CLINICAL STUDY: HEART FAILURE
Exercise performance in patients with end-stage heart failure after implantation of a left ventricular assist device and after heart transplantation
An outlook for permanent assisting?
Nicolaas de Jonge, MDa,
Hans Kirkels, MD, PhDa,
Jaap R. Lahpor, MD, PhD ,
Corinne Klöpping, MDa,
Erik J. Hulzebos, MSc ,
Aart Brutel de la Rivière, MD, PhD and
Etienne O. Robles de Medina, MD, PhD, FACCa
a Heart Failure and Heart Transplantation Unit, University Medical Center Utrecht, Utrecht, The Netherlands
Department of Cardiothoracic Surgery, University Medical Center Utrecht, Utrecht, The Netherlands
Department of Physical Therapy, Heart Lung Center Utrecht, University Medical Center Utrecht, Utrecht, The Netherlands
Manuscript received June 1, 2000;
revised manuscript received February 13, 2001,
accepted February 26, 2001.
Reprint requests and correspondence: Dr. Nicolaas de Jonge, Heart Failure and Heart Transplantation Unit, Heart Lung Center Utrecht, University Medical Center, P.O. Box 85500, 3508 GA Utrecht, The Netherlands
n.dejonge{at}azu.nl
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Abstract
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OBJECTIVES
We sought to study exercise capacity at different points in time after left ventricular assist device (LVAD) implantation and subsequent heart transplantation (HTx).
BACKGROUND
The lack of donor organs warrants alternatives for transplantation.
METHODS
Repeat treadmill testing with respiratory gas analysis was performed in 15 men with a LVAD. Four groups of data are presented. In group A (n = 10), the exercise capacities at 8 weeks and 12 weeks after LVAD implantation were compared. In group B (n = 15), the data at 12 weeks are presented in more detail. In group C (n = 9), sequential analysis of exercise capacity was performed at 12 weeks after LVAD implantation and at 12 weeks and one year after HTx. In group D, exercise performance one year after HTx in patients with (n = 10) and without (n = 20) a previous assist device was compared.
RESULTS
In group A, peak oxygen consumption ( O2) increased from 21.3 ± 3.8 to 24.2 ± 4.8 ml/kg body weight per min (p < 0.003), accompanied by a decrease in peak minute ventilation/carbon dioxide production (E/CO2) (39.4 ± 10.1 to 36.3 ± 8.2; p < 0.03). In group B, peak O2 12 weeks after LVAD implantation was 23.0 ± 4.4 ml/kg per min. In group C, levels of peak O2 12 weeks after LVAD implantation and 12 weeks and one year after HTx were comparable (22.8 ± 5.3, 24.6 ± 3.3 and 26.2 ± 3.8 ml/kg per min, respectively; p = NS). In group D, there appeared to be no difference in percent predicted peak O2 in patients with or without a previous LVAD (68 ± 13% vs. 74 ± 15%; p < 0.37), although, because of the small numbers, the power of this comparison is limited (0.45 to detect a difference of 10%).
CONCLUSIONS
Exercise capacity in patients with a LVAD increases over time; 12 weeks after LVAD implantation, O2 is comparable to that at 12 weeks and one year after HTx. Previous LVAD implantation does not seem to adversely affect exercise capacity after HTx.
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Abbreviations and Acronyms
| | AT | = anaerobic threshold | | CI | = confidence interval | | HTx | = heart transplantation | | LVAD | = left ventricular assist device | | NYHA | = New York Heart Association | | RQ | = respiratory exchange ratio (CO2/O2) | | E | = minute ventilation | | CO2 | = carbon dioxide production | | O2 | = oxygen consumption |
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Aging of the population and prevention of premature death from ischemic heart disease by improved medical and surgical management have led to a growing incidence of heart failure. Improved medical therapy for chronic heart failure has further reduced mortality, thereby expanding the number of patients with heart failure (1). Heart transplantation (HTx) is the ultimate treatment option for end-stage heart failure. However, due to a shortage of donor hearts, this therapy can be offered to only a very small number of patients. Therefore, mechanical circulatory support of the failing heart has been suggested as an alternative to HTx. To date, two types of internal left ventricular assist devices (LVADs) are available: the TCI HeartMate (Thermo Cardiosystems Inc., Woburn, Massachusetts) and the Novacor (World Heart Corporation, Ottawa, Canada), both mainly used as a bridge to transplantation. In selected patients, the results are excellent, with  75% successful bridging to transplantation and LVAD support time lasting up to three to four years (2–4). Based on the growing experience with these devices, it seems realistic to use the LVAD as an alternative to HTx or long-term bridging (in terms of years) to transplantation in youngsters.
One of the prerequisites for permanent or semi-permanent assisting is that exercise performance, although not normal, should be adequate for daily life activities, as is the case after HTx. Until now, few studies are available evaluating exercise capacity after LVAD implantation (5–10). Most studies are based on small numbers of patients from different institutions, with divergent postoperative care, rehabilitation programs and exercise testing protocols, which may limit direct comparison of the results. Only one study compares exercise performance in patients after LVAD implantation and following HTx (6). Data on the ventilatory response to exercise and the progress of exercise performance after long-term LVAD implantation have not yet been reported. Therefore, we performed repeated exercise testing with respiratory gas analysis in a cohort of consecutive patients with end-stage heart failure with an assist device and after HTx. To find an effect of LVAD implantation on exercise performance after HTx, a comparison was made between a group of post-transplant patients with and without a previous LVAD.
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Methods
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Patients with a LVAD.
Fifteen consecutive male patients (age 37 ± 12 years [mean ± SD]) with refractory end-stage heart failure treated in our center with a pneumatic LVAD (TCI HeartMate) were included in the study (Table 1). All patients were in a critical hemodynamic condition, with a mean ejection fraction of 13 ± 5%, cardiac output 3.2 ± 0.5 liter/min and a mean arterial pressure of 62 ± 9 mm Hg, despite high doses of inotropic support and sometimes intra-aortic balloon counterpulsation. All patients were eligible as transplant candidates or were already on the waiting list. After LVAD implantation, patients could be mobilized at an early stage and participated in an intensive rehabilitation program supervised by a physical therapist, aimed at dynamic exercise, strength and endurance (see Appendix).
Heart transplant patients.
A cohort of 20 consecutive male heart transplant recipients (mean age 52 ± 10 years) without a previous HeartMate was used for comparison. All patients were in New York Heart Association (NYHA) functional class I.
Study design.
Maximal exercise capacity 8 weeks after LVAD implantation was compared with that 12 weeks after implantation (group A, n = 10). All patients performed an exercise test 12 weeks after implantation (group B, n = 15). To support the hypothesis that exercise performance, with an assist device, is sufficient and comparable to that after HTx, we performed a sequential analysis of exercise capacity 12 weeks after LVAD implantation, as well as 12 weeks and one year after HTx in the same patients (group C, n = 9). Finally, exercise capacity one year after HTx in the post-LVAD group was compared to that in a cohort of transplant patients without a previous assist device (group D).
Exercise studies.
All patients underwent treadmill exercise testing using a 2-min staged modified Naughton protocol or a modified Bruce protocol. Continuous breath-by-breath respiratory gas analysis (Oxycon, Jaeger Inc., Breda, Netherlands) was performed. Measurements included heart rate, blood pressure, oxygen consumption (O2), carbon dioxide production (CO2), minute ventilation (E) and respiratory exchange ratio  . Peak O2 was defined as the average O2 during the last minute of exercise and is expressed as ml/kg body weight per min, as well as ml/min. In addition, to correct for differences in age among the study groups, the percentage of the predicted values was calculated according to Jones (11,12). Oxygen consumption at the anaerobic threshold (AT) was identified as the oxygen uptake before the systematic increase in the ventilatory equivalent for oxygen (E/O2), without a concomitant increase in the ventilatory equivalent for carbon dioxide (E/CO2), together with the -slope method (13,14). The ventilatory response to exercise was defined as E/CO2 at peak exercise (15). All tests for patients with a LVAD were performed in the automatic mode of the device, allowing LVAD output to follow an increase of venous return.
Statistical analysis.
Data are presented as the mean ± SD. Statistical analysis was performed with two-tailed paired and unpaired Student t tests, as appropriate. For the sequential analysis of the three exercise tests in the same patients, repeated measures analysis of variance (ANOVA) was used. If relevant, 95% confidence intervals (CIs) of differences were calculated. A p value <0.05 was considered statistically significant. All analyses were performed using SPSS version 8.0 for Windows.
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Results
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All 15 patients with a LVAD completed exercise testing 12 weeks after implantation. In 10 of these patients, a test at eight weeks after implantation was available. There were two late deaths: one due to device failure 133 days after implantation and the other due to repeated cerebral embolism 432 days after implantation. Thirteen patients (87%) were successfully bridged to HTx after a mean duration of support of 181 ± 125 days (range 71 to 455 days). Early after HTx, three patients died: one patient died from peri-operative bleeding and two died from intractable infections. One patient was not fit to perform an exercise test 12 weeks after HTx due to pneumonia at that time. Thus, 9 patients completed exercise testing 12 weeks after LVAD implantation, as well as 12 weeks and one year after HTx (Fig. 1). Before LVAD implantation, all 15 patients were in NYHA functional class IV. After LVAD implantation, all patients demonstrated an impressive improvement in general well-being and complete recovery of organ function. Eventually, all cardiac medications could be stopped, even when previous right ventricular failure in the LVAD group was present. Only aspirin, 80 mg/day, was given to prevent thromboembolic complications.
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Figure 1 Flow diagram of patients with a left ventricular assist device (LVAD) and exercise tests. HTx = heart transplantation.
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Group A: Comparison of exercise capacity 8 and 12 weeks after LVAD implantation (table 2).
Peak O2 increased from 21.3 ± 3.8 at 8 weeks to 24.2 ± 4.8 ml/kg per min at 12 weeks after implantation (p < 0.003; 95% CI of the difference: 4.7 to 1.3). Furthermore, the ventilatory response to exercise (E/CO2) decreased from 39.4 ± 10.1 at 8 weeks to 36.3 ± 8.2 at 12 weeks after implantation (p < 0.03; 95% CI of the difference: 0.4 to 5.8).
Group B: Exercise performance 12 weeks after LVAD implantation (Table 3).
Twelve weeks after LVAD implantation, all patients were in NYHA functional class I (apart from the limitations imposed by the drive lines and the console of the device). Peak O2 averaged 23.0 ± 4.4 ml/kg per min (range 17.2 to 32.1), or 58 ± 9% of the predicted value (range 43% to 80%), sufficient for normal acitivities of daily life (14). There was no difference in peak O2 between patients with ischemic heart disease (n = 7) and those with dilated cardiomyopathy (n = 8) (21.6 ± 2.6 vs. 24.2 ± 5.4 ml/kg per min).
Group C: Sequential analysis of exercise performance in 9 patients 12 weeks after LVAD implantation and then at 12 weeks and one year after HTx (Table 4, Fig. 2).
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Table 4 Sequential Analysis of Exercise Performance in Nine Patients at 12 Weeks After Implantation and Then at 12 Weeks and One Year After HTx
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In this group, three exercise tests were compared by repeated measures ANOVA. Peak O2 demonstrated some improvement, although statistically insignificant: 22.8 ± 5.3, 24.6 ± 3.3 and 26.2 ± 3.8 ml/kg per min, respectively (p = 0.26; 95% CI of difference between 12 weeks after LVAD and 12 weeks after HTx: –1.91 to 5.47). Expressed as a percentage of the predicted value, these were 58 ± 12%, 63 ± 10% and 69 ± 13%, respectively (p = NS). The patients’ AT increased from 14.4 ± 4.0 ml/kg per min with a LVAD to 15.9 ± 3.3 and 18.7 ± 2.5 ml/kg per min 12 weeks and one year after HTx, respectively (p = NS). The ventilatory responses (E/CO2 at peak exercise) 12 weeks after LVAD implantation and 12 weeks and one year after HTx were not different (37.2 ± 7.8, 33.0 ± 4.4 and 33.7 ± 4.7, respectively).
Group D: Exercise capacity one year after HTx with and without a previous LVAD.
Because of the large difference in the mean age between both groups, these data were only analyzed as a percentage of predicted peak O2. Without a previous assist device (n = 20), peak O2 one year after HTx was 74 ± 15% versus 68 ± 13% of the predicted value in patients with a previous LVAD (n = 10; p = 0.37 [NS]; 45% power to detect a difference of 10% with a two-sided significance of 0.05).
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Discussion
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The results of this study suggest that exercise performance after implantation of a TCI HeartMate LVAD in a group of patients with end-stage heart failure, who underwent intensive postoperative rehabilitation, is sufficient for activities of normal daily life. An increase in maximal exercise performance from 8 to 12 weeks could be demonstrated (Table 2), most likely due to postoperative convalescence and systematic strenuous training. One may speculate that longer assist times will result in an even better exercise performance. This improvement in exercise performance over time was accompanied by a significant decrease in the ventilatory response to exercise (E/CO2). This variable is often increased in patients with severe heart failure and is considered an independent and even better prognostic marker than reduced peak O2 (15,16). Twelve weeks after implantation, peak O2 averages 23.0 ml/kg per min, compatible with Weber class A (17). Sequential analysis of exercise performance in nine patients at 12 weeks after LVAD implantation and then at 12 weeks and one year after HTx demonstrated similar peak O2, AT and E/CO2 values. Therefore, apart from the limitations imposed by the operating console and the drive line of the HeartMate, activities with placement of a LVAD are expected to be comparable to those after HTx.
Furthermore, exercise performance of patients one year after HTx with a previous LVAD approximates that of post-transplant patients without a previous LVAD. This indicates that the impact of a pre-transplant LVAD, per se, implying a thoracotomy and laparotomy with a diaphragmatic incision through which the inlet conduit of the HeartMate is passed, together with the potential detrimental effects of cardiopulmonary bypass, does not appear to lead to late consequences in terms of diminished exercise capacity. However, the presence of a LVAD certainly complicates the transplant procedure and may lead to excessive bleeding. This may have contributed to the peri-operative death of one of the patients. In contrast, the excellent general condition before HTx, in all probability, is an advantage for post-transplant rehabilitation, although this could not be translated in terms of a shorter hospital stay after HTx.
Comparison with previous studies.
Our results seem to diverge from those of other studies on this subject. In the Experience with left Ventricular Assist Device with Exercise (EVADE) trial (6), peak O2 one to three months after LVAD implantation was reported to be 14.5 ± 3.9 ml/kg per min, compared with 17.5 ± 5.0 ml/kg per min after HTx. Mancini et al. (7) compared exercise performance in 20 patients with a LVAD with that of heart failure patients without mechanical support. Peak O2 in the LVAD group was 16.0 ± 3.8 ml/kg per min.
Several factors may account for these differences. The EVADE study is a multi-center study comprising 18 patients. Because it was a multi-center study, postoperative care, rehabilitation and exercise testing may not have been identical. Exercise testing was performed earlier after implantation (51.9 ± 20.4 days), when peak O2 was not yet optimal (Table 2). Furthermore, the mean age and weight of our patients were considerably lower than those in EVADE (37 ± 12 vs. 48 ± 12 years and 71 ± 8 vs. 76 ± 12 kg, respectively), resulting in a higher predicted peak O2 (39.6 vs. 33.6 ml/min per kg) and a higher normalized O2 per kilogram. Most of the aforementioned differences from the EVADE study also apply to the study of Mancini et al. (7). Furthermore, in that study, exercise testing was by way of a bicycle ergometer instead of a treadmill, as used in our study, which may render 10% to 25% lower values of peak O2 (18). However, several small-scale studies in which exercise performance was measured by way of a treadmill demonstrated much lower peak O2 values (12 to 17 ml/kg per min) than those found in our study (8–10). Despite these differences in study design and patient characteristics, we believe that part of the excellent exercise performance of our patients with a LVAD is due to the intensive post-implantation training program.
Effect of training in heart failure.
The influence of training on exercise capacity may be inferred from evidence showing that impaired exercise performance in patients with chronic heart failure is related not only to an impaired hemodynamic condition, but also to skeletal muscle abnormalities (19–21) like atrophy (22), as well as alterations in muscle histology and biochemistry (23). Sullivan et al. (24) demonstrated, for the first time, that training could improve exercise capacity in patients with severe heart failure. Later, many other studies confirmed this, as reviewed by Piepoli et al. (25). Several studies using muscle biopsies or nuclear magnetic resonance spectroscopy confirmed the improvement in muscle metabolism after training, independent of central hemodynamic changes (26–29). These data demonstrate that in patients with chronic heart failure, despite a limited cardiac output response, an intensive training program can improve exercise capacity significantly.
After HeartMate implantation, total cardiac output will increase significantly, but pump flow is limited to 10 liters/min. At rest, there is usually no contribution of the native heart; however, during exercise, opening of the aortic valve has been observed, contributing to increased total blood flow. Based on a flow of 10 liters/min, a male patient should be able to reach a peak O2 of 1,400 ml/min, if he has a normal hemoglobin and oxygen extraction. Because this value is similar to that found in our study, we assume that exercise performance was optimal for the given blood flow. From the onset of our LVAD program, our policy was aimed at the reversal of skeletal muscle abnormalities by implementation of an intensive training program guided by a physical therapist. The combination of an increase in cardiac output due to LVAD implantation and intensive post-implantation training likely resulted in optimal exercise capacity.
Study limitations.
Due to inherent, considerable mortality and morbidity during the course of LVAD implantation and after successive HTx, only a limited number of patients was available for a complete longitudinal study of exercise performance (n = 9, group C). Therefore, additional analyses were performed on the larger data sets of patients at different time points after LVAD implantation (group A, n = 10) and one year after HTx with and without a previous LVAD (group D, n = 10 and n = 20, respectively). The relatively young age of the patients with a LVAD limits the value of direct comparison of O2 values with those of post-HTx patients without previous LVAD implantation. This can be partly overcome by using a percentage of individually predicted values. Furthermore, as in all small studies, an insignificant difference between two groups will have limited power. The patients in this study were relatively young, very motivated and male, which may have skewed the results and limited generalization to the whole population of patients with end-stage heart failure.
Conclusions.
This study demonstrates that exercise performance in patients with severe heart failure treated with a TCI HeartMate LVAD, combined with an intensive rehabilitation program, increases over time and, at 12 weeks, is fully compatible with activities of normal daily life. Twelve weeks after implantation, exercise capacity is comparable to that at three months and one year after HTx. One year after HTx, there appears to be no difference in exercise performance between patients with and those without a previous LVAD as a bridge to transplantation. Therefore, with regard to exercise performance, a permanent LVAD holds promise as a potential alternative for HTx.
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Appendix
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During the first two weeks after implantation of the HeartMate, the aim of training is to get the patient accustomed to exercise. Thereafter, an interval training program is started, with regular adjustment of the duration and intensity of the workout. Initially, the training regimen consists of 2 to 6 min of low-level activities, alternated with 1 to 2 min of rest. Training includes sessions on the bicycle and treadmill, as well as the rowing machine. Coordination is improved by several games, like badminton, tennis and volleyball. Intensity is adjusted according to the level of perceived exertion (2 to 4 corresponding to "light" to "somewhat hard") on the Borg scale of 0 to 10, with exertional dyspnea not exceeding 2 on a dyspnea scale of 0 to 4. Duration of exercise is gradually increased to 20 to 40 min/day three to five times a week. In addition, strength and endurance training of local muscle groups, according to the 5BX plan of the Royal Canadian Air Force (30), is performed.
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Abstract
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