Central and baroreflex control of heart period during the wake-sleep cycle in spontaneously hypertensive rats

Strength and Latency of the HP-SAP Closed Loop Variability Interactions in Subjects Prone to Develop Postural Syncope

The coupling and latency between heart period (HP) and systolic arterial pressure (SAP) variability can be investigated along the two arms of the HP-SAP closed loop, namely along the baroreflex feedback from SAP to HP, and along the feedforward pathway from HP to SAP. This study investigates the HP-SAP closed loop variability interactions through cross-correlation function (CCF). Coupling strength and delay between HP and SAP variability series were monitored in 13 subjects prone to develop orthostatic syncope (SYNC, 28±9 yrs, 5 males) and in 13 subjects with no history of postural syncope (noSYNC, age: 27±8 yrs, 5 males). Analysis was carried out at rest in supine position (REST) and during head-up tilt (TILT) at 60 degrees. The null hypothesis of HP-SAP uncoupling was tested through a surrogate analysis associating the HP series of a subject with a SAP sequence of a different one. Results showed that during TILT the coupling strength increased along the baroreflex and latency augmented along the mechanical feedforward pathway exclusively in noSYNC subjects. Finally, closed loop HP-SAP interactions were detected in about one third of subjects and the situation of full uncoupling was rarely found. CCF analysis was found to be a straightforward and easily applicable method to investigate HP-SAP control deserving a direct comparison with more sophisticated signal processing tools assessing causality.

Unstable sleep and higher sympathetic activity during late-sleep periods of rats: implication for late-sleep-related higher cardiovascular events

We proposed that the higher incidence of sleep fragmentation, sympathovagal imbalance and baroreceptor reflex impairment during quiet sleep may play a critical role in late-sleep-related cardiovascular events. Polysomnographic recording was performed through wireless transmission using freely moving Wistar-Kyoto rats over 24 h. The low-frequency power of arterial pressure variability was quantified to provide an index of vascular sympathetic activity. Spontaneous baroreflex sensitivity was assessed by slope of arterial pressure-RR linear regression. As compared with early-light period (Zeitgeber time 0-6 h), rats during the late-light period (Zeitgeber time 6-12 h) showed lower accumulated quiet sleep time and higher paradoxical sleep time; furthermore, during quiet sleep, the rats showed a lower δ% of electroencephalogram, more incidents of interruptions, higher σ% and higher β% of electroencephalogram, raised low-frequency power of arterial pressure variability value and lower baroreflex sensitivity parameters. During the light period, low-frequency power of arterial pressure variability during quiet sleep had a negative correlation with accumulated quiet sleep time and δ% of electroencephalogram, while it also had a positive correlation with σ% and β% of electroencephalogram and interruption events. However, late-sleep-related raised sympathetic activity and sleep fragmentation diminished when an α1-adrenoceptor antagonist was given to the rats. Our results suggest that the higher incidence of sleep fragmentation and sympathovagal imbalance during quiet sleep may play a critical role in late-sleep-related cardiovascular events. Such sleep fragmentation is coincident with an impairment of baroreflex sensitivity, and is mediated via α1-adernoceptors.

Snoring effects on the baroreflex: an animal model

Baroreflex sensitivity (BRS) is reduced in humans during snoring, however the mechanisms are unknown. We used an anaesthetised rabbit induced snoring (IS) model, to test: (1) whether IS was associated with reduced BRS; and (2) if snoring related vibration transmission to peri-carotid tissues influenced BRS levels. BRS was quantified using the spontaneous sequence technique. During IS, BRS fell by 40%, without any associated change in blood pressure (BP) but accompanied by an increase in heart rate (HR). Direct application of a snore frequency and intensity matched vibratory stimulus to the peri-carotid tissues of non-snoring tracheostomised rabbits had no effect on BRS, HR or BP. In conclusion, IS induced depression of BRS is likely mediated via a HR driven change in BRS operating point that is unrelated to snoring-related vibration transmission to carotid baroreceptors. The anaesthetised IS rabbit provides a model in which mechanistic interactions between snoring and BRS can be further explored.

Mathematical modeling of cardiovascular coupling: Central autonomic commands and baroreflex control

The cross-correlation function (CCF) yields the correlation coefficient between spontaneous fluctuations of heart period and blood pressure as a function of the time shift between these variables. Two CCF patterns occur in humans: I) positive correlation between heart period and previous pressure values; II) negative correlation between heart period and subsequent pressure values. These patterns may result from the baroreflex and central autonomic commands (CAC), respectively. The aim of this study was to test this interpretation with a non-linear mathematical model of the human cardiovascular system. CAC were modeled as either phasic changes or random fluctuations of vagal and sympathetic activities with opposite sign. CCF pattern I resulted from baroreflex buffering of blood pressure changes elicited by vascular resistance fluctuations. When cardiac baroreflex control was absent or outweighed by CAC to the heart, simulations resulted in CCF pattern II only. In intermediate conditions when cardiac baroreflex interacted with CAC to the heart, CCF patterns I and II coexisted because the coupling between heart period and blood pressure varied with time. CAC to the heart decreased in magnitude the correlation coefficient and lengthened the time shift of CCF pattern I, thus apparently slowing and blunting baroreflex effects. Conversely, the baroreflex decreased in magnitude the correlation coefficient of CCF pattern II, thus blunting CAC effects. These results provide theoretical evidence in favor of application of the CCF analysis to investigate the balance between central autonomic and baroreflex cardiac control.

Dysregulation of heart rhythm during sleep in leptin-deficient obese mice

STUDY OBJECTIVES

sleep deeply affects cardiac autonomic control, the impairment of which is associated with cardiovascular mortality. Obesity entails increased cardiovascular risk and derangements in sleep and cardiac autonomic control. We investigated whether cardiac autonomic control is impaired during sleep in ob/ob mice with morbid obesity caused by congenital leptin deficiency.

DESIGN

indexes of cardiac autonomic control based on spontaneous cardiovascular fluctuations were compared between ob/ob and lean wild-type (+/+) mice during wakefulness, non-rapid eye movement sleep (NREMS), and rapid eye movement sleep (REMS).

SETTING

N/A PATIENTS OR PARTICIPANTS: 7 ob/ob and 11 +/+ male mice.

INTERVENTIONS

instrumentation with electrodes for sleep recordings and a telemetric transducer for measuring blood pressure and heart period.

MEASUREMENTS AND RESULTS

In ob/ob mice, the variability of heart period and cardiac baroreflex sensitivity (sequence technique) were significantly lower than in +/+ mice during each wake-sleep state. The vagal modulation of heart period was significantly weaker in ob/ob than in +/+ mice during NREMS and REMS. In ob/ob mice, the cross-correlation function between heart period and blood pressure suggested that the baroreflex contribution to cardiac control was lower than in +/+ mice during wakefulness and NREMS, whereas the contribution of central autonomic commands was lower than in +/+ mice during NREMS and REMS.

CONCLUSIONS

These data indicate a dysregulation of cardiac autonomic control during sleep in ob/ob mice. Ob/ob mice may represent a useful tool to understand the molecular pathways that lead to cardiac autonomic dysregulation during sleep in obesity.

Increased cerebral activity suppresses baroreflex control of heart rate in freely moving mice

We assessed whether increased cerebral activity suppressed baroreflex control of heart rate (HR) and, if so, whether this occurred prior to the onset of locomotion in daily activity of mice. We measured mean arterial pressure (MAP, arterial catheter), cerebral blood flow in the motor cortex (CBF, laser-Doppler flowmetry), and electroencephalogram in free-moving mice (n = 8) during 12 daytime hours. The contribution of baroreflex control of HR to MAP regulation was determined during a total resting period for approximately 8 h from the cross-correlation function (R(t)) between spontaneous changes in HR (HR) and MAP (MAP) every 4 s and the sensitivity was determined from HR/MAP where R(t) was significant (P < 0.05). The power density ratio of theta to delta wave band in electroencephalogram (theta/delta), determined every 4 s as an index of cerebral activity, was positively correlated with CBF during 73 +/- 3% of the total resting period (P < 0.05) and with R(t) during 59 +/- 2% (P < 0.05). When each measurement during the resting period was divided into seven bins according to the level of theta/delta, CBF was 91 +/- 2% in the lowest bin and 118 +/- 3% in the highest bin (P < 0.001), R(t) was 0.69 +/- 0.06 and 0.27 +/- 0.04 (P < 0.001) and HR/MAP (beats min(1) mmHg(1)) was 12.4 +/- 0.9 and 7.5 +/- 0.9 (P < 0.001), respectively, with significant correlations with theta/delta (all P < 0.002). Moreover, mice started to move in approximately 30 sec after the sequential increases of theta/delta and R(t), mice started to move at 5 times higher probability than after a given time, followed by a rapid increase in MAP by approximately 10 mmHg. These results suggest that increased cerebral activity suppresses baroreflex control of HR and this might be related to the start of voluntary locomotion with a rapid increase in MAP.

Central and baroreflex control of heart period during the wake-sleep cycle in consomic rats with different genetic susceptibility to hypertension

1. In spontaneously hypertensive rats (SHR), the contributions of the baroreflex and central autonomic commands to the control of heart period (HP) vary among wake-sleep states and are impaired during quiet wakefulness and rapid eye movement sleep (REMS), respectively. 2. Dahl salt-sensitive (SS) rats are genetically susceptible to salt-sensitive hypertension, the development of which depends on diet. Substitution of chromosome 13 of SS rats with that of Brown Norway rats confers salt-resistance to consomic SS-13BN rats. 3. In the present study, we tested whether differences in the central and baroreflex contributions to HP control occur among wake-sleep states in SS and SS-13BN rats and reflect genetic susceptibility to hypertension. Rats (n = 5 per group) were fed a prohypertensive diet late during development to minimize hypertension in SS rats and were instrumented with an arterial catheter and electrodes for discriminating wake-sleep states. 4. The cross-correlation function between HP and blood pressure indicated that, in SS and SS-13BN rats, the contributions of the baroreflex and central commands to the control of HP differed significantly among wake-sleep states, with central commands outweighing the baroreflex in REMS. However, these contributions did not differ significantly between SS and SS-13BN rats in any wake-sleep state. 5. The data suggest that differences in the central and baroreflex contributions to HP control among wake-sleep states, which have been demonstrated in SHR, can be generalized to other rat models used in hypertension research. Impairments in the baroreflex and central autonomic control of HP during quiet wakefulness and REMS, respectively, cannot be generalized as an index of genetic susceptibility to hypertension.

Sleep Modulates Hypertension in Leptin-Deficient Obese Mice

Leptin increases sympathetic activity, possibly contributing to hypertension in obese subjects. Hypertension increases cardiovascular mortality, with nighttime (sleep) blood pressure having a substantial prognostic value. We measured blood pressure in male leptin-deficient obese mice (ob/ob; n=7) and their lean wild-type littermates (+/+; n=11) during wakefulness, non–rapid-eye-movement sleep, and rapid-eye-movement sleep to investigate whether, in the absence of leptin, derangements of blood pressure are still associated with obesity and depend on the wake-sleep state. Mice were implanted with a telemetric pressure transducer and electrodes for discriminating wake-sleep states. Mean blood pressure was significantly higher in ob/ob than in +/+ mice during wakefulness (7.3±2.6 mm Hg) and non–rapid-eye-movement sleep (6.7±2.8 mm Hg) but not during rapid-eye-movement sleep (2.6±2.6 mm Hg). In ob/ob and +/+ mice, mean blood pressure was substantially higher during wakefulness than during non–rapid-eye-movement sleep. On passing from non–rapid-eye-movement sleep to rapid-eye-movement sleep, mean blood pressure decreased significantly in ob/ob but not in +/+ mice. The time spent during wakefulness was lower in ob/ob than in +/+ mice during the dark (active) period, whereas the opposite occurred during the light (rest) period. Consequently, mean blood pressure was significantly higher in ob/ob than in +/+ mice during the light (8.2±2.4 mm Hg) but not during the dark (3.0±2.9 mm Hg) period. These data suggest that, in the absence of leptin, obesity may entail hypertensive derangements of blood pressure, which are substantially modulated by the cardiovascular effects of the wake-sleep states.

Physiological sleep-dependent changes in arterial blood pressure: central autonomic commands and baroreflex control

Sleep is a heterogeneous behaviour. As a first approximation, it is subdivided objectively into two states: non-rapid eye movement sleep (NREMS) and rapid eye movement sleep (REMS). The mean value and variability of arterial blood pressure (ABP) decrease physiologically from wakefulness to NREMS. In REMS, there may be a further decrease or increase in mean ABP as well as phasic hypertensive events, which enhance the variability of ABP. The reduced mean ABP during NREMS results from a decrease in either heart rate or sympathetic vasoconstrictor tone. During REMS, sympathetic activity to the different cardiovascular effectors undergoes a substantial repatterning. Thus, the mean ABP in REMS reflects a balance between changes in cardiac output and constriction or dilatation of different vascular beds. In both sleep states, the phasic changes in ABP are driven by bursts of vasoconstriction, which may be accompanied by surges of heart rate. The available evidence supports the hypothesis that the sleep-dependent changes in ABP, either tonic or phasic, result from the integration between cardiovascular reflexes and central autonomic commands that are specific to each sleep state.

Surges of arterial pressure during REM sleep in spontaneously hypertensive rats

STUDY OBJECTIVES

Rapid-eye-movement sleep (REM sleep) physiologically entails arterial pressure surges. Pressure surges may lead to acute cardiovascular events in risk conditions such as arterial hypertension. We investigated whether arterial hypertension alters the rate of occurrence and the characteristics of the pressure surges during REM sleep.

DESIGN

Spontaneously hypertensive rats (SHR) were compared with Wistar-Kyoto normotensive controls (WKY) and a group of SHR, in which hypertension was prevented by long-term enalapril treatment (ena-SHR).

SETTING

N/A.

SUBJECTS

Seven male rats per group.

INTERVENTIONS

Instrumentation with electrodes for polygraphic recordings, a nasal thermistor for measuring ventilatory period, and an arterial catheter for measuring arterial pressure and heart period.

MEASUREMENTS AND RESULTS

SHR showed a significant increase in the rate of occurrence but a similar magnitude of the pressure surges during REM sleep, with respect to WKY and ena-SHR. The pressure surges were associated with a decrease of heart period and an increase of electroencephalographic theta frequency, which were significantly less pronounced in SHR than in either WKY or ena-SHR. The ventilatory period showed only a modest increase before the surges without significant differences among the groups.

CONCLUSIONS

Pressure surges independent of sleep apnea occur during REM sleep at a rate increased in SHR with respect to their controls, supporting a potential role of REM sleep in triggering acute cardiovascular events in arterial hypertension. The characteristics of the pressure surges suggest that, in SHR, the underlying central autonomic commands are increased in frequency, but not in magnitude, by arterial hypertension.

Sleep-dependent changes in the coupling between heart period and blood pressure in human subjects

We investigated whether in human subjects, the pattern of coupling between the spontaneous fluctuations of heart period (HP) and those of systolic blood pressure (SBP) differs among wake-sleep states. Polysomnographic recordings and finger blood pressure measurements were performed for 48 h in 15 nonobese adults without sleep-disordered breathing. The cross-correlation function (CCF) between the fluctuations of HP and SBP at frequencies <0.15 Hz was computed during quiet wakefulness (QW), light (stages 1 and 2) and deep (stages 3 and 4) nonrapid-eye-movement sleep (NREMS), and rapid-eye-movement sleep (REMS). A positive correlation between HP and the previous SBP values, which is the expected result of baroreflex feedback control, was observed in the sleep states but not in QW. In deep NREMS, the maximum CCF value was significantly higher than in any other state, suggesting the greatest baroreflex contribution to the coupling between HP and SBP. A negative correlation between HP and the subsequent SBP values was also observed in each state, consistent with the mechanical feed-forward action of HP on SBP and with central autonomic commands. The contribution of these mechanisms to the coupling between HP and SBP, estimated from the minimum CCF value, was significantly lower in deep NREMS than either in light NREMS or QW. These results indicate that the pattern of coupling between HP and SBP at low frequencies differs among wake-sleep states in human subjects, with deep NREMS entailing the highest feedback contribution of the baroreflex and a low effectiveness of feed-forward mechanisms.