Evolution of heart rate responsiveness after orthotopic cardiac transplantation

Exercise limitation following transplantation

Organ transplantation is one of the medical miracles or the 20th century. It has the capacity to substantially improve exercise performance and quality of life in patients who are severely limited with chronic organ failure. We focus on the most commonly performed solid-organ transplants and describe peak exercise performance following recovery from transplantation. Across all of the common transplants, evaluated significant reduction in VO2peak is seen (typically renal and liver 65%-80% with heart and/or lung 50%-60% of predicted). Those with the lowest VO2peak pretransplant have the lowest VO2peak posttransplant. Overall very few patients have a VO2peak in the normal range. Investigation of the cause of the reduction of VO2peak has identified many factors pre- and posttransplant that may contribute. These include organ-specific factors in the otherwise well-functioning allograft (e.g., chronotropic incompetence in heart transplantation) as well as allograft dysfunction itself (e.g., chronic lung allograft dysfunction). However, looking across all transplants, a pattern emerges. A low muscle mass with qualitative change in large exercising skeletal muscle groups is seen pretransplant. Many factor posttransplant aggravate these changes or prevent them recovering, especially calcineurin antagonist drugs which are key immunosuppressing agents. This results in the reduction of VO2peak despite restoration of near normal function of the initially failing organ system. As such organ transplantation has provided an experiment of nature that has focused our attention on an important confounder of chronic organ failure-skeletal muscle dysfunction.

Understanding exercise-induced hyperemia: central and peripheral hemodynamic responses to passive limb movement in heart transplant recipients

To better characterize the contribution of both central and peripheral mechanisms to passive limb movement-induced hyperemia, we studied nine recent (<2 yr) heart transplant (HTx) recipients (56 ± 4 yr) and nine healthy controls (58 ± 5 yr). Measurements of heart rate (HR), stroke volume (SV), cardiac output (CO), and femoral artery blood flow were recorded during passive knee extension. Peripheral vascular function was assessed using brachial artery flow-mediated dilation (FMD). During passive limb movement, the HTx recipients lacked an HR response (0 ± 0 beats/min, Δ0%) but displayed a significant increase in CO (0.4 ± 0.1 l/min, Δ5%) although attenuated compared with controls (1.0 ± 0.2 l/min, Δ18%). Therefore, the rise in CO in the HTx recipients was solely dependent on increased SV (5 ± 1 ml, Δ5%) in contrast with the controls who displayed significant increases in both HR (6 ± 2 beats/min, Δ11%) and SV (5 ± 2 ml, Δ7%). The transient increase in femoral blood volume entering the leg during the first 40 s of passive movement was attenuated in the HTx recipients (24 ± 8 ml) compared with controls (93 ± 7 ml), whereas peripheral vascular function (FMD) appeared similar between HTx recipients (8 ± 2%) and controls (6 ± 1%). These data reveal that the absence of an HR increase in HTx recipients significantly impacts the peripheral vascular response to passive movement in this population and supports the concept that an increase in CO is a major contributor to exercise-induced hyperemia.

Decreased heart rate and blood pressure in a recent cardiac transplant patient after spinal anesthesia

PURPOSE

To describe the cardiovascular effects of neuraxial blockade in a heart transplant patient.

CLINICAL FEATURES

A 69-yr-old 70-kg male underwent orthotopic heart transplant (bicaval anastomosis technique) for ischemic cardiomyopathy. Five months after transplantation, the patient underwent a transurethral bladder tumour resection under spinal anesthesia. Two millilitres of bupivacaine 0.75% (15 mg) were injected intrathecally at L(3-4) and the patient remained seated for approximately 20 sec prior to assuming the lithotomy position. Subsequently, both blood pressure (BP) and heart rate (HR) diminished gradually (BP and HR immediately pre-spinal: 113 mmHg (mean arterial pressure) and 92 beats x min(-1), respectively; nadir BP and HR: 94 mmHg (16.8% decrease) 30 min postspinal and 73 beats x min(-1) (20.7% decrease) 40 min postspinal, respectively). HR and mean BP were highly correlated (r = 0.9410, P < 0.0001, R(2) = 0.8854). The dermatome level of neuraxial anesthesia, determined by sensitivity to pin prick, was T(8) (five minutes) and T(6) (ten minutes) postinjection of spinal anesthetic. Control patients (n = 10) undergoing elective urological procedures with identical anesthesia management demonstrated very similar cardiovascular responses.

CONCLUSIONS

Although cardiac transplant patients may tolerate neuraxial anesthesia admirably, a fall in HR may ensue which theoretically could have important physiological consequences. It is argued that the change in HR in the transplanted patient was mediated by mechanisms intrinsic to the transplanted heart and/or by reduced catecholamine secretion from the adrenal medulla. It is emphasized that HR changes observed in cardiac transplant patients do not necessarily imply reinnervation of the transplanted organ.

Evolution of heart rate control after transplantation: conduction versus autonomic innervation

In cardiac transplantation, the donor organ is not initially innervated and demonstrates decreased heart rate variability (HRV). However, HRV may improve after several months. The mechanism for HRV improvement has not been elucidated; autonomic "reinnervation" of the donor heart has been proposed. The role of atrioatrial conduction from recipient to donor organ has not been evaluated. We prospectively evaluated cardiac transplant patients with a limited electrophysiology study at the time of their surveillance biopsies. Recordings were made of recipient and donor signals, observing conduction properties between recipient and donor atria. Holter recordings were analyzed and HRV was determined using spectral analysis techniques, recording mean RR interval, low-frequency power (LF), high-frequency power (HF), and the LF/HF ratio. These were compared to published norms. From November 1999 to May 2000, 21 patients (6 female) who underwent cardiac transplantation participated at a median age of 101 months (range, 4.1-217 months). Time posttransplant ranged from 26 days to 71 months. Holter data were available for 20 patients and demonstrated dissociated P waves in 13 (65%). The mean heart rate on Holter was 111 beats per minute (bpm) (range, 85-161 bpm). We were able to record distinct recipient atrial signals in 16 of 21 (76%) patients. The average recipient tissue heart rate was 55% that of the donor heart rate. We documented atrioatrial association in only 1 patient. HRV did not reach normal values for most patients and did not increase with time posttransplantation. The LF values were in the normal range for most patients, whereas 3 patients had normal HF values and 2 patients had values just below normal. Recipients of heart transplantation have a predominantly sympathetic influence of HRV. These preliminary data suggest that atrioatrial conduction does not play a role in reestablishing normal heart rate control following pediatric cardiac transplantation.

Physical activity patterns and exercise performance in cardiac transplant recipients

BACKGROUND

Cardiac transplantation (CTX) improves exercise tolerance, but CTX recipients still achieve only 50% to 70% of normal values for exercise capacity. Among the factors suggested to explain the reduced exercise tolerance in CTX recipients is deconditioning. Little is known about the relation between physical activity patterns and exercise test responses in CTX patients.

METHODS

Forty-seven CTX patients (mean age 47 +/- 12 years; mean 4.8 +/- 3.0 years after CTX) underwent maximal exercise testing and assessment of current and past physical activity patterns using a questionnaire. Energy expenditure from recreational and occupational activities over the last year and for adulthood were expressed in kcal/week and correlated with peak oxygen consumption (VO(2)), VO(2) at the ventilatory threshold, and the percentage of age-predicted peak VO(2) achieved.

RESULTS

The patients reported expending a mean of approximately 1100 kcal/week in recreational activity, suggesting a moderate level of physical activity is maintained after CTX. The mean peak VO(2) achieved for the group was 17.2 +/- 5.2 mL/kg/min, corresponding to 59% +/- 14% of age-predicted exercise capacity. Significant but modest associations were observed between recreational energy expenditure during the last year and percentage of age-predicted peak VO(2) achieved (r = 0.34, P <.01), and VO(2) at the ventilatory threshold (r = 0.45, P <.01). Energy expenditure from blocks walked and stairs climbed per week was modestly associated with peak VO(2) (r = 0.36, P <.05), percentage of predicted peak VO(2) achieved (r = 0.39, P <.01), and VO(2) at the ventilatory threshold (r = 0.42, P <.01). Exercise capacity was poorly related to occupational and recreational activities when expressed as average weekly energy expended throughout adulthood.

CONCLUSION

Post-CTX patients maintain a moderately active lifestyle. Measures of exercise tolerance generally are related to recent daily recreational activities in CTX patients, but these associations are modest. The many physiologic factors unique to CTX recipients likely play a more important role than deconditioning in determining exercise tolerance in these patients.

Functional restitution of cardiac control in heart transplant patients

Cardiovascular control is fundamentally altered after heart transplantation (HT) because of surgical denervation of the heart. The main goal of this work was the noninvasive characterization of cardiac rate control mechanisms after HT and the understanding of their nature. We obtained 25 recordings from 13 male HT patients [age = 28-68 yr, time after transplant (TAT) = 0.5-62.5 mo]. The control group included 14 healthy men (age = 28-59 yr). Electrocardiogram, continuous blood pressure (BP), and respiration were recorded for 45 min in the supine position and then during active change of posture (CP) to standing. The signals were analyzed in the time domain [mean and variance of heart rate (HR) and rise time of HR in response to CP] and the frequency domain [low and high frequency (LF and HF)]. Our principal finding was the consistent pattern of evolution of the HR response to standing: from no response, via a slow response (>40 s, TAT > 6 wk), to a fast increase (<20 s, TAT > 24 mo). HR response correlated with TAT (P < 0.001). LF correlated with HR response to CP (P < 0.0001); HF and HR did not. An important finding was the presence of very-high-frequency peaks in the power spectrum of HR and BP fluctuations. Extensive arrhythmias tended to appear at the TAT that corresponds to the transition from slow to fast HR response to CP. Our results indicate a biphasic evolution in cardiac control mechanisms from lack of control to a first-order control loop followed by partial sympathetic reinnervation and, finally, the direct effect of the old sinoatrial node on the pacemaker cell of the new sinoatrial node. There was no indication of vagal reinnervation.

Vagal reinnervation in the long term after orthotopic heart transplantation

BACKGROUND

Sympathetic reinnervation after orthotopic heart transplantation (HTx) has become an accepted phenomenon, particularly in long-term transplanted patients. Parasympathetic reinnervation, however, still remains questionable.

METHODS

In 38 HTx recipients, mean age of 51.6 +/- 9.7 years (range, 29 to 70 years), with a time span after HTx of 4.6 +/- 2.8 years (0.5 to 10.5 years), we stimulated carotid baroreceptors using periodic neck suction at low (0.1 Hz) and high (0.2 Hz) frequencies to test sympathetic and vagal responses, respectively. Respiratory rate was fixed at 0.25 Hz. We simultaneously recorded surface electrocardiogram, finger blood pressure, respiration and neck pressure signals while patients rested in the supine position. Time series of RR intervals, respiration, and neck and blood pressures were generated and subjected to spectral analysis.

RESULTS

All patients demonstrated a 0.25-Hz peak in RR-interval spectrum, caused by non-autonomic respiratory sinus arrhythmia. Thirteen patients (5. 2 +/- 3.5 years after HTx; range, 0.9 to 10.2 years) showed a baroreflex-induced sharp peak at 0.1 Hz in RR-interval power spectrum during 0.1-Hz neck suction, indicating sympathetic reinnervation. However at 0.2-Hz neck suction, 4 of the 13 sympathetically reinnervated patients displayed a baroreflex-induced 0.2-Hz peak, which could be suppressed with atropine administration-strong evidence for vagal reinnervation.

CONCLUSIONS

Non-invasive carotid baroreflex stimulation is an appropriate tool to prove restoration of autonomic control after orthotopic HTx. Sympathetic reinnervation parallels parasympathetic reinnervation in long-term transplanted patients.

Exercise following heart transplantation

During the past 2 decades, heart transplantation has evolved from an experimental procedure to an accepted life-extending therapy for patients with endstage heart failure. However, with dramatic improvements in organ preservation, surgery and immunosuppressive drug management, short term survival is no longer the pivotal issue for most heart transplant recipients (HTR). Rather, a return to functional lifestyle with good quality of life is now the desired procedural outcome. To achieve this outcome, aggressive exercise rehabilitation is essential. HTR present unique exercise challenges. Preoperatively, most of these patients had chronic debilitating cardiac illness. Many HTR have had prolonged pretransplantation hospitalisation for inotropic support or a ventricular assist device. Decrements in peak oxygen consumption (VO2peak) and related cardiovascular parameters regress approximately 26% within the first 1 to 3 weeks of sustained bed rest. Consequently, extremely poor aerobic capacity and cardiac cachexia are not unusual occurrences in HTR who have required mechanical support or been confined to bed rest. Moreover, HTR must also contend with de novo exercise challenges conferred by chronic cardiac denervation and the multiple sequelae resulting from immunosuppression therapy. There is ample evidence that both endurance and resistance training are well tolerated in HTR. Moreover, there is growing clinical consensus that specific endurance and resistance training regimens in HTR can be efficacious adjunctive therapies in the prevention of immunosuppression-induced adverse effects and the reversal of pathophysiological consequences associated with cardiac denervation and antecedent heart failure. For example, some HTR who remain compliant during strenuous long term endurance training programmes achieve peak heart rate and VO2peak values late after transplantation that approach age-matched norms (up to approximately 95% of predicted). These benefits are not seen in HTR who do not participate in structured endurance exercise training. Rather, peak heart rate and VO2peak values in untrained HTR remain approximately 60 to 70% of predicted indefinitely. However, the mechanisms responsible for improved peak heart rate, VO2peak and total exercise time are not completely understood and require further investigation. Recent studies have also demonstrated that resistance exercise training may be an effective countermeasure for corticosteroid-induced osteoporosis and skeletal muscle myopathy. HTR who participate in specific resistance training programmes successfully restore bone mineral density (BMD) in both the axial and appendicular skeleton to pretransplantation levels, increase lean mass to levels greater than pretransplantation, and reduce body fat. In contrast, HTR who do not participate in resistance training lose approximately 15% BMD from the lumbar spine early in the postoperative period and experience further gradual reductions in BMD and muscle mass late after transplantation.

Anaesthesia for patients with a transplanted organ

Increasing numbers of individuals leading normal lives have transplanted organs. They may appear in any hospital for treatment of trauma or general diseases. Common anaesthesia methods can be used for these patients, but safe conduct of anaesthesia requires knowledge of the immunosuppression, risk factors, and altered physiology or drug actions. This article reviews the anaesthesia-related literature on patients with transplanted organs.

Evidence of time-dependent autonomic reinnervation after heart transplantation

BACKGROUND

Confirming the clinical significance of reinnervation is important in understanding and anticipating how heart rate (HR) responses of transplant recipients to physiologic stress differs early and late after transplant from that of normal individuals.

OBJECTIVES

To evaluate the functional significance of cardiac reinnervation early and late after heart transplantation.

METHODS

Handgrip and deep breathing tests, passive 80 degrees head-up tilt, and heart rate (HR) responsiveness of 33 transplant recipients (n = 16 at < 5 months and n = 17 at > 1 year after transplant) were compared with those of 16 age- and sex-matched control participants.

RESULTS

HR responses to handgrip and passive tilt were absent early after transplant. HR acceleration normalized but was blunted late after transplant. These findings are consistent with late (>1 year) sympathetic reinnervation in transplant recipients.

CONCLUSIONS

When caring for transplant recipients, nurses should consider the time elapsed since transplant in evaluating HR responsiveness to common procedures and interventions.

Stretch-induced changes in heart rate and rhythm: clinical observations, experiments and mathematical models

Clinical and research data indicate that active and passive changes in the mechanical environment of the heart are capable of influencing both the initiation and the spread of cardiac excitation via pathways that are intrinsic to the heart. This direction of the cross-talk between cardiac electrical and mechanical activity is referred to as mechano-electric feedback (MEF). MEF is thought to be involved in the adjustment of heart rate to changes in mechanical load and would help to explain the precise beat-to-beat regulation of cardiac performance as it occurs even in the recently transplanted (and, thus, denervated) heart. Furthermore, there is clinical evidence that MEF may be involved in mechanical initiation of arrhythmias and fibrillation, as well as in the re-setting of disturbed heart rhythm by 'mechanical' first aid procedures. This review will outline the clinical relevance of cardiac MEF, describe cellular correlates to the responses observed in situ, and discuss the role that quantitative mathematical models may play in identifying the involvement of cardiac MEF in the regulation of heart rate and rhythm.

Palpitations and cardiac awareness after heart transplantation

OBJECTIVE

The aim of this study was to examine the awareness of resting heartbeat in heart transplantation recipients, compare it with that found in other medical populations, and determine whether clinical characteristics are associated with accurate heartbeat awareness.

METHODS

Eligible patients underwent a research battery consisting of a heartbeat detection task and self-report questionnaires assessing cardiac symptoms, psychosocial variables, and cognitive function. The accurate awareness of resting heartbeat was determined by presenting the patients with auditory stimuli at each of six different delays following the R wave on the ECG. Patients then selected the tones that they thought coincided with the sensation they had of their heart beating. The patients' physicians rated their cardiac morbidity. The results were contrasted with comparable data obtained in previous work with other ambulatory medical populations.

RESULTS

Forty-one consecutive heart transplantation recipients who survived for at least 3 months after surgery were eligible. Thirty-four (82.9%) of them were studied and complete data were obtained on 26 (63.4%). Nine patients (34.6%) were reliably able to detect their resting heartbeat. When compared with the 17 patients who were not accurately aware of their heartbeat, the two groups did not differ significantly in cardiac morbidity, cognitive brain dysfunction, generalized psychiatric distress, depression, somatization, or hypochondriacal attitudes. A significantly higher proportion of heart transplantation recipients were accurately aware of their heartbeat than was found in a sample of general medical outpatients and in asymptomatic, nonpatient volunteers.

CONCLUSIONS

One-third of heart transplant recipients are accurately aware of resting heartbeat, despite the absence of cardiac innervation.

Electrical connection of native and transplanted sinus nodes via atrial to atrial pacing improves exercise performance after cardiac transplantation

Chronotropic incompetence limits exercise performance in cardiac transplant patients. Electrical linkage of the innervated native sinus node and the denervated donor atrium or direct donor atrium pacing improves exercise performance in patients early after transplant.

Cardiac versus noncardiac limits to exercise after heart transplantation

BACKGROUND

To determine whether the reduced exercise capacity of patients after heart transplantation is primarily a result of decreased cardiac or peripheral vascular factors, we examined the cardiac output (CO) and right atrial pressure (Pra) relation during graded cycle ergometry.

METHODS AND RESULTS

We studied 12 male patients (51.2+/-15.3 years [mean+/-SD]) 35.3+/-12.5 weeks after heart transplantation and 6 young healthy men. Patients had a normal increase in CO with increasing oxygen uptake (VO2) (CO = 0.00597 VO2 + 4.4, r = 0.83). Mean (+/-SEM) heart rate increased from 97.0+/-5.0 beats/min at rest to 146.9+/-6.9 beats/min at peak effort compared with the increase of 67.2+/-1.9 beats/min to 187.2+/-2.5 beats/min in the normal group. Pra in patients increased from 1.6+/-1.0 mm Hg at rest to 8.9+/-1.6 mm Hg during mild exercise but did not increase further at the highest work rates, even though CO continued to increase. In the normal group there was an initial increase in Pra from rest to exercise transition but little further change in Pra with increasing CO. Aerobic capacity (peak VO2) did not increase when cardiac function was increased with dobutamine during exercise in two patients.

CONCLUSIONS

The steep increase in CO relative to Pra during severe exercise in patients who undergo heart transplantation argues against the heart as the sole limiting factor during maximal effort.

Evidence for differential sympathetic and parasympathetic reinnervation after heart transplantation in humans

During heart transplantation (HTX) all neural connections are severed. In humans, signs of autonomic reinnervation have been found. In this study non-invasive tests were used to compare signs of sympathetic and parasympathetic reinnervation. Non-invasive autonomic function tests and heart rate variability parameters (HRV; 24 h electrocardiographic registration) were used to investigate signs of reinnervation. 16 HTX patients (14 males) were compared with age-and sex-matched controls. Parasympathetic heart rate changes in HTX compared to controls were attenuated during the diving test, deep breathing, the Valsalva maneuver and standing up but not during carotid sinus massage. Sympathetic heart rate increases were lower during the cold pressor test and mental stress. The blood pressure responses were comparable to the control group, but not during active standing and tilting. This finding suggests an obligatory 'blood pressure' role for the innervated heart in these two tests. All HRV parameters were lower in HTX. One or more normal parasympathetic responses were found in 13 out of 16 patients versus 4 out of 16 with normal sympathetic responses (p < 0.05). Heart rate variations were less in case of a higher donor age, and higher in case of a longer time after HTX. Parasympathetic signs of reinnervation are more common than sympathetic signs of reinnervation. A higher donor age reduces signs of reinnervation. If the sympatho-vagal balance is a prognostic factor in HTX patients as it is in other cardiac diseases these findings are clinically relevant.

Long-term sequential changes in exercise capacity and chronotropic responsiveness after cardiac transplantation

BACKGROUND

Peak exercise capacity improves early after orthotopic cardiac transplantation. However, the physiological response to exercise remains abnormal, with a reduced rate of heart rate (HR) rise and reductions in peak exercise HR and the increment in HR from rest to peak exercise. This chronotropic incompetence is due in large part to cardiac denervation. If reinnervation occurs after transplantation, it might result in an improvement in both chronotropic responsiveness and maximal exercise capacity. We therefore hypothesized that the chronotropic response to exercise and maximal exercise capacity would improve with time after transplantation.

METHODS AND RESULTS

Peak symptom-limited cardiopulmonary exercise tests performed in 57 clinically stable cardiac transplant recipients (mean age, 45 +/- 2 years) serially for up to 5 years after transplantation and in 33 control subjects without heart disease were analyzed retrospectively. Pretransplantation exercise tests were also performed in 41 patients an average of 4.7 +/- 0.6 months before transplantation. At 1 year after transplantation, peak oxygen consumption was 16.6 +/- 0.9 mL.kg-1.min-1, reflecting a 43% increase versus pretransplantation. Nevertheless, compared with control subjects, maximal exercise capacity and the HR response to exercise were subnormal in transplant recipients. There were no further increases in peak exercise capacity, peak exercise HR, or the peak increment in HR with exercise up to 5 years after transplantation.

CONCLUSIONS

One year after cardiac transplantation, peak exercise capacity and chronotropic responsiveness are subnormal. There is no further improvement in peak exercise capacity or chronotropic responsiveness as late as 5 years after transplantation. These data indicate that with regard to chronotropic responsiveness, functionally significant cardiac reinnervation does not occur between the first and fifth years after transplantation.

Protamine-induced hypotension and bradycardia in a cardiac transplant patient

PURPOSE

The potential for functional reinnervation of the transplanted heart in man is controversial. We report the sudden onset of bradycardia in a cardiac transplant patient following a period of hypotension subsequent to the administration of protamine. Possible mechanisms underlying this response, including reinnervation of the transplanted heart, are assessed.

CLINICAL FEATURES

Eight weeks after cardiac transplantation, a patient returned to hospital for a left femoral-tibial artery bypass vein graft. The patient was anaesthetized using general anaesthesia. Upon completion of the procedure, protamine was administered to reverse the heparin-induced anticoagulation. Although administration of a 5.0 mg "test-dose" appeared to be without cardiovascular effect, after an additional 20.0 mg, blood pressure decreased from 98/52 to 62/40 mmHg. After blood pressure reached its nadir, heart rate decreased precipitously from 57 to 29 beats.min-1.

CONCLUSIONS

This report demonstrates that heart rate can change considerably in patients who have undergone cardiac transplantation. It is argued that the change in heart rate observed in the present report cannot be explained by reinnervation of the transplanted heart, as the patient had undergone transplantation only eight weeks previously. Rather, we suggest that the change was mediated by mechanisms intrinsic to the transplanted heart and extrinsic to the CNS.

Induction of paradoxic bradycardia in rats by inferior vena cava occlusion during the administration of isoproterenol: the essential role of augmented sympathetic tone

INTRODUCTION

Testing human susceptibility for vasodepressor reactions involves combining venous return restriction by passive upright tilting and administering isoproterenol. While sympathetic tone is usually increased by the stimuli that incite a vasodepressor reaction, it is not known if the increased sympathetic tone is an essential or passive component of the mechanism that triggers the reaction. Given that paradoxic bradycardia is a major manifestation of vasodepressor reactions and allowing for the possible extrapolation between paradoxic bradycardia in rats and vasodepressor reactions, we examined the role of sympathetic tone in the paradoxic bradycardia reaction. Paradoxic bradycardia was induced in rats by inferior vena cava occlusion during an isoproterenol infusion. To examine the role of increased sympathetic tone on this reaction, we studied whether carotid artery perfusion (80 to 100 mmHg) during inferior vena cava occlusion, a maneuver that blunts the rise in sympathetic tone, inhibits paradoxic bradycardia.

METHODS AND RESULTS

The maximum changes in R-R were measured during 60 seconds of inferior vena cava occlusion as follows: (a) in control the heart rate accelerated (delta R-R - 10.2 +/- 2.3 msec, P < 0.001); (b) during an infusion of isoproterenol, paradoxic bradycardia occurred (delta R-R + 140.6 +/- 18.2 msec, P < 0.001), and this was inhibited by common carotid artery perfusion (delta R-R - 6.6 +/- 1.5 msec, P < 0.001); and (c) following carotid sinus denervation and during an infusion of isoproterenol, paradoxic bradycardia was induced without and with carotid artery perfusion (delta R-R + 122.6 +/- 12.0 msec, P < 0.001; delta R-R + 151.8 +/- 12.7 msec, P < 0.001, respectively).

CONCLUSIONS

Since carotid artery perfusion during inferior vena cava occlusion inhibits paradoxic bradycardia only when the carotid sinus is innervated, we conclude that carotid artery perfusion blocks the reaction by increasing carotid sinus afferents, thereby limiting the increased sympathetic tone during inferior vena cava occlusion, and not as a result of cerebral perfusion. Thus, the paradoxic bradycardia resulting from inferior vena cava occlusion requires activation of sympathetic tone as a result of carotid sinus hypotension.

Heart rate changes in cardiac transplant patients and in the denervated cat heart after edrophonium

PURPOSE

The effect of edrophonium on heart rate in cardiac transplant patients and in an animal model of acute cardiac denervation were studied, to evaluate the functional state of the peripheral parasympathetic pathway following cardiac denervation.

METHODS

Edrophonium was studied in patients with normally innervated hearts (controls) and in cardiac transplants. Edrophonium was also studied in vagotomized, beta-blocked cats. In Group I animals, the vagus nerve was not stimulated. In Groups 2 & 3 the right vagus nerve was electrically stimulated to produce approximately 20% and 40% reductions in baseline heart rate, respectively.

RESULTS

Maximum heart rate reduction in transplants (7.3 +/- 0.8 beats.min-1 with 0.6 +/- 0.08 mg.kg-1) was less than in controls (13.3 +/- 1.6 beats.min-1 with 0.4 + 0.05 mg.kg-1, P < 0.01). In Group I animals heart rate decreased maximally by 20.9 +/- 2.5 beats.min-1 with 9.0 +/- 1.9 mg.kg-1. In Groups 2 and 3, with doses < 1.5 mg.kg-1, reductions in heart rate were greater than in Group I and maximum reductions were obtained with lower doses (Group 2: maximum reduction by 20.3 +/- 2.8 beats.min-1 with 1.3 +/- 0.1 mg.kg-1; Group 3:22.6 +/- 4.0 beats.min-1 with 0.8 +/- 0.2 mg.kg-1, P < 0.001). Doses > 1.5 mg.kg-1 in Groups 2 and 3 produced increases in heart rate.

CONCLUSION

Edrophonium produced bradycardia in cardiac transplants suggesting spontaneous release of acetylcholine from parasympathetic postganglionic neurons in the transplanted heart. The magnitude of the bradycardia was less in transplant than in control patients. Findings from animal studies suggest that the reduction in transplants can be attributed to diminution or absence of tonic cardiac parasympathetic drive. At high doses, edrophonium may interfere with parasympathetic neuron activation.

Heart rate variability after cardiac transplantation in humans

The reappearance of cardiac innervation after cardiac transplantation remains a matter of debate. We evaluated the ability of heart rate variability (HRV) analysis to detect the extent and time course of functional cardiac allograft reinnervation. Time- and frequency-domain analysis of heart rate was performed on Holter recordings of 120 heart transplant and four heart-lung transplant recipients. A high frequency (HF) component was clearly distinguished on visual inspection of power spectral density in 42 patients. In eight patients an HF component of normal magnitude was detected. The other 34 patients in this group, including all four heart-lung transplants, presented with a very small HF component. The other 82 patients showed a flat spectrum. The group with an HF component of normal amplitude was significantly different, compared to the other groups, for all HRV parameters. Serial plotting of HRV parameters of the patients with an HF component of normal amplitude against time posttransplant, revealed, from 12 months onwards, a progressive increase of parameters denoting HF variability. In five heart transplant patients with acute allograft rejection, the use of HRV analysis for rejection monitoring was unsuccessful. These results suggest that, inasmuch as the HF component of HRV is caused by parasympathetic cardiac innervation, the HF component of normal amplitude, observed in only a minority of cardiac transplant recipients (6%), is a marker for parasympathetic reinnervation. The evolution over time of this HF component is compatible with a biological phenomenon as gradual parasympathetic reinnervation of the sinus node.

The place of perceived exertion ratings in exercise prescription for cardiac transplant patients before and after training

OBJECTIVE

Heart rate provides a poor guide to exercise prescription after cardiac transplantation. This study explores whether the rating of perceived exertion (RPE) provides useful alternative information.

METHODS

Borg's original categoric scale was applied to 36 male patients [T, age 47(SD 9 years] as they performed a progressive cycle ergometer test an average of seven months (range two to 23 months) after cardiac transplantation. The test was repeated after 16(7) months of progressive exercise centred rehabilitation. Sedentary but healthy controls [C, n = 45, age 45(7) years] performed a similar progressive cycle test.

RESULTS

Initially, 13 RPE units corresponded to 66(12)% of peak VO2 in T and 50(11)% in C. Rehabilitation augmented peak VO2 (by 19%) and estimated lean body mass (by 3.5%) in the cardiac transplant patients. The increase of heart rate (HR) at 13 RPE units [delta HR = 10(17) beats.min-1] showed moderate correlations with gains of lean mass (r = 0.72) and gains of peak VO2 (r = 0.58). The relative oxygen intake at 13 RPE units remained unchanged at 68(12)% of peak VO2.

CONCLUSIONS

Large inter-individual variations of RPE at a given VO2 limit the value of perceived exertion in exercise prescription. Ratings seem best restricted to fine tuning fixed distance/fixed speed exercise prescriptions in patients undergoing rehabilitation after cardiac transplantation.

Neostigmine-induced bradycardia following recent vs remote cardiac transplantation in the same patient

PURPOSE

This report describes the effects of neostigmine on heart rate in the same patient following recent and remote cardiac transplantation.

CLINICAL FEATURES

Eighty-six months following the first transplant, neostigmine 5.0 micrograms.kg-1 i.v. produced a 10% reduction in heart rate which was reversed by atropine 1.2 mg. For 24 months prior to this initial study, the patient experienced angina, suggesting cardiac afferent reinnervation. Three months after the second heart transplant, a second study showed that a six-fold increase in the dose of neostigmine, 30.0 micrograms.kg-1, only produced a 3.5% reduction in heart rate which was reversed by atropine 1.2 mg.

CONCLUSIONS

These observations indicate that neostigmine produces bradycardia following cardiac transplantation, and suggest that a greater response may be observed in remotely than in recently transplanted patients.

Neostigmine decreases heart rate in heart transplant patients

PURPOSE

This study evaluated the effect of neostigmine on heart rate in cardiac transplant patients.

METHODS

Neostigmine (2.5-50 micrograms.kg-1) was administered to ASA 1 or 2 patients with normally innervated hearts (controls), and to patients who had undergone recent (< six months before study) or remote (> six months before study) cardiac transplantation.

RESULTS

Baseline heart rate was 66 +/- 3 beats.min-1 in controls (n = 10, mean +/- SEM), which was slower than that observed in recently (95 +/- 4 beats.min-1, n = 15, P < 0.001) and in remotely (88 +/- 3 beats.min-1, n = 16, P < 0.001) transplanted patients. Neostigmine produced a dose-dependent decrease in heart rate in all patients. Controls were the most sensitive to neostigmine, with a 10% decrease in heart rate produced by an estimated dose of 5.0 +/- 1.0 micrograms.kg-1. The recently transplanted group was the least sensitive, with the maximum dose producing only an 8.3 +/- 0.9% reduction. The response to neostigmine of the remotely transplanted patients was variable. The estimated dose to produce a 10% decrease in heart rate in this group was 24 +/- 6 micrograms.kg-1 which was greater than that for controls (P = 0.008). Administration of atropine (1.2 mg) reversed the neostigmine-induced bradycardia in all three groups. Reversal of the bradycardia consisted of a transient peak increase in heart rate in controls to 145 +/- 6% of baseline, a value which was greater than that observed in recent (103 +/- 1%, P < 0.001) and in remote (109 +/- 3%, P < 0.001) transplants.

CONCLUSIONS

Neostigmine produces a dose-dependent bradycardia in heart transplant patients. Some remotely transplanted patients may be particularly sensitive to the bradycardic effects of neostigmine.

Evolution of the chronotropic response to exercise after cardiac transplantation

The chronotropic response to exercise is abnormal in cardiac transplant recipients as a result of autonomic denervation. Differences in the response between recent transplant recipients and longer-term survivors have been described in previous cross-sectional studies. These changes have not been assessed directly using serial studies. The effect of sinus node dysfunction on the chronotropic response has not previously been determined. Thirty-one transplant recipients underwent serial treadmill exercise tests using the chronotropic exercise assessment protocol 3 and 6 weeks and 3 and 6 months after transplantation. Sinus node function was assessed using standard electrophysiologic techniques. The chronotropic response increased between 3 and 6 weeks after transplantation in all subjects. Six months after transplantation, there was a further marked increase in the response in a subgroup of 5 subjects. These subjects also had a dramatic decrease in heart rate on cessation of exercise. Three subjects had abnormal sinus node function. Although heart rates and chronotropic response were below average in these subjects, 2 other subjects with normal sinus node function on electrophysiologic testing had lower heart rates and worse chronotropic responses. Thus, the chronotropic response to exercise evolves over the first 6 weeks after cardiac transplantation in all subjects. In a number of recipients (16%), there is a marked increase in chronotropic response between 3 and 6 months, which suggests efferent sympathetic reinnervation. There was no clear difference in chronotropic response between subjects with and without evidence of sinus node dysfunction.

Spectral analysis of blood pressure variability in heart transplant patients

The cardiac transplant patient provides a unique model for the study of blood pressure variability in the absence of heart rate variability. We examined the harmonic and fractal components of blood pressure variability in 14 heart transplant patients (12 men, 2 women; 21 to 62 years of age) and in age-and sex-matched control subjects during seated rest, supine rest, and supine rest with fixed-pace breathing (12 respirations per minute). Heart rate was faster in transplant patients than in control subjects, with much less heart rate variability (P < .0001). Spectral analysis of blood pressure variability revealed no difference in total power for either systolic or diastolic pressure, but transplant patients had less low-frequency (0 to 0.15 Hz) harmonic spectral power in both systolic (P < .01) and diastolic (P < .03) pressure and more high-frequency power (0.15 to 0.5 Hz) in diastolic pressure than control subjects. The ratio of high-frequency power in diastolic relative to systolic pressure was consistently higher (P < .0001) in the transplant patients (0.29 to 0.51) than in control subjects (0.11 to 0.13). The slope of the fractal component of systolic pressure was approximately 1.8 in both transplant patients and control subjects. This was greater than the slope for heart rate variability (approximately 1.1 in control subjects). These data provide clear evidence of independence of the fractal component of heart rate and blood pressure variabilities in both transplant patients and control subjects. The heart rate component of the arterial baroreflex minimized high-frequency diastolic pressure changes while contributing to low-frequency variations in both systolic and diastolic pressures.

Iodine-123 metaiodobenzylguanidine scintigraphic assessment of the transplanted human heart: evidence for late reinnervation

OBJECTIVES

This study attempted to determine whether cardiac sympathetic reinnervation occurs late after orthotopic heart transplantation.

BACKGROUND

Metaiodobenzylguanidine (MIBG) is taken up by myocardial sympathetic nerves. Iodine-123 (I-123) MIBG cardiac uptake reflects intact myocardial sympathetic innervation of the heart. Cardiac transplant recipients do not demonstrate I-123 MIBG cardiac uptake when studied < 6 months from transplantation. However, physiologic and biochemical studies suggest that sympathetic reinnervation of the heart can occur > 1 year after transplantation.

METHODS

We performed serial cardiac I-123 MIBG imaging in 23 cardiac transplant recipients early (< or = 1 year) and late (> 1 year) after operation. In 16 subjects transmyocardial norepinephrine release was measured late after transplantation.

RESULTS

No subject had visible I-123 MIBG uptake on imaging < 1 year after transplantation. However, 11 (48%) of 23 subjects developed visible cardiac I-123 MIBG uptake 1 to 2 years after transplantation. Only 3 (25%) of 12 subjects with a pretransplantation diagnosis of idiopathic cardiomyopathy demonstrated I-123 MIBG uptake compared with 8 (73%) of 11 with a pretransplantation diagnosis of ischemic or rheumatic heart disease (p = 0.04). All 10 subjects with a net myocardial release of norepinephrine had cardiac I-123 MIBG uptake; all 6 subjects without a net release of norepinephrine had no cardiac I-123 MIBG uptake.

CONCLUSIONS

Sympathetic reinnervation of the transplanted human heart can occur > 1 year after operation, as assessed by I-123 MIBG imaging and the transmyocardial release of norepinephrine. Reinnervation is less likely to occur in patients with a pretransplantation diagnosis of idiopathic cardiomyopathy than in those with other etiologies of congestive heart failure.

Electrocardiographic and electrophysiologic properties of cardiac allografts

The increasing number of heart transplant patients requires that physicians be able to recognize the electrocardiographic (ECG) and electrophysiologic properties of cardiac allografts. Cardiac allografts are characterized by modifications of resting ECGs and frequent arrhythmias in the postoperative period, and the loss of autonomic nervous control illustrated by permanent tachycardia and loss of heart rate variability during 24-hour ambulatory ECG recording. Some clinical and experimental observations suggest a mid-term reinnervation of the cardiac allograft, but this requires histologic confirmation. The electrophysiologic characteristics of the denervated myocardium are similar to those of the innervated myocardium at rest. However, supersensitivity to circulating catecholamines has been observed in cardiac allografts as in experimentally denervated hearts, which is responsible for a progressive increase in heart rate during exercise and a slow decrease during recovery. Supersensitivity of the denervated heart to acetylcholine may explain the high prevalence of donor sinus dysfunction due to impairment of its automaticity. More often, the sinus node dysfunction is transient and can be treated with an adenosine antagonist, such as theophylline, before permanent implantation of a pacemaker. In the case of pacemaker implantation, synchronization of the donor atria with the recipient atria is desirable, and an endocardial lead implantation is preferred. Several electrophysiologic changes have been observed during acute cardiac allograft rejection. From experimental studies, the most important of these are the disturbance of conduction in the atria and the atrioventricular node and a decrease in the amplitude of the ventricular potential. Initial studies on isolated myocytes show profound changes in membrane conductance during experimental cardiac rejection. The development of new noninvasive detection methods of cardiac allograft rejection, such as intramyocardial voltage electrogram monitoring and high-resolution ECG, could help early diagnosis.

Central and peripheral limitations to upright exercise in untrained cardiac transplant recipients

BACKGROUND

Functional capacity and quality of life are subjectively improved after cardiac transplantation. However, the objective improvement in exercise tolerance after transplantation has been disappointing. The extent to which allograft diastolic dysfunction contributes to this exercise intolerance has not been defined.

METHODS AND RESULTS

Thirty cardiac transplant recipients between 3 and 16 months after transplantation and 30 age-matched normal control subjects underwent maximal symptom-limited graded upright bicycle exercise testing with simultaneous radionuclide angiography, invasive hemodynamic monitoring, and breath-by-breath gas analysis. Mean blood pressure was higher in the transplant group at supine rest (112.1 versus 97.7 mm Hg), normalized with upright posture, and became lower than normal at peak exercise (121.1 versus 133.2 mm Hg). Systolic function as measured by ejection fraction was normal in both groups. However, the cardiac transplant recipients had significantly lower exercise tolerance, achieving a mean maximal work rate of 390 kilopond-meters per minute (kpm/min), compared with 825 kpm/min in the normal subjects. Peak oxygen consumption was 12.3 mL.min-1.kg-1 in the transplant group, 46% lower than the normal group's value of 22.9 mL.min-1.kg-1. The transplant patients had a resting tachycardia (94 beats per minute) and a 79% reduction in exercise heart rate reserve compared with normal. Despite this chronotropic incompetence, stroke index response to exercise was consistently lower after transplantation, accounting for a 41% reduction in cardiac index at maximal exercise. The lower stroke index was accompanied by a 32% lower end-diastolic volume index at rest and a 14% lower end-diastolic volume index at peak exercise. Despite the smaller ventricular volumes after transplantation, pulmonary capillary wedge pressure was 35% higher than normal at supine rest and 50% higher at maximal exercise. Right atrial and mean pulmonary arterial pressures were similarly elevated. The ratio of pulmonary capillary wedge pressure to end-diastolic volume index was significantly higher during the postural change and exercise, suggesting allograft diastolic dysfunction. Arteriovenous oxygen difference was similar between groups at rest and with submaximal exercise but was 24% lower at maximal exercise in the transplant group, suggesting an abnormality in peripheral oxygen uptake or utilization.

CONCLUSIONS

Exercise tolerance is severely limited during the first 16 months after cardiac transplantation despite preservation of allograft left ventricular systolic function. This intolerance is due to an inadequate cardiac index response from a combination of chronotropic incompetence and diastolic dysfunction limiting the appropriate compensatory use of the Starling mechanism. In addition, there is a peripheral abnormality in oxygen transport or utilization that may partially reflect the effects of deconditioning.

Heterogeneity of the human myocardial sympathetic innervation: in vivo demonstration by iodine 123-labeled meta-iodobenzylguanidine scintigraphy

The normal pattern of the myocardial sympathetic innervation was studied in 15 subjects using single photon emission computed tomography (SPECT) gamma scintigraphy with iodine 123-labeled meta-iodobenzylguanidine (I123MIBG). Seven young subjects (mean age 29.4 +/- 7.5 years SD) with supraventricular tachycardia and eight older patients (mean age 53.0 +/- 5.1 years SD) with normal coronary arteries at cardiac catheterization and normal thallium-201 scintigrams were studied. MIBG uptake in the hearts of six patients with complete cardiac denervation after orthotopic cardiac transplantation was also studied. MIBG scintigrams were reconstructed into bull's-eye target plots and divided into eight equal sectors. Within each sector, four areas representing the apical, two midventricular, and basal regions were defined. There was a reduction in counts in the older group of subjects with normal coronary anatomy (1218.2 +/- 198.4) as compared with younger subjects with supraventricular tachycardia (1124.4 +/- 317.6), (F = 15.0, df = 1, p < 0.001). The difference was lost after adjustment for age (p = 0.2) by means of analysis of covariance. There was a difference in counts within different sectors of the scan (F = 5.7, df = 7, p < 0.001), with lateral and anterior sectors having higher counts than septal and inferior sectors. There was no difference in the counts within areas of the scan (F = 0.04, df = 3, not significant [NS]) or different areas within the sectors (F = 1.1, df = 21, NS). A small diminution of counts (approximately 10%) in the 10 o'clock region of the bull's-eye target plot was observed in some scans.(ABSTRACT TRUNCATED AT 250 WORDS)

Heart rate variability in patients with orthotopic heart transplantation: long-term follow-up

To evaluate heart rate variability (expressed as the standard deviation of RR intervals) within 5 years of follow-up, we studied 20 patients (14 males, 6 females, mean age 44 +/- 12 years) who underwent orthotopic heart transplantation. Six measurements were taken: one in the first 3 weeks after transplantation, and the others once annually, for 5 years. Twenty healthy subjects (mean age 44 +/- 7 years) constituted the control group. Heart rate variability increased significantly in the first 3 years of follow-up (7.2 +/- 1 vs. 11.1 +/- 4, p < 0.001; 11.1 +/- 4 vs. 15.2 +/- 4, p < 0.01; 15.2 +/- 4 vs. 18.9 +/- 5, p < 0.05); in the following years this trend slackened and values did not reach a statistically significant difference (18.9 +/- 5 vs. 21.4 +/- 5; 21.4 +/- 5 vs. 22.5 +/- 5). The mean standard deviation was invariably greater in the control group (63.6 +/- 12). These findings show that sinus rhythm variability in the denervated heart progressively increased over 5 years of follow-up. The absence of presynaptic uptake, which is responsible for adrenergic hypersensitivity to circulating catecholamines and intrinsic cardiac reflexes, does not appear to cause this phenomenon, since these mechanisms are not able to evolve in time after cardiac transplantation. Therefore, an enhanced beta-adrenergic receptors density or affinity to circulating catecholamines or a limited sympathetic reinnervation may be the more probable underlying mechanism.

Immediate cardiovascular responses to orthostasis in the early and late months after cardiac transplantation

Immediate post-standing (< 30 s) heart rate and blood pressure regulation was studied in patients in the early (2 +/- 2 months, n = 10) and late months (49 +/- 18 months, n = 30) after orthotopic heart or heart and lung transplantation with continuous non-invasive blood pressure (Finapress) and ECG recordings, and was compared to 15 healthy subjects. Heart rate acceleration on standing was standing was absent in the early post-transplantation period. Modest, delayed heart rate acceleration (maximum 12 +/- 8 beats/min) was seen late post-transplantation. Subgroup analysis showed that 15 patients late post-transplantation had limited (maximum 6 +/- 3 beats/min) heart rate acceleration, 11 patients showed maximum heart rate acceleration between 11 and 19 beats/min and 4 patients showed heart rate acceleration comparable in magnitude with that of normal subjects (maximum 28 +/- 5 beats/min). The blood pressure transients were comparable in the 3 groups, with a tendency for greater drop and smaller overshoot in systolic blood pressure in transplant subjects. The findings of normal blood pressure transients in the setting of extensive afferent cardiac denervation questions the role of intracardiac (intraventricular) receptors in reflex blood pressure regulation. The development of heart rate responsiveness is compatible with sympathetic reinnervation in many patients in the late post-transplantation period; however, an intrinsic cardiac mechanism may also be possible.

Normalization of upright exercise hemodynamics and improved exercise capacity one year after orthotopic cardiac transplantation

The mechanisms of improved functional capacity over the first year after cardiac transplantation are not well studied. To assess the contribution of cardiac changes to this improvement, the serial evolution of upright rest and exercise hemodynamics during graded upright bicycle exercise was studied in 17 patients at 3 and 12 months after heart transplantation. Heart rate responsiveness, reflected by rapid heart rate acceleration on sitting and rapid deceleration after exercise, developed in the first year. Pulmonary capillary wedge pressure was lower at 1 year, both at rest and at peak exercise (10 +/- 3 vs 13 +/- 5 mm Hg at rest supine and 14 +/- 6 vs 18 +/- 8 mm Hg at peak exercise, p less than 0.05). Similarly, right atrial pressures were also significantly lower at 1 year (4 +/- 2 vs 6 +/- 3 mm Hg at rest supine and 6 +/- 5 vs 11 +/- 5 mm Hg at peak exercise, p less than 0.05). Cardiac index at peak exercise was greater at 12 months (6.4 +/- 1.3 vs 5.8 +/- 0.8 liters/min/m2, p less than 0.05), mediated primarily by higher exercise heart rate (135 +/- 16 vs 125 +/- 12 beats/min, p less than 0.05). In the first year after heart transplantation, improved rest and exercise hemodynamics and heart rate responsiveness contribute significantly to the improved functional capacity observed in these patients.