Non-invasive model-based estimation of the sinus node dynamic properties from spontaneous cardiovascular variability series

A non-invasive model-based approach to the estimation of sinus node dynamic properties is proposed. The model exploits the spontaneous beat-to-beat variability of heart period and systolic arterial pressure and the sampled respiration, thus surrogating the information from direct measures of neural activity. The residual heart period variability not related to baroreflex, to direct effects of respiration and to low frequency influences independent of baroreflex, is interpreted as the effect of the dynamic properties of the sinus node and modelled as a regression of the RR interval over its previous value. Therefore the sinus node transfer function is modelled by means of a filter with a real pole z = mu (and a zero in the origin). It was found that: first, in young healthy subjects the nodal tissue responded as a low-pass filter with mu = 0.76 +/- 0.12 (mean +/- SD); secondly, ageing did not significantly modify either its shape or gain at 0 Hz; thirdly, in heart transplant recipients, the dynamic transduction properties were lost (all-pass filter, p = 0.06 +/- 0.16, p < 0.001); fourthly, low-dose atropine left the sinus node dynamic properties unmodified; fifthly, high-dose atropine affected the dynamic transduction properties by increasing the gain at 0 Hz and rendering steeper its roll-off (the percent increase of mu with respect to baseline was 18.3 +/- 22.3, p < 0.05).

Quantifying the strength of the linear causal coupling in closed loop interacting cardiovascular variability signals

The coherence function measures the amount of correlation between two signals x and y as a function of the frequency, independently of their causal relationships. Therefore, the coherence function is not useful in deciding whether an open-loop relationship between x and y is set (x acts on y, but the reverse relationship is prevented) or x and y interact in a closed loop (x affects y, and vice versa). This study proposes a method based on a bivariate autoregressive model to derive the strength of the causal coupling on both arms of a closed loop. The method exploits the definition of causal coherence. After the closed-loop identification of the model coefficients, the causal coherence is calculated by switching off separately the feedback or the feedforward path, thus opening the closed loop and fixing causality. The method was tested in simulations and applied to evaluate the degree of the causal coupling between two variables known to interact in a closed loop mainly at a low frequency (LF, around 0.1 Hz) and at a high frequency (HF, at the respiratory rate): the heart period (RR interval) and systolic arterial pressure (SAP). In dogs at control, the RR interval and the SAP are highly correlated at HF. This coupling occurs in the causal direction from the RR interval to the SAP (the mechanical path), while the coupling on the reverse causal direction (the baroreflex path) is not significant, thus pointing out the importance of the direct effects of respiration on the RR interval. Total baroreceptive denervation, by opening the closed loop at the level of the influences of SAP on RR interval, does not change these results. In elderly healthy men at rest, the RR interval and SAP are highly correlated at the LF and the HF. At the HF, a significant coupling in both causal directions is found, even though closed-loop interactions are detected in few cases. At the LF, the link on the baroreflex pathway is negligible with respect to that on the reverse mechanical one. In heart transplant recipients, in which SAP variations do not cause RR interval changes as a result of the cardiac denervation, the method correctly detects a significant coupling only on the pathway from the RR interval to the SAP.

Contrasting effects of phentolamine and nitroprusside on neural and cardiovascular variability

The relative contributions of a central neural oscillator and of the delay in alpha-adrenergic transmission within the baroreflex loop in the predominance of low-frequency (LF) cardiovascular variability during sympathetic activation in humans are unclear. We measured R-R interval (RR), muscle sympathetic nerve activity (MSNA), blood pressure (BP), and their variability in 10 normal subjects during sympathetic activation achieved by BP lowering with sodium nitroprusside (SNP) and alpha-adrenergic blockade using phentolamine. SNP and phentolamine induced comparable reductions in BP (P > 0.25). Despite tachycardia and sympathetic activation with both SNP and phentolamine, LF variability in RR, MSNA, and BP increased during SNP and decreased during phentolamine (SNP: RR +20 +/- 6%, MSNA +3 +/- 5%, systolic BP +9 +/- 6%, diastolic BP +7 +/- 5%; phentolamine: RR -2 +/- 7%, MSNA -34 +/- 6%, systolic BP -16 +/- 8%, diastolic BP -13 +/- 4%, P < 0.05 except systolic BP, where P = 0.09). Thus LF variability is reduced when sympathetic activation is induced by alpha-adrenergic blockade. This suggests that alpha-adrenergic transmission within the baroreflex loop may contribute importantly to the predominance of LF cardiovascular variability associated with sympathetic excitation in humans.

Effects of aging and cardiac denervation on heart rate variability during sleep

BACKGROUND

Cardiac vagal predominance increases the RR interval and RR high-frequency (HF) variability during non-rapid eye movement (non-REM) sleep (stages I through IV) in young subjects. Aging suppresses deep sleep, but effects of age-related changes in sleep architecture on RR are unknown. Whether mechanical effects of changes in the breathing pattern on the sinus node during sleep affect RR variability is unclear.

METHODS AND RESULTS

Polygraphic sleep recordings and RR and RR spectral profiles were determined in 8 young (22.5+/-3.3 years) and 8 older (55.0+/-7.3 years) healthy volunteers. HF oscillations in RR of 8 cardiac-denervated heart transplant recipients determined mechanical effects of respiration on the sinoatrial node during sleep. Transition from wakefulness to non-REM sleep increased the RR interval in young and older subjects and increased the HF variability of RR in the young (P:<0.05) but not in the older subjects. Older subjects disclosed a faster RR (P:<0.01) and a lower HF variability (P:<0.05) during non-REM sleep than the young subjects. Aging did not affect light and REM sleep but decreased deep sleep (stage IV) from 39+/-23 to 6+/-6 minutes (P:<0.001). Reduction in sleep stage IV with aging blunted the increase in RR and in RR HF variability during non-REM sleep (r>0.55, P:<0.05). Transition from wakefulness to non-REM sleep doubled the markedly reduced HF variability of RR in the heart transplant recipients (P:<0.05).

CONCLUSIONS

Disappearance of deep sleep with aging impairs nocturnal increase in cardiac vagal activity. Mechanical effects of changes in breathing pattern during sleep favor increases in HF oscillations of the RR interval during non-REM sleep.

[Mechanisms controlling blood pressure and heart rate variability: physiological and physiopathological aspects]

These studies explore the mechanisms controlling the 24-hour, the ultradian and the instantaneous variability in blood pressure and RR interval in humans. Several studies are undertaken to determine the contribution of environment, genetics, sleep-wake cycle and specific sleep stages on the blood pressure and RR interval variability in normal subjects. The analysis of the effects of cardiac failure, of renal failure and of organ transplantation provides further information on the mechanisms controlling blood pressure and RR interval variability in pathological conditions.

Power spectral analysis of cardiovascular variability in critically ill neurosurgical patients

OBJECTIVE

Patients with brain damage exhibit a number of changes in heart rate and cardiovascular control. The aim of this study was to relate changes in autonomic cardiovascular control seen in critically ill neurosurgical patients to the quality of subsequent outcome and survival.

DESIGN

Prospective, longitudinal, outcome study.

SETTING

Intensive care department of a university teaching hospital.

PATIENTS

A total of 29 consecutive neurosurgical patients admitted for > or =2 days to the intensive care department with a Glasgow Coma Scale score < 13 who needed electrocardiographic and invasive arterial monitoring.

INTERVENTIONS

Sampling of the electrocardiogram, respiratory rate, and arterial pressure into a personal computer was carried out for > or =60 mins. Power spectral analysis was then applied to the data by using a fast Fourier transformation. Arterial baroreflex sensitivity was determined as the gain of the transfer function between systolic arterial blood pressure and electrocardiograph R-R interval (RRI) variability. All surviving patients were followed up at 3 months postadmission to measure quality of outcome.

MEASUREMENTS AND MAIN RESULTS

There were reductions in the total power (p < .01) of RRI variability in those who subsequently died compared with those who survived. This was significant for very low frequency (p < .001) and low-frequency (LF) (p < .05) but not high-frequency (HF) bands (p = .11). Blood pressure variability, however, did not change between groups. Baroreflex sensitivity was 8.7+/-2.2 msecs/mm Hg for patients with a good later outcome and 4.4+/-1.5 msecs/mm Hg for patients who subsequently died (p = .03). Patients who recovered to a good quality outcome also had a raised LF/HF ratio in RRI (p = .05).

CONCLUSION

A reduction in the total power variability of RRI and a lowered LF/HF ratio of the RRI are associated with a poor quality recovery or death after neurosurgical illness. A reduction in the baroreflex was specifically associated with death in this patient group.

Ambulatory blood pressure and neuroendocrine control after diet-assisted gastric restrictive surgery

BACKGROUND

Long-term weight control after conventional diet is disappointing but may be improved when diet is assisted by gastric restrictive surgery (GRS).

OBJECTIVE

To determine the effects of GRS on ambulatory blood pressure (ABP) and neuroendocrine BP control in 28 morbidly obese subjects.

METHODS

A BP and heart rate were recorded every 10 min for 25 h before and 4 months after GRS. Effects of marked reductions in body weight on the renin-angiotensinaldosterone system, on plasma insulin and on sympathetic activity were also determined.

RESULTS

Body mass index decreased from 43 +/- 1 to 34 +/- 1 kg/m2 and systolic (S) BP decreased by 7 +/- 2 mmHg during daytime (P=0.01) and by 8 +/- 3 mmHg during the night (P=0.02). Pulse pressure, a marker of reduced arterial compliance, decreased by 5 +/- 1 mmHg throughout the 24 h period (P < 0.001). Diastolic BP remained unchanged. Heart rate decreased more during the night (-13 +/- 2 bpm, P<0.0001) than during daytime (-5 +/- 2 bpm, P=0.03). Reductions in SBP were largest in subjects with highest initial BP values (r = -0.63, P<0.001) but were unrelated to weight loss. GRS decreased fasting glycaemia, plasma insulin, plasma C peptide and 24 h urine sodium (n=20) and noradrenaline (n=19) excretion (P<0.01).

CONCLUSIONS

Diet-assisted GRS favourably affects neuroendocrine BP control in obese patients. Reductions in sodium intake, insulin levels and sympathetic tone combined with possible improvements in arterial compliance induce persistent 24 h reductions in SBP and pulse BP. Reductions in BP are largest in subjects with highest initial BP values and are unrelated to the amount of weight loss, thereby emphasizing the importance of even moderate reductions in weight on BP control.

Arterial baroreflex control of the sinus node during dobutamine exercise stress testing

The contributions of increases in circulating catecholamines, changes in central command, and muscle afferents on baroreflex control of the sinus node during exercise are unclear. We used a dobutamine infusion to induce hemodynamic changes comparable to those of moderate physical exercise in the absence of changes in central command and muscle afferents in 13 healthy subjects. Dobutamine (up to 9 microg/kg body weight per minute) increased systolic blood pressure, shortened the RR interval, increased systolic blood pressure variability, but blunted RR interval variability (P<0.05 versus placebo). Consequently, dobutamine decreased the coherence between variations in systolic blood pressure and RR interval and decreased arterial baroreflex sensitivity from 12+/-2 to 3+/-1 ms/mm Hg (P<0.01). The largest increases in systolic blood pressure with dobutamine were paralleled by the greatest impairments in arterial baroreflex sensitivity (0. 50<r<0.56, P<0.01). The chronotropic effects of dobutamine prevented a reflex bradycardia in response to the blood pressure increase. However, less predominant low-frequency oscillations in systolic blood pressure (P<0.0001) suggested preserved sympathetic withdrawal in response to the blood pressure increase induced by dobutamine. In conclusion, this study revealed that a shift in the operating point of the arterial baroreceptors and the chronotropic effects of adrenergic stimulation impair baroreflex control of the sinus node during dobutamine exercise stress testing. Baroreflex control of the sinus node is not reset when hemodynamic characteristics of exercise are reproduced in the absence of modifications in central command and muscles afferents.

Sympathetic rhythmicity in cardiac transplant recipients

BACKGROUND

Variability of R-R interval and muscle sympathetic nerve activity (MSNA) occurs predominantly at a low frequency (LF, +/-0.1 Hz) and a high frequency (HF, +/-0.25 Hz) in normal humans. Increased sympathetic drive in normal humans is associated with an increased LF component of the R-R interval and MSNA. Patients with severe heart failure have high sympathetic activity but decreased or absent LF power of both R-R and MSNA. We tested the hypothesis that this dysfunction in autonomic modulation in heart failure can be reversed by heart transplantation.

METHODS AND RESULTS

We performed spectral analysis of resting MSNA, R-R interval, and respiration in 9 patients with heart transplants, 9 chronic heart failure patients, and 9 normal control subjects, all closely matched for age, sex, and body mass index. MSNA (bursts per minute) was higher in patients with heart transplants (74+/-3) than either patients with heart failure (56+/-6) or normal subjects (40+/-4) (P<0.001). LF variability in the R-R interval was reduced in both heart transplant recipients and heart failure patients compared with the control subjects (P<0.01). The LF variability in MSNA was also nearly absent in the heart failure patients (P<0.01). However, the LF and HF oscillations in MSNA in patients with heart transplants were comparable to those evident in the control subjects.

CONCLUSIONS

Cardiac transplantation does not reduce MSNA. However, LF oscillations in sympathetic activity are restored after transplantation such that the MSNA oscillatory profile is similar to that observed in normal subjects.

Evidence for a central origin of the low-frequency oscillation in RR-interval variability

BACKGROUND

Short-term variability of RR interval and blood pressure occurs predominantly at low frequency (LF; approximately 0.1 Hz) and high frequency (approximately 0.25 Hz). The arterial baroreflex is thought to be the predominant determinant of the LF component of RR variability. Patients with severe congestive heart failure (CHF) have an attenuated or absent LF oscillation in RR variability. The left ventricular assist device (LVAD) offers a unique possibility for analysis of spectral oscillations in RR interval independent of any effects of blood pressure that influence these oscillations via the baroreflex.

METHODS AND RESULTS

We performed spectral analysis of RR, blood pressure, and respiration in 2 patients with CHF before and after LVAD implantation. LF components of the RR-interval and blood pressure variability were absent in both CHF patients before LVAD implantation. After LVAD implantation, spectral analysis of the RR interval showed restoration of a clear and predominant LF oscillation in the native hearts of both patients, with no such oscillation evident in the blood pressure profile.

CONCLUSIONS

During total circulatory support with the LVAD, the LF oscillation in RR interval of the native heart, absent in CHF, is restored. This LF oscillation in RR interval occurs in the absence of LF oscillations in blood pressure and thus is unlikely to be explained by baroreflex mechanisms. Hence, the absence of LF oscillation in the RR interval in CHF is functional and is reversible by LVAD circulation. The presence of a predominant LF oscillation in RR interval independent of any oscillation in blood pressure suggests that the LF oscillation is a fundamental property of central autonomic outflow.

Dopamine depresses minute ventilation in patients with heart failure

BACKGROUND

Low-dose dopamine is frequently used in patients in the intensive care setting. Dopamine may inhibit chemoreceptor afferents and hence decrease chemoreflex sensitivity to hypoxia.

METHODS AND RESULTS

In a double-blind, randomized, crossover study, we determined the effects of dopamine (5 microg x kg(-1) x min(-1)) and placebo infusion on oxygen saturation, minute ventilation, and sympathetic nerve activity during normoxia and 5 minutes of hypoxia in 10 normal young subjects. We further investigated the effects of dopamine and placebo on minute ventilation during normoxic breathing in 8 patients with severe heart failure and in 8 age-matched control subjects. Dopamine did not decrease minute ventilation during normoxia in normal subjects. During hypoxia, minute ventilation was 12.9+/-1.3 L/min on dopamine and 15.8+/-1.5 L/min on placebo (P<0.0001). Oxygen saturation during hypoxia was lower with dopamine (78+/-3%) than placebo (84+/-2%; P<0.0001). Sympathetic nerve activity during hypoxia was not enhanced with dopamine despite the lower O2 saturation. Subjects were able to maintain a voluntary apnea to a lower oxygen saturation on dopamine than on placebo (P<0.05). In heart failure patients breathing room air, but not in age-matched control subjects, dopamine decreased minute ventilation despite decreased oxygen saturation and increased PETCO2 during dopamine (all P< or =0.02).

CONCLUSIONS

Dopamine inhibits chemoreflex responses during hypoxic breathing in normal humans, preferentially affecting the ventilatory response more than the sympathetic response. Dopamine also depresses ventilation in normoxic heart failure patients breathing room air. Ventilatory inhibition by low-dose dopamine may adversely influence outcome in hypoxic patients, especially in patients with heart failure.

Effect of Cheyne-Stokes respiration on muscle sympathetic nerve activity in severe congestive heart failure secondary to ischemic or idiopathic dilated cardiomyopathy

Severe congestive heart failure (CHF) is associated with Cheyne-Stokes (C-S) respiration, which may be an index of poorer prognosis. The mechanisms linking C-S respiration to poorer functional status and prognosis in patients with CHF are unknown. We tested the hypothesis that C-S respiration increases muscle sympathetic nerve activity (MSNA) in 9 patients with CHF. Oxygen saturation was 96 +/- 1% during normal breathing and 91 +/- 1% after the apneic episodes (p < 0.05). Mean blood pressure was 79 +/- 8 mm Hg during normal breathing and 85 +/- 8 mm Hg during C-S respiration (p = 0.001). C-S respiration increased MSNA burst frequency (from 45 +/- 5 bursts/min during normal breathing to 50 +/- 5 bursts/min during C-S respiration; p < 0.05) and total integrated nerve activity (to 117 +/- 7%; p < 0.05). We also studied an additional 5 patients in whom C-S breathing was constant, without any periods of spontaneous normal breathing. In these patients, MSNA was higher (65 +/- 5 bursts/min) than MSNA in patients in whom C-S breathing was only intermittent (45 +/- 5 bursts/min; p < 0.05). In all 14 patients, the effects of different phases of C-S respiration were examined. MSNA was highest during the second half of each apnea (increasing to 152 +/- 14%; p < 0.01) and blood pressure was highest during mild hyperventilation occurring after termination of apnea (p < 0.0001). We conclude that C-S respiration decreases oxygen saturation, increases MSNA, and induces transient elevations in blood pressure in patients with CHF.

Relationship between repeated measures of hemodynamics, muscle sympathetic nerve activity, and their spectral oscillations

BACKGROUND

We determined the intraclass correlation coefficients (ICC) of repeated measures of the mean levels and variability of RR and muscle sympathetic nerve activity (MSNA) in 7 normal subjects. We examined whether spontaneous fluctuations in RR and MSNA over repeated measurements were mirrored by changes in spectral components of RR and MSNA.

METHODS AND RESULTS

Twenty-minute recordings of respiration, RR, blood pressure (BP), and MSNA were performed at day 1, 1 week, 1 month, and 3 months and divided into two 10-minute periods for the analysis of short-term reliability. Comparison between these recordings also determined the long-term reliability. Linear regressions examined the relationship between changes in these measurements and changes in spectral components of RR and MSNA. All analyses were carried out blinded to subject and session. Short-term ICC of RR, BP, MSNA and of the variabilities of RR and MSNA (in % of total power) ranged between .98 and .70 and indicated a good short-term reliability. The long-term reliability of RR variability was comparable to MSNA variability (range of ICC, .34 to .52). Spontaneous decreases in RR during the repeated recordings were accompanied by increases in sympathetic drive, as evidenced by increases in the ratio of low-frequency to high-frequency variability (LF/HF ratio) of RR interval (r=-.43, P<.01) and by increases in MSNA (r= -.36, P=.01). The changes in the LF/HF ratio of RR were mirrored by parallel changes in the LF/HF ratio of MSNA (r=+.30, P<.05). Spontaneous decreases in BP were accompanied by increases in the LF/HF ratio of MSNA (r=-.52; P=.0001).

CONCLUSIONS

Heart rate, MSNA, and their variability are stable in the short-term, but less so over the long term. Spontaneous changes in repeated measurements of RR interval and blood pressure over the long term are accompanied by parallel changes in the normalized spectral components of RR and MSNA variability. Thus even over an extended period, there is a synchrony between changes in absolute cardiovascular measures and changes in their spectral components.

Heart rate variability before and after treatment with electroconvulsive therapy

It has been suggested that depression may be associated with decreased parasympathetic activity. Based on this work, we tested the hypothesis that treatment of depression with electroconvulsive therapy (ECT) would result in a relative increase in cardiac vagal (parasympathetic) activity. Changes in respiratory sinus arrhythmia, a marker of cardiac parasympathetic activity, were examined in nine patients with depressive episodes before and after ECT using spectral analysis. Hamilton Depression Rating Scale scores decreased significantly. In terms of the heart rate measures, RR interval tended to decrease and the amplitude of respiratory sinus arrhythmia decreased significantly following the course of ECT. This reduction in respiratory sinus arrhythmia contributed to the overall decrease in RR interval variability. Additionally, the magnitude of symptom improvement as measured by the Hamilton Scale correlated with the decrease in amplitude of the respiratory sinus arrhythmia. We report that treatment of depression with ECT was associated with a relative decrease in parasympathetic activity, in contrast to our initial hypothesis of a relative increase. This finding may not be related to the ECT per se but rather to the resolution of depression, as there was a significant correlation between the decrease in Hamilton Depression Rating Scale scores and decrease in parasympathetic activity. Further work is necessary to better understand the autonomic changes associated with depressive illness and the clinical risks and benefits associated with various treatment modalities.

Effects of alcohol on sympathetic activity, hemodynamics, and chemoreflex sensitivity

Alcohol intake has been shown to worsen obstructive sleep apnea and increase nocturnal hypoxemia. The mechanisms of this action are unclear. Animal studies suggest that a reduction in chemoreflex sensitivity may be implicated. Using a double-blind, randomized, vehicle-controlled design, we tested the hypothesis that oral alcohol intake depresses chemoreflex sensitivity in humans. We examined the effects of oral alcohol intake (1.0 g/kg body wt) on blood pressure, heart rate, heart rate variability, muscle sympathetic nerve activity, forearm vascular resistance, and minute ventilation in 16 normal male subjects. Peripheral and central chemoreflex sensitivity were measured in response to hypoxia (n = 10) and hypercapnia (n = 6), respectively. Plasma alcohol increased from 0 to 23.2 +/- 1.5 mmol/L (107 +/- 7 mg/dL) at 60 minutes and 20.2 +/- 1 mmol/L (93 +/- 4 mg/dL) at 85 minutes after alcohol intake (P < .0001). Alcohol induced an increase in heart rate from 59 +/- 2 to 66 +/- 2 beats per minute (P < .01) and increased the ratio of low- to high-frequency variability of heart rate (P < .05). Although alcohol increased sympathetic nerve activity by up to 239 +/- 22% of baseline values (P < .01), forearm vascular resistance after alcohol was lower than that after vehicle (P < .05). Blood pressure did not increase compared with the vehicle session. Oxygen saturation during hypoxia after alcohol was 4 +/- 1% lower than it was during hypoxia after vehicle (P < .05) although arterial blood PO2 was unchanged. Alcohol did not affect the cardiovascular, sympathetic, or ventilatory responses to either hypoxia or hypercapnia. Acute increases in plasma alcohol increase heart rate and sympathetic nerve activity; blood pressure is not increased, probably because of vasodilator effects of alcohol. Alcohol does not alter chemoreflex responses to hypoxia or hypercapnia; thus, alterations in chemoreflex sensitivity are unlikely to explain the effects of alcohol on sleep apnea. Alcohol may reduce the affinity of hemoglobin for oxygen.

Absence of low-frequency variability of sympathetic nerve activity in severe heart failure

BACKGROUND

In normal humans, variability of blood pressure, RR interval, and sympathetic activity occurs predominantly at a low frequency (LF; 0.04 to 0.14 Hz) and a high frequency (HF; +/-0.25 Hz). In conditions that increase sympathetic activation in normal humans, the LF component is increased relative to the HF component. Patients with heart failure have high levels of sympathetic activity. We tested the hypothesis that the LF component of sympathetic nerve activity variability is increased in heart failure.

METHODS AND RESULTS

We performed spectral analysis of simultaneous recordings of resting muscle sympathetic nerve activity (MSNA) and RR interval in 21 patients with chronic heart failure and 12 age-matched control subjects. MSNA was higher in patients with heart failure (62 +/- 4 bursts per minute) than in the normal subjects (39 +/- 4 bursts per minute; P < .01). LF components of RR interval and MSNA variability were lower in the heart failure patients versus the control subjects (P < .01). HF variability of RR interval and MSNA was preserved, at least in part, in heart failure. There was close coherence between variability patterns of RR interval and MSNA. Furthermore, in 14 heart failure patients who had no LF variability in MSNA compared with 7 heart failure patients who did manifest LF variability in MSNA, RR interval was shorter, the variance of RR interval was lower, MSNA was higher, respiratory rate was faster, and left ventricular ejection fraction was lower (all P < .05). At a median follow-up of 12 months, 4 heart failure patients had died, all of whom had had absent LF oscillations in MSNA and RR interval.

CONCLUSIONS

The LF variability of sympathetic nerve activity is absent in patients with severe heart failure. This disturbed pattern of variability is closely coherent with the abnormal variability of RR interval. These disturbances of rhythmic oscillations of autonomic outflow, evident in both RR interval and MSNA, suggest a central autonomic regulatory impairment in heart failure and may have important prognostic implications.

Tonic chemoreflex activation does not contribute to elevated muscle sympathetic nerve activity in heart failure

BACKGROUND

Sympathetic activation in heart failure may be due to an increase in sympathetic excitatory influences or to a decrease in inhibitory signals to the brain stem. Chemoreflex sensitivity may be increased in patients with heart failure. The present study tested the hypothesis that tonic activation of excitatory chemoreceptor afferents contributes to the elevated sympathetic activity in heart failure.

METHODS AND RESULTS

We recorded sympathetic nerve activity to muscle circulation from the peroneal nerve of 12 chronic heart failure patients while the patients were breathing room air and during deactivation of the chemoreceptors while the patients were breathing a 100% O2 gas mixture. All patients except 2 were in class III of the New York Heart Association functional classification. Left ventricular ejection fraction defined by radionuclide ventriculography was 24 +/- 2% (mean +/- SE). We also obtained measurements of resting sympathetic nerve activity in 9 healthy control subjects to document that sympathetic nerve activity was elevated in heart failure subjects. Resting sympathetic nerve activity was 59 +/- 5 bursts/min in heart failure patients versus 36 +/- 4 bursts/min in control subjects (P < .01). In heart failure patients, oxygen administration increased oxygen saturation from 94 +/- 0.9% to 99 +/- 0.3% (P < .0001). This increase in oxygen saturation did not affect resting muscle sympathetic nerve activity (798 +/- 122 U/min while patients breathed room air and 824 +/- 35 U/min during 100% O2 breathing) or blood pressure.

CONCLUSIONS

Increased efferent sympathetic activity to muscle circulation in patients with heart failure is not explained by tonic activation of excitatory chemoreflex afferents.

Does cardiac denervation affect the short-term blood pressure variability in humans?

OBJECTIVE

To explore the repercussion of cardiac denervation on the short-term blood pressure variability in humans, in order to assess the extent to which the variability of blood pressure is linked to the variability of heart rate.

METHODS

Beat-to-beat blood pressure and RR interval time were recorded in 16 heart-transplanted patients and were compared with those of 10 healthy control subjects in the resting supine, sitting and standing positions. Blood pressure and RR interval variabilities were assessed by spectral analysis.

RESULTS

The total blood pressure power and the sitting very low-frequency, low-frequency, low-frequency and high-frequency blood pressure variability were similar in the heart-transplanted patients and in the controls, despite a marked reduction in the RR interval variability in the heart-transplanted patients. However, the heart-transplanted patients had lower standing low-frequency blood pressure variability than the control subjects. Moreover, very low-frequency and low-frequency RR interval variabilities reappeared in the long-term heart-transplanted patients but not in the short-term heart-transplanted patients (range of time after transplantation 53-124 and 3-25 months, respectively).

CONCLUSIONS

Short-term RR interval fluctuations are not mandatory for the maintenance of normal blood pressure variability in the supine and sitting positions, but may contribute to the increase in the low-frequency blood pressure variability which occurs normally in the standing position. Moreover, the long-term heart-transplanted patients had increased RR interval variability, which may have been caused by the reappearance of limited autonomic cardiac modulation. However, this increased RR interval variability did not affect the corresponding blood pressure variability.

Twenty-four-hour blood pressure and heart rate profiles in humans. A twin study

To delineate the relative roles of genetic and environmental factors on physiological variations of blood pressure and heart rate, we performed 24-hour ambulatory blood pressure monitorings with simultaneous polygraphic sleep recordings in 28 monozygotic and 16 dizygotic healthy young male twin pairs investigated in a standardized physical and social environment. Blood pressure and heart rate were measured every 10 minutes for 24 hours. A best-fit curve based on the periodogram method was used to quantify changes in blood pressure and heart rate over the 24-hour span. Surprisingly, monozygotic twins as a group tended to have higher blood pressure values than dizygotic twins, and this difference reached the level of significance for daytime systolic blood pressure (P < .005). Although environmental influences largely controlled the mean levels and characteristics of the 24-hour systolic blood pressure variations, significant genetic effects were demonstrated for the mean levels and 24-hour patterns of diastolic blood pressure and heart rate. For both diastolic blood pressure and heart rate, the genetic effects concerned largely the same characteristics of the 24-hour profiles: the 24-hour mean, the daytime mean, the value of the evening acrophase, and the value of the major acrophase. Moreover, there was a strong genetic influence for the amplitude of the 24-hour rhythm of heart rate.

Bisoprolol and atenolol in essential hypertension: effects on systemic and renal hemodynamics and on ambulatory blood pressure

The acute and short-term responses to bisoprolol and to atenolol on systemic and renal hemodynamics and on ambulatory blood pressure (BP) were compared in a randomized double-blind cross-over study including 14 patients with mild to moderate essential hypertension. After a 4-week placebo period, the patients received either bisoprolol (10 mg once daily, o.d.) or atenolol (100 mg o.d.) for 4 weeks and were switched to the other drug after a new 4-week placebo period. Cardiac output (CO) was measured by Doppler echography, and renal blood flow (RBF) and glomerular filtration rate (GFR) were measured by constant infusion techniques using [123I]iodohippurate and [51Cr]EDTA, respectively. Bisoprolol and atenolol decreased diurnal and nocturnal blood pressure (BP). Both drugs decreased heart rate (HR) and BP both acutely and after 4 weeks. During short-term treatment, CO was maintained with bisoprolol but reduced by atenolol (by 17%). RBF decreased after the first drug intake (by 9 and 12%, respectively) but returned to its baseline value after 4 weeks, so that calculated renal vascular resistance (RVR) was reduced (by 12 and 15%, respectively). Overall, GFR was not affected by treatment. Bisoprolol and atenolol are effective antihypertensive agents that preserve renal hemodynamics during short-term treatment.

Sleep quality and continuous, non-invasive beat-to-beat blood pressure recording

OBJECTIVE

To investigate the effects of continuous, non-invasive, beat-to-beat finger blood pressure monitoring on sleep in healthy men.

DESIGN

After 1 night of habituation to the laboratory environment, which consisted of the placement of electroencephalographic equipment without recording, polygraphic sleep recordings were performed during two consecutive nights (nights 1 and 2) in 15 healthy men (mean +/- SD age 25 +/- 6 years). Blood pressure was recorded continuously for 24 h from the end of night 1 to the end of night 2.

RESULTS

The blood pressure recording procedure caused a decrease in the amount of rapid eye movement (REM) sleep and increased the duration of nocturnal awakenings. Consequently, sleep efficiency was decreased by approximately 5%. The blood pressure measurements did not affect the duration of light and of deep sleep. Although the respective predominance of deep sleep and of REM sleep at the beginning and at the end of the sleep period were preserved during the night of blood pressure recording, the blood pressure recording procedure hampered the rise in REM sleep during the final two thirds of the sleep period.

CONCLUSION

In healthy young men continuous, non-invasive, beat-to-beat finger blood pressure monitoring induced modest reductions in sleep efficiency of similar magnitude to those observed previously with non-invasive ambulatory blood pressure monitoring.

Twenty-four-hour blood pressure and heart rate patterns in chronic hemodialysis patients

To test the assumption that the previously reported abnormal 24-hour blood pressure (BP) variations in hemodialysis (HD) patients might be due to associated complications of chronic renal failure, we recorded the 24-hour noninvasive BP and heart rate (HR) patterns in 14 carefully selected chronic HD patients. No of the patients suffered from orthostatic hypotension, and all were free of any previously reported associated condition known to affect the 24-hour BP profiles. The BP and HR profiles were quantitatively analyzed using the periodogram method. The results were compared those from 14 controls matched for age, sex, and casual BP. No difference could be found between the HD patients and the controls, either in the absolute and relative amplitudes of the 24-hour systolic BP and diastolic BP variations or in the timings and levels of their respective acrophases and nadirs (except for the timings of the second systolic BP and diastolic BP acrophases; P < 0.05). The absolute levels of the HR acrophases and nadirs were higher in the HD patients than in the controls (P < 0.05 and P < 0.01, respectively), while both the absolute and relative amplitudes of the 24-hour HR profiles and the relative values and timings of the HR acrophases and nadirs were similar in both groups. Consequently, the 24-hour HR pattern was similar in the HD patients than in the controls, except that the entire profile was shifted upward.(ABSTRACT TRUNCATED AT 250 WORDS)

Circadian rhythms of blood pressure after liver transplantation

Twenty-four-hour systolic blood pressure, diastolic blood pressure, and heart rate profiles were recorded in 17 liver-transplanted patients by noninvasive ambulatory monitoring and were analyzed with the periodogram method. These recordings were compared with those of control subjects matched for age, sex, and daytime ambulatory blood pressure. Abnormal blood pressure patterns were found in seven of the 17 patients, whereas the other 10 patients had circadian blood pressure profiles that were not different from those of control subjects. These two groups of liver-transplanted patients did not differ in age, sex, oral dose of cyclosporine, specific serum cyclosporine level, and proportion of patients receiving azathioprine and antihypertensive medications. In contrast, the daily oral dose of prednisolone was significantly higher (p < 0.001) in the seven patients with abnormal circadian blood pressure patterns. Moreover, only the daily oral dose of prednisolone was inversely correlated with the magnitude of the nighttime systolic and diastolic blood pressure decrease (r = -0.64 and r = -0.66, p < 0.01). In contrast to blood pressure, patients and control subjects had similar circadian heart rate variations. We conclude that exogenous glucocorticoid administration may have a dose-dependent effect on the nighttime blood pressure fall and may play an important role in the pathogenesis of the abnormal circadian blood pressure profiles observed in liver-transplanted patients.