Atrial Overdrive Pacing in Patients with Sleep Apnea with Implanted Pacemaker
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美国呼吸和危急护理医学 2005年第7期
Department of Cardiology and Pneumology, Georg-August-Universitt, Gttingen, Germany
ABSTRACT
Rationale: Atrial overdrive pacing markedly improved sleep-disordered breathing in a recent study. Objectives: Using a single-blind, randomized, crossover design, we aimed to reproduce these findings and investigate the possible underlying mechanisms. Methods: Twenty ambulatory patients with an implanted pacemaker or cardioverter defibrillator were studied by polysomnography on 3 consecutive nights in a randomized, single-blind, crossover study in which devices were programmed for nonpacing or for overdrive pacing at 7 or 15 beats/minute faster than the mean nocturnal heart rate. Ventilation and biomarkers (urinary norepinephrine excretion, amino-terminal portion of the precursor of brain natriuretic peptide, or NT-proBNP, were also evaluated. Measurements and Main Results: Neither the primary endpoint apneaeChypopnea index, nor the apnea index, oxygen desaturation, ventilation, or biomarkers were affected by the nocturnal atrial overdrive pacing. A small, clinically insignificant, rate-dependent reduction in the hypopnea index was evoked by pacing (nonpacing, 13.4 ± 1.4; pacing 7, 12.9 ± 1.4; pacing 15, 10.9 ± 1.0; p < 0.01, analysis of variance). Conclusions: The lack of effect on the apneaeChypopnea index means that atrial overdrive pacing is inappropriate for treating sleep-disordered breathing.
Key Words: pacing randomized trial sleep apnea
Obstructive sleep apnea (OSA) has a high prevalence within the general population, which will rise further given the obesity epidemic at hand (1). OSA is associated with arterial hypertension and increased cardiovascular morbidity (2eC4). Continuous positive airway pressure is an effective therapy for OSA. However, this therapy is often difficult to tolerate, and patients frequently stop using it because of discomfort. The nasal mask interface may cause pressure sores, claustrophobia, nasal congestion, and other side effects that lead to suboptimal compliance (5eC7).
The finding that atrial overdrive pacing reduces the number of central sleep apnea and OSA episodes by approximately 50% (8) is tantalizing, because it might give rise to a new therapeutic concept (9). Two key underlying mechanisms of the effect of atrial overdrive pacing in sleep apnea have been suggested: (1) overdrive pacing can improve cardiac output (10) as well as pulmonary congestion and thereby reduce hyperventilation and central apneas (11) and (2) overdrive pacing counteracts nocturnal hypervagotonia by influencing cardiac vagal or sympathetic afferent neurons, thus affecting ventilation and stabilizing respiration (11). However, these concepts have not been proven (9).
Biomarkers of neurohumoral activation, such as plasma and urinary norepinephrine as well as brain natriuretic peptide (BNP) plasma concentration, are elevated in OSA (12eC14) and are inversely correlated with left ventricular function and prognosis in patients with heart failure (15, 16). It is possible that the increase in heart rate caused by overdrive pacing impacts on systolic or diastolic left ventricular function and thus on these biomarkers.
Garrigue and colleagues (8) performed atrial overdrive pacing with a rate that was arbitrarily set at 15 beats/minute faster than the mean nocturnal heart rate. It remains to be investigated whether overdrive pacing at a lower rate has the same beneficial effect on sleep-disordered breathing.
Using a single-blind, randomized, crossover design, we investigated the effects of nocturnal atrial overdrive pacing on sleep-disordered breathing, minute ventilation, and biomarkers in patients with an implanted pacemaker (PM) or implanted cardioverter defibrillator (ICD). Overdrive pacing was performed with a rate of 7 as well as 15 beats/minute faster than the mean nocturnal heart rate. Some of the results of these studies have been previously reported in the form of abstracts (17, 18).
METHODS
Patient Selection
Patients in our outpatient clinic with PMs and ICDs with stable sinus rhythm and a dual-chamber device implanted were screened for sleep apnea with an ambulatory device (Somnocheck Effort; Weinmann, Hamburg, Germany), regardless of any symptoms suggestive of sleep apnea. To evaluate the mean nocturnal heart rate by Holter ECG, the PM/ICD was programmed to 40 beats/minute (dual-chamber AV synchronous sensing and pacing). The inclusion criterion was an apneaeChypopnea index (AHI) greater than 15/hour with associated oxygen desaturations of more than 4%. The exclusion criteria were as follows: chronic atrial arrhythmias, myocardial infarction within 1 month of the study, decompensated heart failure, and age groups younger than 18 years or older than 75 years. Written, informed consent was obtained from each patient, and the study was approved by the University of Gttingen Institutional Review Board.
Protocol
Patients underwent full-night polysomnography for 3 consecutive nights in a randomized single-blind crossover design. In the 3 nights, the PM/ICD was programmed either to a backup rate of 40 beats/minute (nonpacing) or to an atrial overdrive pacing rate of 7 or 15 (pacing 7 or 15) beats higher than the mean nocturnal heart rate of the screening night. Before the first night, an ECG, lung function tests, and echocardiography were performed. For further information, see the online supplement.
Polysomnography
An EEG, electrooculogram, EMG, and ECG were recorded as previously described (19). Airflow was recorded by nasal pressure, whereas thorax and abdominal wall motion was monitored by Respitrace (Ambulatory Monitoring Inc., Ardsley, NY), as detailed later. Arterial oxygen saturation was measured transcutaneously by pulse oximetry (Healthdyne Technologies, Inc., Marietta, GA). The polysomnogram was visually analyzed with a computer system (ALICE IV; Heinen and Lwenstein, Bad Ems, Germany) as already described. An apnea was considered obstructive when nasal flow was absent in the presence of abdominal or thoracic movements, and central when movements were absent as well. Central hypopneas were defined as a 50% or greater reduction in VT from the baseline value for at least 10 seconds with proportional in-phase reductions in ribcage and abdominal movements. Obstructive hypopneas were similarly defined, except that out-of-phase thoracoabdominal motion had to be present (20). Sleep stages and arousals were evaluated according to standard criteria (21, 22).
Ventilation, Holter Monitoring, Blood Pressure, and Biomarkers
Respiratory rate and VT were registered by calibrated respiratory inductive plethysmography (Respitrace) as previously described (23). Blood pressure was measured noninvasively by sphygmomanometry (Dinamap XL monitor, Model 9302; Johnson and Johnson Medical, Inc., Tampa, FL) once every hour. Blood samples were taken each morning directly after waking. Urine was collected overnight. For details, see online supplement.
Statistical Analysis
Variables are given as mean ± SEM. The primary endpoint was the AHI. Repeated-measure analysis of variance was used for comparison of the 3 nights. For the secondary endpoints (apnea index and hypopnea index), analysis of variance with Bonferroni's correction was applied. If the analysis of variance revealed significant differences, a paired t test with Bonferroni's correction as a post hoc test was performed. Two-tailed tests were used, and significance was recognized at a value of p < 0.05.
RESULTS
Subject Characteristics
From May to December 2003, 655 patients visited our PM and ICD outpatient clinic. Of these, 189 patients were excluded by the age criterion, and 215 patients were excluded because they had a single-chamber device implanted. Of the remaining 251 patients, 130 gave written, informed consent and fulfilled the remaining inclusion criteria. Suspected sleep apnea in the ambulatory measurement (AHI > 15/hour with associated oxygen desaturations > 4%) appeared in 28 cases. Of these, eight patients failed an inclusion criterion or withdrew consent after screening.
Twenty predominately male and overweight patients were included in the study (Table 1). Ten patients had PMs implanted, and 10 had implanted ICDs. Indications for implantation were ventricular tachycardia/fibrillation in nine, atrioventricular block in seven, sick sinus syndrome in three, and prophylactic indication in one patient. An underlying heart disease was apparent in 13 patients: namely, coronary artery disease in 10, dilated cardiomyopathy in two, and a Brugada syndrome in one patient. Diuretics were prescribed to 10 patients, -blockers to nine, and angiotensin-converting enzyme inhibitors/angiotensin II receptor antagonists to 12 patients. Amiodarone was taken by six patients, and digitalis by two patients.
Heart Rate
In the pacing nights, an effective stimulation with a significant increase in mean heart rate was revealed by the 24-hour Holter monitoring (Table 2). The minimum heart rate in the nonpacing night was 50.0 ± 3.1 beats/minute; with pacing 7, it was 54.5 ± 2.5 beats/minute; and with pacing 15, it was 60.6 ± 2.7 beats/minute (p < 0.001). Maximum heart rate with nonpacing was 66.2 ± 2.6 beats/minute, increased to 72.7 ± 2.4 beats/minute by pacing 7, and to 75.8 ± 2.5 beats/minute during pacing 15 (p < 0.01). The percentage of atrial pacing was 10.8 ± 6.1% during nonpacing; it increased during pacing 7 to 87.7 ± 3.2% and during pacing 15 to 94.6 ± 1.5% (p < 0.0001).
Respiratory Events
The patients suffered from moderate sleep apnea. The predominant type was obstructive (Table 1). The hypopneas were both central (central hypopnea index, 8.2 ± 1.6/hour) and obstructive (obstructive hypopnea index, 5.6 ± 1.1/hour). Central apneas rarely occurred (central apnea index, 1.8 ± 0.9/hour); obstructive apneas were detected more frequently (obstructive apnea index, 5.2 ± 0.8/hour). Pacing did not result in a significant change of either the AHI (p = 0.07, analysis of variance) or of the apnea index. There was a small but significant decrease of the hypopnea index (p < 0.05 after Bonferroni's correction for multiple comparisons; Figure 1 and Table 2). The paired t test (again with Bonferroni's correction) revealed a significant difference only between baseline and pacing 15. When obstructive and central apneas as well as obstructive and central hypopneas were analyzed separately, no significant effects of pacing were seen.
Ventilation, Sleep, Biomarkers, and Blood Pressure
As shown in Table 2, pacing did not affect sleep, nocturnal ventilation, or biomarkers. Similarly, the mean nocturnal blood pressure showed no significant difference.
DISCUSSION
In this randomized, single-blind, crossover study, nocturnal atrial overdrive pacing did not affect the primary endpoint AHI nor did it improve oxygen desaturation. Nevertheless, there was a significant but minor, and thus therapeutically not relevant, reduction in the hypopnea index. Other novel findings were that higher pacing rates had a stronger effect on hypopneas as compared with lower rates, and ventilation as well as biomarkers were not affected by pacing.
Our results appear to differ from those in a previously published article by Garrigue and colleagues (11), who described a reduction of over 50% in apneas, hypopneas, and oxygen desaturations. These discrepancies might be explained by the differences in patient characteristics. As compared with the study by Garrigue and colleagues, our patients were slightly younger (63 vs. 69 years) and had a lower ejection fraction (47 vs. 54%), and more of them had predominant OSA (18 of 20 vs. 7 of 15 patients) (11). Body mass index as well as underlying heart disease cannot be compared, because these data were not given in the former study. It thus seems possible that the higher proportion of predominant obstructive apneas in our population might account for the differences. Moreover, in contrast to Garrigue and colleagues' population, only 10 of the 20 patients investigated in our study had an indication for device implantation for bradycardia. Accordingly, our patients had a higher mean nocturnal heart rate in the nonpacing night. As mentioned later, a low nocturnal heart rate might contribute to central apnea.
Effects of Pacing on Heart Rate and Hemodynamics
Our knowledge of the effects of pacing dates back to 1871, when Bowditch described the positive forceeCfrequency relation in isolated hearts. Further work with animals and humans clearly revealed improvements in left ventricular contractility and diastolic filling, particularly after atrial pacing (24). However, more recent work by us and others suggests a low or even negative forceeCfrequency relation with impaired left ventricular systolic and diastolic function and calcium handling in older subjects as well as in patients with heart failure (24eC26). These findings were independent of the pacing site and thus cannot be explained by pacing-induced ventricular desynchronization. Of note, there are no human studies evaluating the long-term effects of pacing-induced, slightly accelerated heart rates. It is known, however, that increasing periods of ventricular pacing cause increased risk of heart failure, probably from ventricular desynchronization by right ventricular pacing (27). In the study by Garrigue and colleagues (8), the mean nocturnal heart rate was 51 beats/minute as compared with 55 beats/minute in our patient population. The acute hemodynamic effect of pacing depends largely on the basal heart rate. The same absolute increase in heart rate with pacing will induce a higher increase in cardiac output if the basal heart rate is low as compared with a higher basal heart rate (10). Thus, the difference in nocturnal basal heart rate might contribute to the more pronounced effect of pacing in the study by Garrigue and coworkers. Furthermore, pacemaker implantation in six patients with pronounced bradycardia but normal ejection fraction was effective in reducing mainly Cheyne-Stokes respiration in an uncontrolled case series (28).
Effects on Central and Obstructive Events and Ventilation
When explaining the effects of nocturnal overdrive pacing on sleep-disordered breathing, two key mechanisms linking overdrive pacing with ventilation were put forward (11): (1) Pacing might counteract nocturnal hypervagotonia by influencing cardiac vagal or sympathetic afferent neurons (11). Furthermore, pulmonary vagal afferents to the medullary respiratory control center stimulate ventilation. However, whether cardiac afferents impact on ventilation is unknown (11). (2) Overdrive pacing might improve cardiac function, and thus pulmonary congestion might be ameliorated in patients with heart failure or bradycardia. In patients with heart failure, pulmonary congestion causes activation of pulmonary J receptors, thereby inducing hyperventilation with hypocapnia, and thus destabilizing ventilatory control and favoring central sleep apnea (29). However, in our patients, we were unable to prove the hypothesis that overdrive pacing evokes a significant ventilatory response.
It was speculated that pacing—by impacting on cardiac function and thereby on the ventilatory control loop as discussed previously (30)—might affect predominantly central hypopnea and apnea (9). Recently published data support this hypothesis (31). In our patients, central apneas only rarely occurred. Thus, the effects of pacing on these events could not be clarified. Central hypopneas were more common, but their reduction after pacing did not reach statistical significance. Further studies using more elaborate tools to distinguish between obstructive and central events might verify the concept that pacing reduces mainly central respiratory events. This is of interest because pacing is frequently applied in the aging population where central respiratory events are common (27).
Sleep
Garrigue and colleagues (11) reported no change in total sleep time, with a clear reduction in the AHI and accordingly in arousals from disordered breathing. Sleep stages and overall arousals were not reported. In our study, sleep stages as well as arousals were not affected by overdrive pacing, thus confirming that pacing per se has no negative effects on sleep.
Neurohumoral Activation
In the present study, overdrive pacing did not influence urinary norepinephrine excretion or amino-terminal proBNP concentration, suggesting that no major negative or positive effects on sympathetic activation or ventricular filling occurred. This is reassuring given the possibility of impaired ventricular function after tachycardia in heart failure or aged myocardium as discussed previously (25, 26). In accord with previous studies, amino-terminal proBNP was increased in our patients as compared with 48 healthy elderly subjects investigated previously in our department (median, 42 [range, 10eC118] pg/ml).
Limitations
Limitations include, first, the single-blind study design. In mitigation, this approach was adopted so as to maximize patient safety. Also, even though the data were obtained in a single-blind fashion, quantification of ventilation, sleep, and biomarkers was made by two observers blinded to subject and intervention (L.L., D.D.). Second, we used calibrated respiratory inductance plethysmography. This method extrapolates semiquantitative measures of chest wall movement to derive quantitative, approximate measures of minute ventilation. In previous studies by others and by our group using the same method, changes in minute ventilation of approximately 15% were detected (23, 32). Thus, we cannot rule out minor effects of pacing on ventilation. More obtrusive methods, such as a tightly fitting face mask, would have been necessary. Furthermore, besides blood pressure and heart rate, no hemodynamic data were obtained; thus, the impact of pacing on hemodynamics has not thoroughly been investigated.
Conclusion
Clinically, the lack of effect on the AHI and oxygen desaturations renders atrial overdrive pacing inappropriate for treating sleep-disordered breathing. Nevertheless, regarding pathophysiology, the heart rateeCdependent reduction in hypopneas sheds light on the complexity of the respiratory control mechanisms and mandates further investigation.
This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org
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ABSTRACT
Rationale: Atrial overdrive pacing markedly improved sleep-disordered breathing in a recent study. Objectives: Using a single-blind, randomized, crossover design, we aimed to reproduce these findings and investigate the possible underlying mechanisms. Methods: Twenty ambulatory patients with an implanted pacemaker or cardioverter defibrillator were studied by polysomnography on 3 consecutive nights in a randomized, single-blind, crossover study in which devices were programmed for nonpacing or for overdrive pacing at 7 or 15 beats/minute faster than the mean nocturnal heart rate. Ventilation and biomarkers (urinary norepinephrine excretion, amino-terminal portion of the precursor of brain natriuretic peptide, or NT-proBNP, were also evaluated. Measurements and Main Results: Neither the primary endpoint apneaeChypopnea index, nor the apnea index, oxygen desaturation, ventilation, or biomarkers were affected by the nocturnal atrial overdrive pacing. A small, clinically insignificant, rate-dependent reduction in the hypopnea index was evoked by pacing (nonpacing, 13.4 ± 1.4; pacing 7, 12.9 ± 1.4; pacing 15, 10.9 ± 1.0; p < 0.01, analysis of variance). Conclusions: The lack of effect on the apneaeChypopnea index means that atrial overdrive pacing is inappropriate for treating sleep-disordered breathing.
Key Words: pacing randomized trial sleep apnea
Obstructive sleep apnea (OSA) has a high prevalence within the general population, which will rise further given the obesity epidemic at hand (1). OSA is associated with arterial hypertension and increased cardiovascular morbidity (2eC4). Continuous positive airway pressure is an effective therapy for OSA. However, this therapy is often difficult to tolerate, and patients frequently stop using it because of discomfort. The nasal mask interface may cause pressure sores, claustrophobia, nasal congestion, and other side effects that lead to suboptimal compliance (5eC7).
The finding that atrial overdrive pacing reduces the number of central sleep apnea and OSA episodes by approximately 50% (8) is tantalizing, because it might give rise to a new therapeutic concept (9). Two key underlying mechanisms of the effect of atrial overdrive pacing in sleep apnea have been suggested: (1) overdrive pacing can improve cardiac output (10) as well as pulmonary congestion and thereby reduce hyperventilation and central apneas (11) and (2) overdrive pacing counteracts nocturnal hypervagotonia by influencing cardiac vagal or sympathetic afferent neurons, thus affecting ventilation and stabilizing respiration (11). However, these concepts have not been proven (9).
Biomarkers of neurohumoral activation, such as plasma and urinary norepinephrine as well as brain natriuretic peptide (BNP) plasma concentration, are elevated in OSA (12eC14) and are inversely correlated with left ventricular function and prognosis in patients with heart failure (15, 16). It is possible that the increase in heart rate caused by overdrive pacing impacts on systolic or diastolic left ventricular function and thus on these biomarkers.
Garrigue and colleagues (8) performed atrial overdrive pacing with a rate that was arbitrarily set at 15 beats/minute faster than the mean nocturnal heart rate. It remains to be investigated whether overdrive pacing at a lower rate has the same beneficial effect on sleep-disordered breathing.
Using a single-blind, randomized, crossover design, we investigated the effects of nocturnal atrial overdrive pacing on sleep-disordered breathing, minute ventilation, and biomarkers in patients with an implanted pacemaker (PM) or implanted cardioverter defibrillator (ICD). Overdrive pacing was performed with a rate of 7 as well as 15 beats/minute faster than the mean nocturnal heart rate. Some of the results of these studies have been previously reported in the form of abstracts (17, 18).
METHODS
Patient Selection
Patients in our outpatient clinic with PMs and ICDs with stable sinus rhythm and a dual-chamber device implanted were screened for sleep apnea with an ambulatory device (Somnocheck Effort; Weinmann, Hamburg, Germany), regardless of any symptoms suggestive of sleep apnea. To evaluate the mean nocturnal heart rate by Holter ECG, the PM/ICD was programmed to 40 beats/minute (dual-chamber AV synchronous sensing and pacing). The inclusion criterion was an apneaeChypopnea index (AHI) greater than 15/hour with associated oxygen desaturations of more than 4%. The exclusion criteria were as follows: chronic atrial arrhythmias, myocardial infarction within 1 month of the study, decompensated heart failure, and age groups younger than 18 years or older than 75 years. Written, informed consent was obtained from each patient, and the study was approved by the University of Gttingen Institutional Review Board.
Protocol
Patients underwent full-night polysomnography for 3 consecutive nights in a randomized single-blind crossover design. In the 3 nights, the PM/ICD was programmed either to a backup rate of 40 beats/minute (nonpacing) or to an atrial overdrive pacing rate of 7 or 15 (pacing 7 or 15) beats higher than the mean nocturnal heart rate of the screening night. Before the first night, an ECG, lung function tests, and echocardiography were performed. For further information, see the online supplement.
Polysomnography
An EEG, electrooculogram, EMG, and ECG were recorded as previously described (19). Airflow was recorded by nasal pressure, whereas thorax and abdominal wall motion was monitored by Respitrace (Ambulatory Monitoring Inc., Ardsley, NY), as detailed later. Arterial oxygen saturation was measured transcutaneously by pulse oximetry (Healthdyne Technologies, Inc., Marietta, GA). The polysomnogram was visually analyzed with a computer system (ALICE IV; Heinen and Lwenstein, Bad Ems, Germany) as already described. An apnea was considered obstructive when nasal flow was absent in the presence of abdominal or thoracic movements, and central when movements were absent as well. Central hypopneas were defined as a 50% or greater reduction in VT from the baseline value for at least 10 seconds with proportional in-phase reductions in ribcage and abdominal movements. Obstructive hypopneas were similarly defined, except that out-of-phase thoracoabdominal motion had to be present (20). Sleep stages and arousals were evaluated according to standard criteria (21, 22).
Ventilation, Holter Monitoring, Blood Pressure, and Biomarkers
Respiratory rate and VT were registered by calibrated respiratory inductive plethysmography (Respitrace) as previously described (23). Blood pressure was measured noninvasively by sphygmomanometry (Dinamap XL monitor, Model 9302; Johnson and Johnson Medical, Inc., Tampa, FL) once every hour. Blood samples were taken each morning directly after waking. Urine was collected overnight. For details, see online supplement.
Statistical Analysis
Variables are given as mean ± SEM. The primary endpoint was the AHI. Repeated-measure analysis of variance was used for comparison of the 3 nights. For the secondary endpoints (apnea index and hypopnea index), analysis of variance with Bonferroni's correction was applied. If the analysis of variance revealed significant differences, a paired t test with Bonferroni's correction as a post hoc test was performed. Two-tailed tests were used, and significance was recognized at a value of p < 0.05.
RESULTS
Subject Characteristics
From May to December 2003, 655 patients visited our PM and ICD outpatient clinic. Of these, 189 patients were excluded by the age criterion, and 215 patients were excluded because they had a single-chamber device implanted. Of the remaining 251 patients, 130 gave written, informed consent and fulfilled the remaining inclusion criteria. Suspected sleep apnea in the ambulatory measurement (AHI > 15/hour with associated oxygen desaturations > 4%) appeared in 28 cases. Of these, eight patients failed an inclusion criterion or withdrew consent after screening.
Twenty predominately male and overweight patients were included in the study (Table 1). Ten patients had PMs implanted, and 10 had implanted ICDs. Indications for implantation were ventricular tachycardia/fibrillation in nine, atrioventricular block in seven, sick sinus syndrome in three, and prophylactic indication in one patient. An underlying heart disease was apparent in 13 patients: namely, coronary artery disease in 10, dilated cardiomyopathy in two, and a Brugada syndrome in one patient. Diuretics were prescribed to 10 patients, -blockers to nine, and angiotensin-converting enzyme inhibitors/angiotensin II receptor antagonists to 12 patients. Amiodarone was taken by six patients, and digitalis by two patients.
Heart Rate
In the pacing nights, an effective stimulation with a significant increase in mean heart rate was revealed by the 24-hour Holter monitoring (Table 2). The minimum heart rate in the nonpacing night was 50.0 ± 3.1 beats/minute; with pacing 7, it was 54.5 ± 2.5 beats/minute; and with pacing 15, it was 60.6 ± 2.7 beats/minute (p < 0.001). Maximum heart rate with nonpacing was 66.2 ± 2.6 beats/minute, increased to 72.7 ± 2.4 beats/minute by pacing 7, and to 75.8 ± 2.5 beats/minute during pacing 15 (p < 0.01). The percentage of atrial pacing was 10.8 ± 6.1% during nonpacing; it increased during pacing 7 to 87.7 ± 3.2% and during pacing 15 to 94.6 ± 1.5% (p < 0.0001).
Respiratory Events
The patients suffered from moderate sleep apnea. The predominant type was obstructive (Table 1). The hypopneas were both central (central hypopnea index, 8.2 ± 1.6/hour) and obstructive (obstructive hypopnea index, 5.6 ± 1.1/hour). Central apneas rarely occurred (central apnea index, 1.8 ± 0.9/hour); obstructive apneas were detected more frequently (obstructive apnea index, 5.2 ± 0.8/hour). Pacing did not result in a significant change of either the AHI (p = 0.07, analysis of variance) or of the apnea index. There was a small but significant decrease of the hypopnea index (p < 0.05 after Bonferroni's correction for multiple comparisons; Figure 1 and Table 2). The paired t test (again with Bonferroni's correction) revealed a significant difference only between baseline and pacing 15. When obstructive and central apneas as well as obstructive and central hypopneas were analyzed separately, no significant effects of pacing were seen.
Ventilation, Sleep, Biomarkers, and Blood Pressure
As shown in Table 2, pacing did not affect sleep, nocturnal ventilation, or biomarkers. Similarly, the mean nocturnal blood pressure showed no significant difference.
DISCUSSION
In this randomized, single-blind, crossover study, nocturnal atrial overdrive pacing did not affect the primary endpoint AHI nor did it improve oxygen desaturation. Nevertheless, there was a significant but minor, and thus therapeutically not relevant, reduction in the hypopnea index. Other novel findings were that higher pacing rates had a stronger effect on hypopneas as compared with lower rates, and ventilation as well as biomarkers were not affected by pacing.
Our results appear to differ from those in a previously published article by Garrigue and colleagues (11), who described a reduction of over 50% in apneas, hypopneas, and oxygen desaturations. These discrepancies might be explained by the differences in patient characteristics. As compared with the study by Garrigue and colleagues, our patients were slightly younger (63 vs. 69 years) and had a lower ejection fraction (47 vs. 54%), and more of them had predominant OSA (18 of 20 vs. 7 of 15 patients) (11). Body mass index as well as underlying heart disease cannot be compared, because these data were not given in the former study. It thus seems possible that the higher proportion of predominant obstructive apneas in our population might account for the differences. Moreover, in contrast to Garrigue and colleagues' population, only 10 of the 20 patients investigated in our study had an indication for device implantation for bradycardia. Accordingly, our patients had a higher mean nocturnal heart rate in the nonpacing night. As mentioned later, a low nocturnal heart rate might contribute to central apnea.
Effects of Pacing on Heart Rate and Hemodynamics
Our knowledge of the effects of pacing dates back to 1871, when Bowditch described the positive forceeCfrequency relation in isolated hearts. Further work with animals and humans clearly revealed improvements in left ventricular contractility and diastolic filling, particularly after atrial pacing (24). However, more recent work by us and others suggests a low or even negative forceeCfrequency relation with impaired left ventricular systolic and diastolic function and calcium handling in older subjects as well as in patients with heart failure (24eC26). These findings were independent of the pacing site and thus cannot be explained by pacing-induced ventricular desynchronization. Of note, there are no human studies evaluating the long-term effects of pacing-induced, slightly accelerated heart rates. It is known, however, that increasing periods of ventricular pacing cause increased risk of heart failure, probably from ventricular desynchronization by right ventricular pacing (27). In the study by Garrigue and colleagues (8), the mean nocturnal heart rate was 51 beats/minute as compared with 55 beats/minute in our patient population. The acute hemodynamic effect of pacing depends largely on the basal heart rate. The same absolute increase in heart rate with pacing will induce a higher increase in cardiac output if the basal heart rate is low as compared with a higher basal heart rate (10). Thus, the difference in nocturnal basal heart rate might contribute to the more pronounced effect of pacing in the study by Garrigue and coworkers. Furthermore, pacemaker implantation in six patients with pronounced bradycardia but normal ejection fraction was effective in reducing mainly Cheyne-Stokes respiration in an uncontrolled case series (28).
Effects on Central and Obstructive Events and Ventilation
When explaining the effects of nocturnal overdrive pacing on sleep-disordered breathing, two key mechanisms linking overdrive pacing with ventilation were put forward (11): (1) Pacing might counteract nocturnal hypervagotonia by influencing cardiac vagal or sympathetic afferent neurons (11). Furthermore, pulmonary vagal afferents to the medullary respiratory control center stimulate ventilation. However, whether cardiac afferents impact on ventilation is unknown (11). (2) Overdrive pacing might improve cardiac function, and thus pulmonary congestion might be ameliorated in patients with heart failure or bradycardia. In patients with heart failure, pulmonary congestion causes activation of pulmonary J receptors, thereby inducing hyperventilation with hypocapnia, and thus destabilizing ventilatory control and favoring central sleep apnea (29). However, in our patients, we were unable to prove the hypothesis that overdrive pacing evokes a significant ventilatory response.
It was speculated that pacing—by impacting on cardiac function and thereby on the ventilatory control loop as discussed previously (30)—might affect predominantly central hypopnea and apnea (9). Recently published data support this hypothesis (31). In our patients, central apneas only rarely occurred. Thus, the effects of pacing on these events could not be clarified. Central hypopneas were more common, but their reduction after pacing did not reach statistical significance. Further studies using more elaborate tools to distinguish between obstructive and central events might verify the concept that pacing reduces mainly central respiratory events. This is of interest because pacing is frequently applied in the aging population where central respiratory events are common (27).
Sleep
Garrigue and colleagues (11) reported no change in total sleep time, with a clear reduction in the AHI and accordingly in arousals from disordered breathing. Sleep stages and overall arousals were not reported. In our study, sleep stages as well as arousals were not affected by overdrive pacing, thus confirming that pacing per se has no negative effects on sleep.
Neurohumoral Activation
In the present study, overdrive pacing did not influence urinary norepinephrine excretion or amino-terminal proBNP concentration, suggesting that no major negative or positive effects on sympathetic activation or ventricular filling occurred. This is reassuring given the possibility of impaired ventricular function after tachycardia in heart failure or aged myocardium as discussed previously (25, 26). In accord with previous studies, amino-terminal proBNP was increased in our patients as compared with 48 healthy elderly subjects investigated previously in our department (median, 42 [range, 10eC118] pg/ml).
Limitations
Limitations include, first, the single-blind study design. In mitigation, this approach was adopted so as to maximize patient safety. Also, even though the data were obtained in a single-blind fashion, quantification of ventilation, sleep, and biomarkers was made by two observers blinded to subject and intervention (L.L., D.D.). Second, we used calibrated respiratory inductance plethysmography. This method extrapolates semiquantitative measures of chest wall movement to derive quantitative, approximate measures of minute ventilation. In previous studies by others and by our group using the same method, changes in minute ventilation of approximately 15% were detected (23, 32). Thus, we cannot rule out minor effects of pacing on ventilation. More obtrusive methods, such as a tightly fitting face mask, would have been necessary. Furthermore, besides blood pressure and heart rate, no hemodynamic data were obtained; thus, the impact of pacing on hemodynamics has not thoroughly been investigated.
Conclusion
Clinically, the lack of effect on the AHI and oxygen desaturations renders atrial overdrive pacing inappropriate for treating sleep-disordered breathing. Nevertheless, regarding pathophysiology, the heart rateeCdependent reduction in hypopneas sheds light on the complexity of the respiratory control mechanisms and mandates further investigation.
This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org
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