Individual patient meta-analysis of exercise training effects on systemic brain natriuretic peptide expression in heart failure

BACKGROUND

Brain natriuretic peptide (BNP) predicts exercise performance and exercise training may modulate BNP and its N-terminal portion (NT-pro-BNP), we therefore conducted an individual patient analysis of exercise training effects on BNP and NT-pro-BNP.

AIMS

To use an individual patient meta-analysis to relate changes in BNP, NT-pro-BNP, and peak VO(2); to link these changes to volume parameters of exercise training programmes (intensity etc.); and to identify patient characteristics likely to lead to greater improvements in BNP, NT-pro-BNP, and peak VO(2).

DESIGN

Individual patient meta-analysis.

METHODS

A systematic search was conducted of Medline (Ovid), Embase.com, Cochrane Central Register of Controlled Trials, and CINAHL (until July 2008) to identify randomized controlled trials of aerobic and/or resistance exercise training in systolic heart failure patients measuring BNP and/or NT-pro-BNP. Primary outcome measures were change in BNP, NT-pro-BNP, and peak VO2. Subanalyses were conducted to identify (1) patient groups that benefit most and (2) exercise programme parameters enhancing favourable changes in primary outcome measures.

RESULTS

Ten randomized controlled studies measuring BNP or NT-pro-BNP met eligibility criteria, authors provided individual patient data for 565 patients (313 exercise and 252 controls). Exercise training had favourable effects on BNP (-28.3%, p < 0.0001), NT-pro-BNP (-37.4%, p = < 0.0001), and peak VO(2) (17.8%, p < 0.0001). The analysis showed a significant change in primary outcome measures; moreover, change in BNP (r = -0.31, p < 0.0001) and NT-pro-BNP (r = -0.22, p < 0.0001) were correlated with peak VO(2) change.

CONCLUSION

Exercise training has favourable effects on BNP, NT-pro-BNP, and peak VO(2) in heart failure patients and BNP/NT-pro-BNP changes were correlated with peak VO(2) changes.

Characterization of the non-linear rate-dependency of QT interval in humans

AIMS

Repolarization has rate-dependent and rate-independent components. A function considering such components separately was validated in canine Purkinje fibres and applied to the QT/RR relation in humans.

METHODS AND RESULTS

Action potential duration (APD) was measured in Purkinje fibres during steady-state pacing at different cycle lengths (CL) and after prolonged quiescence (APD(0)). The APD/CL relationship was expressed by this function: APD=APD(max)(*)CL(S)/(CL(50)(S)+CL(S)), where APD(max) (APD extrapolated at infinite CL) is a rate-independent measure of repolarization, CL(50) (CL at which 50% of APD(max) is achieved) and S evaluates the rate dependency of APD. The same function was used to fit the QT/RR relation in 46 normal subjects (20 males, 26 females) and in 7 amiodarone-treated subjects undergoing a bicycle stress test. RR and QT (V(5)) were measured at the end of each load step; QT(c) (Bazett's formula) was obtained at rest. The APD/CL and QT/RR relations were equally well expressed by the function with high correlation coefficients (R>or=0.90). In Purkinje fibres, APD(max) was 461+/-37 ms, CL(50) was 394+/-54 ms and S was 0.98+/-0.11. APD(max) and APD(0) correlated (R=0.96) and were similar. The corresponding values in humans were: QT(max) 432+/-63 ms, RR(50) 345+/-60 ms and S 2.6+/-0.8. While QT(c) and QT(max) were longer in females, RR(50) and S were similar between genders. Amiodarone increased QT(c), QT(max) and RR(50) and decreased S. In QT(max) and QT(c) distributions generated by pooling data from treated and untreated subjects, 86% of treated subjects were correctly identified by QT(max) and 28% by QT(c).

CONCLUSIONS

Canine and human repolarization showed a saturating dependency on cycle length, described by the proposed function. Gender and amiodarone independently affected QT(max), RR(50) and S: therefore they might reflect specific ionic mechanisms. Finally, QT(max) identified drug-induced repolarization abnormalities in individual subjects better than QT(c).

Changes of autonomic cardiac profile after a 3-week integrated body weight reduction program in severely obese patients

The autonomic control of the heart is abnormal in obese subjects due to a prevalence of sympathetic over parasympathetic limb of the autonomic balance. We evaluated the effects of a short-term (3 weeks) integrated body weight reduction program (consisting of energy restricted diet and high-intensity exercise training) on heart rate variability (HRV) in severely obese, normotensive patients. The HRV was evaluated both in the time and frequency domain over a 18-hour Holter recording period obtained before and at the end of the third week. Three-week body weight reduction program reduced BMI (from 41.4 +/- 4.6 to 39.5 +/- 4.3 kg/m2, -4.6%, p<0.0001) and heart rate (from 77.8 +/- 8.6 to 73.6 +/- 8.7 b/min, p=0.0003). Significant changes in the autonomic profile were observed both in the time and frequency domain (SD of RR interval, SDRR: +16.1%; mean squared successive difference: (MSSD) +16.7%; percentage of RR intervals differing more than 50 msec from the preceding one, pNN50: +31.8%; low frequency oscillation, LF: +17.1%; high frequency oscillation, HF: +/- 18.2%). In conclusion, this study demonstrates that a short-term, integrated body weight reduction program is able to favorably modify the autonomic profile in a population of normotensive, severely obese subjects. The reduction of heart rate and the increase in parasympathetic activity may consistently contribute to a reduction of the risk of cardiovascular morbidity and of sudden cardiac death, still high in this patients' group.

Different baseline sympathovagal balance and cardiac autonomic responsiveness in ischemic and non-ischemic congestive heart failure

BACKGROUND

A profound autonomic unbalance is present in heart failure: its correlation with the etiology of the disease has never been investigated.

AIMS

We characterized the sympatho-vagal balance and autonomic responsiveness of 42 patients (21 with ischemic heart failure, 21 with idiopathic dilated cardiomyopathy). Patients had comparable NYHA class, ejection fraction, exercise pVO(2), exercise ventilatory response, incidence of beta-blocking treatment. None showed periodic breathing or nocturnal arterial desaturation.

METHODS

Heart rate variability was assessed in the time and frequency domain during: (1) 10 min of quiet supine resting and free breathing; (2) 10 min of regular breathing at a frequency of 20 acts/min (=parasympathetic stimulus); and (3) 10 min of active standing (=sympathetic stimulus). The ratio of the low- to high-frequency components of each autospectrum obtained in the frequency domain (LF/HF) was used as an index of sympathovagal balance.

RESULTS

Patients with ischemic heart failure had a greater baseline sympathetic activation (higher LF/HF) than those with idiopathic dilated cardiomyopathy, maintaining some parasympathetic responsiveness as well (reduced LF/HF with regular breathing).

CONCLUSIONS

There is a distinct autonomic control according to the etiology of heart failure, a finding that may help understanding its pathophysiology, and could be useful in the clinical management of patients.

Long-term lifestyle changes maintain the autonomic modulation induced by rehabilitation after myocardial infarction

The altered autonomic balance observed after myocardial infarction is shifted toward a higher parasympathetic tone by rehabilitation. This effect persists after 1 year, however we observed a discrete variability in the long-term sympathovagal balance among patients. We postulated that such variability derives from the disparate adherence of patients to lifestyle prescriptions regarding exercise continuance and smoking avoidance. To test this hypothesis, we reviewed the data of 40 patients, who completed with a favourable autonomic modulation the initial rehabilitation phase after myocardial infarction and underwent the annual follow-up. One year after infarction, 23 patients complied to the advice about regular exercise and smoking avoidance (adherent, Group 1); 17 did not (non-adherent, Group 2). Groups were similar for age, site of infarction, left ventricular function, stress test duration and therapy. The ratio LF/HF, derived from the power spectral density of RR intervals variability, was used as an index of the sympathovagal balance. It was obtained from 15 min of ECG at rest, assessed 1 month after MI (baseline), and repeated 3 months (rehabilitation) and 1 year (follow-up) afterwards. Rehabilitation increased parasympathetic tone in all patients, reducing LF/HF by 33%. At follow-up, this potentially favourable autonomic profile persisted only in Group 1 patients. In conclusion, after a first myocardial infarction, the persistence of the potentially beneficial effect of rehabilitation on the sympathovagal balance depends on the compliance to the lifestyle changes proposed during the initial phase.

Markers of electrical instability in hypertensive patients with and without ventricular arrhythmias. Are they useful in identifying patients with different risk profiles?

BACKGROUND

Markers of electrical instability of the ventricular myocardium, namely abnormal repolarization and late potentials, are frequently observed in patients with hypertension when both ventricular arrhythmias and left ventricular hypertrophy are present. This information cannot be extrapolated to the population of hypertensive patients with ventricular arrhythmias but without left ventricular hypertrophy.

OBJECTIVE

To evaluate QT duration, QT dispersion and the incidence of ventricular late potentials in patients with essential hypertension, already on anti-hypertensive therapy, both with and without non-sustained ventricular arrhythmia.

DESIGN

The study population consisted of 49 patients with essential hypertension who were compared to 89 control normotensive subjects both with and without frequent (> 30 per h) ventricular ectopic beats (VPBs). Patients were divided into four groups: (1) hypertensive patients without VPBs (H, n = 19), (2) hypertensive patients with VPBs (HA, n = 30), (3) normotensive subjects without VPBs (C, n = 28), and (4) normotensive subjects with VPBs (CA, n=61).

METHODS

Echocardiographic parameters, QT interval, QT dispersion and signal-averaged ECG were evaluated without withdrawing anti-hypertensive drugs.

RESULTS

In no case was left ventricular hypertrophy documented. The number of VPBs during 24 h Holter recording (median 11 343 versus 7617) and the incidence of repetitive VPBs (37 versus 46% of patients) were similar in the two groups of patients (HA versus CA). Signal-averaged ECG parameters were normal and not different between the four groups. QT interval was longer in hypertensive patients compared to controls irrespective of the presence of VPBs. QT dispersion was slightly greater in subjects with VPBs, both hypertensive and normotensive, compared to subjects without arrhythmias.

CONCLUSIONS

In patients with hypertension well-controlled by drug therapy and without left ventricular hypertrophy, frequent VPBs are not associated with markers indicating an electrophysiological substrate for re-entrant arrhythmias. However, QT prolongation suggests the persistence of a higher risk of cardiovascular mortality that is independent of the presence of VPBs.

Role of the input/output relation of sinoatrial myocytes in cholinergic modulation of heart rate variability

INTRODUCTION

Modulation of sinus rate may be viewed as the transduction of an input signal (receptor stimulation) into an output signal (cycle length [CL]) by the sinus node. This study analyzes the input/output (I/O) relation of sinoatrial pacemaking elements and tests its impact on cholinergic modulation of heart rate variability.

METHODS AND RESULTS

Spontaneous activity of isolated rabbit sinoatrial myocytes was recorded by patch clamp techniques. CL and all the action potential parameters determining CL were automatically measured from >100 consecutive cycles. Acetylcholine (ACh, 5 to 50 nM) increased CL by decreasing diastolic depolarization rate (DDR) only. This was associated with a substantial increase in the coefficient of variation of CL and minor changes in the coefficient of variation of other parameters. A simple function relating CL to action potential parameters accurately described CL response to ACh (I/O relation). Numerical simulations based on this I/O relation showed that ACh-induced depression of DDR might, by its own, increase CL variability.

CONCLUSION

Time-domain measurements of CL variability may not necessarily reflect variability of the neural input to the sinoatrial node, but also may be affected by its tonic level. Properties of the I/O relation of sinoatrial myocytes may fully account for the dependency of CL variability on mean heart rate, previously described in humans. Any condition depressing DDR may enhance CL variability, independent of changes in the pattern of neural activity.

Relationship between baseline sympatho-vagal balance and the autonomic response to cardiac rehabilitation after a first uncomplicated myocardial infarction

BACKGROUND

After a first uncomplicated myocardial infarction, cardiac rehabilitation increases the parasympathetic tone, in a direction linked to a reduced risk of sudden cardiac death. This change in sympatho-vagal balance may be related to other clinical variables. The aim of this study was to define the factors implicated in determining the autonomic response to cardiovascular rehabilitation after myocardial infarction.

METHODS

In 55 patients (39-80 years) we evaluated the modulation of the autonomic profile induced by 8 weeks of rehabilitation: we analyzed the changes in pNN50 derived from time-domain analysis (deltapNN50) and in the low frequency/high frequency (LF/HF) ratio derived from autoregressive power spectral analysis (deltaLF/HF). A control group of 15 patients not undergoing rehabilitation was also studied. Variables considered at 4 weeks postinfarction and related to deltapNN50 and deltaLF/HF ratio were: age, site of myocardial infarction, previous thrombolysis, ejection fraction, stress test duration, baseline LF/HF ratio.

RESULTS

Patients not undergoing rehabilitation did not change their autonomic profile. On the contrary, rehabilitation induced a higher vagal tone (pNN50 from 6.5 +/- 1.5 to 16.2 +/- 3.1; LF/HF ratio from 8.3 +/- 5.2 to 5.1 +/- 2.9, p < 0.05). Eleven patients (20%) had baseline LF/HF ratio exceeding the mean value by 1.5 SD (19.4 +/- 1.4): in this subgroup, pNN50 was very low. In these patients, rehabilitation increased pNN50 and decreased LF/HF ratio. Indeed, both deltapNN50 and deltaLF/HF ratio were significantly related to their baseline values (p < 0.001), even considering thrombolysis, site of myocardial infarction, age, and beta-blocker therapy.

CONCLUSIONS

After a first uncomplicated myocardial infarction, sympatho-vagal balance may be very disturbed in some patients, despite a preserved ventricular function, good exercise capability and beta-blockers. These patients should be encouraged to undergo rehabilitation, since the significant improvement in the parasympathetic tone may protect them against subsequent arrhythmic events.

Increased left ventricular dimensions in patients with frequent nonsustained ventricular arrhythmia and no evidence of underlying heart disease

INTRODUCTION

To test the hypothesis that frequent nonsustained ventricular premature beats (VPBs) in patients without underlying heart disease are the first marker of mild systolic dysfunction of the left ventricle, we evaluated whether a subclinical abnormality of left ventricular function and/or an intraventricular conduction defect was present at the first clinical documentation of the arrhythmia.

METHODS AND RESULTS

We compared 57 patients (mean age 46 +/- 14 years) with > 30 VPBs/hour and no heart disease (A) to 32 healthy volunteers (mean age 42 +/- 12 years) without arrhythmia (B). Left ventricular echocardiographic parameters and signal-averaged ECG were evaluated. Filtered QRS duration (98 +/- 10 msec in A vs 98 +/- 7 msec in B) was similar in the two groups. End-diastolic left ventricular diameter (EDLVD) was 50 +/- 6 mm in A versus 47 +/- 3 mm in B (P < 0.005); 15 patients (26%) and none of the controls had EDLVD > or = 55 mm (P < 0.005). Filtered QRS interval was longer in the subgroup of patients (n = 15) with increased EDLVD (> or = 55 mm) compared with the subgroup (n = 42) with EDLVD < 55 mm (106 +/- 9 msec vs 95 +/- 9 msec; P < 0.001) and was related to greater left ventricular mass.

CONCLUSION

We documented a subclinical but significant increase of left ventricular dimensions that suggests that frequent VPBs may be an initial marker of mild systolic dysfunction of the left ventricle. However, an effect of VPBs per se in modifying left ventricular dimensions cannot be excluded.

Effects of cardiac rehabilitation and beta-blocker therapy on heart rate variability after first acute myocardial infarction

After acute myocardial infarction (AMI), rehabilitation with physical training increases parasympathetic tone. It is unknown whether such a favorable effect of exercise on the sympathovagal balance interacts with effects of other widespread therapies, such as beta blockers. In 53 patients after a first, uncomplicated AMI, we studied the combined short- and long-term influence on heart rate variability (HRV) of rehabilitation and beta blockade. Patients were divided into 3 groups: group 1 (n = 19) underwent rehabilitation with physical training; group 2 (n = 20) was taking beta blockers and underwent rehabilitation; group 3 (n = 14) was taking beta blockers and did not enter the rehabilitation program for logistic reasons. Patients were similar as to age, site of infarction, ejection fraction, left ventricular diameter, and baseline stress test duration. Measures of HRV (obtained from a 15-minute resting electrocardiogram) were the standard deviation of the mean RR interval (RRSD), the mean squared successive differences (MSSD), the percent of RR intervals differing >50 ms from the preceding one (pNN50), the low-(LF) and high-(HF) frequency components of the autoregressive power spectrum of the RR intervals and their ratio (LF/HF). Four weeks after AMI, there was less sympathetic predominance in groups 2 and 3 (i.e., patients taking beta blockers [p <0.05]). Rehabilitation modified HRV in groups 1 and 2 (p <0.05), with signs of increased parasympathetic tone (group 1: MSSD +25%, pNN50 +69%, LF/HF -40%; group 2: MSSD +41%, pNN50 +48%, LF/HF -39%). These changes persisted in the long term. In group 3, HRV was unchanged over time. Hence, after AMI, the effects of rehabilitation and beta blockers on HRV are not redundant: their association induces a more favorable sympathovagal balance, accelerating the recovery of a normal autonomic profile.

Adults with childhood-onset growth hormone deficiency: effects of growth hormone treatment on cardiac structure

OBJECTIVES

To evaluate the effects of growth hormone deficiency (GHD) and of growth hormone (GH) therapy on cardiac structure in adults with childhood-onset GHD.

SETTING

Out-patient clinic in the Italian Institute for Auxology, Milan.

SUBJECTS

Eight adults with childhood-onset GHD and eight healthy controls, matched for sex, age, exercise and body mass index.

INTERVENTIONS

Recombinant GH (Saizen Serono, Italy), administered in a conventional dose of 0.5 IU kg-1 week-1 for 6 months.

MAIN OUTCOME MEASURES

Cardiac structure parameters, evaluated by two-dimensional, M-mode and Doppler echocardiograms, and stress test, by means of a modified Bruce protocol with a bicycle ergometer, were determined before and after 6 months GH therapy.

RESULTS

Before treatment, mean (+/- SE) intraventricular septal thickness (IVST: 7.1 +/- 0.2 mm), LV posterior wall thickness (LVPT: 5.2 +/- 0.1 mm), LV mass (LVM: 94.6 +/- 5.0 g), LV mass index (LVM/body surface area, LVMI: 65.1 +/- 3.0 g m-2) and left ventricular end-diastolic diameter (LVED: 41.4 +/- 0.6 mm) of patients were significantly lower (P < 0.01) than in controls, whilst LV end-systolic diameter (LVES) of patients (25.5 +/- 0.7 mm) was similar to controls (27.5 +/- 0.7). GH treatment significantly (P < 0.01) increased LVPT (6.8 +/- 0.2 mm), LVM (111.6 +/- 4.6 g) and LVMI (80.5 +/- 3.5 g m-2); no significant changes were observed in LVED, LVES and IVST values. The stress test showed a significant improvement of cardiac performance, as demonstrated by the reduction of blood pressure x heart rate product at the same workload (basal: 32,722.5 +/- 897.4 vs. after: 25,574.6 +/- 439.7).

CONCLUSIONS

GH plays a role in the maintenance of a normal cardiac structure in adulthood. The present study suggests that GH treatment might be able to improve the cardiac structure of patients with childhood-onset GHD.

Muscarinic effects on action potential duration and its rate dependence in canine Purkinje fibers

Studies of the autonomic influence on action potential duration (APD) in the ventricles show direct effects of muscarinic stimulation on epicardial, but not endocardial, APD and conflicting results regarding direct vagal effects on the conduction system. In canine Purkinje fibers, we analyzed the action of the M2 agonist oxotremorine (OXO, 0.1 microM) on APD and on its cycle length (CL) dependence. Fibers were impaled with glass microelectrodes and superfused with Tyrode's solution. APD90 was measured after 3 minutes of drive at CL between 0.3 and 5 seconds. The best fit for the APD/CL relationship at steady state was a hyperbole: APD = APDmax*CL/(CL+CL50), where APDmax (APD at infinite CL) is a rate independent measure of APD, and CL50 (CL at which 50% APDmax is reached) is an index of the rate dependence of APD. In five fibers, OXO reduced APD at all CL (P < 0.05), APDmax was also reduced to 377 +/- 41 ms from 447 +/- 34 ms (P < 0.05), while CL50 was unchanged (405 +/- 46 ms from 437 +/- 28 ms). No effects of OXO on APD and APDmax were seen in two fibers obtained from dogs pretreated with pertussis toxin (PTX). In conclusion, stimulation of M2 receptors in intact, and not PTX treated, Purkinje fibers affects APD but not its CL dependence. This may reflect the activation of a rate independent, background current through a GTP binding protein-linked pathway, such as, IK,ACh. These data differ from those obtained in endocardial and epicardial muscle, stressing the regional differences in vagal modulation of ventricular electrophysiological properties.

Short and long term effects of exercise training on the tonic autonomic modulation of heart rate variability after myocardial infarction

We studied the effects of cardiac rehabilitation on the sympathovagal control of heart rate variability in 30 patients after a first, uncomplicated myocardial infarction. Twenty-two patients completed 8 weeks of endurance training (trained), while eight decided not to engage in the rehabilitation programme for logistical reasons, and were taken as untrained controls. Age, site of infarction, ejection fraction, ventricular diameter and stress test duration were similar in the two groups at baseline. Heart rate variability was evaluated 4 weeks after infarction before starting rehabilitation, and repeated 8 weeks and one year later in both trained and untrained patients. Measures of heart rate variability, obtained from both time- and frequency-domain analysis of a 15 min ECG recording in resting conditions, were as follows: mean RR interval and its standard deviation (RRSD), the mean square successive differences (MSSD), the percent of RR intervals differing > 50 ms from the preceding RR (pNN50), the low and high frequency components of the autoregressive power spectrum of the RR intervals and their ratio (LF/HF). At baseline, heart rate variability was similar in trained and untrained patients. In the short term (8 weeks after infarction), training increased RRSD by 25% (P < 0.01), MSSD by 69% (P < 0.01), pNN50 by 120% (P < 0.01), and reduced LF/HF ratio by 30% (P < 0.01). The effects persisted after one year in trained patients. In untrained patients, the autonomic control of heart rate variability did not change 8 weeks after myocardial infarction and was only slightly modified by time. Thus, exercise training, performed for 8 weeks after a myocardial infarction, modifies the sympathovagal control of heart rate variability toward a persistent increase in parasympathetic tone, known to be associated with a better prognosis. This may partly account for the favourable outcome of patients who undergo rehabilitation.

Effects on atrial repolarization of the interaction between K+ channel blockers and muscarinic receptor stimulation

We have tested, in guinea pig atria, how muscarinic stimulation by oxotremorine (Oxo) modifies the effects on action potential duration (APD) of two iK blockers: d-sotalol (5 microM) and ambasilide (1 microM). APD was prolonged by d-sotalol (+34.8 +/- 2.9%) and ambasilide (+54.2 +/- 5.5%). Simultaneous superfusion with Oxo 0.5 microM markedly shortened APD; this effect was larger in the presence of d-sotalol than in the presence of ambasilide (-69 +/- 2% vs. -37.4 +/- 5%; P < .05). Moreover ambasilide, but not d-sotalol, antagonized APD shortening induced by Oxo. The basis for such a difference between the two drugs was studied in patch-clamp experiments on isolated rabbit atrial and sinoatrial myocytes. Besides blocking iK (half-effective concentration: EC50 = 2 microM), ambasilide almost completely inhibited iKACh (-86 +/- 2% at 10 microM; EC50 = 1.6 microM), which was minimally affected by d-sotalol. Ambasilide 2 microM increased 10-fold the acetylcholine (ACh) required for 50% iKACh activation, and reduced maximally activated iKACh by 18.8 +/- 6.3%. When iKACh was activated through a receptor-independent mechanism, 10 microM ambasilide reduced this current by only 18.7 +/- 1.4% of its control value. Moreover, ambasilide, although not affecting the current i(f), in basal conditions, reversed its inhibition by ACh. Thus, 1) the effect of K+ channel blockers on atrial APD may be blunted by ACh; ambasilide effects are less sensitive to ACh than those of d-sotalol; 2) ambasilide, but not d-sotalol, inhibits iKACh; this probably occurs largely, although not exclusively, through muscarinic receptor antagonism. Inhibition of iKACh may account for the persistence of the effects of this drug on atrial APD despite muscarinic stimulation.

Modulation of the electrical restitution of canine Purkinje fibers by local anesthetic drugs: a study with flecainide and propafenone

Some antiarrhythmic drugs that depress conduction also influence action potential duration (APD). This could modify the time course of changes in the APD of premature stimuli (electrical restitution) and affect dispersion of repolarization and homogeneity of refractoriness. The effects of two potent local anesthetic drugs, i.e., flecainide and propafenone, on electrical restitution were studied in canine Purkinje fibers, superfused with Tyrode's solution and impaled with glass microelectrodes. APD was measured at 90% repolarization (APD90): fibers were stimulated for 3 minutes at cycle lengths (CLs) between 350 msec and 1.5 seconds, and kept quiescent for 5 minutes in between. For each run we calculated the percent ratio of the second APD (APD test = APDt) to the first APD after quiescence (APDo) (APDt/APDo x 100). The ratio was correlated with the CL of the run (i.e., the coupling interval between APDo and APDt = CI) by the monoexponential function APDt/APDo x 100 = 100-exp(-tau/CI), whose time constant tau indicates the speed of electrical restitution. At 1 microM, flecainide decreased tau by 26% +/- 4% (to 310 +/- 41 msec from 245 +/- 30 msec, n = 6, P < 0.05), while propafenone did not change it. Thus, unlike propafenone, flecainide slows the process of electrical restitution in Purkinje fibers. This may derive from the drug's action on currents other than Na current (i.e., IK), relevant to the duration of action potential. In a reentrant circuit, despite similar effects on conduction, APD and refractoriness of the first beat would be differently modulated by flecainide and propafenone.

Diverse electrophysiologic effects of propafenone and flecainide in canine Purkinje fibers: implications for antiarrhythmic drug classification

Propafenone and flecainide are assigned to class Ic of the Campbell-Vaughan Williams classification because of their effects on ventricular muscle. The authors compared the use-dependent local anesthetic properties and the effects on repolarization of these drugs (1 and 5 microM) in Purkinje fibers. A reduction in maximum upstroke velocity was used as an index of the local anesthetic action. The rate dependency of the drug's effects on repolarization was evaluated by analyzing the relationship between action potential duration during steady-state stimulation and cycle length (CL). Tonic block was higher for propafenone (n = 10) than for flecainide (n = 7) at both concentrations tested (19 +/- 3% vs. 4 +/- 1% at 1 microM; 59 +/- 10% vs. 24 +/- 4% at 5 microM). Use-dependent block onset and dissipation were significantly slower for flecainide than for propafenone (e.g., at 1 microM and CL = 500 ms, time constant of block onset = 31 +/- 6 vs. 9 +/- 1 beats; time constant of recovery from block = 7.7 +/- 0.2 vs. 2.8 +/- 0.2 sec; P < .05). Steady-state block measured at each CL was compared with that predicted by a theoretical model of use dependency. Predictions approximated the experimentally results only for flecainide. At 1 microM, propafenone shortened action potential duration at all rates; flecainide had biphasic effects. At 5 microM, the effect of flecainide was similar to that of 1 microM propafenone. Thus, in Purkinje fibers, the kinetics of use-dependent local anesthetic effects and the effects on repolarization discriminate flecainide from propafenone.

Quantitative analysis of T wave abnormalities and their prognostic implications in the idiopathic long QT syndrome

OBJECTIVES

We evaluated the diagnostic and prognostic value of morphologic abnormalities of the T wave (mainly notched or biphasic T waves) in patients affected by the idiopathic long QT syndrome.

BACKGROUND

In the long QT syndrome, these abnormalities in T wave morphology are often observed and are of uncertain significance.

METHODS

The T wave abnormalities in the electrocardiogram (ECG) of 53 patients with the long QT syndrome and 53 control subjects of similar age and gender were analyzed, and their association with major cardiac events was defined.

RESULTS

Notched or biphasic T waves were defined according to morphologic criteria. They were present in 33 (62%) of 53 patients with the long QT syndrome and in 8 (15%) of 53 control subjects (p < 0.001). Moreover, among patients with the long QT syndrome they were much more frequent in symptomatic (history of syncope or cardiac arrest) than in asymptomatic subjects (30 [81%] of 37 vs. 3 [19%] of 16, p < 0.001). The same distribution was observed within families with the long QT syndrome, in which symptomatic members had more pronounced T wave abnormalities than did their asymptomatic siblings or parents. In symptomatic patients, the occurrence of T wave abnormalities was independent of the length of repolarization (corrected QT). These T wave abnormalities were associated with the presence of a specific pattern of abnormal left ventricular wall motion.

CONCLUSIONS

This study has quantified an ECG pattern typical of the long QT syndrome and provides the first evidence that morphologic analysis of T wave abnormalities may contribute to the diagnosis of the long QT syndrome and the identification of patients at higher risk for syncope or cardiac arrest.

Experimental evidence for beta adrenergic blocking properties of propafenone and for their potential clinical relevance

The magnitude and biological relevance in vivo of the beta-blocking properties, observed in vitro, of propafenone remains controversial. We studied the effects of propafenone on the heart rate response to graded stimulation of the right stellate ganglion in 13 anaesthetized, vagotomized cats. The decentralized ganglion was stimulated (3 ms, 10 to 15 V) for periods of 45 s at frequencies from 1 to 14 Hz. The effects of sympathetic stimulation on heart rate were evaluated by the relationship between stimulation frequency and changes in heart rate from control. The relationship was significantly displaced downward and to the right by propafenone (4 mg.kg-1, n = 7), indicating a blunted response to sympathetic stimulation. This effect was lost at the highest frequencies. The frequency range at which propafenone was effective is the same as that elicited in cardiac sympathetic nerves by acute myocardial ischaemia. With propranolol (0.5 mg.kg-1 n = 4), the effects on the heart rate response were similar but of greater magnitude. Another sodium channel blocker (flecainide, 4 mg.kg-1 n = 2) was administered, and no changes were observed in the heart rate response. Thus, propafenone has significant beta-blocking effects, which would be of clinical relevance during the reflex sympathetic activation consequent to myocardial ischaemia.

Parasympathetic control of cycle length dependence of endocardial ventricular repolarisation in the intact feline heart during steady state conditions

OBJECTIVE

Parasympathetic modulation of the rate dependence of ventricular repolarisation in vivo during steady state conditions was investigated by analysing the effects of removing vagal activity in three groups of anaesthetised cats.

METHODS

Bilateral vagotomy was performed while different levels of background sympathetic tone were present in the control periods of each group: in group 1 (n = 6), cardiac sympathetic nerves were intact; in group 2 (n = 7), bilateral stellate ganglionectomy was performed; and in group 3 (n = 7), beta adrenergic blockade (propranolol, 0.5 mg.kg-1) was performed after stellate ganglionectomy. The duration of a left ventricular endocardial monophasic action potential (APD) was measured during atrial pacing at 7-10 cycle lengths (CL). The APD/CL relation was fitted to a hyperbolic function: APD = CL/[(axCL)+b]. Two parameters were considered: APDmax (1/a, ie, APD extrapolated at infinite cycle length, a rate-independent measure of APD) and CL50 (bxAPDmax, ie, the cycle length at which 50% of APDmax is reached).

RESULTS

In control conditions, APDmax and CL50 were longer in groups 2 and 3, when cardiac sympathetic effects were reduced or absent. Vagotomy reduced APDmax and CL50 similarly in groups 1 and 2 (APDmax, -24% and -18%; CL50, -36% and -27%) (p < 0.05 v control). In group 3, vagotomy did not affect APDmax and CL50.

CONCLUSIONS

No direct parasympathetic influence on the rate dependence of endocardial ventricular repolarisation was observed. The vagal modulation of sympathetic effects may take place either through vago-sympathetic reflexes or via the antagonism of circulating catecholamines distal to the beta receptor.

Sympathetic modulation of the relation between ventricular repolarization and cycle length

Sympathetic influences on ventricular repolarization are not yet fully elucidated, despite their relevance to arrhythmogenesis. The sympathetic control of repolarization, measured from an endocardial monophasic action potential duration (APD) and from the QT interval, was investigated in 24 anesthetized cats. The effects of right and left stellectomy and of subsequent bilateral stellectomy or beta-blockade on the relation between APD (or QT) and cycle length (CL) at steady state, and on the kinetics of adaptation of APD to a sudden change in cycle length were studied separately. Steady-state APD/CL (or QT/CL) relations were obtained by atrial pacing at different cycle lengths. The kinetics of APD adaptation were evaluated for a sudden decrease of approximately 100 msec in pacing cycle length. The steady-state APD/CL (QT/CL) relation was fitted by the hyperbolic function APD = CL/[(a. CL) + b]. From this, two parameters were computed: 1) 1/a, that is, APD (QT) extrapolated at infinite cycle length (APDmax or QTmax) and 2) the cycle length at which 50% of the total change in APD (or QT) occurred (CL50 = b/a). Right stellectomy reduced APDmax and CL50, an effect reversed by subsequent left stellectomy or beta-blockade (propranolol, 0.5 mg/kg). Left stellectomy prolonged APDmax and CL50. Bilateral stellectomy, in both groups, caused a further increase in these variables. Results were similar for the QT/CL relation. The adaptation kinetics of APD to cycle length was described by the sum of two exponentials. The first time constant (tau fast, about three beats) was unchanged by any intervention; the second (tau slow) was shortened by right stellectomy and prolonged by left stellectomy. The further removal of the remaining stellate ganglion had the same effect in both groups, that is, an increase in tau slow. Thus, sympathetic innervation modulates both the steady-state dependence on cycle length and the kinetics of adaptation to sudden rate changes of ventricular repolarization. The effects of sympathetic influence are asymmetrical. Right stellectomy shortens APDmax and QTmax, reduces CL50, and accelerates APD adaptation to a new steady state. Because these effects are reversed by beta-blockade or left stellectomy, they are likely to be due to a reflexly enhanced sympathetic outflow to the ventricles through the left-sided nerves.

Beta adrenergic modulation of cardiac rhythm in a rat model of altered sympathetic neural development

We previously have shown that treatment of neonatal rats (days 1-10) with Nerve Growth Factor (NGF) or its antibody (Ab) modifies alpha-adrenergic receptor-effector coupling, such that innervated hearts at day 10 show high levels of a 41 kDa GTP regulatory protein (G protein) that is a substrate for pertussis toxin and that links the alpha 1-receptor to the Na/K pump. This receptor-effector pathway results in alpha adrenergic-induced decreases in automaticity. In contrast, non-innervated hearts at day 10 show lower levels of the pertussis toxin sensitive G-protein and increases in automaticity induced by alpha-agonist. We now report the effects of administration of NGF, Ab or placebo on beta-adrenergic receptor-effector coupling in neonatal rats. Rats were administered NGF, Ab or placebo on days 1-10 of life. On day 10, the beta-receptor number and affinity and the stimulatory G-protein, Gs, were equivalent across groups. Moreover, the ventricular automatic response to beta-adrenergic receptor stimulation was equivalent across groups suggesting there was no change in receptor-effector coupling as a result of the difference in innervation. These results on beta-adrenergic receptor-effector coupling considered in light of our prior studies on alpha-adrenergic coupling suggest that the development of sympathetic innervation is more a determinant of alpha than beta adrenergic modulation of ventricular rhythm.

Specific alpha 1-adrenergic receptor subtypes modulate catecholamine-induced increases and decreases in ventricular automaticity

Fifty percent of adult canine Purkinje fibers manifest a decrease in automaticity in response to alpha 1-adrenergic stimulation with 10(-10)-10(-8) M norepinephrine (NE), and 50% manifest an increase. In contrast, most neonatal Purkinje fibers show an increase in automaticity in response to these concentrations of NE. We studied the modulation of NE effects, using the subtype selective alpha 1-adrenergic antagonists chloroethylclonidine (CEC) and WB 4101. CEC selectively antagonized the decrease in automatically such that, in both age groups, all Purkinje fibers showed NE-induced increases in automaticity. In Purkinje fibers from dogs treated with pertussis toxin, NE no longer induced a CEC-sensitive decrease in automaticity. In contrast, WB 4101 selectively antagonized the NE-induced increase in automaticity in both age groups. In the presence of WB 4101, NE decreased automaticity uniformly in adult Purkinje fibers and tended to induce no change in automaticity in neonatal Purkinje fibers. In the presence of prazosin (10(-6) M) or combined CEC (10(-7) M) and WB 4101 (10(-7) M), no alpha-agonist-induced increase or decrease in rate was observed. Pretreatment of membranes from newborn and adult dog and rat ventricles with CEC resulted in a selective and irreversible inactivation of 25% of specific binding sites labeled with [125I]IBE2254. In cultured neonatal rat ventricular myocytes, exposure to CEC resulted in a 35% decrease in the density of specific binding sites labeled with [125I]IBE2254 but did not influence alpha-adrenergic stimulation of inositol phosphate accumulation. In contrast, WB 4101 inactivated NE-stimulated inositol phosphate accumulation. Our results suggest that 1) at least two distinct alpha 1-adrenergic receptor subtypes are present in neonatal and adult cardiac tissue, 2) the CEC-sensitive subtype is linked to a decrease in automaticity via a pertussis toxin-sensitive substrate, 3) the WB 4101-sensitive subtype is linked to an increase in automaticity (possibly via a mechanism related to phosphoinositide breakdown), and 4) although CEC- and WB 4101-sensitive alpha 1-adrenergic receptor subtypes are present in the neonate, only the WB 4101-sensitive subtype is expressed functionally to induce effects on ventricular automaticity.

Sympathetic neural modulation of cardiac impulse initiation and repolarization in the newborn rat

We injected neonatal rats with nerve growth factor, the antiserum to nerve growth factor, or placebo for the first 10 days of life. Our goal was to determine the relation between sympathetic innervation of the developing heart, the electrocardiographic expression of cardiac rhythm, and the response of the heart to alpha-adrenergic stimulation with phenylephrine. We were especially interested in the latter area because of the prior demonstration in isolated cell systems of sympathetic neural modulation of a 41-kDa GTP regulatory protein and alpha-adrenergic responsiveness. Ten- to 11-day-old rats treated with nerve growth factor had more complete sympathetic innervation, faster heart rates, and higher levels of the 41-kDa protein than the placebo group. Electrophysiological studies were performed on isolated ventricular septa superfused with Tyrode's solution at 37.0 degrees-37.5 degrees C. The electrophysiological response of septa to 10(-9) and 10(-8) M phenylephrine from the 10-11-day-old nerve growth factor group was comparable with that of 3-week-old control animals. In contrast, 10-11-day-old antiserum-treated rats had an abnormal innervation pattern, lower levels of the 41-kDa protein, and a more immature electrophysiological response to alpha-adrenergic stimulation than the placebo group. In addition, antiserum-treated rats had an abnormally prolonged electrocardiographic QT interval. Our results demonstrate for the first time in intact animals a direct link between sympathetic innervation and alpha-adrenergic receptor-effector coupling as well as the dependence on innervation of the modulation of impulse initiation by alpha-agonists. This sequence of developmental events may be important not only in the regulation of normal cardiac rhythm but also in the expression of certain pathological entities such as the congenital long QT syndrome and the sudden infant death syndrome.

Electrophysiologic effects of ketanserin on canine Purkinje fibers, ventricular myocardium and the intact heart

We studied the actions of ketanserin (KT) on transmembrane action potentials (AP) of canine Purkinje fibers (PF) and ventricular muscle (VM) and on rhythm in vivo. PF AP duration (APD) was increased by KT (10(-8) to 10(-6) M) and shortened at 10(-5) M. KT effect on APD was greater during stimulation at longer cycle lengths and KT induced early afterdepolarizations in two of six PF at [K+]0 = 2.7 mM. Maximum diastolic potential, AP amplitude and Vmax were not changed by KT. In VM, KT (10(-8) to 10(-6) M) prolonged APD; but 10(-5) M KT did not shorten APD, reducing the difference in APD between VM and PF. KT had no effect on slow response Vmax or amplitude but prolonged APD. To analyze whether changes in Na plateau current or transient outward current contributed to KT effects on APD, we used tetrodotoxin (TTX) and 4-aminopyridine. TTX shortened APD and in its presence, KT (10(-5) M) induced no further shortening. In contrast, the effect of KT persisted in the presence of 4-aminopyridine. In six anesthetized, open chest dogs, KT prolonged the QT interval, but did not modify QRS duration and epicardial conduction time or induce arrhythmias. KT facilitated the onset of torsades de pointes during epicardial aconitine application.(ABSTRACT TRUNCATED AT 250 WORDS)

Spectral analysis of heart rate variability in the assessment of autonomic diabetic neuropathy

We studied heart rate variability in 49 uncomplicated diabetics (27 with insulin therapy; 22 with oral hypoglycemic agents) and in 40 age-matched controls. An automatic autoregressive algorithm was used to compute the power spectral density (PSD) of beat by beat RR variability derived from the surface ECG. The PSD contains two major components (a low frequency approximately 0.1 Hz (LF) and a high frequency, respiratory linked, approximately 0.25 Hz (HF] that provide, respectively, quantitative markers of sympathetic and vagal modulatory activities and of their balance. As compared to controls, in diabetics, besides a reduced RR variance at rest (2722 +/- 300 and 1436 +/- 241 ms2, respectively), we observed during passive tilt an altered response of spectral indices of sympathetic activation and vagal withdrawal, suggestive of a complex modification in the neural control activities. In addition, we compared this approach to the commonly used clinical tests score, and observed that the latter provides overall results similar to those obtained with spectral changes induced by tilt (r = 0.42; P less than 0.01). Of potential clinical importance is that the data obtained with spectral analysis appear more thoroughly quantifiable and do not require the active collaboration of the patients.

Electrophysiologic, inotropic and antiarrhythmic effects of propafenone, 5-hydroxypropafenone and N-depropylpropafenone

We compared the electrophysiologic, inotropic and antiarrhythmic properties of propafenone and two metabolites, 5-hydroxy (5-OH) propafenone and N-depropyl (N-DP) propafenone. In 18 canine Purkinje fibers with normal maximum diastolic potentials, all drugs (1 x 10(-8) to 1 x 10(-5) M) reduced action potential amplitude and duration. However, propafenone and 5-OH propafenone reduced Vmax in a use-dependent fashion at a lower concentration than N-DP propafenone. In 16 Purkinje fibers, slow response action potentials were induced by 22 mM K+ and isoproterenol, 1 x 10(-6) M. Vmax was comparably reduced by all compounds at 1 x 10(-5) M, but action potential amplitude was not affected by 5-OH propafenone. In 16 other Purkinje fibers in which automaticity at low levels of membrane potential was induced by BaCl2 (0.25 mM), only 5-OH propafenone was effective in slowing the automatic rate at therapeutic concentrations (3 micrograms/ml). In 15 guinea pig papillary muscles, all three drugs had negative inotropic effects at concentrations greater than or equal to 1 x 10(-6) M. In conscious dogs with sustained ventricular tachycardia 24 hr after infarction, we injected propafenone or a metabolite through an atrial cannula. At similar plasma levels, neither propafenone (n = 6) nor N-DP propafenone (n = 6) suppressed the arrhythmia, whereas 5-OH propafenone eliminated ventricular tachycardia in four of six dogs, and was more effective against monomorphic than polymorphic ventricular tachycardia. Hence, the two major metabolites of propafenone have important electrophysiologic effects, and 5-OH propafenone is more potent than the parent compound as a antiarrhythmic drug in the 24-hr Harris dog.

The response to overdrive pacing of triggered atrial and ventricular arrhythmias in the canine heart

Although triggered activity has been identified in isolated atrial tissue with the use of cellular electrophysiologic techniques, there has been no identification of triggered atrial arrhythmias in situ. Moreover, it is unclear whether triggered rhythms of different causes and sites of origin in the heart exhibit uniform responses to pacing that might aid in their identification. We therefore studied arrhythmias induced by overdrive pacing in three canine preparations, and based the analysis of our results on guidelines derived from microelectrode studies. We studied ventricular tachycardias induced by ouabain or by anterior wall myocardial infarction and atrial (coronary sinus) arrhythmias induced by the infusion of epinephrine into the great cardiac vein. In the ouabain and postinfarction preparations, right ventricular epicardial pacing induced ventricular premature beats or tachycardias whose recovery intervals after cessation of pacing shortened and showed overdrive acceleration as pacing rate increased. The first postpacing beat displayed progressive fusion with the paced beats but transient entrainment could not be induced. In the coronary sinus, the recovery intervals of impulses induced by epinephrine and pacing decreased as the drive rate increased, and inducibility of the paced rhythms increased at faster drive rates. Thus, the recovery intervals of triggered activity induced in the coronary sinus are phenomenologically similar to those of infarct- and digitalis-induced triggered rhythms. This is the first demonstration of consistent behavior in response to pacing of diverse types of triggered activity. Considered in light of the failure to induce transient entrainment, the results emphasize the potential utility of pacing in clinical identification of triggered rhythms and their differentiation from reentry.

Attenuation of baroreceptive mechanisms by cardiovascular sympathetic afferent fibers

By sectioning spinal dorsal roots from C8 to T6, we analyzed the contribution of sympathetic cardiovascular afferent fibers to the reflex bradycardia induced by arterial pressure rises in 24 anesthetized and in 21 decerebrate cats. In anesthetized cats, the reflex bradycardia was obtained in 16 animals with occlusions of the thoracic aorta and in 8 animals with phenylephrine injections (25-75 micrograms/kg). In both experimental conditions, the dorsal root section enhanced the bradycardia response, which thus increased from 15 +/- 3 to 20 +/- 3% during aortic constrictions and from 11 +/- 3 to 19 +/- 6% during phenylephrine injections (P less than 0.05). The enhancement, after rhizotomy of the reflex bradycardia during aortic occlusion, was more pronounced in eleven decerebrate cats as it increased from 21 +/- 4 to 34 +/- 4%, P less than 0.05. In five vagotomized and decerebrate cats, the reflex bradycardia was also increased after rhizotomy despite the overall reduction of the reflex response. In five decerebrate cats with beta-adrenergic receptor blockade (propranolol 0.2-0.4 mg/kg iv), aortic occlusion resulted in a small reduction in heart rate which was not significantly affected by dorsal root section. Our data indicate that excitatory reflexes mediated by dorsal roots are likely to modulate the inhibitory supraspinal reflexes that determine the heart rate reduction during acute rises in arterial blood pressure.

Effects of sympathetic activation on ventricular ectopic beats in subjects with and without evidence of organic heart disease

In a selected group of 10 apparently healthy subjects and 22 patients with organic heart disease, all with frequent ventricular ectopic beats on Holter monitoring, we assessed the influence of sympathetic activation by comparing the arrhythmogenic effects of a symptom-limited bicycle exercise stress test and 90 degree head up tilt. Tilting reduced ventricular arrhythmias in the normal subjects (-48 +/- 18% from 9 +/- 2 beats min-1, P less than 0.05). Exercise stress testing caused small and insignificant changes in arrhythmias during the early (50-75 W) phases and an almost complete suppression of ventricular ectopic beats in the final stages (-99 +/- 1%, P less than 0.01). In six of the 10 subjects, ventricular arrhythmias reappeared in the early recovery phase. In the 22 patients with organic heart disease, tilting increased ventricular ectopic beats (43 +/- 17% from 9 +/- 3 beats min-1, P less than 0.05); augmented repetitive forms in 12 patients (179 +/- 88% from 1.4 +/- 0.6 per 3 min) and produced repetitive forms in six of the 10 remaining patients who did not show repetitive forms during control conditions. Exercise stress testing caused a marked increase in ectopic activity in the early phase (84 +/- 35%) while the response during the maximal phase of exercise as well as during recovery was related to the effort capabilities. Arrhythmias were increased in 12 patients with limited exercise duration and were reduced in 10 patients with good exercise tolerance. These data indicate that sympathetic activation has different effects on ventricular arrhythmias depending on the clinical setting and that tilting is a useful maneuver to evaluate the arrhythmogenic effects of increased sympathetic activity.

Power spectral analysis of heart rate and arterial pressure variabilities as a marker of sympatho-vagal interaction in man and conscious dog

In 57 normal subjects (age 20-60 years), we analyzed the spontaneous beat-to-beat oscillation in R-R interval during control recumbent position, 90 degrees upright tilt, controlled respiration (n = 16) and acute (n = 10) and chronic (n = 12) beta-adrenergic receptor blockade. Automatic computer analysis provided the autoregressive power spectral density, as well as the number and relative power of the individual components. The power spectral density of R-R interval variability contained two major components in power, a high frequency at approximately 0.25 Hz and a low frequency at approximately 0.1 Hz, with a normalized low frequency:high frequency ratio of 3.6 +/- 0.7. With tilt, the low-frequency component became largely predominant (90 +/- 1%) with a low frequency:high frequency ratio of 21 +/- 4. Acute beta-adrenergic receptor blockade (0.2 mg/kg IV propranolol) increased variance at rest and markedly blunted the increase in low frequency and low frequency:high frequency ratio induced by tilt. Chronic beta-adrenergic receptor blockade (0.6 mg/kg p.o. propranolol, t.i.d.), in addition, reduced low frequency and increased high frequency at rest, while limiting the low frequency:high frequency ratio increase produced by tilt. Controlled respiration produced at rest a marked increase in the high-frequency component, with a reduction of the low-frequency component and of the low frequency:high frequency ratio (0.7 +/- 0.1); during tilt, the increase in the low frequency:high frequency ratio (8.3 +/- 1.6) was significantly smaller. In seven additional subjects in whom direct high-fidelity arterial pressure was recorded, simultaneous R-R interval and arterial pressure variabilities were examined at rest and during tilt. Also, the power spectral density of arterial pressure variability contained two major components, with a relative low frequency:high frequency ratio at rest of 2.8 +/- 0.7, which became 17 +/- 5 with tilt. These power spectral density components were numerically similar to those observed in R-R variability. Thus, invasive and noninvasive studies provided similar results. More direct information on the role of cardiac sympathetic nerves on R-R and arterial pressure variabilities was derived from a group of experiments in conscious dogs before and after bilateral stellectomy. Under control conditions, high frequency was predominant and low frequency was very small or absent, owing to a predominant vagal tone. During a 9% decrease in arterial pressure obtained with IV nitroglycerin, there was a marked increase in low frequency, as a result of reflex sympathetic activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of propranolol on the impulse activity of cardiovascular sympathetic afferent fibers

The influence of beta-adrenergic receptor blockade on the impulse activity of 21 cardiovascular sympathetic afferent nerve fibers (11 from the thoracic aorta, 10 from the pulmonary veins), isolated from the left sympathetic rami communicantes T-3 and T-4 was studied in anesthetized, vagotomized cats. Aortic pressure, heart rate, and neural discharge were recorded during control conditions and during brief aortic occlusions of comparable amplitude and duration. Administration of dl-propranolol (0.2-0-4 mg/kg) did not modify aortic pressure or neural discharge of the fibers during control conditions, although, as expected, heart rate was diminished. dl-Propranolol administration did change the response of cardiovascular sympathetic afferents to similar aortic pressure increases. Before drug administration, aortic occlusion caused a significant increase in neural discharge of both aortic and pulmonary vein sympathetic afferent fibers, from 0.52 +/- 0.12 to 1.64 +/- 0.31 and from 0.67 +/- 0.10 to 2.08 +/- 0.25 impulses/sec, respectively (p less than 0.05). After dl-propranolol administration, comparable increases in aortic pressure resulted in slight but not significant increases in neural discharge of aortic and pulmonary vein fibers. Administration of d-propranolol (0.4-0.6 mg/kg), which possesses only membrane-stabilizing properties, did not modify the firing rate of four pulmonary sympathetic afferents, which subsequently decreased their response to pressure rises after administration of dl-propranolol. These results indicate that beta-adrenergic receptor blockade reduces the responsiveness to hemodynamic stimuli of sympathetic cardiovascular afferent fibers that are capable of mediating excitatory pressor reflexes.

Power spectral density of heart rate variability as an index of sympatho-vagal interaction in normal and hypertensive subjects

Instantaneous heart rate reflects sympatho-vagal influences on pace-maker activity. Hence computer analysis of heart rate variability might provide a quantitative index of that interaction. The power spectral density (PSD) estimate of heart rate variability was obtained in normal controls and in uncomplicated hypertensives, both at rest and during a non-hypotensive sympathetic stimulus (tilting). In normal controls PSD shows three major peaks of frequencies P1 = 0.07, P2 = 0.12, P3 = 0.25 cycles/beat. P1, which is associated with sympathetic activity, represents only a minor portion of total variability at rest, while becoming predominant with tilting. P2 and P3 are associated with vagal activity, and represent the major part of variability at rest, while they are reduced by tilting. In hypertensive patients PSD is altered, as P1 is already predominant at rest and increases only slightly with tilting. Thus PSD of heart rate variability is capable of detecting an early alteration in sympatho-vagal balance of cardiac control present in uncomplicated hypertension.

Global versus regional myocardial ischaemia: differences in cardiovascular and sympathetic responses in cats

The cardiovascular and sympathetic responses to occlusions of the left main ("global" ischaemia) or distal left anterior descending ("regional" ischaemia) coronary artery were studied in 19 anaesthetised cats with chronic sinoaortic baroreceptor denervation. "Global" ischaemia, before vagotomy, resulted in a significant reduction of mean arterial pressure (MAP), left ventricular pressure (LVP), and LVdP/dtmax while sympathetic efferent impulse activity was significantly augmented during the initial 15 +/- 2 s of occlusion (early phase) and, vice versa inhibited during the subsequent 20 +/- 2 s of occlusion (late phase). Vagotomy did not modify the haemodynamic responses, however, a significant increase in sympathetic discharge was detectable during the whole occlusion period (early and late phases). "Regional" ischaemia, before vagotomy, resulted in a significant increase in sympathetic neural discharge and MAP, with no changes in left ventricular function. After vagotomy the occlusion elicited a significant increase in MAP, LVP, LVdP/dtmax and efferent sympathetic neural activity. These excitatory responses were abolished after the interruption of a large part of the cardiac sympathetic afferents. Thus coronary artery occlusion induced haemodynamic and sympathetic reflex responses that were dependent upon the interaction of opposite influences mediated by the simultaneous activation of cardiac vagal and sympathetic afferents. The extent of "ischaemic myocardium" represented a determinant factor for the prevailing type of neural response.