Left ventricular distensibility does not explain impaired exercise capacity in pediatric heart transplant recipients

http://www.jhltonline.org

Left ventricular distensibility does not explain impaired exercise capacity in pediatric heart transplant recipients Luis A. Altamirano-Diaz, MD,a,b Michael D. Nelson, PhD,c Lori J. West, MD,a Nee S. Khoo, MD,a Ivan M. Rebeyka, MD,a and Mark J. Haykowsky, PhDd From the aStollery Children’s Hospital, University of Alberta, Edmonton, Alberta, Canada; bDepartment of Pediatrics, Children’s Hospital, London Health Science Centre, University of Western Ontario, London, Ontario, Canada; cHeart Institute, Cedars-Sinai Medical Center, Los Angeles, California; and the dFaculty of Rehabilitation, University of Alberta & Alberta Cardiovascular and Stroke Research Centre, Mazankowski Alberta Heart Institute, Edmonton, Alberta, Canada.

KEY WORDS: pediatric heart transplant; aerobic capacity; tilt-table positioning; diastolic dysfunction; left ventricular distensibility

BACKGROUND: Despite improved ventricular function after heart transplantation, the aerobic capacity, as measured by peak oxygen consumption (VO2peak) of pediatric heart transplant recipients (HTRs), remains 30% to 50% lower than age-matched healthy individuals. Research in adult HTRs suggests that diastolic dysfunction is a major determinant of exercise intolerance; however, it is unknown whether the impaired VO2peak in younger HTRs is due to reduced left ventricular (LV) distensibility. METHODS: Eight HTRs (mean age, 15 years; mean time post-transplant, 7 years) and 8 matched healthy controls were studied. To evaluate LV distensibility, echocardiographic measurements of ventricular volumes were obtained in 3 positions: supine, head-up tilt, and head-down tilt. Subsequently, participants underwent exercise stress testing to evaluate VO2peak. RESULTS: As expected, VO2peak was 26% lower in HTRs (p o 0.05). Ventricular volumes in each position were small in HTRs (p ¼ 0.01); however, the percentage change in LV end-diastolic volume indexed (EDVi) to body surface area after the transition from supine to head-up tilt and from head-up tilt to head-down tilt were similar between HTRs (p ¼ 0.956) and controls (p ¼ 0.801). The change in EDVi during the transition from head-up tilt to head-down tilt (LV distensibility) strongly predicted VO2peak in patients (R2 ¼ 0.614, p ¼ 0.021) and controls (R2 ¼ 0.510, p ¼ 0.047). Importantly, the slope of this relationship did not differ between HTRs (1.01) and controls (0.977; p ¼ 0.951). CONCLUSIONS: LV distensibility does not appear to be a major determinant of exercise intolerance in young HTR. J Heart Lung Transplant 2013;32:63–69 r 2013 International Society for Heart and Lung Transplantation. All rights reserved.

Heart transplantation is a lifesaving intervention in children and adolescents with end-stage refractory heart failure. Despite improved ventricular function after surgery, the aerobic capacity, as measured by peak oxygen consumption (VO2peak) of pediatric heart transplant recipiReprint requests: Luis A. Altamirano-Diaz, MD, Rm B1-146, Children’s Hospital, London Health Science Centre, 800 Commissioners Rd East, London, ON, N6A 5W9, Canada. Telephone: 519-685-8500, ext. 56061. Fax: 519-685-8156. E-mail address: [email protected]

ents (HTRs), remains 30% to 50% lower than age-matched healthy individuals.1,2 Prior research in adult HTRs suggests that diastolic dysfunction is a major determinant of exercise intolerance3; however, it is unknown if the impaired VO2peak in younger HTRs is due to reduced left ventricular (LV) distensibility. Passive postural changes by tilt-table positioning can non-invasively produce a relatively wide range of alterations in venous return and has been successfully used to evaluate LV distensibility in healthy individuals and cardiac patients.4–6 We hypothesized that pediatric HTRs would

1053-2498/$ - see front matter r 2013 International Society for Heart and Lung Transplantation. All rights reserved. http://dx.doi.org/10.1016/j.healun.2012.09.029

64

The Journal of Heart and Lung Transplantation, Vol 32, No 1, January 2013

have reduced changes in end-diastolic volume in response to cardiac loading and unloading during tilt-table positioning as a result of impaired LV distensibility.

Material and methods Ethics approval for this study was obtained from the University of Alberta Institutional Review Board, and all participants provided written informed consent.

Patients Eight HTRs with normal global systolic function (ejection fraction 4 55%) were recruited from the heart transplant clinic at the Stollery Children’s Hospital and the University of Alberta Hospital. All patients were free of rejection at the time of the study. One patient had a history of significant rejection (International Society for Heart and Lung Transplantation [ISHLT] 3R) more than 6 months before the study. No other patients had a history of significant rejection (clinically or by biopsy). One patient had mild coronary allograft vasculopathy (ISHLT CAV1) using the ISHLT recommended nomenclature.7 Eight healthy controls, matched for age and sex, were recruited from the community. All participants were asked to arrive well hydrated and nourished and to avoid caffeine and vigorous exercise for at least 24 hours.

Echocardiographic assessment Participants were evaluated in 3 postures, in the following order: (1) supine (01), (2) 301 head-up tilt (HUT), after 5 minutes of equilibration, and (3) 301 head-down tilt (HDT), after 5 minutes of equilibration. Heart rate was monitored continuously, and blood pressure was measured in each posture after equilibration using an automated blood pressure device. LV chamber quantification and systolic and diastolic function were assessed by transthoracic 2dimensional (2D) echocardiography and Doppler ultrasound imaging using commercially available ultrasound machines (Vivid 7 or Vivid Q; GE Medical Systems, Milwaukee, WI), as previously described.8 LV volumes were calculated and averaged over 3 cardiac cycles, according to the area-length method [5/6  LV cavity area (cm2)  LV length (cm)], in accordance with the current recommendations for quantification of LV volumes by 2D echocardiography.9 Pulsed Doppler was used to quantify early and late diastolic inflow velocities. Tissue Doppler was used to assess mitral valve lateral annular tissue velocities during systole, early diastole, and late diastole, reported as an average of at least 3 cardiac cycles. Mean arterial blood pressure was calculated as pulse pressure divided by 3, plus diastolic blood pressure. Cardiac output was calculated as LV end-diastolic volume (LVEDV) minus LV end-systolic volume (LVESV) multiplied by heart rate. Systemic vascular resistance was calculated as mean arterial pressure divided by cardiac output.10

individual prevented the use of the cycle ergometer; therefore, the HTR and the respective control were tested on a motorized treadmill. Workload progressively increased every 2 minutes until ventilatory threshold, after which workload increased every minute until volitional exhaustion. Ventilatory threshold was defined as having a respiratory exchange ratio (RER) 4 1.0 and a systematic rise in the ventilatory equivalent of Co2 (VE/VCO2) of O2 (VE/VO2), while the ventilatory equivalent of CO2 remained constant or declined slightly.11 Respiratory gas exchange was measured using a commercially available metabolic gas analyzer (Vmax Encore VIASYS Healthcare Inc, Yorba Linda, CA). VO2peak was determined from the highest 30-second average. Criteria for reaching VO2peak were a plateau in VO2, RER 4 1.10, and rating of perceived exertion equal to 20, or achievement of age-predicted maximal heart rate. All participants reached VO2peak by at least 2 of these criteria.

Statistical analysis Comparisons between groups were made using a Student’s t-test. A 2-way repeated-measures analysis of variance was used to compare the response to tilt-table positioning. Where main effects were found, Holm-Sidak post hoc comparisons were made to define discrete differences. To assess the relationship between peak aerobic capacity and LV distensibility, linear regression analysis was performed, relating the change in end-diastolic volume during the transition from HUT to HDT (extreme of volume loading) to VO2peak for both groups. The difference in the slope of the linear regression between groups was compared using Student’s t-test analysis. Data are reported as means ⫾ standard error, unless otherwise specified. The a-level for all statistical analysis was set at 0.05.

Results Participant characteristics The median age at heart transplant was 7 years, and the average time to study participation after heart transplant was 7 years (Table 1). Peak heart rate and heart rate reserve were lower in HTR (p ¼ 0.06) than in controls (p o 0.01, Table 2). As expected, VO2peak was 26% lower in HTR than in controls (p o 0.05, Table 2). HTRs and controls provided maximal effort during exercise stress testing, achieving a RER 4 1.10 (Table 2). Peak ventilation tended to be reduced in HTRs compared with controls, owing to a lower tidal volume, although these differences did not reach statistical significance (Table 2). At supine rest, HTRs and controls had similar LV ejection fraction, septal and posterior wall thickness, and LV mass (Table 1). Heart rate was higher in patients than in controls, with no differences in arterial blood pressure (Table 1). HTRs had reduced lateral annular systolic tissue velocities (Table 1). Diastolic function was normal in both groups (Table 1).

Assessment of aerobic capacity Aerobic capacity was assessed using a progressive cardiopulmonary graded exercise test to volitional exhaustion, designed to elicit a maximal response in 8 to 12 minutes. Participants were tested on an electrically braked cycle ergometer (Monark 894E, Varberg, Sweden), with the first stage beginning at 10 to 50 W, depending on each individual’s age, height, body mass, and physical abilities (at the discretion of a highly experienced investigator). The height of one

LV function and hemodynamics in response to tilttable positioning All patients and controls tolerated the tilting procedures well. In HTRs compared with controls, the LVEDV indexed to body surface area (LVEDVi; p o 0.01), LVESVi (p ¼ 0.08), and LV stroke volume index (LVSVi;

Altamirano-Diaz et al.

Pediatric HTX LV Distensibility

Table 1 Variables

65

Characteristics of the Study Population a

HTR (n ¼ 8)

Controls (n ¼ 8)

p-value

15 ⫾ 5 7 (3 days-16 years) 7 (3–20) 4 46.9 ⫾ 20.8 152.4 ⫾ 21.7 1.4 ⫾ 0.4 89 ⫾ 10

15 ⫾ 5 y y 4 52.7 ⫾ 23.1 155.3 ⫾ 20.4 1.5 ⫾ 0.4 70 ⫾ 11

0.949 y y y 0.604 0.787 0.65 o0.01b

102 ⫾ 7 58 ⫾ 6

103 ⫾ 8 54 ⫾ 6

0.731 0.253

0.54 ⫾ 0.09 0.58 ⫾ 0.07 68.7þ 21.8 0.85 ⫾ 0.19 63 ⫾ 8 9.1 ⫾ 2.0 2.48 ⫾ 0.82 3.1 ⫾ 0.7

0.61 ⫾ 0.10 0.57 ⫾ 0.10 87.7 ⫾ 32.4 0.68 ⫾ 0.11 60 ⫾ 6 11.0 ⫾ 1.2 2.34 ⫾ 0.80 3.1 ⫾ 0.8

0.144 0.822 0.191 0.024b 0.487 0.035b 0.737 0.925

Characteristics Age, years (median) Age at transplant Years post-transplant (mean) Female sex Weight, kg Height, cm Body surface area, m2 Heart rate, beats/min Blood pressure, mm Hg Systolic Diastolic Left ventricle Wall thickness, cm Septal Posterior Mass, g Concentricity index Ejection fraction, % Lateral S’, cm/s Mitral E-to-A ratio Lateral E’-to-A’ ratio

HTR, heart transplant recipient. a Continuous data are reported as mean ⫾ standard deviation or median (range). b

Statistically significant.

p o 0.01) were reduced in each posture (Figure 1). CO was similar in HTRs compared with controls at each posture (p ¼ 0.984; Figure 1). During the transition from supine to HUT, HTRs and controls had a similar percentage change in LVEDVi Table 2

(–22.9% ⫾ 5.6% vs –23.4% ⫾ 6.8%; p ¼ 0.956), LVESVi (–11.8% ⫾ 7.3% vs –17.4% ⫾ 9.9%; p ¼ 0.654), LVSVi (–31.2% ⫾ 8.4% vs –27.7% ⫾ 6.2%; p ¼ 0.739), and cardiac output index (–24.0% ⫾ 8.7% vs –22.3% ⫾ 9.7%; p ¼ 0.893). During the transition from HUT to HDT, HTRs

Cardiopulmonary Exercise Testing Results

Variablea

HTR

Controls

p-value

Peak VO2, ml/kg/min Peak VO2, liters/min Predicted VO2, % Peak VCO2, ml/kg/min Peak VCO2, liters/min Peak VE, liters/min Peak Vt, liters Peak RR, breaths/min Peak HR, beats/min HRR, beats/min Peak RER VE/VO2 VE/VCO2 Peak O2 pulse, ml/beat VET, ml/kg/min VET, % VO2peak

32.0 ⫾ 3.1 1.42 ⫾ 0.21 72 ⫾ 17 36.4 ⫾ 3.7 1.60 ⫾ 0.29 62.8 ⫾ 10.2 1.11 ⫾ 0.20 62.1 ⫾ 3.8 171 ⫾ 6 82 ⫾ 20 1.19 ⫾ 0.03 44.0 ⫾ 1.9 40.8 ⫾ 1.9 8.39 ⫾ 1.2 23.18 ⫾ 3.4 71 ⫾ 5

43.4 ⫾ 2.7 2.35 ⫾ 0.45 95 ⫾ 14 41.9 ⫾ 5.9 2.64 ⫾ 0.51 98.7 ⫾ 21.8 1.73 ⫾ 0.39 62.5 ⫾ 5.0 188 ⫾ 6 118 ⫾ 22 1.18 ⫾ 0.0 40.9 ⫾ 3.2 36.4 ⫾ 2.4 12.3 ⫾ 2.2 27.9 ⫾ 2.4 65 ⫾ 5

0.014b 0.086 o0.01b 0.440 0.101 0.158 0.186 0.953 0.060 o0.01b 0.689 0.422 0.182 0.145 0.283 0.411

HR, heart rate; HRR, heart rate reserve; HTR, heart transplant recipient; RER, respiratory exchange ratio; RR, respiratory rate; VCO2, volume of carbon dioxide production; VE, ventilation; VET, ventilatory threshold expressed in VO2 and as a percentage of peak VO2; VO2, volume of oxygen uptake; Vt, tidal volume. a Values presented as mean ⫾ standard error. b Statistically significant.

66

The Journal of Heart and Lung Transplantation, Vol 32, No 1, January 2013

Figure 1 (A) End-diastolic volume index (EDVi), (B) end-systolic volume index (ESVi), (C) stroke volume index (SVi), and (D) cardiac output index (CI) at supine, head-up tilt (HUT) and head-down tilt (HDT) postures for heart transplant recipients (solid markers) vs controls (open markers). Plot of mean values ⫾ standard error. p o 0.05 main effect for posture.

had similar percentage changes compared with controls in LVEDVi (26.5% ⫾ 4.4% vs 25.0% ⫾ 4.1%; p ¼ 0.801), LVESVi (25.7% ⫾ 3.7% vs 13.4% ⫾ 6.2%; p ¼ 0.111), LVSVi (26.6% ⫾ 6.0% vs 30.8% ⫾ 3.9%; p ¼ 0.558), and cardiac output index (21.4% ⫾ 6.1% vs 23.1% ⫾ 5.1%; p ¼ 0.837; Figure 2). Heart rate and blood pressure remained higher in HTRs than in controls throughout postural changes, without significant intergroup differences in diastolic parameters (Table 3). Systolic annular tissue velocity was lower in HTR than in controls throughout tilt-table positioning. As expected, transitioning from supine to HUT slightly increased heart rate and systemic vascular resistance, whereas early mitral inflow and annular tissue velocities tended to decrease (Table 3).

Correlation between LV volumes and exercise capacity We found no relationship between VO2peak and supine LVEDVi in patients (R2 ¼ 0.164, p ¼ 0.320) or controls

(R2 ¼ 0.290, p ¼ 0.169). VO2peak was, however, related to the change in EDVi during the transition from HUT to HDT (LV distensibility) in HTRs (R2 ¼ 0.614, p ¼ 0.021) and controls (R2 ¼ 0.510, p ¼ 0.047; Figure 3). The slope of this relationship did not differ between HTRs (1.01) and controls (0.977; p ¼ 0.951).

Discussion This study used tilt-table positioning to assess the relationship between LV distensibility and exercise tolerance in pediatric and adolescent HTRs. Contrary to findings of prior studies in adult HTRs,3,12 our data in younger HTRs suggest that LV distensibility is not a major limiting factor contributing to reduced VO2peak. The severe and marked excise intolerance in pediatric HTR has previously been attributed to chronotropic incompetence secondary to cardiac allograft denervation. However, we, and others,12 have found that peak heart rate is very close to that of age-predicted values in healthy individuals and only marginally different between HTRs and controls. Indeed, allograft

Altamirano-Diaz et al.

Pediatric HTX LV Distensibility

67

Figure 2 Percent change in (A) end-diastolic volume index (EDVi), (B) end-systolic volume index (ESVi), (C) stroke volume index (SVi), and (D) cardiac output index (CI) in response to postural changes from supine (SUP) to head-up tilt (HUT) and HUT to head-down tilt (HDT) for heart transplant recipients (solid bars) vs controls (open bars). Data reported as mean ⫾ standard error. p o 0.05 between groups.

cardiac reinnervation appears to be age-related13,14 and likely plays a significant role in younger HTRs. In contrast to the traditional chronotropic hypothesis, we postulated that impaired LV distensibility and concomitant decline in stoke volume reserve would be a primary contributor to exercise intolerance in pediatric HTRs. Using non-invasive tilt-table positioning, we show similar LV distensibility in pediatric HTRs compared with controls during cardiac loading and unloading (Figure 2). In addition, peak aerobic capacity was significantly related to LV distensibility in patients and controls (Figure 3), but this relationship did not differ between groups. Therefore, although LV distensibility appears to be an important contributor to aerobic capacity, the impairment in VO2peak could not be explained by differences in LV distensibility. These results are in direct contrast with previous work in adult HTRs. For example, Kao et al3 demonstrated impaired stroke volume reserve secondary to reduced LV distensibility in response to postural change and exercise stress in adult HTRs. It remains unclear why a difference exists

between adult and pediatric HTRs. In adults, recurrent rejection episodes and subsequent fibrosis are believed to augment LV stiffening and diastolic dysfunction. Infant HTRs, however, are known to have better graft ‘‘acceptance’’ because of their immunologic immaturity. The present results may reflect this advantage. Despite similar LV distensibility between patients and controls, peak aerobic capacity was 26% lower in HTRs. One possible explanation for these results may be related to LV development. For example, supine LV volumes were significantly lower in HTRs than in controls. Indeed, reduced LV volumes, coupled with a lower peak heart rate, would ultimately reduce peak exercise cardiac output and subsequent convective oxygen delivery to the active muscles. Alternatively, non–cardiac-related abnormalities, including decreased diffusive oxygen transport, abnormal peripheral oxygen uptake, and/or use, or pulmonary limitations may also have contributed to the reduced aerobic capacity. Indeed, evidence of abnormal skeletal muscle metabolism and impairments in peripheral blood flow regulation

68

The Journal of Heart and Lung Transplantation, Vol 32, No 1, January 2013 Table 3

Hemodynamic and Doppler Ultrasound Variables

Variable a Heart rate, beats/min Supine Head-up tilt Head-down tilt Blood, mm Hg Systolic Supine Head-up tilt Head-down tilt Diastolic Supine Head-up tilt Head-down tilt Mean arterial pressure, mm Hg Supine Head-up tilt Head-down tilt SVRi, mm Hg  L-1  min-1 Supine Head-up tilt Head-down tilt E velocity, cm/sec–1 Supine Head-up tilt Head-down tilt A velocity, cm/sec–1 Supine Head-up tilt Head-down tilt E’ velocity, cm/sec–1 Supine Head-up tilt Head-down tilt A’ velocity, cm/s–1 Supine Head-up tilt Head-down tilt S’ velocity, cm/sec–1 Supine Head-up tilt Head-down tilt

p-value HTR

Controls Posture

Group

89 ⫾ 3 94 ⫾ 4 88 ⫾ 4

70 ⫾ 4 74 ⫾ 5 67 ⫾ 4

o0.001

0.003

102 ⫾ 3 105 ⫾ 3 107 ⫾ 3

103 ⫾ 3 103 ⫾ 3 103 ⫾ 4

0.072

0.705

58 ⫾ 2 63 ⫾ 3 64 ⫾ 2

54 ⫾ 2 55 ⫾ 1 55 ⫾ 2

0.009

0.018

72 ⫾ 2 77 ⫾ 2 79 ⫾ 2

70 ⫾ 2 71 ⫾ 2 71 ⫾ 2

0.004

0.058

23 ⫾ 1 30 ⫾ 2 23 ⫾ 2

23 ⫾ 1 28 ⫾ 3 21 ⫾ 1

o0.001

0.473

83.4 ⫾ 6.9 77.0 ⫾ 6.1 89.3 ⫾ 6.3

83.3 ⫾ 4.6 71.2 ⫾ 4.2 81.6 ⫾ 4.8

0.008

0.517

35.9 ⫾ 3.6 44.9 ⫾ 6.7 39.0 ⫾ 4.7

37.4 ⫾ 2.5 37.4 ⫾ 5.0 36.6 ⫾ 2.8

0.336

0.598

16.5 ⫾ 1.4 13.5 ⫾ 0.9 16.5 ⫾ 1.3

19.4 ⫾ 1.3 17.2 ⫾ 1.2 18.3 ⫾ 1.0

o0.001

0.094

5.3 ⫾ 0.2 5.0 ⫾ 0.3 5.4 ⫾ 0.3

6.5 ⫾ 0.5 5.1 ⫾ 0.5 6.5 ⫾ 0.4

0.005

0.117

9.1 ⫾ 0.7 10.0 ⫾ 0.9 8.1 ⫾ 0.8

11.0 ⫾ 0.4 10.9 ⫾ 0.5 10.6 ⫾ 0.7

0.079

0.047

HTR, heart transplant recipient; SVRi, systemic vascular resistance index. a Values are mean ⫾ standard error. Main effects for posture and group are reported.

have been reported in adult HTRs.15 Likewise, long-term use of immunosuppressive medications could also affect convective and diffusive oxygen transport,16,17 whereas previous sternotomies and scarring could significantly limit pulmonary gas exchange. Lastly, independent of transplantspecific factors contributing to reduced peripheral oxygen utilization, simple deconditioning may also significantly contribute to the reduced aerobic capacity. Although we do not have quantitative evidence, qualitatively we found a general resistance to physical activity in our patient population, originating primarily with the parents. Perhaps

the greatest barrier for these children is ill-advised stereotypes limiting exposure to physical activity. Our study has some limitations. Interpretation of the present results is limited by our small sample size. Nevertheless, despite limited access to an incredibly rare patient population, we are confident that the sample size used was adequate to test our specific hypothesis. Direct measurement of end-diastolic pressure would also have significantly added to the interpretation of our results; however, our patients were not scheduled for cardiac catheterization during the study period, and our control group comprised healthy volunteers without an

Altamirano-Diaz et al.

Pediatric HTX LV Distensibility

69

References

Figure 3 Relationship between the change in end-diastolic volume index (EDVi) in response to transitioning from head-up tilt (HUT) to head-down tilt (HDT) and peak aerobic capacity assessed by volume of oxygen consumption (VO2peak) in heart transplant recipients (solid circle) and healthy controls (open circle). Note that the change in EDVi is significantly related to VO2peak for patients (R2 ¼ 0.614, p ¼ 0.021) and controls (R2 ¼ 0.510, p ¼ 0.047). In addition, note that the relationship (slope) is not different between groups (p ¼ 0.951) but that the controls are shifted to the right, reflecting improved aerobic capacity (þ26%).

indication for cardiac catheterization. In addition, our results point to non–cardiac-related abnormalities. Admittedly, we did not measure peripheral oxygen delivery and/or utilization, nor did we measure pulmonary function because these were not the primary focus of this investigation. Future studies are indeed needed to address this limitation. In conclusion, aerobic capacity in pediatric and adolescent HTRs is diminished compared with agematched healthy controls. LV distensibility, by tilt-table positioning, does not appear to play a significant role. Further research examining the possibility of non-cardiac contributors, such as peripheral oxygen delivery and utilization, is warranted.

Disclosure statement The authors thank the volunteers for their time and dedication and also thank Dr Lisa Hornberger and the echocardiology department at the Stollery Children’s Hospital for their invaluable contribution to this study. M.D. Nelson was funded by the Natural Sciences and Engineering Research Council of Canada. None of the authors has a financial relationship with a commercial entity that has an interest in the subject of the presented manuscript or other conflicts of interest to disclose.

1. Hsu D, Garofano R, Douglas J, et al. Exercise performance after pediatric heart transplantation. Circulation 1993;88:ll238-42. 2. Nixon PA, Fricker FJ, Noyes BE, Webber SA, Orenstein DM, Armitage JM. Exercise testing in pediatric heart, heart-lung, and lung transplant recipients. Chest 1995;107:1328-35. 3. Kao AC, Van Trigt P, Shaeffer-McCall GS, et al. Central and peripheral limitations to upright exercise in untrained cardiac transplant recipients. Circulation 1994;89:2605-15. 4. Stefadouros M, El Shahawy M, Stefadouros F, Witham A. The effect of upright tilt on the volume of the failing human left ventricle. Am Heart J 1975;90:735-43. 5. John JM, Haykowsky M, Brubaker P, Stewart K, Kitzman DW. Decreased left ventricular distensibility in response to postural change in older patients with heart failure and preserved ejection fraction. Am J Physiol Heart Circ Physiol 2010;299:H883-9. 6. Higginbotham MB, Morris KG, Williams RS, McHale PA, Coleman RE, Cobb FR. Regulation of stroke volume during submaximal and maximal upright exercise in normal man. Circ Res 1986;58:281-91. 7. Mehra MR, Crespo-Leiro MG, Dipchand A, et al. International Society for Heart and Lung Transplantation working formulation of a standardized nomenclature for cardiac allograft vasculopathy–2010. J Heart Lung Transplant 2010;29:717-27. 8. Nelson MD, Altamirano-Diaz LA, Petersen SR, et al. Left ventricular systolic and diastolic function during tilt-table positioning and passive heat stress in humans. Am J Physiol Heart Circ Physiol 2011;301: H599-608. 9. Lang RM, Bierig M, Devereux RB, et al. Recommendations for Chamber quantification: a report from the American Society of Echocardiography’s Guidelines and Standards Committee and the Chamber Quantification Writing Group, Developed in Conjunction with the European Association of Echocardiography, a Branch of the European Society of Cardiology. J Am Soc Echocardiogr 2005;18: 1440-63. 10. Borlaug BA, Melenovsky V, Redfield MM, et al. Impact of arterial load and loading sequence on left ventricular tissue velocities in humans. J Am Coll Cardiol 2007;50:1570-7. 11. Wasserman K, Whipp BJ, Koyal SN, Beaver WL. Anaerobic threshold and respiratory gas-exchange during exercise. J Appl Physiol 1973; 35:236-43. 12. Scott JM, Esch BT, Haykowsky MJ, et al. Cardiovascular responses to incremental and sustained submaximal exercise in heart transplant recipients. Am J Physiol Heart Circ Physiol 2009;296:H350-8. 13. Marconi C, Marzorati M, Fiocchi R, et al. Age-related heart rate response to exercise in heart transplant recipients. Functional significance. Pflugers Arch 2002;443:698-706. 14. Bengel FM, Ueberfuhr P, Hesse T, et al. Clinical determinants of ventricular sympathetic reinnervation after orthotopic heart transplantation. Circulation 2002;106:831-5. 15. Marconi C, Marzorati M. Exercise after heart transplantation. Eur J Appl Physiol 2003;90:250-9. 16. Faure J, Causse X, Bergeret A, Meyer F, Neidecker J, Paliard P. Cyclosporine induced hemolytic anemia in a liver transplant patient. Transplant Proc 1989;21:2242-3. 17. Abu-Elmagd K, Bronsther O, Kabayaski M, et al. Acute hemolytic anemia in liver and bone marrow transplant patients under FK 506 therapy. Transplant Proc 1991;23:3190-2.