HEART FAILURE
Lack of evidence for peripheral alpha1- adrenoceptor blockade during long-term treatment of heart failure with carvedilol
Toshihiko Kubo, MD, PhDa,
Eduardo R. Azevedo, MDa,
Gary E. Newton, MD, FACCa,
John D. Parker, MD, FACCa and
John S. Floras, MD, DPhil, FACC*,a
a Division of Cardiology, Mount Sinai Hospital and the University of Toronto, Toronto, Canada
Manuscript received April 6, 2001;
revised manuscript received June 21, 2001,
accepted July 13, 2001.
* Reprint requests and correspondence to: Dr. John S. Floras, 600 University Avenue, Suite 1614, Toronto, Ontario M5G 1X5, Canada
john.floras{at}utoronto.ca
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Abstract
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OBJECTIVES
The purpose of this study was to determine whether carvedilol’s alpha1-adrenoceptor antagonism persists during long-term therapy of patients with congestive heart failure (CHF).
BACKGROUND
Carvedilol and metoprolol differ in that carvedilol also antagonizes beta2- and alpha1-adrenoceptors. We hypothesized that in contrast to metoprolol, carvedilol would increase calf vascular conductance (CVC), blunt neurally mediated vasoconstriction and attenuate neuroeffector transfer function gain.
METHODS
We randomized 36 patients with CHF (age 55 ± 1 years, ejection fraction 19 ± 1%, means ± SE) to either drug. Blood pressure (BP), heart rate, muscle sympathetic nerve activity (MSNA) and CVC were assessed before and after four months of treatment. The variability of BP and MSNA was determined using fast Fourier transformation.
RESULTS
Paired data were obtained in 23 (carvedilol, 13; metoprolol, 10) subjects. Both beta-blockers decreased heart rate, but neither affected mean BP or CVC (carvedilol: 0.016 ± 0.002 to 0.018 ± 0.003 U; metoprolol: 0.020 ± 0.002 to 0.020 ± 0.004 U). Isometric handgrip exercise (30% of maximum) increased heart rate, mean BP and MSNA. The calf vasoconstrictor response to handgrip exercise was not affected by carvedilol (from 16 ± 6 resistance U to 25 ± 10 resistance U, NS). The gain of the transfer of oscillations in MSNA into BP under resting conditions was not attenuated by carvedilol.
CONCLUSIONS
Carvedilol did not increase CVC, blunt the calf vasoconstrictor response to handgrip or attenuate the gain of the neuroeffector transfer function, indicating the absence of functionally important peripheral alpha1-adrenoceptor antagonism during long-term treatment of CHF.
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Abbreviations and Acronyms
| | ANOVA | = analysis of variance | | BP | = blood pressure | | CBF | = calf blood flow | | CHF | = congestive heart failure | | CVC | = calf vascular conductance | | CVR | = calf vascular resistance | | ECG | = electrocardiogram | | HF | = high frequency | | LF | = low frequency | | MSNA | = muscle sympathetic nerve activity | | VLF | = very low frequency |
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Sympathetic nervous system activation is a well-characterized marker of adverse prognosis in patients with congestive heart failure (CHF) (1–3). Placebo-controlled trials of carvedilol, which blocks beta2- and alpha1- as well as beta1-adrenoceptors, provided the first definitive evidence for mortality reduction with beta-blockade (4–8). However, the selective antagonists, metoprolol and bisoprolol, were subsequently also shown to improve outcome (9,10), suggesting that beta1-antagonism may be the primary mechanism by which this benefit is achieved. Whether the beta2-, and alpha1-antagonistic properties of carvedilol, or its antioxidant action, confer any therapeutic advantage beyond that of beta1-selective agents is currently the subject of a large randomized clinical trial (11). However, in a recent trial, bucindolol, another nonselective beta-antagonist, did not show mortality benefit compared with a placebo control (12), and in a prospective study metoprolol and carvedilol had similar effects on an indirect marker of oxidative stress (13). These negative findings have redirected attention at the potential benefits of concomitant alpha1-blockade.
Whether carvedilol’s alpha1-adrenoceptor antagonism persists during long-term therapy and contributes to its effects on hemodynamics is unclear. Some investigators report a reduction in systemic vascular resistance with chronic treatment (14,15), but this has not been a consistent finding (16,17). Symptomatic orthostatic hypotension can occur when carvedilol is introduced, but this usually subsides with time. Regional circulatory responses to neurogenic vasoconstrictor stimuli have not been reported.
The objective of this study was to determine whether significant alpha1-adrenoceptor blockade could be detected in patients with chronic stable CHF after four months of carvedilol. We applied both time and frequency domain analysis to compare the effects of carvedilol and metoprolol on blood pressure (BP), muscle sympathetic nerve activity (MSNA) and calf vascular conductance (CVC). We hypothesized, first, that carvedilol, by blocking alpha1-receptors, would increase resting CVC. To exclude withdrawal of efferent sympathetic vasoconstrictor discharge as a cause of local vasodilation during long-term treatment, MSNA was measured simultaneously in the opposite leg (18,19). Our second hypothesis was that carvedilol would blunt alpha1-mediated neurogenic vasoconstriction induced by handgrip exercise. Finally, we hypothesized that carvedilol, by blocking the postjunctional action of neurally released norepinephrine at the level of the alpha1-receptor, would blunt the BP response to changes in sympathetic outflow, as evidenced by a decrease in the gain of the transfer function between the variability of MSNA and the variability of BP, as an index of neuroeffector transduction (20).
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Methods
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Subjects.
Patients with stable CHF secondary to ischemic heart disease or idiopathic cardiomyopathy were recruited. Inclusion criteria included symptoms (New York Heart Association functional class II or class III), stable medical therapy for >1 month, documented systolic ventricular dysfunction with a left ventricular ejection fraction <35% and no prior exposure to beta-blockade. Exclusion criteria included peripheral neuropathy, an acute coronary syndrome within three months, primary valvular heart disease, contraindications to beta-blockade, systolic BP <85 mm Hg and resting heart rate <60 beats/min. This study was one aspect of a larger protocol approved by the University of Toronto Human Subjects Review Committee. Informed, written consent was obtained from all participants.
Procedures.
All studies were performed in the same quiet, temperature-controlled room at the same time of the day. Subjects rested supine. In a subgroup, maximal voluntary contraction was predetermined using a handgrip dynamometer (right arm) (Lafayette Instrument, model 78010, Lafayette, Indiana). Blood pressure was measured each minute from the right arm by an automatic cuff recorder (Critikon, Dinamap 1846 SX, Johnson and Johnson, Tampa, Florida) and also continuously from a left finger (Ohmeda 2300 Finapres, Englewood, Colorado) to permit power spectral analysis. Heart rate was derived from lead II of the electrocardiogram (ECG). Blood flow in the left calf (CBF) (ml/min/100 ml of calf volume) was estimated by venous occlusion plethysmography (Hokanson, Bellevue, Washington). The MSNA was recorded from the right peroneal nerve (18,19).
Protocol.
After instrumentation, subjects lay quietly for 15 min to achieve steady state. The ECG, MSNA and the Finapres BP signals were then recorded continuously onto paper and by computer over a 9-min baseline period (20). The CBF was determined every 20 s over the last 2 min of baseline. In 18 subjects, there was a further 5-min rest period, then 2 min of prehandgrip recording followed by 2 min of isometric exercise at 30% of maximal voluntary contraction. This protocol was replicated after four months of beta-blockade.
After the first experimental session, patients were randomized in a double-blind fashion to receive either carvedilol or metoprolol tartrate, in addition to their previous treatment. Those allocated to carvedilol received an initial dose of 3.125 mg twice daily. Those randomized to metoprolol began with 6.25 mg twice daily. Patients were reassessed weekly. Doses were doubled if there was no clinical evidence of worsening CHF, heart rate was >65 beats/min and systolic BP was >85 mm Hg. Target dosages were carvedilol 25 mg twice daily or metoprolol 50 mg twice daily. Once these were achieved, patients were reassessed monthly in clinic. The assigned dose was taken on the morning of the second study.
Data analysis.
The MSNA was expressed as burst frequency (bursts/min), burst incidence (bursts/100 cardiac cycles) and integrated nerve activity (units; the product of burst frequency and mean burst amplitude). For the purpose of between-subject comparisons, the latter was normalized, for each subject, against the maximum burst amplitude recorded during the 9-min baseline period. Calf vascular resistance (CVR) was calculated as the quotient of mean arterial pressure and the average of four to six calf blood flow measurements, with CVC the inverse of CVR. All analyses were performed and calculated prior to unblinding of the principal investigators.
Frequency domain analysis was performed using previously published methods (20). Both MSNA and continuous BP signals were submitted to fast Fourier transformation. The following conventional frequency bands were applied to both signals: very low frequency (VLF), 0.0098 to 0.05 Hz; low frequency (LF), 0.05 to 0.15 Hz; and high frequency (HF), 0.15 to 0.5 Hz. To calculate the gain of the transfer function from MSNA to BP, power in the cross-spectrum of MSNA and BP was divided by power in the autospectrum of MSNA. Gain and coherence values presented are those calculated as average values across entire prescribed frequency bands (20).
Statistical analysis.
Values are expressed as means ± SE. Physiologic and pharmacologic characteristics of the two groups of subjects were compared by the unpaired t test and the Fisher exact test. When data were distributed normally, paired t tests were used for within-group comparisons of pre- and four-month beta-blockade values. The Wilcoxon rank-sum test was applied whenever tests of normality failed. Two-way analysis of variance (ANOVA) was used for between-group comparisons of baseline data before and with beta-blockade. Two-way ANOVA with repeated measures was used to evaluate the effect of each drug on responses to handgrip exercise. A p value <0.05 was required for statistical significance.
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Results
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Subjects.
Thirty-six patients with a mean age of 54.6 ± 1.8 years and with a mean left ventricular ejection fraction of 19 ± 1% were recruited. Of these, 18 were randomized to each drug. High-quality mean voltage neurograms could not be obtained in four subjects (two per group); they were not retested. One carvedilol patient died suddenly four weeks after randomization. One patient in the metoprolol group was withdrawn owing to poor drug compliance. Five (one carvedilol and four metoprolol) patients refused restudy. The remaining 25 patients (14 in the carvedilol group and 11 in the metoprolol group) participated in both study sessions. However, high-quality MSNA could not be obtained during the second study in two subjects (one assigned carvedilol and one metoprolol) and CBF could not be measured accurately in two subjects allocated to carvedilol. Thus, paired analysis of MSNA was possible in 13 carvedilol- and 10 metoprolol-treated subjects, and paired calf hemodynamics at rest were available for 11 patients in the carvedilol group and 10 in the metoprolol group.
Of the 23 subjects who completed the study, 18 participated in the handgrip exercise (10 carvedilol and 8 metoprolol). Paired measurements of CBF were available for comparison of calf vascular responses to handgrip exercise before and after beta-blockade in eight patients in each group.
Effect of beta-blockade.
No significant differences existed in age, gender or other baseline characteristics between carvedilol- and metoprolol-treated subjects (Table 1). Underlying medical therapy for CHF was stable throughout the study. At the time of the second study the mean doses prescribed were 43 ± 4 mg/day of carvedilol and 65 ± 10 mg/day of metoprolol.
Table 2 summarizes the effect of four months of beta-blockade on systemic and calf hemodynamics and on MSNA. Neither drug altered BP, but both lowered heart rate (p < 0.0001, carvedilol; p = 0.017, metoprolol). No significant difference was seen between this effect of carvedilol and metoprolol on heart rate. Carvedilol did not alter sympathetic vasoconstrictor traffic to the calf: MSNA burst frequency, burst incidence and integrated activity were unchanged. Metoprolol tended to decrease MSNA burst frequency and integrated activity (p < 0.07) but not burst incidence. Neither drug altered resting CBF, resistance or conductance.
Response to handgrip exercise.
Handgrip increased BP and heart rate significantly; four months of beta-adrenoceptor blockade had no effect on these responses to exercise (Table 3). Before carvedilol, handgrip increased MSNA (p < 0.01, p = 0.04, p < 0.01 for burst frequency, burst incidence, and integrated activity, respectively) and CVR (p = 0.007), and decreased CVC (p = 0.01). As can be seen in Figure 1, MSNA increased in all subjects (including four with diabetes). Carvedilol had no effect on the reflex sympathoneural response to isometric exercise, or on the calf hemodynamic response to this vasoconstrictor stimulus. Metoprolol tended to decrease integrated MSNA both before and during handgrip (p = 0.051), but not burst frequency (p = 0.15) or burst incidence (p = 0.76), and there was no interaction term between treatment and handgrip responses.
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Figure 1 Effects of carvedilol on muscle sympathetic nerve activity (MSNA) (A, n = 10) and calf hemodynamics (B and C, n = 8) in response to handgrip (HG) exercise. Before treatment, HG significantly increased MSNA and calf vascular resistance (CVR), and significantly decreased calf vascular conductance (CVC). Carvedilol had no effect on the reflex sympathoneural response to HG, or on calf hemodynamic responses to this vasoconstrictor stimulus. Individual data and means ± SE values are expressed. *p < 0.01 compared with control values;  p < 0.05 compared with control values.
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Gain of the neuroeffector transfer function.
The transfer function from MSNA to BP can be considered a representation of signal transduction at the level of the resistance vessels in muscular beds (20). Transfer function analysis was possible in 13 patients in the carvedilol group and 8 patients in the metoprolol group. Figure 2 illustrates the gain of the transfer function from MSNA to BP before and after four months of carvedilol. As documented previously, this gain is highest within the VLF range, with a progressive reduction from VLF to LF to HF bands (20). After four months of carvedilol, there was no change in either gain or coherence in any frequency band (gain: from 0.55 ± 0.08 to 0.71 ± 0.08 mm Hg/U in VLF, p = 0.120; from 0.38 ± 0.06 to 0.46 ± 0.11 in LF, p = 0.457; from 0.19 ± 0.03 to 0.24 ± 0.04 mm Hg/U in HF, p = 0.231). After four months of metoprolol, VLF and LF gain increased (both p < 0.05), whereas HF gain was unchanged. Both VLF and HF coherence also increased (both p < 0.05). However, two-way ANOVA indicated no between-group difference with regard to these values for gain and coherence.
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Figure 2 Gain of the transfer function from muscle sympathetic nerve activity (MSNA) to blood pressure (BP) before and four months after carvedilol treatment (n = 13). Carvedilol did not alter neuroeffector transfer function gain in any frequency band. Values are means ± SE. Open bars = before treatment; solid bars = after four months treatment. Very low frequency (VLF) = 0–0.05 Hz; low frequency (LF) = 0.05–0.15 Hz; high frequency (HF) = 0.15–0.5 Hz.
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Discussion
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Major findings.
In addition to augmenting ventricular function, the proven benefits of long-term beta-blockade for patients with CHF now include reductions in mortality, morbidity and need for hospitalization. Carvedilol differs from metoprolol as it is also an alpha1- and a beta2-receptor antagonist. Whether its acute effect on alpha1-adrenoceptors persists during long-term therapy of CHF is uncertain. In the present study, carvedilol did not increase CVC at rest, did not attenuate the reflex vasoconstrictor response to increased sympathetic outflow to the calf elicited by handgrip exercise and had no effect on the gain of the transfer function between the variability of MSNA and the variability of BP, a marker of neuroeffector transduction. Moreover, there was no significant difference between responses to carvedilol and responses to metoprolol with respect to any of these variables.
Effect of beta-blockade on calf hemodynamics.
There was no evidence for a functionally important effect of alpha1-adrenoceptor blockade with carvedilol on CVR or CVC under resting conditions. Calf vasodilation after long-term therapy could arise from a number of factors, including a reduction in sympathetic nerve traffic to skeletal muscle with improvement in clinical status, blockade of alpha1-receptors accessible to norepinephrine released from calf sympathetic nerve endings, decreased interstitial fluid or vascular remodeling.
In hypertensive subjects, Svensson et al. (21) were unable to detect any change in CVR after six months of metoprolol. Wallin et al. (22) observed reductions in both BP and MSNA in hypertensive subjects after four months of monotherapy with metoprolol (mean dose 187.5 mg/day). Because the decline in BP should have elicited a baroreflex-mediated increase in MSNA, this reduction appeared to arise from a central sympathoinhibitory action of this lipophilic beta1-adrenergic antagonist (22). Calf vascular hemodynamics were not obtained. In a previous study of patients with dilated cardiomyopathy, we observed a 50% reduction in sympathetic outflow to calf muscle and a corresponding 62% decrease in CVR after an average of 20 months of treatment with metoprolol (19)—an observation also consistent with a central neural action. However, in the present analysis, neither drug (after four months) significantly affected MSNA or resistance in the calf, the vascular bed distal to the recording electrode.
Muscle sympathetic nerve activity in CHF, recorded at rest, is inversely related to peak oxygen uptake during bicycle exercise (23). Exercise evokes a further increase in MSNA. This exercise pressor reflex is activated at lower workload, and is of greater magnitude in those with moderate to severe left ventricular systolic dysfunction than in age-matched healthy subjects. Patients with the lowest peak oxygen uptake achieve the highest MSNA during exercise (24). Augmentation of neurogenic vasoconstriction in skeletal muscle may limit further exercise capacity in CHF. Conversely, peripheral alpha1-antagonism could augment skeletal muscle blood flow and improve exercise performance. In young healthy subjects, both MSNA and CVR are tightly coupled during isometric exercise (25). Neurogenic vasoconstriction, evoked by exercise or coronary ischemia, can be blocked by intra-arterial infusion of alpha1-adrenoceptor antagonists (26, 27). However, in the present study, carvedilol did not alter the reflex increase in MSNA, or the alpha1-adrenoceptor–mediated calf vasoconstrictor response induced by handgrip exercise; before treatment, MSNA burst frequency and CVR increased by 19% and by 24%, respectively. After four months of treatment, MSNA and CVR increased by 20% and by 30%, respectively.
This lack of evidence for alpha1-antagonism over the long term is consistent with previous experience with other alpha1-blockers, prazosin and doxazosin, in CHF. We did not test for the presence of acute alpha1-blocking effects of carvedilol, which might allow for the initial tolerance of this nonspecific beta-blocker. However, hemodynamic responses to acute administration of prazosin dissipate rapidly (28). Tolerance to doxazosin develops during long-term CHF treatment, even when combined with beta-blockade (29). When added to a background regimen of digoxin and diuretics, prazosin had no long-term effect on left ventricular function or on mortality (30).
Effects of carvedilol on neuroeffector transfer function.
The transfer function from sympathetic nerve activity to arterial resistance behaves as a low-pass filter, in that signal transduction at the level of the neuroarterial junction diminishes at higher frequencies (31). Cross-spectral analysis in humans indicates that the gain of the transfer function from MSNA to BP is also highest in the VLF range, intermediate in the LF range and lowest in the HF spectral band (20). Transfer function gain within the VLF and LF ranges is significantly lower in patients with CHF than in healthy control subjects. We reasoned that if carvedilol did block the postjunctional action of neurally released norepinephrine at the level of the alpha1-adrenoreceptor, modulation of vascular smooth muscle responses to changes in sympathetic outflow would have been blunted. However, carvedilol had no effect on the gain or on the coherence of this neuroeffector transfer function, providing no evidence for alpha1-antagonism. The results of this transfer function analysis (from MSNA to BP) might be confounded by the "closed-circuit" nature of the baroreflex. However, the frequency response characteristics of the arterial baroreceptors cause the BP  MSNA component of this loop to act as a high-pass filter (20,32) that opens lower frequencies to study.
Conclusions.
After four months of therapy, neither carvedilol nor metoprolol had any effect on calf hemodynamics, either at rest, or in response to neurogenic vasoconstriction assessed by handgrip exercise. The gain of the transfer function relating oscillations in MSNA to oscillations in BP was unchanged after four months of carvedilol. Therefore, the peripheral alpha1-blocking property of carvedilol does not appear to be functionally important during long-term treatment of CHF.
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Acknowledgments
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This work was supported by Operating Grants from the Heart and Stroke Foundation of Ontario (T4050, T3696). Dr. Kubo received a Research Fellowship from the Department of Medicine, Mount Sinai Hospital, and additional unrestricted fellowship support from Merck Frosst Canada, SmithKline Beecham and Roche Pharmaceuticals. Dr. Azevedo held an AstraZeneca/Heart and Stroke Scientific Research Corporation of Canada Research Fellowship Award. Dr. Newton holds a Research Scholarship Award from the Heart and Stroke Foundation of Canada. Dr. Floras is the recipient of a Career Investigator Award from the Heart and Stroke Foundation of Ontario.
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K. Hryniewicz, A. S. Androne, A. Hudaihed, and S. D. Katz
Comparative Effects of Carvedilol and Metoprolol on Regional Vascular Responses to Adrenergic Stimuli in Normal Subjects and Patients With Chronic Heart Failure
Circulation,
August 26, 2003;
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[Abstract]
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T. J. Vittorio, C. C. Lang, S. D. Katz, M. Packer, D. M. Mancini, and U. P. Jorde
Vasopressor Response to Angiotensin II Infusion in Patients With Chronic Heart Failure Receiving {beta}-Blockers
Circulation,
January 21, 2003;
107(2):
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[Abstract]
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J. S. Floras
The "Unsympathetic" Nervous System of Heart Failure
Circulation,
April 16, 2002;
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Abstract
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