INTRODUCTION

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IT HAS LONG BEEN KNOWN that sleep results in profound state-dependent alterations in heart rate (2, 9, 12, 19, 29, 32). Slow-wave sleep (SWS) is associated with a relatively stable pattern of reduced heart rate characterized by respiratory sinus arrhythmia. Rapid eye movement (REM) sleep results in more labile heart rate and is characterized by abrupt fluctuations (2, 8, 9, 14-16, 23). The most common is a surge in heart rate that is generally accompanied by the baroreflex-mediated deceleration in rate in response to the initial tachycardia (2, 8, 9, 23).

The REM sleep-induced increases in heart rate are accompanied by a striking increase in coronary arterial blood flow that, in canines, achieves 35% over baseline and lasts for 15-20 s (15). These heart rate surges occur mainly during phasic REM sleep (8) and appear to be mediated by the sympathetic nervous system because they are abolished by chronic stellectomy (15). In canines with coronary stenosis, the surge in heart rate results in a decrease in coronary flow through the stenosed vessel (16). REM sleep may have an important impact on arrhythmogenesis clinically, because ventricular arrhythmias (6, 32, 33) and angina (19, 21) are more pronounced during this phase of sleep. The importance of changing autonomic tone to cardiac vulnerability during sleep is further underscored by the recent clinical evidence of a nonuniform distribution of atrial fibrillation (25), sudden death, myocardial infarction, and implantable cardioverter/defibrillator discharge (18).

In the present study, we describe a novel phenomenon of a primary deceleration in heart rate that occurs predominantly during tonic REM sleep. This pattern is distinct from the previously described baroreflex-mediated decelerations in that it is characterized by an abrupt decrease in heart rate without antecedent or subsequent changes in rate.

The main goals of our study were to characterize the primary decelerations in heart rate and to shed light on the underlying central and peripheral autonomic mechanisms. These objectives were accomplished by recording from several CNS structures and by administration of selective autonomic blocking agents that do not cross the blood-brain barrier.

	METHODS

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The study was conducted according to National Institutes of Health standards, and protocols were approved by the Harvard Medical Area Standing Committee on Animal Use. The animals were housed in 1.2 × 1.2-m cages, and food and water were provided ad libitum. A 12:12-h light-dark cycle was maintained.

Surgical preparation. Seven adult male cats weighing between 2 and 2.5 kg were anesthetized with halothane (1-2%) and chronically implanted with electrodes to monitor electroencephalogram (EEG), pontogeniculooccipital (PGO) wave activity in lateral geniculate nucleus (LGN) [6.5 anterior (A), 10.0 lateral (L), +12.0 ventral (V)], hippocampal CA1 theta activity (3.3 A, 5.5 L, +17.0 V), electromyogram (EMG), electrooculogram (EOG), respiration, and electrocardiogram (ECG). The stereotaxic coordinates are according to Berman (4). EMG and EOG were monitored from electrodes placed in the nuchal muscle and posterior wall of the orbit, respectively. Respiration was monitored with a pair of diaphragmatic electrodes inserted through a midline incision in the peritoneum and sutured directly onto the costal margin of the muscle. Precordial ECG electrodes were placed subcutaneously. The noncephalic electrode leads were tunnelled subcutaneously to emerge with the cephalic leads in an amphenol pin connector at the top of the head. In three animals, a jugular intravenous catheter was inserted subcutaneously for later autonomic blockade. In two additional animals, a catheter was placed in the carotid artery for blood pressure measurement, tunnelled subcutaneously, and affixed by acrylic to the headcap. Postsurgical antibiotic treatment was administered as needed after consultation with a veterinarian.

Recording procedures. Monitoring was initiated 10-14 days after surgery as the cats slept spontaneously in a sound-attenuated, darkened, 1 × 1 × 1.2-m recording chamber at room temperature (23°C) with a window for behavioral observation. Data from the 4-h afternoon sessions were recorded on a Grass 78 multichannel polygraph with 7P511 amplifiers for alternating current channels and 7P1/7DA amplifiers for direct current channels (Grass Instruments, Quincy, MA). The cable from the amphenol pin connector allowed 360° of rotation so that the animal could move freely in the chamber.

Peripheral autonomic blockade. After control recording of at least one REM episode, the ₁-adrenergic blocker atenolol (0.3 mg/kg iv) and the muscarinic blocker glycopyrrolate (0.1 mg/kg iv) were administered in alternating sequence with a 90- to 120-min interval through the jugular catheter without disturbing the animals. Six repetitions of the protocol were carried out in each of the three cats. The blood-brain barrier is impermeable to these two compounds (11, 17). Dosages were selected to achieve a relatively high degree of receptor blockade without disrupting sleep state.

Data analysis. The criteria for selecting a heart rate deceleration were 1) 20% increase in the R-R interval over the mean for the preceding 6 s and 2) duration 1.4 s. A paper printout was used for visual scanning and reference, and an optical disk copy was analyzed on a Compaq DeskPro Pentium 90 MHz computer under Windows 95. Our customized software provides color-coded plots of the R-R intervals, which make it feasible to identify decelerations that fit the criteria. The thresholds for the decelerations were entered before analysis. Any increase in R-R interval >20% was plotted by the computer in a different color from baseline R-R intervals. The program could highlight a set time surrounding the deceleration so that heart rate preceding and after the event could be measured. For each animal, the number of control and peripherally blocked decelerations during quiet waking, SWS, and phasic and tonic REM sleep was totaled and the duration of each deceleration was measured. The mean R-R intervals for the 6 s immediately preceding and after the deceleration were evaluated. The effects of respiration and of normal sinus arrhythmia were eliminated from the analysis by discarding the decelerations that coincided with diaphragmatic inflections and those that appeared in rhythmic series with respiration rate. In addition, typical decelerations linked to respiration were found to have a duration <1.4 s and were thus excluded by our criterion of duration 1.4 s.

Statistical methods. The two-way ANOVA was used to calculate differences in the relative incidence of events by sleep stage. Bonferroni post-test was used to compare the results among the sleep states. Values are means ± SE (P < 0.05).

	RESULTS

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Our findings demonstrate that tonic REM sleep is associated with marked, transient reductions in heart rate that are not preceded by increases in heart rate (Fig. 1). This event is characterized as an R-R interval increase >20% over the R-R mean value for the preceding 6 s. A second criterion was a duration of 1.4 s. The 44 h of control records analyzed yielded 27.4 h of sleep, with 18.5 h of SWS and 8.8 h of REM sleep. There were 179 heart rate events during sleep that satisfied the deceleration criteria. The mean R-R increase was 34 ± 9%, and the greatest increase was 58%. The return to normal rhythm after the deceleration episode was unremarkable, showing no significant alteration in rate compared with the predeceleration rate (Fig. 2). The great majority (86%) of all decelerations lasted between 1.5 and 3 s, with a mean duration of 2.25 ± 0.7 s.

View larger version (55K): [in this window] [in a new window]   Fig. 1.   Representative polygraphic recording of a primary heart rate deceleration during tonic rapid eye movement sleep (T-REM). During this deceleration, heart rate decreased from 150 to 105 beats/min, or 30%. Deceleration occurred during a period devoid of pontogeniculooccipital (PGO) spikes in lateral geniculate nucleus (LGN) or theta rhythm in hippocampal (CA1) leads. Deceleration is not a respiratory arrhythmia, because it is independent of diaphragmatic movement. Abrupt decreases in amplitude of CA1, PGO waves (LGN), and respiratory amplitude and rate (DIA) are typical of transitions from phasic to tonic REM. EKG, electrocardiogram; EMG, electromyogram; DIA, diaphragm.

View larger version (27K): [in this window] [in a new window]   Fig. 2.   Heart rates before, during, and after decelerations as a function of sleep state. For all sleep states, heart rates during decelerations are statistically different from heart rates for the 6 s preceding and after decelerations (P < 0.001). For all states, there was no statistical difference between heart rates 6 s before and 6 s after each deceleration. During decelerations within slow-wave sleep (SWS), heart rate decreased from 145.6 ± 3.9 before to 102.7 ± 4.4 during decelerations and then recovered to 142.8 ± 1.9 beats/min. In phasic REM sleep (P-REM), heart rate decreased from 140.8 ± 3.1 before to 89.1 ± 2.7 during deceleration and then recovered to 141.0 ± 6.8 beats/min. For T-REM sleep, heart rate decreased from 144.1 ± 3.7 before to 97.3 ± 4.2 during deceleration and then recovered to 137.7 ± 2.6 beats/min. NS, not significant.

In the two cats instrumented for arterial blood pressure measurement, we found that this parameter was relatively stable before and after the heart rate decelerations of tonic REM (Fig. 3). On cessation of PGO activity and concomitant with the heart rate deceleration, there was a moderate but statistically significant reduction in arterial blood pressure, which returned to the predeceleration value on resumption of PGO wave firing. The fact that the pressure change was relatively minor probably accounts for the absence of an appreciable reflex compensatory heart rate increase.

View larger version (33K): [in this window] [in a new window]   View larger version (16K): [in this window] [in a new window]   Fig. 3.   A: representative polygraphic recording of decrease in arterial blood pressure during a primary heart rate deceleration during T-REM. During this deceleration, heart rate decreased 20% from 175 to 138 beats/min and then recovered to 180 beats/min. As shown on blood pressure channel (BP), there is no rise in arterial blood pressure before onset of deceleration, indicating that deceleration is due to central nervous system activation of vagus nerve and is not a reflexly mediated compensatory phenomenon. Deceleration occurred during a period devoid of PGO spikes in LGN. B: mean arterial blood pressure before, during, and after 2 T-REM-induced heart rate decelerations in each of 2 felines. Arterial blood pressure decreased from 65.67 ± 0.58 and 65.72 ± 0.73 mmHg, at 20-11 and 10-1 beats before the deceleration, respectively, to 58.53 ± 0.96 mmHg (* P < 0.001) during deceleration. Blood pressure returned to 64.65 ± 1.00 mmHg (P < 0.001 compared with deceleration BP). Arterial pressure before and after the event did not differ significantly (means ± SE).

All heart rate decelerations included in this analysis are distinct from respiratory sinus arrhythmia. Examination of respiratory-linked decelerations detected through the diaphragmatic tracing during both REM and SWS revealed that the mean R-R increase for sinus arrhythmia was 18%, with a mean duration <1.4 s. Furthermore, heart rate decelerations coinciding with expiratory diaphragmatic inflections or appearing in close rhythmic series were excluded from the analyses. The criteria used in selecting decelerations thus excluded most heart rate decelerations linked to respiratory sinus arrhythmia.

The relationship between heart rate decelerations and sleep states was also explored. SWS composed 42% of the total recording time and 68% of the total sleep period. REM sleep composed 20% of the total recording time and 32% of the total sleep period. The mean proportion of REM sleep spent in tonic activity was ~30%. SWS averaged 1 deceleration per 43.5 min; phasic REM sleep averaged 1 deceleration per 16 min. During tonic REM sleep, decelerations occurred at a rate of 1 per 74 s, a 13-fold increase over phasic REM sleep (P = 0.0002) (Fig. 4). Significantly more decelerations occurred during tonic REM compared with either phasic REM or SWS (P < 0.001, Bonferroni multiple-comparison test). The difference between the number of decelerations during phasic REM and SWS was not significant (P > 0.05).

View larger version (12K): [in this window] [in a new window]   Fig. 4.   Number of heart rate decelerations per minute as a function of sleep state. There were 0.023 ± 0.007 decelerations/min in SWS and 0.063 ± 0.030 decelerations/min in P-REM (NS). Number of decelerations/min in T-REM (0.806 ± 0.100) is significantly greater compared with other sleep states (P < 0.001).

Peripheral autonomic blockade with the mixed muscarinic antagonist glycopyrrolate both alone (Fig. 5) and 90-120 min after -blockade with atenolol immediately abolished heart rate decelerations during REM sleep. At the dosage used, glycopyrrolate produced a mean rate elevation of 49% and atenolol caused a mean heart rate depression of 16%. The effect of glycopyrrolate at this dose also persisted beyond one half-life of the drug. Administration of the -adrenergic antagonist atenolol did not affect the frequency of decelerations (Fig. 6). In two cats, we compared the proportion of recording time spent in REM sleep and the mean duration of REM epochs during 2 h before and after glycopyrrolate and/or atenolol administration. These parameters were not significantly different (P > 0.05), indicating that sleep structure had not been disrupted, perhaps due to the remote intravenous access and the lack of lipophilicity of the two agents.

View larger version (12K): [in this window] [in a new window]   Fig. 5.   Blockade with a single dose of the M2 muscarinic antagonist glycopyrrolate (0.1 mg/kg iv) resulted in immediate elimination of REM sleep-induced heart rate decelerations in all 3 repetitions of this protocol in each of 3 cats. Pretreatment with atenolol did not alter this effect in 3 repetitions in each of 3 cats (data not shown). Sleep architecture was not affected by glycopyrrolate administration. Data analysis was initiated 30 min after cats were placed in sleep chamber. By this time, animals were uniformly asleep.

View larger version (17K): [in this window] [in a new window]   Fig. 6.   -Adrenergic blockade with atenolol (0.3 mg/kg iv) did not affect frequency of deceleration events (P > 0.05). Three repetitions of this protocol were performed in each of three cats. Sleep architecture was not affected by atenolol administration. Data analysis was initiated 30 min after cats were placed in sleep chamber. By this time, animals were uniformly asleep.

	DISCUSSION

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This study describes a novel phenomenon of a primary, abrupt deceleration in heart rhythm that occurs predominantly during tonic REM sleep. It is distinct from previously reported sleep state-dependent perturbations in heart rhythm inasmuch as it occurs in the absence of antecedent acceleration or subsequent change in heart rate or arterial blood pressure. In earlier reports by Baust and Bohnert (2) and Dickerson and co-workers (7), decelerations were almost invariably accompanied by prior accelerations in rate, with the likely involvement of baroreceptor activation. Another distinctive feature is that the deceleration occurs primarily during tonic REM sleep. This differs from the previously observed baroreceptor-mediated decelerations, which occur mainly during transition from SWS to desynchronized sleep and more frequently during phasic REM than any other stage of REM sleep (7). It is surprising that this phenomenon of a primary deceleration has not been previously described. This omission may have been in part due to the fact that previous investigations have relied predominantly on ECG recordings with a highly compressed time scale or on tachograph recordings, which have a relatively long time constant and do not provide accurate representation of beat-to-beat changes. In the present study, the customized software, with preprogrammed, objective criteria for acceleration and deceleration, made it possible to track in exquisite detail the subtle dynamics of beat-to-beat fluctuations throughout the recording period.

Comparison of new observations with previous reports of cardiac decelerations in REM. This phenomenon appears to be distinct from the reductions in heart rate observed by Dickerson and co-workers (7). They found heart rate pauses accompanied by increases in coronary blood flow that were concentrated during transition from SWS to desynchronized sleep and in REM sleep, where they occurred more frequently during phasic than tonic REM sleep. Because the heart rhythm pause was almost invariably preceded by tachycardia and elevations in arterial blood pressure, it is likely that the deceleration in rate was the result of reflex vagal activation secondary to baroreceptor stimulation. This differs from the present findings of decelerations that occur primarily during tonic REM sleep and are not associated with any preceding or subsequent change in heart rate or arterial blood pressure. The only point of similarity is that the phenomena appear to have a common mediator, namely enhanced vagal activity. In the heart rhythm pauses described by Dickerson and co-workers (7), however, vagal activity appears to be due to a reflex response after sinus tachycardia and hypertension. In the present study, the involvement of the vagus appears to be directly initiated by central influences, because there is no antecedent or subsequent change in resting heart rate or arterial blood pressure. The REM sleep-related decelerations reported by Baust and Bohnert (2) in their classic study were also heralded by rate accelerations and were therefore likely to have been part of a reflex response. The heart rate decelerations in the present study were not preceded by a rise in arterial blood pressure, and therefore the present phenomenon does not appear to be attributable to baroreflex phenomenon. Rather, the main mechanism appears to be central activation of the vagus nerve, which slows the heart directly. This surmise is further supported by the fact that the decelerations were completely eliminated by muscarinic blockade with glycopyrrolate.

Probable CNS origins of heart rate decelerations in tonic REM. The primary involvement of CNS activation is demonstrated by the consistent, antecedent abrupt cessation of PGO activity and concomitant interruption of hippocampal theta rhythm, which are salient features of the tonic REM sleep decelerations. This finding had been anticipated based on the established positive relationship between PGO activity and hippocampal theta activity in cats (5, 26). How these changes in CNS activity lead to the tonic REM sleep-induced increase in vagal tone to suppress sinus node activity remains unknown. The literature on the relationship between PGO activity during sleep and cardiac function is sparse. Baust and colleagues (3) found only a relatively minor, baseline rate-dependent, variable response in heart rate (<80 ms R-R interval change) to PGO activity in cats. In normal human volunteers, Taylor and colleagues (30) observed heart rate decelerations during REM sleep that preceded eye bursts by 3 s and suggested that the phenomenon reflects an orienting response at the onset of dreaming. However, because the decelerations were not illustrated nor their magnitude described, their similarity to those we characterize is debatable. Notwithstanding extensive studies of the physiological and anatomic basis for PGO activity, little is known about the conductivity and functional relationship to heart rhythm control during sleep (1, 10, 27, 28).

Possible mechanisms of heart rate decelerations in tonic REM. The most likely basis for the abrupt deceleration in heart rate during tonic REM sleep is a change in the centrally induced pattern of autonomic activity to the heart. This could be the result of a decrease in sympathetic activity or an enhancement of vagal tone, either alone or in combination. We found that cardioselective ₁-adrenergic blockade with atenolol did not affect the incidence or magnitude of decelerations but muscarinic blockade with glycopyrrolate completely abolished the phenomenon. These observations suggest that the tonic REM sleep-induced decelerations are primarily mediated by bursting of cardiac vagal efferent fiber activity. It is well known that enhanced vagal activity can abruptly and markedly affect sinus node firing rate (22). The quaternary ammonium structure of glycopyrrolate limits its passage through the blood-brain barrier, thus minimizing possible confounding CNS effects (11). The agent did not affect REM sleep structure. Therefore, it is unlikely that indirect effects of the drug on brain state contributed to this effect. Because -adrenergic blockade exerted no effect on the frequency or magnitude of decelerations, it does not appear that withdrawal of cardiac sympathetic tone is an important factor in the observed rate changes. Rather, a primary surge in vagal activity is implicated. This does not preclude the fact that cardiorespiratory interactions may moderate heart rate, because this influence operates throughout sleep (14). However, our results suggest that respiratory interplay is not an essential component of the deceleration, because the phenomenon often occurred in the absence of a temporal association with inspiratory effort, as is evident in Fig. 3.

Perspectives