between pulmonary hypertension and obstructive or central sleep apnea in obese patients with congestive heart failure (CHF). We propose that a pulmonary artery systolic pressure of 35 mm Hg or greater in ambulatory patients with CHF may signify an increased risk of sleep apnea.

Patients with congestive heart failure and their conceptions of their sleep situation

AIM

To describe, from a nursing perspective, how patients with CHF conceived their sleep situation.

BACKGROUND

Sleep disturbances are very common in patients with congestive heart failure (CHF). Polysomnographic studies have shown that the total duration of sleep is shorter and the sleep structure disturbed, with frequent arousals and sleep stage changes.

METHODS

A qualitative descriptive design inspired by a phenomenographic approach was employed. Conceptions were collected through interviews with 20 strategically chosen CHF patients.

FINDINGS

The findings showed that the patients' sleep was affected by their daily activities, the disease itself and cardiac symptoms. The sleep disturbances gave effects such as fatigue, listlessness, loss of concentration and loss of temper. These effects led to a need for daytime sleep, seclusion, counselling and information. Patients handled their sleep disturbances through coping mechanisms related to developed patterns of daily life and through support from their psychosocial environment.

CONCLUSIONS

Through an increased awareness of the causes of sleep disturbances in CHF patients, nurses can more effectively meet their caring needs and reduce the psychological stressors that patients develop. Information and education, both to patients and the next of kin, about the disease and the sleep situation, especially good sleeping habits, can help patients to better cope with sleep disturbances.

New graphical method for evaluating gas exchange in congestive heart failure

PURPOSE

The purpose of this study was to determine the usefulness of a new graphical method for evaluating gas exchange (GE), as applied to three common averaging techniques used during rest, exercise, and recovery in patients with congestive heart failure (CHF).

METHODS

Fifty patients' resting, exercise, and recovery GE graphs were evaluated using a new graphical method to determine oxygen consumption (VO2), ventilation (VE), and respiratory exchange ratio (RER). Precision of the new method was evaluated by repeated measures using the same investigator, an additional investigator, and an untrained student. A 1-min rolling time average (RTA) updated every 20 s was used as the reference to the graphical method and the metabolic cart's computer analysis. Breath-by-breath data were evaluated using three averaging techniques, average 5 of 7 breaths (5/7), average of 8 breaths (AVG 8), and 30-s average (30-s). Differences between the computer and the graphical method were assessed by two-sided t-tests; ANOVA was used between the three investigators and also between the three averaging techniques.

RESULTS

There were no significant differences of repeated measures between trained and untrained investigators. The graphical method results were significantly different than the computer results. The only difference with RTA involved computer resting values, which were also different than the graphical method.

CONCLUSION

In conclusion, the graphical method may be used to standardize GE evaluation because it displays excellent intrainvestigator precision and good interinvestigator precision between experienced and inexperienced investigators. Averaging techniques less than 30 s have greater bias when using computer-derived values, although when incorporating the graphical method the averaging technique chosen has little influence on all measures.

Sleep apnea in heart transplant recipients: type, symptoms, risk factors, and response to nasal continuous positive airway pressure

BACKGROUND

We determined the type, symptoms, and risk factors for sleep apnea in heart transplant recipients and the response to nasal continuous positive airway pressure.

METHODS

A retrospective study on heart transplant recipients with sleep apnea was conducted in a tertiary care medical center with follow-up telephone interviews. Between February 1988 and August 1998, 147 patients underwent orthotopic heart transplantation at our institution. Seventeen patients (11.6%) who were suspected of having sleep apnea underwent polysomnography at a mean interval of 17.5 months after transplantation.

RESULTS

All were diagnosed with sleep apnea: 13 had obstructive sleep apnea and 4 had mixed sleep apnea. Mean age at polysomnography was 50.8 years (range, 24-67 years). The patients presented with snoring (100%), excessive daytime somnolence (65%), witnessed apneas (53%), and morning fatigue (53%). Sixteen (94%) had a mean weight gain of 10.4 kg after transplantation, and 1 patient lost 14.6 kg. In the 11 patients with obstructive sleep apnea who underwent nasal continuous positive airway pressure titration, significant improvements occurred in the apnea-hypopnea index (decreased from 37.6 to 10.4; p = 0.01) and mean arousal index (decreased from 44.5 to 19.4; p = 0.01). Only 2 of the 8 patients with sleep apnea for whom nasal continuous positive airway pressure was recommended continued to use it at the time of telephone follow-up.

CONCLUSIONS

Sleep apnea, especially obstructive sleep apnea, occurs frequently in heart transplant recipients. Obstructive sleep apnea appears to present in the typical manner, and although a positive response to nasal continuous positive airway pressure can be documented by polysomnography, long-term use of nasal continuous positive airway pressure may be low.