Spontaneous cardiac baroreflex in humans. Comparison with drug-induced responses

Altered heart rate and blood pressure variability in mice lacking the Mas protooncogene

Heart rate variability is a relevant predictor of cardiovascular risk in humans. A significant genetic influence on heart rate variability is suggested, although the genes involved are ill-defined. The Mas-protooncogene encodes a G-protein-coupled receptor with seven transmembrane domains highly expressed in testis and brain. Since this receptor is supposed to interact with the signaling of angiotensin II, which is an important regulator of cardiovascular homeostasis, heart rate and blood pressure were analyzed in Mas-deficient mice. Using a femoral catheter the blood pressure of mice was measured for a period of 30 min and 250 data values per second were recorded. The mean values and range of heart rate and blood pressure were then calculated. Neither heart rate nor blood pressure were significantly different between knockout mice and controls. However, high resolution recording of these parameters and analysis of the data by non-linear dynamics revealed significant alterations in cardiovascular variability in Mas-deficient animals. In particular, females showed a strong reduction of heart rate variability. Furthermore, the data showed an increased sympathetic tone in knockout animals of both genders. The marked alterations detected in Mas-deficient mice of both genders suggest that the Mas-protooncogene is an important determinant of heart rate and blood pressure variability.

Baroreflex failure following radiation therapy for nasopharyngeal carcinoma

The authors present a 51-year-old man with right-sided nasopharyngeal carcinoma who was treated for regional lymph node involvement by bilateral radiation therapy of the neck. Six years later he presented with episodic complaints of headache, flushing, and palpitations accompanied by elevations of blood pressure. Examination of arterial baroreflex function indicated selective afferent carotid sinus denervation. Cross spectral analysis of spontaneous heart rate and blood pressure variability showed decreased arterial baroreflex sensitivity (6.5 ms/mm Hg). Twenty-four hour measurements of blood pressure and heart rate variability showed labile hypertension during normal daytime activities. Baroreflex failure in this patient probably represents a late complication of bilateral radiation therapy of the neck.

Determination of baroreflex gain using auto-regressive moving-average analysis during spontaneous breathing

The heart rate component of the arterial baroreflex gain (BRG) was determined with auto-regressive moving-average (ARMA) analysis during each of spontaneous (SB) and random breathing (RB) protocols. Ten healthy subjects completed each breathing pattern on two different days in each of two different body positions, supine (SUP) and head-up tilt (HUT). The R-R interval, systolic arterial pressure (SAP) and instantaneous lung volume were recorded continuously. BRG was estimated from the ARMA impulse response relationship of R-R interval to SAP and from the spontaneous sequence method. The results indicated that both the ARMA and spontaneous sequence methods were reproducible (r = 0.76 and r = 0.85, respectively). As expected, BRG was significantly less in the HUT compared to SUP position for both ARMA (mean +/- SEM; 3.5 +/- 0.3 versus 11.2 +/- 1.4 ms mmHg-1; P < 0.01) and spontaneous sequence analysis (10.3 +/- 0.8 versus 31.5 +/- 2.3 ms mmHg-1; P < 0.001). However, no significant difference was found between BRG during RB and SB protocols for either ARMA (7.9 +/- 1.4 versus 6.7 +/- 0.8 ms mmHg-1; P = 0.27) or spontaneous sequence methods (21.8 +/- 2.7 versus 20.0 +/- 2.1 ms mmHg-1; P = 0.24). BRG was correlated during RB and SB protocols (r = 0.80; P < 0.0001). ARMA and spontaneous BRG estimates were correlated (r = 0.79; P < 0.0001), with spontaneous sequence values being consistently larger (P < 0.0001). In conclusion, we have shown that ARMA-derived BRG values are reproducible and that they can be determined during SB conditions, making the ARMA method appropriate for use in a wider range of patients.

Kidney transplantation improves baroreceptor sensitivity

Kidney transplant recipients as well as patients on hemodialysis frequently share an increased risk of cardiovascular diseases. Besides other factors, modulations in central neural blood pressure regulation have to be considered as a pathogenetic factor. In this study, baroreceptor function as a possible modulator of blood pressure and the activity of the generating components of the sympathetic nervous system were estimated in 20 kidney transplant recipients, 20 normotensive patients on hemodialysis and 20 age-matched volunteers using the sequence analysis technique and Fast Fourier Transformation (FFT). No blood pressure differences could be measured (83.7+/-2.5 vs. 82.5+/-3.8 vs. 79.2+/-2.4 mm Hg). Pulse interval-blood pressure sequences and the slope of delta pulse interval/delta mean arterial blood pressure of these sequences, representing baroreceptor sensitivity, did not differ between kidney transplant recipients and controls (11.2+/-1.4 vs. 13.4+/-1.3 ms/mm Hg), whereas in hemodialysis patients a reduced baroreceptor sensitivity (5.2+/-1.2 ms/mm Hg) was detected. The 66-129 mHz component (Mayer waves) of heart rate and blood pressure spectrum in normals (650+/-57 and 630+/-70 modulus) significantly (p<0.05) exceeded its equivalent in kidney transplant recipients (430+/-32 and 452+/-27 modulus) and patients on hemodialysis (375+/-38 and 394+/-40 modulus). In conclusion, our study provided evidence that both in kidney transplant and dialysis patients a decreased activity of the generating compounds of the sympathetic nervous system can be detected as compared to healthy volunteers. In contrast to hemodialysis patients, the baroreceptor sensitivity is unaffected in kidney transplant recipients and may, therefore, not contribute to the development of cardiovascular diseases.

Reduced dietary fat intake increases parasympathetic activity in healthy premenopausal women

1. Hypercholesterolaemia has been associated with decreased heart rate variability, a measure of cardiac parasympathetic activity. However, the effect of perturbation of the lipid profile on autonomic function has not been examined systematically. 2. The effects of short-term dietary lipid modification on autonomic function are studied in 25 normotensive, non-smoking, premenopausal women with normal bodyweight. Subjects consumed either a low (L, 25%) or high fat (H, 40%) diet for 2 weeks in an open, randomized, cross-over manner with a 2 week washout. 3. Baroreflex sensitivity was determined by gating beat-to-beat heart period (RR) interval and continuous non-invasive blood pressure recordings. Heart rate variability measures of cardiac parasympathetic nervous system activity were obtained in the time (standard deviation of RR intervals, root mean square of successive differences (rMSSD)) and frequency (high frequency power) domains. All assessments were made at the same timepoint in the menstrual cycle. 4. Both low-density lipoprotein-cholesterol and high-density lipoprotein-cholesterol decreased significantly (P < 0.05) with increased dietary fat intake (H, 2.7 +/- 0.1 vs L, 2.2 +/- 0.1; H, 1.3 +/- 0.1 vs L, 1.1 +/- 0.1 mmol/L, respectively) as did mean arterial pressure (H, 78.1 +/- 1.5 vs L, 74.3 +/- 1.5 mmHg). Weight was unchanged by dietary lipid intake (H, 62.6 +/- 8.5 vs L, 62.3 +/- 8.3 kg, P = NS). 5. There was a significant increase in rMSSD (H, 29.6 +/- 3.4 vs L, 38.8 +/- 6.4 msec, P < 0.05) and natural logarithm of high frequency power following low fat diet (H, 4.4 +/- 0.2 vs L, 4.8 +/- 0.3 msec2, P = 0.01). Baroreflex sensitivity also increased following the low fat diet (H, 13.91 +/- 2.2 vs L, 16.9 +/- 3.2 msec/mmHg, P = 0.23). 6. Short-term dietary lipid modification can significantly increase cardiac parasympathetic nervous system activity in healthy premenopausal women. These changes in autonomic status appear to be independent of changes in bodyweight and may be of clinical relevance considering the prognostic implications of heart rate variability in cardiovascular disease.

Abnormal autonomic cardiovascular control in ankylosing spondylitis

OBJECTIVE

This study was aimed at assessing the contribution of the autonomic nervous system to adjustments of cardiovascular function in patients with ankylosing spondylitis (AS).

METHODS

In 18 AS patients (mean age: 34.9; mean disease duration: 6.4 years) and 13 healthy controls (mean age: 31.7) the changes of heart rate (HR) with deep breathing (E/I ratio) and standing up (30/15 ratio) were recorded. The slope of cardiac baroreflex, the times series of blood pressure and HR values upon lying and standing, and venous plasma concentrations of catecholamines were also analysed. Erythrocyte sedimentation rate (ESR), plasma C reactive protein (CRP) concentration and a clinical index (BASDAI score) were used to assess the degree of disease activity in patients.

RESULTS

In the standing patients, blood pressure was found to decrease progressively (p< 0.001). Furthermore, the patients with a BASDAI score > 5 had a higher heart rate than patients with a BASDAI score < 5 (p<0.02), and there was a trend for a similar difference when patients were classified according to their ESR and CRP. Plasma catecholamine concentrations and the E/I ratio were not different in patients from controls. The 30/15 ratio and the slope of the spontaneous baroreflex during standing were both lower in AS patients than controls (p< 0.01).

CONCLUSIONS

This study demonstrated a change in autonomic nervous system function of AS patients, with a decreased parasympathetic activity, as evidenced by higher HR and lower baroreflex slope. As these significant deviances were mainly observed in patients with more active (or more inflammatory) disease, the autonomic nervous system involvement could be related to the inflammatory process. This autonomic strain may be related to the cardiac involvement in AS patients.

Noninvasive assessment of the cardiac baroreflex: response to downward tilting and comparison with the phenylephrine method

OBJECTIVES

We studied the relation between changes in systolic blood pressure and RR interval during downward tilting in comparison with assessment of baroreflex sensitivity (BRS) measured by the phenylephrine method (Phe-BRS) and with measures of heart rate variability (HRV).

BACKGROUND

The method most extensively used for assessing BRS involves bolus injections of phenylephrine. Several noninvasive methods proposed to assess BRS have not been widely applied in the clinical setting.

METHODS

Sixteen healthy male volunteers were studied (mean age +/- SD 27.5+/-4.6 years). Arterial blood pressure using tonometry and electrocardiogram was simultaneously recorded. After 20 min of 70 degrees upright tilting, the table was returned to supine position at a speed of 3.2 degrees/s. Subsequently, BRS was assessed using an intravenous bolus injection of phenylephrine (2 to 3 microg/kg). Heart rate variability under resting conditions also was analyzed.

RESULTS

In all subjects, a beat to beat systolic blood pressure increase associated with corresponding RR interval lengthening was observed during downward tilting as well as during phenylephrine administration. During both testing procedures, these two variables showed linear correlation, and the slope of regression line during downward tilting (DT-BRS) correlated significantly with Phe-BRS (r = 0.79, p = 0.0003). The DT- and Phe-BRS also correlated significantly with the high frequency component of resting HRV (r = 0.70, p = 0.0023 for DT-BRS; r = 0.58, p = 0.0185 for Phe-BRS).

CONCLUSIONS

We conclude that in a small homogeneous group DT-BRS provided an assessment of reflex cardiac vagal function comparable to that obtained by the phenylephrine method.

Effect of sinoaortic denervation on frequency-domain estimates of baroreflex sensitivity in conscious cats

In animals and humans, baroreceptor modulation of the sinus node in daily life can be studied by identification of the number of sequences in which systolic blood pressure (SBP) and pulse interval (PI) linearly decrease or increase for several beats. It is also studied by power spectral analysis of SBP and PI in regions where their powers are coherent, although, in contrast to the sequence method, whether this frequency-domain method specifically reflects the baroreceptor-heart rate reflex has not been adequately tested. We recorded intra-arterial BP for approximately 3.5 h in eight conscious cats, first intact and then 7-10 days after sinoaortic denervation (SAD). Sensitivity of baroreceptor-heart rate reflex was assessed in 120-s segments by the square root of the ratio of PI and SBP spectral powers (alpha) in the regions around 0.1 (MF) and 0.3 (HF) Hz, and coherence between PI and SBP spectral powers in MF and HF regions was computed. SAD increased overall SBP variability and reduced PI variability throughout the frequency range examined. SAD markedly reduced (P < 0.01) both alpha-MF (-65.6%) and alpha-HF (-79. 9%) and consistently reduced the number of coherent segments [i.e., where coherence (K2) > 0.5] and average coherence values in the MF region. In the HF region, however, SAD did not alter the number of coherent segments, and although average coherence value throughout the HF band was reduced, in restricted portions of the band (different between animals), a high coherence value survived denervation. No significant changes were seen in any measured variables in five sham-operated cats. Thus the frequency-domain method specifically reflects baroreflex modulation of heart rate in the MF region only. In the HF region, in contrast, baroreflex and nonbaroreflex influences on the sinus node both contribute to a variable degree to determination of heart rate responses to BP oscillations. If used to study baroreflex function in daily life, this method should use the coefficient derived from MF data.

Human sympathetic and vagal baroreflex responses to sequential nitroprusside and phenylephrine

We evaluated a method of baroreflex testing involving sequential intravenous bolus injections of nitroprusside followed by phenylephrine and phenylephrine followed by nitroprusside in 18 healthy men and women, and we drew inferences regarding human sympathetic and vagal baroreflex mechanisms. We recorded the electrocardiogram, photoplethysmographic finger arterial pressure, and peroneal nerve muscle sympathetic activity. We then contrasted least squares linear regression slopes derived from the depressor (nitroprusside) and pressor (phenylephrine) phases with 1) slopes derived from spontaneous fluctuations of systolic arterial pressures and R-R intervals, and 2) baroreflex gain derived from cross-spectral analyses of systolic pressures and R-R intervals. We calculated sympathetic baroreflex gain from integrated muscle sympathetic nerve activity and diastolic pressures. We found that vagal baroreflex slopes are less when arterial pressures are falling than when they are rising and that this hysteresis exists over pressure ranges both below and above baseline levels. Although pharmacological and spontaneous vagal baroreflex responses correlate closely, pharmacological baroreflex slopes tend to be lower than those derived from spontaneous fluctuations. Sympathetic baroreflex slopes are similar when arterial pressure is falling and rising; however, small pressure elevations above baseline silence sympathetic motoneurons. Vagal, but not sympathetic baroreflex gains vary inversely with subjects' ages and their baseline arterial pressures. There is no correlation between sympathetic and vagal baroreflex gains. We recommend repeated sequential nitroprusside followed by phenylephrine doses as a simple, efficientmeans to provoke and characterize human vagal and sympathetic baroreflex responses.

Sympathoinhibitory effects of clonidine are transmitted by the caudal ventrolateral medulla in cats

We examined mechanisms of the central sympathoinhibitory actions of systemically administered clonidine in anesthetized cats. To avoid influences of sympathetic chemo- and baroreflexes, the animals were deafferentated by cutting the carotid sinus and vagal nerves bilaterally. Intravenous (i.v.) injections of clonidine (25-250 nmol/kg) caused significant (50-90%) decreases in preganglionic sympathetic nerve activity (SNA) recorded from the white ramus of the third thoracic segment. Microinjections (500 nl) into the rostral ventrolateral medulla (RVLM) of clonidine at doses (50-500 pmol in 500 nl), which probably produced higher local concentrations than produced by systemic administration, caused only slight reductions of SNA and small decreases in arterial blood pressure (BP). Furthermore, sympathoinhibition and hypotension caused by intravenous clonidine was almost unaffected by prior microinjection of alpha2-receptor antagonist rauwolscine (500 pmol) into the RVLM. Microinjections of clonidine into the caudal ventrolateral medulla (CVLM), which provides important inhibitory input to the RVLM, had no significant effects. However, chemical lesions of the CVLM with kainate (5.0 nmol), effectively blocked the sympathoinhibitory effects of subsequently administered intravenous clonidine. The results suggest that the central sympathoinhibitory effects of therapeutically relevant doses of systemically administered clonidine may be primarily mediated by pathways that activate the CVLM rather than by direct actions within the RVLM.

Cyclosporine A modulates baroreceptor function in kidney transplant recipients

Cyclosporine has been described to increase the sympathetic tone. Alterations in sympathetic tone may contribute to baroreceptor dysfunction. Therefore, in this study baroreceptor function in 20 kidney transplant recipients was investigated under both low and high cyclosporine whole blood concentrations using the sequence analysis technique. The sympathetic nerve activity was estimated by calculating the low frequency oscillation of heart rate and blood pressure following Fast Fourier Transformation (FFT). Besides cyclosporine, azathioprine and prednisolone no other drugs were used. The increase in cyclosporine whole blood levels (from 101+/-13.4 ng/ml to 469+/-52 ng/ml) did not change mean arterial blood pressure significantly (83.7+/-2.5 vs. 82.2+/-2.0 mm Hg). Baroreflex sensitivity in +PI/+RR (+pulsinterval/+blood pressure) sequences, however, increased from 11.2+/-0.4 to 13.0+/-0.5 ms/mm Hg, whereas it was reduced in -PI/-RR (-pulsinterval/-blood pressure) sequences (14.4+/-0.3 to 12.5+/-1.1 ms/mm Hg). The increase in cyclosporine whole blood concentrations was associated with an increase in low frequency oscillation of heart rate (430+/-12 to 461+/-13) and blood pressure (452+/-9 to 469+/-12), indicating an enhanced sympathetic tone. Our results provide evidence that cyclosporine A by itself alters baroreceptor function. An imbalance between the sympathetic and parasympathetic nervous system due to an enhanced sympathetic tone may explain the reduction in -PI/-RR and the increase in +PI/+RR sequence baroreflex sensitivity.

Noninvasive assessment of spontaneous baroreflex sensitivity in patients with liver cirrhosis

AIMS/BACKGROUND

An impairment of baroreceptor sensitivity has been found in liver cirrhosis. Noninvasive and spontaneous estimates of baroreflex sensitivity are obtained from beat-to-beat blood pressure and heart rate recordings by means of cross-spectrum analysis and calculation of alpha-index (as a measure of baroreflex gain). The aim of the present study was to investigate the function of the spontaneous baroreflex sensitivity related to clinical Child score in liver cirrhosis.

METHODS

The alpha-index was evaluated in 40 cirrhotic patients (18 with and 22 without ascites) and 17 healthy subjects by analysing finger arterial pressure recorded noninvasively with the Portapres device.

RESULTS

Baroreflex sensitivity was significantly lower in cirrhotic patients with and without ascites compared with healthy subjects (p<0.01). Furthermore, in patients with ascites the baroreflex gain was significantly related to plasma sodium (p<0.01). A significant inverse relationship was present between baroreflex gain and grade of Child score and the severity of ascites (p<0.01). There were no significant relationships between hormonal parameters (catecholamines, renin, aldosterone, arginine-vasopressin, atrial natriuretic peptide and nitric oxide) and baroreflex gain. No significant differences were found between healthy subjects and cirrhotic patients with respect to systolic and diastolic blood pressure total variability in a supine position, whilst it was lower in cirrhotic patients with ascites in a tilted position (p<0.05).

CONCLUSION

Our findings showed that baroreflex sensitivity was significantly impaired in cirrhotic patients when compared with healthy subjects. In addition, there was a significant trend toward lower baroreflex sensitivity values with the grade score of Child class (p<0.01). Spectral analysis of the alpha-index provides viable alternatives to the pharmacological approach for estimation of baroreflex sensitivity and may represent a prognostic tool to identify cirrhotic patients at increased risk of adverse events.

Changes in the sympathetic nervous system induced by 42 days of head-down bed rest

Changes in autonomic nervous system activity could be linked to the orthostatic intolerance (OI) that individuals suffer after a spaceflight or head-down bed rest (HDBR). We examined this possibility by assessing the sympathetic nervous system activity during 42 days of HDBR in seven healthy men. Heart rate variability was studied with the use of power spectral analysis, which provided indicators of the sympathetic (SNSi) and parasympathetic (PNSi) nervous system influences on the heart. Urinary catecholamines and the spontaneous baroreflex sensitivity were measured. Urinary catecholamines decreased by 21.3%, showing a decrease in SNSi. Heart rate variability was greatly reduced during 42 days of HDBR with a drop in PNSi but with no significant changes in SNSi. The baroreflex sensitivity was greatly reduced (30.7%) on day 42 of HDBR. These results suggest a dissociation between the catecholamine response and the SNSi of the heart rate. This dissociation could be the consequence of an increase in beta-adrenergic receptor density and/or activity induced by a decrease in catecholamines during HDBR. The subjects who suffered from OI also had a greater sympathetic response and much lower baroreflex sensitivity when supine than those who finished the stand test. However, the mean response of all subjects indicated that the sympathetic activity (catecholamine excretion) was probably slightly inhibited during HDBR and could contribute to OI.

Low-dose transdermal scopolamine decreases blood pressure in mild essential hypertension

BACKGROUND

Increasing cardiovascular parasympathetic nervous activity could have antihypertensive effects. Low-dose transdermal scopolamine increases vagal-cardiac modulation of sinus node and baroreflex sensitivity in healthy subjects and in cardiac patients.

OBJECTIVE

To study the short-term effects of transdermal scopolamine on blood pressure and cardiovascular autonomic control in patients with mild essential hypertension.

DESIGN

A randomized, double-blind, placebo-controlled crossover trial with 12 untreated middle-aged [aged 39+/-5 years (mean+/-SD)] patients with mild essential hypertension.

METHODS

We recorded the electrocardiogram, auscultatory sphygmomanometric and continuous photoplethysmographic finger arterial pressure, and spirometry signals with patients supine and 70 degrees tilted during controlled (0.25 Hz) breathing. Cardiovascular autonomic regulation was analyzed with power spectrum analysis of R-R interval and arterial pressure variability and a spontaneous sequence method for baroreflex sensitivity. In addition, a deep-breathing test was performed to assess maximal breathing-related sinus arrhythmia.

RESULTS

Transdermal scopolamine treatment significantly decreased blood pressure both when patients lay supine and when they were in the 70 degrees tilted position. Scopolamine also slowed heart rate and increased baroreflex sensitivity and R-R interval high-frequency variability for both body positionings. In addition, scopolamine accentuated respiratory sinus arrhythmia during deep breathing and blunted the tilt-induced increase in heart rate. Scopolamine did not affect blood pressure variability.

CONCLUSIONS

Transdermal scopolamine decreases arterial pressure, increases baroreflex sensitivity and accentuates vagal-cardiac modulation of sinus node in patients with mild hypertension. Our study supports the hypothesis that increasing cardiovascular parasympathetic activity could have antihypertensive effects in essential hypertension.

The reproducibility of cardiac baroreceptor activity assessed non-invasively by spectral sequence techniques

Baroreceptor sensitivity (BRS) is increasingly used as a prognostic indicator in cardiovascular disease. Traditionally it has been measured using invasive techniques with pharmacological manipulation of blood pressure (BP). With the advent of newer methods to measure pulse interval and beat-to-beat changes in BP it is now possible, using sophisticated mathematical modelling techniques, to calculate cardiac BRS non-invasively. However, there are virtually no data on the reproducibility of these newer techniques and what factors may affect the repeatability of these measurements. We studied 39 subjects, aged 22-82 years, with a supine systolic BP range 97-160 mmHg and a diastolic BP range 57-94 mmHg on two occasions between 1 week and 6 months apart. Cardiac BRS was measured by power spectral analysis using Fast Fourier Transformation (FFT), sequence analysis (using up, down and combined sequences) and from phase IV of the Valsalva manoeuvre. There was no significant difference between visits for any of the methods for measuring cardiac BRS. Mean BRS values were similar for FFT (16.7 +/- 11.2 ms/mmHg) and sequence analysis (15.8 +/- 11.4 ms/mmHg); however, results using phase IV of the Valsalva manoeuvre were significantly lower (8.1 +/- 2.9 ms/mmHg, p < 0.0001). The coefficient of variation for the five measures of cardiac BRS varied from 16.8% for Valsalva-derived values to 26.1% for 'down' sequence analysis. However, in ten subjects BRS could not be calculated from the Valsalva manoeuvre. None of the independent variables tested (including age, BP levels and time between testing) significantly influenced the degree of repeatability. In summary, there appears to be little difference between these non-invasive methods in their degree of reproducibility. These techniques would seem suitable for longitudinal studies of changes in cardiac BRS and overcome many of the problems associated with the invasive pharmacological methods.

Spectrum analysis of cardiovascular time series

The demand for noninvasive assessment of cardiovascular control parameters has promoted the use of spectrum analysis. These techniques have been applied on a broad basis; however, because of the abstract mathematical approach, spectrum analysis in physiology is still not fully accepted by some circles in the scientific community. Thus it is the goal of the following review to focus on the rationale for applying spectrum analysis in different fields of circulation research, which range from determining arterial baroreceptor reflex sensitivity to the early detection of heart allograft rejection. Within this scope, major findings regarding the physiological and pathophysiological regulation of the cardiovascular system are discussed. In addition, inherent limitations of these methods are made clear. Toward the end of this survey, a perspective is provided for the general readership.

Cross spectral analysis in assessment of baroreflex gain in patients with coronary artery disease

BACKGROUND

Interest in determination of baroreflex sensitivity in clinical practice is growing because of its prognostic information in patients with heart disease. The purpose of the present study was to assess the feasibility of cross spectral analysis in the determination of baroreflex gain from spontaneous RR interval and systolic pressure fluctuations, and to compare the results to the traditional pharmacological method in patients with coronary artery disease. Methods. We measured the gain and time lag between RR interval and systolic pressure variabilities in the frequency domain, and compared baroreflex indexes obtained by this technique with standard phenylephrine tests in 32 patients with coronary artery disease. Results. Cross spectral analysis by fast Fourier transform techniques yielded acceptable (> 0.5) coherence between systolic pressure and RR interval in the mid- (0.07-0.15 Hz) and in the respiratory-frequency (0.15-0.40 Hz) band fluctuations in 30 patients (94%), with mean coherences of 0.69 and 0.74. The mean phase difference in the mid-frequency hand was greater than in the respiratory-frequency band (-83 vs -23 degrees, P < 0.001), suggesting that the mid-frequency fluctuations of RR intervals followed nearly 2 seconds after pressure changes, while respiratory-frequency fluctuations of RR intervals occurred nearly concomitantly with systolic pressure. The mean baroreflex slope derived from the bolus phenylephrine technique was 6.2 ms/mmHg (range 1.6-16.0), 5 patients had an abnormally low (<3 ms/mmHg) baroreflex sensitivity. Baroreflex gain determined by cross spectral analysis from the mid-frequency band correlated significantly (r = 0.60, P < 0.001, n = 27) with the baroreflex gain determined by the phenylephrine test, while the correlation in the respiratory-frequency band was not significant (r = 0.35, P = 0.09, n = 26). Conclusions. Baroreflex slopes derived from cross spectral techniques provide reliable (but not perfect) information regarding baroreflex gain derived from the clasic phenylephrine technique, even in patients with depressed baroreflex responses. Cross correlation calculation of spontaneous baroreflex slopes should be limited to data in the mid-frequency range, where the slopes are likely to reflect simple baroreflex physiology.

Effects of inhaled albuterol and ipratropium bromide on autonomic control of the cardiovascular system

STUDY OBJECTIVE

Systemic administration of beta-agonist and anticholinergic drugs markedly impair normal autonomic heart rate control. The purpose of this study was to quantify and compare the effects of therapeutic doses of inhaled albuterol and ipratropium on autonomic control of the cardiovascular system.

DESIGN

Randomized, double-blind, placebo-controlled, crossover design study.

SETTING

Tertiary-care hospital.

SUBJECTS

Twelve healthy male volunteers.

INTERVENTIONS

Subjects self-administered four puffs through a spacer device from one of three identical inhalers containing albuterol (100 microg per puff), ipratropium (20 microg per puff), or placebo in three different testing sessions.

MEASUREMENTS

ECG and noninvasive continuous BP traces were recorded at baseline and from 45 to 75 min after administration of the drug. Autonomic control of the cardiovascular system was quantified by analysis of spontaneous baroreflex sensitivity and power spectral analysis of heart rate variability.

RESULTS

Neither albuterol nor ipratropium caused a significant alteration in baroreflex sensitivity, normalized low-power frequency, or normalized high-power frequency. No adverse effects were reported by subjects.

CONCLUSIONS

Inhalation of four puffs of albuterol (400 microg) or ipratropium (80 microg) does not alter the autonomic control of the cardiovascular system in young, healthy male subjects.

Assessment of spontaneous baroreflex sensitivity in neonates

AIMS

To determine whether it is possible to assess baroreflex sensitivity in neonates by studying only spontaneous variation in systolic blood pressure and heart rate.

METHODS

ECG and non-invasive blood pressure signals were continuously studied in 14 preterm neonates (term 29-32 weeks) and five term neonates (term 40-41 weeks). Non-invasive blood pressure measures were obtained using a Finapres placed around the child's wrist. Both signals (ECG and blood pressure), sampled at 400 Hz, were digitised by an A/D converter and stored in a binary mode on magnetic disk. An inhouse software QRS detection algorithm was used to define R peaks of the QRS complexes with an accuracy greater than 2 ms. Four 4 minute periods were recorded in each infant. The slope of the linear regression of RR intervals versus systolic blood pressure was calculated in each period and the mean value of the four slopes was then considered as the index of baroreflex sensitivity (in ms/mm Hg) in each neonate.

RESULTS

Spontaneous baroreflex sensitivity was lower in preterm neonates than in term neonates (mean(SD): 4.07 (2.19) ms/mm Hg vs 10.23 (2.92) ms/mm Hg).

CONCLUSION

Baroreflex sensitivity can be assessed in term and preterm neonates by studying spontaneous variations in systolic blood pressure alone. This method could be useful for studying the ontogeny of baroreflex sensitivity and might therefore provide information about the maturation of the autonomic nervous system.

The effects of reversal of neuromuscular blockade on autonomic control in the perioperative period

Impaired parasympathetic control of heart rate is associated with increased incidence of cardiac dysrhythmias and ischemia. Anticholinergic drugs suppress parasympathetic control and could be detrimental in the early postoperative period in high-risk patients. In this double-blind randomized trial, 30 ASA physical status I and II patients undergoing minor surgery received either atropine 20 micrograms/kg and neostigmine 50 micrograms/kg (Group A), glycopyrrolate 8 micrograms/kg and neostigmine 50 micrograms/kg (Group G), or placebo (Group P) for reversal of neuromuscular blockade. Two indices of parasympathetic modulation of heart rate, spontaneous baroreflex sensitivity, and high-frequency heart rate variability, were assessed. At 2 h after reversal, Group A showed persisting impairment of baroreflex sensitivity with respect to Group P (7.12 +/- 0.86 vs 12.71 +/- 1.38 ms/mm Hg, P = 0.022) as well as decreased high-frequency heart rate variability (280.8 +/- 30.1 vs 569.2 +/- 115.2 ms2/Hz, P = 0.015). Groups A and G showed a borderline decrease in normalized high-frequency variability at 2 h (P = 0.05 for Groups A and G versus Group P). Anticholinergic drugs with neostigmine cause impairment of parasympathetic control of heart rate which persists into the early postoperative period. The effects of glycopyrrolate appear to be of shorter duration; this drug may thus be preferable in patients at risk of cardiovascular complications.

The duration of impairment of autonomic control after anticholinergic drug administration in humans

Impaired parasympathetic control of heart rate is associated with increased incidence of cardiac dysrhythmias and ischemia. Anticholinergic drugs, commonly administered during reversal of neuromuscular blockade, suppress parasympathetic control in the early postoperative period. This could potentially be detrimental in patients at risk of cardiovascular complications. The duration of parasympathetic impairment by two anticholinergic drugs were compared in this double-blind randomized cross-over study. Fourteen healthy volunteers received a single intravenous injection of atropine 20 micrograms/kg or glycopyrrolate 8 micrograms/kg during two different study sessions. The methods of spontaneous baroreflex analysis and spectral analysis of heart rate variability generated indices of beat-by-beat parasympathetic modulation of heart rate. Both drugs resulted in a marked decrease in baroreflex sensitivity and high-frequency heart rate variability. The times to return to baseline values were approximately doubled after atropine compared to glycopyrrolate (177 +/- 22 vs 82 +/- 8 min for baroreflex sensitivity, 212 +/- 16 vs 111 +/- 14 min for high-frequency power, and 171 +/- 18 vs 95 +/- 18 min for high-frequency power normalized to total power; P < 0.01 for all variables). Atropine leads to more prolonged impairment of parasympathetic control than equipotent doses of glycopyrrolate, and its use may thus be less desirable in high-risk patients in the early postoperative period.

Non-invasive assessment of baroreflex sensitivity and relation to measures of heart rate variability in man

1. We have developed a simple noninvasive method to assess the spontaneous baroreflex (BRS; reflex heart rate response to change in systemic blood pressure) in man. The BRS is impaired in many cardiovascular disorders and is known to be of major prognostic significance; however, the invasive nature of traditional BRS assessment has thus far limited its clinical application. 2. Sixteen healthy subjects (7M, 9F; age 28 +/- 3 years) were studied at baseline and during stepwise i.v. infusion of phenylephrine and bolus i.v. dosing of nitroprusside. Invasive BRS was derived from linear regression of the change in systolic blood pressure (SBP) and RR interval from baseline following these perturbations. Noninvasive BRS was derived from approximately 1500 gated beat-to-beat SBP and RR interval data points acquired by Finapres BP and continuous ECG monitoring. Slopes were derived from linear regression of three consecutive baroreflex-mediated data points (following a phase shift of one RR interval from its accompanying SBP value) and were averaged to determine BRS. Correlations between invasive and noninvasive BRS measures were found to be highly significant (r = 0.78; P = 0.0009; slope = 0.87). 3. Significant correlations between noninvasively determined BRS and heart rate variability derived measures of autonomic activity (24h standard deviation of normal RR intervals, root mean square of successive RR intervals, low frequency and high frequency power) were also observed (r = 0.79-0.83; P = 0.003-0.008). 4. These results support the validity of this noninvasive method of measurement of BRS in man and of parasympathetic contribution to spontaneous baroreflex.