a Nuklearmedizinische Klinik und Poliklinik der Technischen Universität München, Klinikum rechts der Isar, Ismaninger Str. 22, München 81675, Germany
b Herzchirurgische Klinik der Ludwig-Maximilians Universität München, Munich, Germany
Received March 21, 2004; revised June 24, 2004; accepted July 1, 2004 * Corresponding author. Tel.: +49 89 4140 2971; fax: +49 89 4140 4950 (E-mail: frank.bengel{at}lrz.tu-muenchen.de).
![]() |
Abstract |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
METHODS AND RESULTS: Myocardial catecholamine storage capacity was determined in 12 non-rejecting transplant recipients using PET and C-11 adrenaline (epinephrine). Haemodynamics and left ventricular function were measured using radionuclide angiography at rest and during symptom-limited exercise before and after non-selective ß-blockade (propranolol iv). Exercise time and stress-induced increases of heart rate and LVEF before ß-blockade were significantly higher in reinnervated compared to denervated recipients. While resting LVEF remained unchanged, heart rate and blood pressure were generally reduced by ß-blockade, which was well tolerated by all patients. Exercise time and increases of heart rate and LVEF were more attenuated in reinnervated recipients. Differences of chronotropic and inotropic response to exercise between groups were no longer present following ß-blockade. Correlations between myocardial adrenaline retention, peak heart rate and increase of global, as well as regional ejection fraction during exercise were observed before, but not during ß-blockade.
CONCLUSION: Acute, non-selective ß-blockade is well tolerated by transplant recipients, but significantly attenuates beneficial functional effects of sympathetic reinnervation on exercise performance. The data suggest that reappearance of sympathetic nerve terminals is associated with reestablishment of intact pre-/postsynaptic interaction.
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Reinnervation of the transplanted heart has been described in various studies using invasive measurements of transcardiac noradrenaline (norepinephrine) spillover, non-invasive imaging with radiolabelled catecholamine analogues, electrophysiologic measurements of heart rate variability, or clinical reappearance of anginal symptoms.913 Presence and extent of reinnervation are determined by multiple factors, including time after transplantation, duration of surgery, frequency of rejection, donor age and recipient age.14 Reinnervation remains incomplete and regionally limited,15,16 but physiological effects on myocardial blood flow and metabolism have been demonstrated.1719 Recently, we have shown that reinnervation is associated with improved chronotropic and inotropic response to exercise and thereby enhances exercise performance.20 Although this and other studies21,22 documented the functional relevance of presynaptic sympathetic reinnervation for the transplanted heart, little is known about the interaction between restored presynaptic nerve terminals and postsynaptic adrenergic signal transduction.
Thus, we sought to non-invasively evaluate effects of acute ß-adrenergic receptor blockade on allograft performance, and to correlate these effects with presence and extent of sympathetic reinnervation as quantified by positron emission tomography (PET). We speculated that, if reappearance of presynaptic nerve terminals is associated with restoration of pre-/postsynaptic interaction, ß-blockade would attenuate the previously observed beneficial chronotropic and inotropic effects of allograft reinnervation during exercise. On the contrary, if attenuation of exercise performance would turn out to be more pronounced in denervated, relative to reinnervated hearts, previous notions of a higher sensitivity to ß-blockade after denervation due to reliance on circulating catecholamines could be confirmed.
![]() |
Methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Using electrocardiographically gated equilibrium radionuclide angiography, allograft performance at rest and in response to standardized, symptom-limited exercise was determined before and after intravenous non-selective ß-adrenoceptor blockade on the same day. Presence and regional extent of reinnervation were quantified non-invasively on the next day using PET and C-11 adrenaline. Heart rate, blood pressure and 12-lead electrocardiogram were recorded continuously throughout all procedures. Prior to study inclusion, all patients signed written informed consent forms approved by the ethical committee of the medical faculty of the TU München.
Baseline assessment of left ventricular function and haemodynamics
Autologous erythrocytes were labelled with 8001000 MBq of Tc-99m by combined in vivo/in vitro technique, and reinjected after purification. Following 5 min to allow for equilibrium, patients were positioned in a semi-upright position on a cycle ergometer table. Planar gated bloodpool images at rest (frame mode, 24 time bins, 3 min acquisition) were acquired in "best septal" left anterior oblique view using a small field of view gamma camera (Basicam, Siemens, Erlangen, Germany). Then, symptom-limited cycle ergometer exercise was started following a standardized protocol with 50 W initial workload. Imaging (parameters similar to rest) was commenced after 1 min in the stage to allow for haemodynamic stabilization, and continued for the remaining 3 min. Workload was increased by 50 W every 4 min until exhaustion. Similar to the first stage, imaging was performed in each stage after stabilization phase of 1 min. During the post-stress recovery phase, 3 min after exercise cessation, a final image was acquired.
ß-Adrenergic blockade and reassessment of left ventricular performance
Baseline measurements were followed by a 60 min break to allow for recovery. Then, patients were repositioned, and 0.1 mg/kg body weight of propranolol were administered intravenously as a short infusion over 10 min. Dosage was chosen based on previous results showing significantly attenuated heart rate response to isoprenaline as an indicator of sufficient ß-adrenergic blockade.23 A second series of radionuclide angiographic images at rest, under symptom-limited exercise and in recovery phase was commenced 10 min later.
PET
C-11 adrenaline was synthesised as previously described.24 PET imaging was performed using an ECAT EXACT scanner (CTI/Siemens, Knoxville, TN, USA). After adequate positioning, a transmission scan of 15 min was acquired for correction of photon attenuation. To measure perfusion, a 15 min static scan was performed 5 min after injection of 250300 MBq of N-13 ammonia. After a break of 20 min to allow for radioactivity decay, 200450 MBq of C-11 adrenaline were injected, and a dynamic imaging sequence (14 frames, 6x30, 2x60, 2x150, 2x300, 2x600 s) was acquired. To determine contribution of C-11 labelled metabolites to blood activity, venous blood samples were drawn at 1, 5, 10, 20 and 40 min after injection.
Data analysis
Left ventricular function
Radionuclide angiography was analysed according to international standards25 using commercial software (Gaede, Freiburg, Germany). Based on semi-automatic definition of regions of interest for left ventricle and background in end-systolic and end-diastolic frames, ventricular count rates throughout the cardiac cycle were obtained. Global left ventricular ejection fraction, regional ejection fraction in anteroseptal, lateral and inferoapical segments, and peak filling rate, a parameter of diastolic relaxation, were obtained.
PET
Attenuation-corrected transaxial PET images were reconstructed by filtered back projection. Using volumetric sampling26 in static N-13 ammonia perfusion images, myocardial radioactivity was defined in 460 left ventricular segments, depicted in a polar map. Polar maps were normalized to maximum and used for qualitative assessment of regional perfusion. Myocardial segments were transferred to dynamic imaging sequence for C-11 adrenaline, and time activity curves were obtained. Arterial input function was derived from a small circular region of interest in left ventricular cavity and corrected for presence of C-11 labelled adrenaline metabolites, which were assayed from serial blood samples using Sep-Pak cartridges as previously described.27 Adrenaline retention was calculated as myocardial activity at 40 min divided by the integral of metabolite-corrected arterial blood curve. Global extent of reinnervation was quantified as percentage of polar map showing retention within 2.5 standard deviations of average segmental value of a normal database, composed of 7 healthy volunteers.27 Adrenaline retention was also assessed regionally in the anteroseptal, lateral and inferior wall, reflecting vascular territories of left anterior descending (LAD), circumflex (LCX) and right coronary artery (RCA).
Statistical analysis
Values are expressed as means±standard deviation. Relationship between pairs of continuous variables was described by Spearman's correlation co-efficient. Non-parametric Mann-Whitney U test was applied to compare results in denervated and reinnervated transplant recipients. Wilcoxon's signed rank test was applied to compare results before and after ß-blockade. Two-sided P-values <0.05 were initially defined as significant. To correct for inflation of type I error due to multiple testing, which may be an issue in serial testing of the same parameter (e.g., heart rate at rest, during stress, during recovery, at rest during ß-blockade, during stress during ß-blockade, during recovery during ß-blockade), the significance level for P was reduced to half its initial value in a stepwise fashion with each additional comparison (0.050.025, then to 0.0125 etc.).
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Mean global left ventricular adrenaline retention ranged from 4.2 to 14.6%/min, and maximal individual retention ranged from 5.8 to 30.3%/min. Regionally, myocardial adrenaline retention was highest in LAD territory (9.7±5.2%/min), followed by LCX (6.2±2.4%/min) and RCA (5.8±1.9%/min). Signs of reinnervation, defined as regional retention within 2.5 standard deviations of the average of healthy normals, were found in 8 patients. Reinnervated area ranged from 1% to 49% of left ventricle, and was mainly located in the anteroseptal wall (LAD territory). Four transplant recipients remained completely denervated. Characteristics of denervated and reinnervated transplant recipients are summarized in Table 1.
|
|
|
|
Systemic haemodynamics, global left ventricular function and exercise performance during ß-Blockade
During ß-blockade, baseline correlation between global maximal adrenaline retention and exercise response of global ejection fraction (r=0.16; P=0.59), as well as peak heart rate (r=0.52; P=0.09) was no longer observed. Also, correlation between regional adrenaline retention and ejection fraction was no longer present (r=0.04; P=0.82; Fig. 1). Interestingly, significant correlation between global adrenaline retention and heart rate increase (r=0.70; P=0.02) as well as peak rate pressure product (r=0.62; P=0.04) continued to exist.
Group results of haemodynamic and functional parameters during ß-blockade are summarized in Table 3. ß-Blockade resulted in significant reduction of resting heart rate (P=0.002) and rate-pressure product (P=0.002) by 10±7% and 16±10%, respectively. Resting ejection fraction remained unchanged. Exercise time was reduced by 15±8% (P=0.003). Additionally, peak heart rate (22±8%; P=0.002), heart rate increase (49±25%; P=0.002), peak rate pressure product (34±12%; P=0.002) and peak ejection fraction (8±10%; P=0.037) during exercise were attenuated. Lower heart rate (19±9%; P=0.002) and rate pressure product (20±2%; P=0.012) were also observed during recovery. Significant differences in haemodynamics and ejection fraction between reinnervated and denervated recipients, which were observed at baseline, were no longer present during ß-blockade (Fig. 2). Reduction of exercise time (P=0.022), peak heart rate (P=0.017), peak rate pressure product (P=0.017) and ejection fraction increase (P=0.034) during exercise was significantly higher in reinnervated compared to denervated transplant recipients, explaining the absence of significant differences in exercise performance during ß-blockade between groups. Additionally, no differences in regional ejection fraction between groups were observed for all segments. The innervation-specific pattern (increase from rest to stress, then mild decrease during recovery) in anteroseptal segments of reinnervated patients was no longer present.
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Several previous studies have investigated effects of denervation on postsynaptic adrenergic components and catecholamine responsiveness. An increased sensitivity of denervated hearts to catecholamines has been described in animal studies28 and early human studies.23,29 This was initially thought to be consequence of ß-receptor upregulation. Studies in human myocardium, however, showed normal ß-receptor density,5 and other studies confirmed that adrenergic supersensitivity is due to lack of presynaptic neuronal uptake capacity rather than upregulation of ß-receptors.4,30 Despite normal overall ß-receptor density, shift from ß1 to ß2 subtype expression has been suggested.31 ß2-Adrenoceptors are thought to be mainly responsible for response to circulating catecholamines, and significantly contribute to chronotropic responsiveness in transplant recipients.31 All of these previous studies have in common, however, that the presence of myocardial reinnervation was not identified. Reinnervation has been shown to influence regional regulation of metabolism19 and contractility,20 but it has not been confirmed that these effects are mediated via ß-adrenergic receptors. Although the relative contribution of ß1- vs ß2-adrenoceptors and other steps of postsynaptic signal transduction were not investigated in detail, the present study is the first to demonstrate that restored presynaptic nerve terminals after cardiac transplantation are associated with re-established interaction of pre- and postsynaptic adrenergic components.
Because attenuating effects of nonselective ß-blockade were more pronounced in reinnervated than in denervated patients, the present data do not confirm previous notions that ß-blockade may be especially detrimental in denervated hearts due to dependency on circulating catecholamines.68 This hypothesis of early reports has already been challenged by other studies, which could not reproduce adverse effects of ß-blockade on exercise performance after transplantation.29,32 All patients tolerated ß-blockade well and concerns against the therapeutic use of ß-blockers in cardiac transplant recipients are not supported by our data, although it needs to be considered that acute high-dose ß-blockade was used in this study. Extrapolation to chronic treatment may be difficult, but interestingly, significant improvement of systolic allograft dysfunction during chronic ß-blocker therapy was described in a recent report.33
In contrast to previous studies where C-11-labelled hydroxyephedrine, a false neurotransmitter, was used for PET to determine sympathetic reinnervation,10,14,15,1820 the recently introduced, more physiological tracer C-11 adrenaline was applied in this study. Its retention fraction in normal myocardium is higher than that of C-11 hydroxyephedrine, but correlates closely with the latter.27 While C-11 hydroxyephedrine is thought to primarily reflect capacity of presynaptic uptake-1 catecholamine transporter, C-11 adrenaline is thought to reflect not only neuronal uptake, but also metabolism and storage, and has thus been proposed as a more suitable and sensitive probe for the study of neuronal integrity.27 Consistently, previous findings of significant beneficial effects of sympathetic reinnervation, determined by C-11 hydroxyephedrine on chronotropic and inotropic response to exercise,20 were reproducible in the present study before ß-blockade in an albeit smaller group of transplant recipients.
In addition to effects of ß-blockade, this study includes information on haemodynamics and ventricular function in the early recovery phase as another aspect of exercise adaptation which has not been investigated in our previous studies. A specific pattern with continuously elevated heart rate and a further increase of ejection fraction after termination of exercise was observed in denervated, but not in reinnervated transplant recipients before ß-blockade. This may reflect the delayed response to circulating catecholamines, as suggested by previous observations.22 However, this pattern was also observed to some degree after ß-blockade in both groups, suggesting that other mechanisms such as ventricular unloading are also involved. Notably, heart rate in reinnervated recipients decreased to a larger extent than global ejection fraction during early recovery before ß-blockade. This may indicate a differential response of chronotropy and inotropy to ventricular unloading, but other factors such as regionally limited reinnervation, or technical issues related to acquisition of images over a period of 3 min which may attenuate maximally measurable functional parameters need to be considered. To which degree the chronotropic and inotropic adaptation in the recovery phase in reinnervated transplant recipients was comparable to normals cannot be answered in the present study due to a lack of healthy controls, but may be the topic of future investigations.
The dichotomous approach applied in this and previous studies,10,14,15,1820 which divides patients into reinnervated and denervated, bears the limitation that a variety of different degrees of innervation is combined into one group. Results nevertheless are conclusive and correlation analysis on a continuous scale was performed to confirm group results. Finally, peak filling rates as diastolic parameters are co-registered with innervation for the first time, suggesting lusitropic effects of reinnervation. These parameters, however, should be interpreted with caution because they are highly sensitive to changes of heart rate during image acquisition, which impair reliability in the recovery phase, but also during exercise due to an often slow, but steady increase in transplant recipients.22 Other potential limitations related to the applied study protocol should be considered: firstly, individuals had to undergo a maximal exercise test twice within 12 h so that fatigue may have limited exercise performance in the second test. The complex study protocol and radionuclide decay did not allow for a longer break between both tests. Secondly, progressive exercise consisted of 4 minute stages to allow for stable and reproducible radionuclide image quality. Such relatively long stages may also contribute to leg fatigue before maximal cardiovascular stress. No objective measures of peak cardiovascular exercise were obtained, but individuals were encouraged to exercise until absolute subjective exhaustion. Although fatigue may have contributed to some of the general reduction of maximal haemodynamic response observed in the second test after ß-blockade, it cannot explain the significant interindividual variations related to sympathetic innervation. Additionally, functional results were also analysed at a standardized workload of 50 W to correct for differences in maximal exercise capacity.
In conclusion, results of this study give further insights into exercise physiology of the transplanted heart. Beneficial effects of sympathetic reinnervation on exercise performance are predominantly mediated via ß-adrenergic receptors, and restored presynaptic innervation is paralleled by intact pre-/postsynaptic interaction.
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|