Evidence for a central site of action to explain the negative chronotropic effect of atropine: studies on the human transplanted heart

Oscillatory behavior of P wave duration and PR interval in experimental congestive heart failure: a preliminary study

OBJECTIVE

The relationship between the autonomic nervous system (ANS) modulation of the sinus node and heart rate variability has been extensively investigated. The current study sought to evaluate, in an animal experimental model of pacing-induced tachycardia congestive heart failure (CHF), a possible ANS influence on the P wave duration and PR interval oscillations.

APPROACH

Short-term (5 min) time and frequency domain analysis has been obtained in six dogs for the following electrocardiographic intervals: P wave duration (P), from the onset to peak of P wave (P _p), from the onset of P wave to the q onset (PR) and from the end of P wave to the onset of q wave (P _e R). Direct vagal nerve activity (VNA), stellate ganglion nerve activity (SGNA) and electrocardiogram (ECG) intervals have been evaluated contextually by implantation of three bipolar recording leads.

MAIN RESULTS

At the baseline, multiple regression analysis pointed out that VNA was strongly positively associated with the standard deviation of PP and P _e R intervals (r ²:0.997, p  <  0.05). The same variable was also positively associated with high-frequency (HF) of P expressed in normalized units, of P _p, and of P _e R (b: 0.001) (r ²: 0.993; p  <  0.05). During CHF, most of the time and frequency domain variability significantly decreased from 20% to 50% in comparison to the baseline values (p  <  0.05) and SGNA correlated inversely with the low frequency (LF) obtained from P _e R (p  <  0.05) and PR (p  <  0.05) (r ²:0.899, p  <  0.05). LF components, expressed in absolute and normalized power, obtained from all studied intervals, were reduced significantly during CHF. Any difference between the RR and PP spectral components was observed.

SIGNIFICANCE

The data showed a significant relationship between ANS and atrial ECG variables, independent of the cycle duration. In particular, the oscillations were vagal mediated at the baseline, while sympathetic mediated during CHF. Whereas P wave variability might have a clinical utility in CHF management, it needs to be addressed in specific studies.

The effect of oxybutynin on cardiac autonomic modulation in healthy subjects

PURPOSE

The aim of this study was to assess the various doses of oral oxybutynin on cardiac autonomic modulation by measuring short-term heart rate variability (HRV) indexes during supine rest position.

METHODS

Eight male healthy subjects (20-23 years) participated in the double-blind crossover randomized study. The single dose of oxybutynin (2.5, 5 and 10 mg) or placebo was given to the volunteers in four sessions within 5-day intervals. Before and minutes of 30, 60, 90 and 120 after administration, lead II electrocardiogram (ECG) was recorded for 5 min. ECG extracted RR intervals data became the base of the calculation of time domain and frequency domain HRV parameters, which indicate cardiac autonomic activity. Statistical analysis was done by using the nonparametric Wilcoxon and Kruskal-Wallis tests.

RESULTS

The data analysis has revealed that MNN (P < 0.001), SDNN (P < 0.05), PNN50% (P < 0.01), RMSSD (P < 0.001), HFnu (P < 0.05) and LF/HF ratio (P < 0.05) values were significantly increased relative to baseline at various time points in all the groups except in placebo group. LFnu (P < 0.05) values were significantly increased relative to baseline at various time points in all the groups except in placebo group.

CONCLUSIONS

Our findings have revealed that acute consumption of 2.5, 5 and 10 mg oxybutynin (an anticholinergic compound) in the juvenile healthy male subjects produces a cholinergic effect according to time and frequency domain of HRV indexes.

The risk of adverse events associated with atropine administration during dobutamine stress echocardiography in cardiac transplant patients: a 28-year single-center experience

BACKGROUND

Although dobutamine stress echocardiography (DSE) is performed in heart transplant patients, the safety profile of atropine administration in DSE in this setting is unclear.

METHODS AND RESULTS

We identified heart transplant patients who received atropine during DSE from January 1984 to August 2011 at our institution and compared them with a propensity-scored matched control group of heart transplant patients who underwent DSE without atropine. Adverse events were defined as significant arrhythmias (sinus arrest, Mobitz type II heart block, complete heart block, ventricular tachycardia, or ventricular fibrillation), hypotension requiring hospitalization, syncope or presyncope, myocardial infarction, and death. Forty-five heart transplant patients (median age 62 years, 82% male) received 0.2-1 mg atropine during DSE. Of these, 1 patient (2.2%) developed temporary complete heart block. No adverse events were identified in the control group of 154 patients who received dobutamine without atropine.

CONCLUSIONS

Our findings suggest that complete heart block can occur infrequently with the administration of atropine in heart transplant patients undergoing DSE. Therefore, patients should be appropriately monitored for these adverse events during and after DSE.

Does respiratory sinus arrhythmia serve a buffering role for diastolic pressure fluctuations?

Though many consider the magnitude of respiratory sinus arrhythmia as an index of cardiac vagal control, its physiological origins remain unclear. One influential model postulates that the systolic pressure rise within a given beat stimulates the baroreflex arc to adjust the following heart period such that diastolic pressure is "stabilized" and hence displays lesser fluctuation. Accordingly, the magnitude of diastolic pressure fluctuations with respiration should change reciprocally after augmentation or inhibition of respiratory sinus arrhythmia. To test this, we augmented and subsequently inhibited respiratory sinus arrhythmia with vagotonic and vagolytic atropine administration in 19 healthy young volunteers to assess the relation between respiratory R-R interval and diastolic pressure fluctuations. Respiratory diastolic pressure fluctuations showed parallel rather than inverse changes in relation to those in respiratory sinus arrhythmia: they increased with augmented respiratory sinus arrhythmia (138 and 190% of baseline in the frequency and time domains, both P < 0.05) and tended to decrease with inhibited respiratory sinus arrhythmia (82 and 93% of baseline in frequency and time domains, P = 0.20 and P = 0.07). Furthermore, >60% of the change in diastolic pressure fluctuations was explained by the change in respiratory sinus arrhythmia (R(2) = 0.62; P < 0.001), that is, an approximately 50-ms increase or decrease in respiratory sinus arrhythmia resulted in a parallel approximately 1-mmHg change in diastolic pressure fluctuations. Thus, in young healthy individuals during supine rest, respiratory fluctuations in R-R interval do not buffer against diastolic pressure fluctuations but actually cause diastolic pressure fluctuations. Therefore, our data provide little evidence for a predominant role of a baroreflex feedback mechanism underlying respiratory sinus arrhythmia during supine rest.

Incongruous changes in heart period and heart rate variability with vagotonic atropine: implications for rehabilitation medicine

OBJECTIVE

To describe the relationship between vagally mediated bradycardia and heart rate variability indices in young and older healthy individuals.

DESIGN

Cross-sectional, physiologic study.

SETTING

Outpatient cardiovascular research laboratory.

PARTICIPANTS

A total of 34 young (mean age 24 years) and 27 older (mean age 63 years) healthy adults.

METHODS

Eight bolus injections of atropine sulfate were given intravenously to participants while in a supine position (cumulative doses from 0.4 to 7.2 microg/kg). Each dose was followed by a 3-minute data collection period in which subjects controlled their breathing frequency at a rate of 15 breaths per minute.

MAIN OUTCOME MEASUREMENTS

Chronotropic responses were assessed from average RR interval and blood pressure was assessed by automated brachial cuff. Heart rate variability (HRV) indices were calculated to represent both time domain measures (RR interval standard deviation and root mean squared of successive differences) and frequency domain measures (respiratory sinus arrhythmia and total power).

RESULTS

RR interval responses exhibited the expected curvilinear pattern to atropine administration with all subjects exhibiting a bradycardia with at least one dose and RR interval returning to baseline or decreasing in most subjects as atropine dosing progressed. RR interval was closely related to vagotonic atropine dose with an r(2) greater than 0.70 in 89% of subjects. Heart rate variability indices were not consistently correlated with the bradycardic effect of vagotonic atropine and ranged from highly positive to highly negative with almost one-fifth of correlations less than 0.5.

CONCLUSIONS

The relationship between HRV and vagal tone is likely complex and has a large interindividual variation.

Effects of fitness and age on the response to vagotonic atropine

Previous work indicates compromised cardiac vagal control plays a prominent role in reducing arterial baroreflex gain with age, however older fit individuals display cardiovagal baroreflex responses similar to young individuals. The purpose of this study was to test the hypothesis that chronic aerobic exercise mitigates against age-related declines in cardiac parasympathetic receptor function. In forty-four young and old (fit and unfit) individuals, we used the parasympathomimetic responses to low doses of atropine to probe cardiac cholinergic receptor responses. Data were collected before and after eight doses of atropine sulfate from 0.4 to 7.2 microg/kg. Chronotropic responses were assessed from average RR intervals and heart rate variabilities were derived in time and frequency domains. All subjects exhibited bradycardia with at least one dose of atropine and peak bradycardia occurred at a similar dose in each group. However, changes in heart rate variability did not consistently track the chronotropic responses within subjects (r-square from 0.90 down to 0). As expected, basal RR interval was longer in the fit groups and was unaffected by age. However, the degree of RR interval lengthening with parasympathomimetic atropine was unaffected by physical fitness and was significantly less in all older subjects. These data indicate there are certain prepotent age-related declines in the cardiac parasympathetic system that cannot be prevented by regular physical activity.

Sympathoexcitation increases the QT/RR slope in healthy men: differential effects of hypoxia, dobutamine, and phenylephrine

INTRODUCTION

Dynamic ventricular repolarization assessed by QT/RR slopes studies the effects of modifications in cardiac repolarization independently of variations in RR interval (RR). The effects of changes in sympathetic and vagal activity on the QT/RR slope are controversial. We tested the hypothesis that sympathoexcitation is an important determinant of the QT/RR slope.

METHODS AND RESULTS

We compared the effects of a reflex sympathetic activation in response to hypoxia, to the direct effects of the infusion of the beta-adrenergic agent dobutamine, on the QTa (apex) and QTe (end)/RR slopes. Dobutamine was titrated to obtain similar increases in cardiac output than with hypoxia. Cardiac vagal activity was estimated by rMSSD and pNN50. In a second group of healthy subjects, we assessed the effect of a reflex cardiac vagal activation in response to phenylephrine infusion on the same variables. We observed a similar increase in QTa and QTe slopes during hypoxia and dobutamine (both P < 0.017 vs. normoxia), despite divergent changes in cardiac vagal activity, as rMSSD and pNN50 decreased with hypoxia compared to normoxia (P < 0.001) but increased during dobutamine infusion compared to hypoxia (P < 0.017). In contrast, these slopes did not change during the rises in rMSSD and pNN50 elicited by phenylephrine (P > 0.7).

CONCLUSION

Beta-adrenergic stimulation induces comparable increases in the QT/RR slopes than hypoxia, but in the presence of a larger cardiac vagal activity. Vagal cardiac activation by phenylephrine does not change the QT slopes. This reveals that the sympathetic system is an important determinant of QT/RR dynamicity in healthy men.

Negative chronotropic response to low-dose atropine is associated with parasympathetic nerve-mediated cardiovascular response in postoperative patients with congenital heart disease

OBJECTIVES

To investigate the negative chronotropic response (NCR) to low-dose atropine in postoperative patients with congenital heart disease (CHD).

BACKGROUND

Low-dose atropine causes a NCR through the central nervous system muscarinic receptor and is attenuated in adult heart failure patients. It has never been evaluated in CHD patients.

METHODS

NCR corrected for basal heart rate (HR) (minimal HR/basal HR=cNCR) was determined after low-dose atropine (3 microg/kg) administration in 124 postoperative CHD patients (97 biventricular repair and 27 Fontan patients) and 11 controls and was compared with the cardiac autonomic nervous and functional status.

RESULTS

The cNCR in simple CHD (post atrial or ventricular septal defect closure), complex biventricular CHD, and Fontan patients were 0.92+/-0.04, 0.94+/-0.04 and 0.96+/-0.04, respectively, and higher than in controls (0.87+/-0.03, p<0.001). In the complex CHD patients, higher cNCR was mainly associated with the lower pharmacologically determined cardiac parasympathetic nervous tone (PST), HR variability, high atrial natriuretic peptide, and lower right ventricular ejection fraction (p<0.0001). In Fontan patients, the lower beta sensitivity of the sinus node and the PST mainly determined the higher cNCR (p<0.01) and the cNCR did not correlate with either hemodynamics or exercise capacity.

CONCLUSIONS

NCR is attenuated in proportion to the impaired cardiac parasympathetic nervous system and hemodynamics in postoperative complex biventricular CHD patients. In addition to PST, beta sensitivity of the sinus node significantly influences the NCR in Fontan patients.

Orthostatic hypotension in aging humans

We tested the hypothesis that hypotension occurred in older adults at the onset of orthostatic challenge as a result of vagal dysfunction. Responses of heart rate (HR) and mean arterial pressure (MAP) were compared between 10 healthy older and younger adults during onset and sustained lower body negative pressure (LBNP). A younger group was also assessed after blockade of the parasympathetic nervous system with the use of atropine or glycopyrrolate and after blockade of the beta(1)-adrenoceptor by use of metoprolol. Baseline HR (older vs. younger: 59 +/- 4 vs. 54 +/- 1 beats/min) and MAP (83 +/- 2 vs. 89 +/- 3 mmHg) were not significantly different between the groups. During -40 Torr, significant tachycardia occurred at the first HR response in the younger subjects without hypotension, whereas significant hypotension [change in MAP (DeltaMAP) -7 +/- 2 mmHg] was observed in the elderly without tachycardia. After the parasympathetic blockade, tachycardiac responses of younger subjects were diminished and associated with a significant hypotension at the onset of LBNP. However, MAP was not affected after the cardiac sympathetic blockade. We concluded that the elderly experienced orthostatic hypotension at the onset of orthostatic challenge because of a diminished HR response. However, an augmented vasoconstriction helped with the maintenance of their blood pressure during sustained LBNP.

Nine months in space: effects on human autonomic cardiovascular regulation

We studied three Russian cosmonauts to better understand how long-term exposure to microgravity affects autonomic cardiovascular control. We recorded the electrocardiogram, finger photoplethysmographic pressure, and respiratory flow before, during, and after two 9-mo missions to the Russian space station Mir. Measurements were made during four modes of breathing: 1) uncontrolled spontaneous breathing; 2) stepwise breathing at six different frequencies; 3) fixed-frequency breathing; and 4) random-frequency breathing. R wave-to-R wave (R-R) interval standard deviations decreased in all and respiratory frequency R-R interval spectral power decreased in two cosmonauts in space. Two weeks after the cosmonauts returned to Earth, R-R interval spectral power was decreased, and systolic pressure spectral power was increased in all. The transfer function between systolic pressures and R-R intervals was reduced in-flight, was reduced further the day after landing, and had not returned to preflight levels by 14 days after landing. Our results suggest that long-duration spaceflight reduces vagal-cardiac nerve traffic and decreases vagal baroreflex gain and that these changes may persist as long as 2 wk after return to Earth.

Differential effects of parasympathetic blockade and parasympathetic withdrawal on heart rate variability

INTRODUCTION

Low heart rate variability (HRV) has been shown to have important prognostic significance in multiple settings. Although this is believed to reflect reduced parasympathetic tone, the physiology of reduced parasympathetic tone has not been elucidated.

METHODS AND RESULTS

To evaluate whether parasympathetic withdrawal and partial parasympathetic blockade result in similar changes in HRV, 27 normal volunteers underwent complete beta-adrenergic blockade and then were given (1) graded doses of nitroprusside to achieve baroreflex-mediated parasympathetic withdrawal and (2) low-dose atropine (0.01 mg/kg) to achieve partial parasympathetic blockade. Five-minute ECG recordings were obtained for HRV analysis. In 19 subjects, paired 5-minute recordings from each condition were available with mean RR intervals that differed by < 50 msec (low-dose atropine: 869 +/- 96 msec and nitroprusside 875 +/- 99 msec). The root mean square of the successive RR interval differences was lower following low-dose atropine than following parasympathetic withdrawal with nitroprusside (16 +/- 11 msec vs 22 +/- 15 msec; P < 0.02). During parasympathetic withdrawal, the low-frequency (LF) power was 0.917 +/- 0.602 bpm2 and the high-frequency (HF) power was 0.501 +/- 0.521 bpm2. During partial parasympathetic blockade, the LF and HF powers were significantly lower (0.443 +/- 0.474 bpm2, P < 0.005; and 0.198 +/- 0.207 bpm2, P < 0.02).

CONCLUSION

These data confirm that HRV reflects the character of parasympathetic modulation of the heart rate rather than parasympathetic tone per se. Furthermore, this study identifies two distinct physiologic explanations for the finding of low HRV, namely, diminished vagal discharge and resistance of cardiac muscarinic receptors to vagal discharge. Further delineation of the relationships between parasympathetic tone and HRV will allow for better understanding of the pathophysiologic derangements associated with low HRV.

Nonpharmacologic validation of the intrinsic heart rate in cardiac transplant recipients

BACKGROUND

The maximum sinus rate during exertion in humans is inversely related to age. However, the sinus rate at rest is quite variable. The intrinsic heart (IHR) following pharmacologic blockade of autonomic tone with propranolol and atropine has been proposed as a test of sinus node function and is related to age by the linear regression equation: IHR = 118.1 - (0.57 x age). Whether this relationship exists for transplanted hearts for which the donor sinus node is denervated has not been determined.

METHODS

The relationship between the resting heart rate and the age of the donor heart was examined in 103 patients 1 year following orthotopic cardiac transplantation in the absence of rejection or intercurrent illness. Patients receiving beta-blockers, calcium blockers, antiarrhythmic drugs, digitalis, theophylline, or with biopsy evidence of rejection or abnormal coronary arteriograms were excluded from analysis.

RESULTS

The recipient age, left ventricular ejection fraction, pulmonary capillary pressure, cardiac index, donor heart ischemic time and cardiopulmonary bypass time did not correlate with the rate of the resting donor sinus node. The resting heart rate was inversely related to age of the donor heart by the linear regression equation: HR = 112.0 - (046 x age).

CONCLUSION

The resting rate of the denervated sinus node is related to donor age with a regression equation that is similar, though slightly slower, than that predicted after pharmacologic autonomic blockade.

Central vagotonic effects of atropine modulate spectral oscillations of sympathetic nerve activity

BACKGROUND

Low-dose atropine causes bradycardia either by acting on the sinoatrial node or by its effects on central muscarinic receptors increasing vagal activity. Any central muscarinic effects of high-dose atropine on RR interval are masked by peripheral muscarinic blockade at the sinoatrial node, which causes tachycardia. Effects of central parasympathetic activation on sympathetic activity are not known.

METHODS AND RESULTS

Using power spectral analysis of RR interval, intra-arterial blood pressure, respiration, and muscle sympathetic nerve activity (MSNA), we examined the effects of both low (2 microgram/kg IV) and high (15 microgram/kg IV) doses of atropine. After low-dose atropine, RR increased by 9+/-1% (P<0.0001), the low-frequency (LF) component (in normalized units, NU) of RR variability decreased by -32+/-8%, and the high-frequency (HF)NU component increased (+74+/-19%); hence, LF/HF of RR variability fell by 52+/-10% (all P<0.01). Although overall MSNA did not change, LFNU of MSNA decreased (-15+/-5%), HFNU of MSNA increased (+31+/-3%), and LF/HF of MSNA fell (-41+/-8%) (all P<0.01). After high-dose atropine, LFNU of MSNA decreased (-17+/-12%), HFNU of MSNA increased (+22+/-3%), and LF/HF of MSNA fell (-51+/-21%) (all P<0.02).

CONCLUSIONS

Increasing central parasympathetic activity with low-dose atropine is associated with an increase in the HF and a decrease in the LF oscillations of both RR interval and MSNA variability. High-dose atropine similarly induces an increase in the HF and a decrease in the LF components of MSNA variability. Thus, central parasympathetic activation is able to modulate the oscillatory characteristics of sympathetic nerve traffic to peripheral blood vessels.

Cardiac muscarinic receptors decrease with age. In vitro and in vivo studies

The M1 muscarinic receptor antagonist pirenzepine in low doses decreases resting heart rate; this effect declines with age (Poller, U., G. Nedelka, J. Radke, K. Pönicke, and O.-E. Brodde. 1997. J. Am. Coll. Cardiol. 29:187-193). To study possible mechanisms underlying this effect, we assessed (a) in six young (26 yr old) and six older volunteers (61 yr old), pirenzepine effects (0.32 and 0.64 mg intravenous [i.v.] bolus) on isoprenaline-induced heart rate increases; (b) in five heart transplant recipients, pirenzepine effects (0.05-10 mg i.v. bolus) on resting heart rate in the recipient's native and transplanted sinus nodes; and (c) in right atria from 39 patients of different ages (5 d-76 yr) undergoing open heart surgery, M2 muscarinic receptor density (by [3H]N-methyl-scopolamine binding) and adenylyl cyclase activity. (a) Pirenzepine at both doses decreased heart rate in young volunteers significantly more than in older volunteers; (b) pirenzepine (< 1 mg) decreased resting heart rate in the recipient's native but not transplanted sinus node; and (c) M2 receptor density and carbachol-induced inhibition of forskolin-stimulated adenylyl cyclase activity decreased significantly with the age of the patients. We conclude that pirenzepine decreases heart rate via inhibition of presynaptic M1 autoreceptors, thereby releasing endogenous acetylcholine, and that the heart rate-decreasing effect of acetylcholine declines with age because right atrial M2 receptor density and function decrease.

Usefulness of low doses of atropine to quantify the vagal stimulus-response relation in patients with congestive heart failure

The response of low doses of atropine is reported to be attenuated in patients with congestive heart failure (CHF). Judging from the main site of action of low doses of atropine, we may be able to assess the functional state of the vagal center in the central nervous system. This study examines the clinical significance of heart rate (HR) response to a low dose of atropine in patients with CHF. Low and high doses of atropine were administered intravenously in 72 patients with CHF. HR after a low (parasympathomimetic) dose injection was assessed by the ratio Rm (minimal HR/basal HR), and after a high (parasympatholytic) dose by the ratio R1 (augmented HR/basal HR). Rm and R1 were related to indexes of CHF. Rm increased with progression of CHF (0.92 +/- 0.03 in New York Heart Association functional class I, 0.98 +/- 0.05 in class II, and 1.00 +/- 0.04 in class III). It also correlated with ejection fraction (r = -0.48, p <0.01) and more importantly, with peak oxygen uptake (r = -0.59, p <0.01). R1 exhibited weak correlation with basal HR (r = -0.33, p <0.05) and ejection fraction (r = 0.31, p <0.05), but had no correlation with other indexes. The vagal center may be already blunted in New York Heart Association class II with respect to increased Rm, which may be related to depressed exercise capacity. A low dose of atropine injection offers a simple and safe method for providing important information on the functional state of the vagal center in the central nervous system in patients with CHF.

Atrioventricular block after administration of atropine in patients following cardiac transplantation

BACKGROUND

Atropine is widely used as a parasympatholytic agent during diagnostic and therapeutic procedures. We observed an unexpected paradoxical response to atropine after cardiac transplantation.

METHODS

In a study investigating the occurrence of autonomic reinnervation after cardiac transplantation, atropine, at 0.015 mg/kg body weight, was given intravenously to 23 patients (mean age, 56+/-8 years) 98 days to 6.4 years after transplantation.

RESULTS

Two patients experienced a witnessed syncope 40 and 150 min after administration of atropine. Second-degree atrioventricular (AV) block was documented in the first patient immediately afterward, and third-degree AV block was seen on 24-hr electrocardiogram monitoring in the second patient. A third patient developed documented AV block 15 min after atropine but experienced no sequelae because of a previously implanted pacemaker.

CONCLUSIONS

Although the underlying mechanism is not clear, these findings suggest that atropine may paradoxically cause high-degree AV block in patients after transplantation. Accordingly, it should be used with caution and appropriate monitoring in these patients.

Age-dependent changes in cardiac muscarinic receptor function in healthy volunteers

OBJECTIVES

This study was conducted to determine possible age-dependent changes in the responsiveness of human cardiac muscarinic receptors.

BACKGROUND

It is well known that the baroreflex activity decreases with aging. However, the mechanisms underlying this phenomenon are not completely understood at present.

METHODS

In six healthy young (mean [+/-SEM] age 26 +/- 2 years) and six healthy older volunteers (mean age 60 +/- 2 years), we determined 1) the effects of graded doses of atropine (bolus application, six doses, each for 20 min, range 0.03 to 0.96 mg) and the M1-cholinoceptor selective antagonist pirenzepine (bolus application, eight doses, each for 20 min, range 0.04 to 10 mg) on heart rate, blood pressure and systolic time intervals (as measure of inotropism); and 2) the baroreflex activity by assessing the bradycardic response to phenylephrine.

RESULTS

Atropine and pirenzepine caused biphasic effects on heart rate: At lower doses (< 0.12 mg for atropine, < 5 mg for pirenzepine) they decreased heart rate, whereas at higher doses they increased heart rate. Heart rate decreases induced by both antimuscarinic drugs were significantly larger in the young volunteers than in the older volunteers, whereas heart rate increases were not significantly different for both drugs. Atropine and pirenzepine did not significantly affect blood pressure and systolic time intervals. Infusion of graded doses of phenylephrine (four doses ranging from 0.1 to 1.0 microgram/kg body weight per min for 15 min each) caused a higher increase in systolic blood pressure and a smaller decrease in heart rate at each dose in the older volunteers than in the young volunteers. The slopes of the regression lines were 16 +/- 2.3 ms/mm Hg for the young and 6 +/- 0.5 ms/mm Hg for the older volunteers (p < 0.01).

CONCLUSIONS

Human cardiac muscarinic receptor activity is diminished with increasing age; such decreased cardiac muscarinic receptor activity could contribute to the decrease in baroreflex activity with aging. In contrast, antimuscarinic drugs seem to have no effect on human cardiac contractility.

Relation between bronchial and cardiac vagal tone in healthy humans

A generally increased vagal nervous activity was suggested to exist in bronchial asthma; however, this contention has never been fully tested. Before studying vagal activity in asthma, we raised the question if there existed any relation between vagal outflows to different target organs in the physiologic state. The specific aim of this study was to determine the relation between the resting level of bronchial and cardiac vagal activity in healthy volunteers. Bronchial and cardiac vagal tone were measured in 12 subjects as the change of airway resistance (Raw) and heart period (HP) in response to complete cholinergic blockade. It was found that after intravenous atropine administration, Raw was reduced from 1.30 +/- 0.41 cm H2O/L/s to 0.51 +/- 0.22 cm H2O/L/s and HP was reduced from 789 +/- 84 ms to 506 +/- 41 ms. All the values are given as mean +/- SD. Both the bronchial and cardiac responses exhibited considerable interindividual variability, but there was no significant correlation between them (R = 0.22, p = 0.50). It was concluded that under resting conditions, vagal control of bronchial tone and HP were not related in healthy human subjects.

Relation between tonic sympathetic and vagal control of human sinus node function

BACKGROUND

Clinical conditions, such as heart failure or myocardial infarction are associated with enhanced sympathetic and reduced parasympathetic activity as compared to normal controls. The reciprocal alteration in cardiac autonomic tone likely contributes to the electrical instability of the myocardium. Little information is available on the relation between sympathetic and vagal cardiac control in healthy human subjects.

METHODS AND RESULTS

Heart period changes in response to autonomic blockades were measured in 16 young, healthy human subjects. Adrenergic and cholinergic blockades were induced by i.v. propranolol (0.2 mg/kg) and atropine (0.04 mg/kg) in two opposite orders on two occasions; interindividual correlations were performed between the R-R interval responses to propranolol and to atropine obtained under the various blockade conditions, and the magnitude of the responses were compared by a drug x order two factorial ANOVA design. It was found, that previous adrenergic blockade did not reduce the extent of cardioacceleration produced by subsequent cholinergic blockade and that the R-R interval responses to atropine and to subsequently given propranolol did not share significant variance across subjects (r = 0.22, P = 0.234). Also, no interindividual correlation was found between the R-R interval responses to propranolol and to atropine, with the influence of the other, respective, autonomic division already blocked (r = 0.42, P = 0.114).

CONCLUSIONS

Under resting conditions, activity levels of cardiac vagal and sympathetic outflows are not related across young, healthy human subjects and peripheral interaction is not manifest between the autonomic divisions.

Effects of consecutive administration of central and peripheral anticholinergic agents on respiratory sinus arrhythmia in normal subjects

Respiratory sinus arrhythmia is thought to be vagally mediated, since it disappears after atropine, but the site of action of the drug (central vs. peripheral) accounting for this effect has not been elucidated. To investigate the effects of anticholinergic agents on respiratory arrhythmia, ten healthy subjects received an intravenous bolus of tropatepine (a presumed central antagonist) at a dose of 0.08 mg per kg of body weight, then, 7 min later, prifinium (a peripheral antagonist) at a dose of 0.1 mg per kg of body weight. Respiratory sinus arrhythmia during controlled breathing was evaluated as the area under the high-frequency peak of the heart rate variability spectrum coinciding with the respiratory frequency +/- 0.02 Hz. The power of this high-frequency peak decreased by 55% after tropatepine (P less than 0.05) with a concomitant increase of the mean RR interval from 930 to 1072 ms (P less than 0.01). When prifinium was added, a further but non-significant decrease of respiratory arrhythmia was observed, while the mean RR interval decreased from 1072 to 714 ms (P less than 0.01). The low-frequency components (0.05 to 0.15 Hz) of the power spectrum, significantly decreased (P less than 0.05) after infusion of both drugs. In conclusion, tropatepine depresses respiratory sinus arrhythmia with a paradoxical concomitant bradycardia. This suggests that tropatepine acts like a pure central muscarinic antagonist, in support of the hypothesis that a central cholinergic receptor is involved in the respiratory modulation of heart rate.

Sympathetic and vagal influences on rate-dependent changes of QT interval in healthy subjects

Dependence of QT interval duration on cardiac heart rate has been well established and is considered to be an intrinsic property of ventricular myocardium. Conclusive results of autonomic influences on such phenomena are lacking. To evaluate whether rate-dependent changes of QT interval are conditioned by the autonomic nervous system, 28 normal subjects with no heart disease and a normal QT interval were electrophysiologically assessed. The QT interval was calculated at 6 paced cycle lengths (600, 540, 500, 460, 430 and 400 ms) during the basal state, and after beta blockade (propranolol 0.2 mg/kg) and autonomic blockade (propranolol plus atropine 0.04 mg/kg). Because of atrioventricular nodal conduction limits, intrapatient cross-comparisons were performed in 10 subjects (aged 42 +/- 15 years). Single regression lines, evaluated in each subject, showing correlation between pacing cycle length and QT duration at each of the 3 states were analyzed. The mean slope observed after autonomic blockade (b = 0.10 +/- 0.04) was significantly lower than that seen during the basal state (b = 0.22 +/- 0.12, p less than 0.05) and after beta blockade (b = 0.23 +/- 0.08, p less than 0.05); nonsignificant differences were found between slopes during the basal state and after beta blockade. Results showed that vagal tone increased intrinsic dependence of QT at increasing cycle length, whereas sympathetic tone did not seem to interfere significantly. Since (in each subject) beta blockade was performed--or achieved--before atropine administration, the vagal influences are likely to be directly exerted on the ventricular electrophysiologic substrate.