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Cardiac Output Responses During Exercise in Volume-Expanded Heart Transplant Recipients 1
Cardiac Output Responses During Exercise in Volume-Expanded Heart Transplant Recipients Randy W. Braith,
PhD,
Mary B. Plunkett,
MS,
and Roger M. Mills, Jr.,
MD
The mechanisms responsible for immediate adjustments in cardiac output at onset of exercise, in the absence of neural drive, are not well defined in heart transplant (HT) recipients. Seven male HT recipients (mean 6 SD 57 6 6 years) and 7 age-matched sedentary normal control subjects (mean age 57 6 5 years) performed constant load cycle exercise at 40% of peak power output (Watts). Cardiac output and plasma norepinephrine were determined at rest and every 30 seconds during the first 5 minutes of exercise and at minutes 6, 8, and 10. All subjects were admitted to the General Clinical Research Center for determination of plasma volume. After 3 days of equilibration to a controlled and standardized diet, plasma volume was measured using a modified Evans Blue Dye (T-1824) dilution technique. Heart rate at rest was higher in the HT group (105 6 12 vs 74 6 6 beats/min), but during submaximum exercise, heart rates in the control group increased more rapidly (p <0.05) and to a greater magnitude (54 6 7% vs 17 6 4% above rest). Stroke volume at rest was lower in HT recipients (45 6 4 vs 68 6 9 ml) but was signifi-
cantly augmented immediately after onset of exercise (30 seconds) and the relative increase was greater than controls at peak exercise (61% vs 38% greater than baseline). Cardiac output at rest was within the normal range in both groups (4.58 6 0.27 vs 4.94 6 0.40 L/min). Relative increases in cardiac output were similar (p >0.05) for the HT (106 6 12%) and control groups (97 6 10%). Plasma norepinephrine did not become significantly greater than resting values until approximately 4 minutes after onset of exercise in both groups. Blood volume, normalized for body weight, was 12% greater in the HT group. Thus, HT recipients with expanded blood volume (12%) augment stroke volume immediately after the onset of exercise. Plasma norepinephrine levels contribute negligibly to the rapid adjustment in cardiac output. Rather, we speculate that abrupt on-transit increases in stroke volume are due to augmented venous return, secondary to expanded blood volume. Q1998 by Excerpta Medica, Inc. (Am J Cardiol 1998;81:1152–1156)
troke volume augmentation is known to occur during upright exercise in heart transplant (HT) S recipients, despite the absence of cardiac innerva-
METHODS
tion.1–10 It has been suggested that increased stroke volume could be due to the direct effect of enhanced plasma catecholamine concentration,11 or increased catecholamine sensitivity.12,13 However, abrupt increases in stroke volume could also be due to augmented venous return in HT recipients who have expanded extracellular fluid volume.14,15 This study examines the temporal pattern of stroke volume and cardiac output augmentation at the onset of exercise in HT recipients late after transplantation. In an attempt to clarify the mechanisms responsible for adjustments in stroke volume, we measured plasma norepinephrine (NE) levels during the transition from rest to exercise and extracellular fluid volume under controlled and standardized conditions.
From the Department of Exercise and Sport Sciences, and the Department of Medicine, University of Florida, Gainesville, Florida; and the Division of Cardiovascular Medicine, University of Kentucky, Lexington, Kentucky. This work was supported by National Institutes of Health Clinical Research Center Grant RR00082 and Dr. Braith was supported by a NIH National Research Service Award (HL08777), Bethesda, Maryland. Manuscript received June 4, 1997; revised manuscript received and accepted January 28, 1998. Address for reprints: Randy W. Braith, PhD, P.O. Box 118206, Center for Exercise Science, University of Florida, Gainesville, Florida 32611.
1152
©1998 by Excerpta Medica, Inc. All rights reserved.
Subjects: Descriptive characteristics of the HT recipients (n 5 7) and normal control subjects (n 5 7) are listed in Table I. The 2 groups did not differ (p $0.05) with respect to age, height, weight, and body composition. All of the HT recipients had bi-atrial anastomosis at transplantation and received immunosuppressive therapy with cyclosporine, prednisone, and azathioprine. The maintenance prednisone dose was 5 to 10 mg/day at the time of the study and no HT patient required enhanced corticosteroid doses within 6 months of the study. No a or b blockers or other cardiac medications were used by the HT recipients. All HT recipients received antihypertensive agents for management of hypertension. The HT recipients underwent endomyocardial biopsy within 2 months of the study: 6 of 7 patients were biopsied within 1 month of the study and 1 was biopsied within 2 months of the study. There was no evidence of rejection or allograft vascular disease at the time of biopsy in any of the HT recipients. All HT recipients participated in self-monitored walking programs. Healthy, age-matched control subjects were sedentary and had no cardiac or pulmonary disease as determined by clinical examination and graded exercise testing, and none were receiving prescription medication. The protocol was approved by the Institutional Review Board for the protection of human subjects at the University of Florida and all subjects 0002-9149/98/$19.00 PII S0002-9149(98)00113-1
TABLE I Physical Characteristics of the Normal Control and Heart Transplant (HT) Groups Variable Age (yrs) Height (cm) Weight (kg) Body fat (%) VO2† (ml/kg/min) Months post-Tx
Controls (n 5 7) 57 6 179 6 90 6 27 6 30 6 —
5 4 11 4 6
HT (n 5 7) 57 179 90 27 18 15
6 6 6 6 6 6
6 5 10 5 4.1* 5
Values are mean 6 SD. *p #0.5 HT group versus control group. † Peak systemic oxygen consumption. Tx 5 transplantation.
provided written informed consent for participation in the study. Maximum graded exercise test: During the first visit to the laboratory, subjects underwent a physical examination and a 12-lead electrocardiogram at rest. Following screening, they completed a graded exercise test on an electronically braked cycle ergometer. Initial power output was 20 W and power output was increased every 2 minutes by 20 or 40 W for the HT or control subjects, respectively, until the subject reached voluntary maximum exertion and could not maintain 60 pedal revolutions per minute. Peak oxygen consumption was determined by analysis of expired air using a metabolic cart (Medical Graphics, St. Paul, Minnesota). Submaximum exercise test: After 1 week, subjects returned to the laboratory for submaximum exercise testing on the cycle ergometer used for the graded exercise test. Submaximum tests were performed at the same time of day (12:00 to 3:00 P.M.) as the graded exercise test to control for a possible effect of circadian rhythms. To study stroke volume, cardiac output, heart rate, and plasma NE responses during the onset of exercise, each subject performed 10 minutes of cycle exercise at 40% of the peak power output (Watts) achieved during the graded exercise test. Before the test, subjects rested for 10 minutes while seated on the cycle. After determination of baseline heart rate, stroke volume, and cardiac output, subjects immediately began pedaling at 60 revolutions per minute at 40% of peak power output. Cardiac output: Cardiac output was estimated in duplicate by impedance cardiography using a Minnesota Impedance Cardiograph (Surcom Inc., Minneapolis, Minnesota) at rest and every 30 seconds during the first 5 minutes of exercise and at minutes 6, 8, and 10. Impedance cardiography measurement of cardiac output in HT recipients during submaximum bicycle exercise has previously been validated using the thermodilution technique as the reference method of measurement, with close agreement between the 2 techniques at rest and during exercise.16 Blood samples were drawn from a forearm vein for determination of plasma NE levels at rest and at 30-second intervals during the first 5 minutes of exercise and at minutes 6, 8, and 10.
Norepinephrine analysis: Plasma NE was measured by high performance liquid chromatography (ESA Coulochem, Bedford, Massachusetts). NE was extracted by adsorbing plasma samples onto alumina. After washing the adsorbed alumina with a buffer solution, NE was eluted from the alumina by treating it with an acid solution. 3, 4-Dihydroxybenzylamine was used as an internal standard, and extraction efficiency of NE was based on the extraction of known standards. All samples were corrected for percent recovery. After extraction, the samples were injected onto a reverse-phase C18 column. An ESA 16-channel Coulochem multielectrode array detector was used to determine the concentration of NE in the samples. The within-assay coefficient of variation (CV) was 1.3% and the between assay CV was 3.0%. Plasma volume measurement: After completion of the exercise protocol, HT recipients discontinued diuretics and all antihypertensive agents for 8 days. Subjects were then admitted to the Clinical Research Center at Shands Hospital, at the University of Florida, for a period of 4 days to measure plasma volume under controlled and standardized conditions. The first 3 days in the Center were an equilibration to the controlled diet that consisted of sufficient calories to maintain their current weight and provided 87 mEq/24 hours of Na1 and 80 mEq/24 hours of K1. Water intake was standardized to 1,000 ml on day 3 but provided ad libitum on all other days of the study. All subjects were 610% of Na1 balance before the plasma volume measurements. Plasma volume was determined on the morning of day 4 using a modified Evans Blue Dye (T-1824, New World Trading Corp., DeBary, Florida) dilution technique.17 Plasma samples were drawn at 10, 20, and 30 minutes following injection of blue dye and the data were extrapolated to 0-time in calculating the plasma volume. The dye from the plasma samples was extracted onto a wood-cellulose powder (Solka Floc SW 40A, Sigma Chemical, St Louis, Missouri) chromatographic column after it had been separated from albumin by the action of a detergent (Teepol 610 in 2% Na2HPO4, Sigma Chemical, St. Louis, Missouri). Interfering substances such as pigments, proteins, and chylomicrons were washed from the column with 2% Na2HPO4. The dye was then eluted from the column with a 1:1 acetone-water mixture. The addition of KH2PO4 buffered the pH of the eluate to 7.0; absorbance of the eluate was read at 615 nm. Duplicate microhematocrit determinations were made using a microhematocrit centrifuge and a Micro-Capillary Tube Reader (Fisher Scientific, Pittsburgh, Pennsylvania). Raw microhematocrit values were corrected (30.91) for trapped plasma and for whole body microhematocrit.18 Blood volume was calculated as PV/ (1 2 corrected microhematocrit). Statistical analysis: Descriptive characteristics and plasma volume values were compared between groups using analysis of variance. Analysis of covariance (ANCOVA) with repeated measures was used to analyze all response variables during submaximum exercise. When a significant group by time interaction MISCELLANEOUS/HEART TRANSPLANT CARDIAC OUTPUT
1153
group (145 6 10 vs 169 6 13 beats/ min). Heart rate at rest was higher in the HT recipients than in the control group (105 6 12 vs 74 6 6 beats/min). During submaximum exercise at 40% of peak power, heart rate at all measurement periods was significantly greater than rest for both groups. However, heart rate in the control group increased more rapidly (p #0.05) and to a greater magnitude than heart rate responses in the HT recipients (54 6 7% vs 17 6 4% above rest). The temporal pattern of relative changes in heart rate during submaximum exercise is presented in Figure 1. Stroke volume: Stroke volume at rest was lower in the HT recipients (45 6 4 vs 68 6 9 ml). During submaximum exercise, the stroke volume in HT recipients was augmented immediately after onset of exercise, becoming significantly greater than baseline values after 30 seconds of exercise and reached peak values that were 61% greater than baseline. In contrast, stroke volume in the normal control group did not become significantly greater than baseline until 2 minutes after the onset of exercise and reached peak values that were 38% above baseline. The temporal pattern of relative changes in stroke volume during submaximum exercise is presented in Figure 1. Cardiac output: Cardiac output at rest was within the normal range in the HT recipients (4.58 6 0.27 L/min) but FIGURE 1. Temporal pattern of relative changes (percent change from rest) in carsignificantly less than the control group diac output, heart rate, and stroke volume during 10 minutes of constant load cy(4.94 6 0.40 L/min). Absolute cardiac cle exercise at 40% of peak power output in the HT and normal control groups. Values are expressed as mean value 6 SEM. *p <0.05 rest versus exercise. †p output in the control group during ex<0.05 HT group versus control group. ercise was greater (p #0.05) than the HT recipients at all measurement periods. However, 40% of peak power outwas observed, within-group comparisons and be- put represented a greater absolute work rate (Watts) in tween-group comparisons were performed using AN- the control group and required a greater absolute carCOVA with contrast analysis for obtaining appropri- diac output. In contrast, relative increases in cardiac ate post-hoc custom hypothesis tests. All statistical output (percent change from rest) were similar (p analyses were performed using the SAS statistical $0.05) for HT recipients (106 6 12%) and the control program (SAS Institute Inc., Cary, North Carolina). group (97 6 10%) during 10 minutes of cycle exercise An a level of p #0.05 was required for statistical at 40% of peak power output. The temporal pattern of relative changes in cardiac output is presented in Figsignificance. ure 1. Norepinephrine: Absolute changes in plasma NE RESULTS Peak oxygen uptake: Peak oxygen uptake achieved are presented in Figure 2. NE levels at rest were by the HT recipients during the graded exercise test similar (p $0.05) in the HT (0.467 6 0.058 ng/ml) was 60% of that attained by the control group (18.2 6 and control groups (0.387 6 0.037 ng/ml). NE levels 4.1 vs 30.2 6 5.6 ml/kg21/min21) (p #0.05). Peak increased slowly and did not become significantly power output (Watts) in the HT recipients was 57% of greater than resting values until 3.5 and 4 minutes the value achieved by the control group (115 6 15 vs after the onset of exercise in the HT and control groups, respectively. The magnitude of NE increase 200 6 30 W) (p #0.05). Heart rate: Peak heart rate achieved by the HT during exercise was not significantly different in the recipients was 85% of that attained by the control HT and control groups. 1154 THE AMERICAN JOURNAL OF CARDIOLOGYT
VOL. 81
MAY 1, 1998
study also confirm our previous finding that blood volume is expanded (12%) in clinically stable HT recipients, when compared with age-matched controls.14 Stroke volume: Factors that mediate stroke volume in HT recipients include preload, afterload, compliance of the ventricles, and the inotropic state. The heart rate at rest is elevated in HT recipients and the intrinsic rate of the allograft sinoatrial node is approximately 100 beats/min in the absence of parasympathetic innervation.4,7,10 The increased rate and decreased diastolic period contribute to reduced stroke volume values at rest. Additionally, absence of sympathetic innervation appears to alter the inotropic state and decreases cardiac compliance, resulting in further diastolic dysfunction FIGURE 2. Temporal pattern of absolute changes in plasma NE concentrations dur(leftward shift in the pressure–volume ing 10 minutes of constant load cycle exercise at 40% of peak power output in curve).2,6 the HT and normal control groups. Values are expressed as mean value 6 SEM. *p <0.05 rest versus exercise. Blood volume: Although an altered inotropic state and decreased cardiac compliance contribute to an attenuated stroke volume response and diminished cardiac output TABLE II Fluid Volume and Plasma Electrolytes in Normal reserve in HT recipients,1– 6 our data indicate that Control and Heart Transplant Groups these patients are able to augment stroke volume imControls HT mediately after the onset of cycle exercise. One partial Variable (n 5 7) (n 5 7) explanation for the stroke volume response in HT recipients versus normal controls is that the lower Plasma volume (ml) 2,805 6 321 3,698 6 399* Plasma volume (ml/kg) 38 6 5 43 6 6* stroke volume at rest permits greater relative increases Blood volume (ml) 4,613 6 732 5,703 6 990* in stroke volume during exercise. However, our data Blood volume (ml/kg) 60 6 8 67 6 11* also suggest that an increase in venous return, facili1 Plasma Na (mEq/L) 140 6 2 142 6 2 tated by the “skeletal muscle pump,” during the restPlasma osmolality (mOsm/L) 300 6 25 302 6 20 to-exercise transition in volume-expanded HT recipiValues are mean 6 SD. ents may offset the altered inotropic state and diastolic *p #0.05 HT group versus control group. dysfunction of the denervated heart and permit acute augmentation of stroke volume. Our finding of a 12% expansion in blood volume in this and previous studPlasma volume: Blood volume, plasma volume, and ies,14 raises the possibility that volume expansion in electrolyte data are presented in Table II. Blood vol- these patients is a compensatory adaptation to cardiac ume and plasma volume, normalized for body weight denervation and that HT recipients may require an (milliliters per kilogram), were significantly greater in expanded blood volume to maintain cardiac output the HT recipients. The 12% blood volume expansion and blood pressure in the absence of efferent cardiac in the HT recipients was not associated with signifi- autonomic nerves. Norepinephrine: We previously reported that plasma cantly different concentrations in plasma Na1 or NE levels in HT recipients are significantly elevated at plasma osmolality. exercise intensities .70% of peak power output when compared with age-matched controls.20 Others have DISCUSSION The main finding of this study was that circulating speculated that high humoral levels of NE are responNE in HT recipients contributes negligibly to the rapid sible for the progressive increases in stroke volume adjustment in cardiac output during the rest to exercise observed in HT recipients (but not normals) at work transition. HT recipients augmented stroke volume intensities .50%.7,9,11 Certainly, high NE levels immediately at the onset of exercise (30 seconds) and would enhance venous tone (venous return) and inachieved stroke volume values that were 61% greater crease cardiac contractility. than baseline values at rest. However, this adjustment The present study was the first investigation to in cardiac output was too rapid to be ascribed to serially measure NE levels during the transition from elevated plasma NE concentrations. Indeed, plasma rest to steady-state exercise in HT recipients. We NE levels did not increase significantly above baseline observed no difference between groups in NE concenvalues in either group until approximately 4 minutes trations at rest or the absolute NE response to exercise after the onset of cycle exercise. The results of this at 40% of peak power output. Plasma NE in both MISCELLANEOUS/HEART TRANSPLANT CARDIAC OUTPUT
1155
groups did not increase significantly above resting values until approximately the fourth minute of constant load exercise. Therefore, we doubt that immediate augmentation of stroke volume in HT recipients at the onset of exercise is due to the effect of humoral catecholamines. Rather, we speculate that abrupt increases in stroke volume are likely due to augmented venous return, secondary to expanded blood volume. 1. Younis LT, Melin JA, Schoevaerdts JC, Van Dyck M, Detry JM, Robert A, Chalant C, Goenen M. Left ventricular systolic function and diastolic filling at rest and during upright exercise after orthotopic heart transplantation: comparison with young and aged normal subjects. J Heart Transpl 1990;9:683– 692. 2. Rudas L, Pflugfelder PW, Kostuk WJ. Comparison of hemodynamic responses during dynamic exercise in the upright and supine postures after orthotopic cardiac transplantation. J Am Coll Cardiol 1990;16:1367–1373. 3. Rudas L, Pflugfelder PW, Mckenzie FN, Menkis AH, Novick RJ, Kostuk WJ. Normalization of upright exercise hemodynamics and improved exercise capacity one year after orthotopic cardiac transplantation. Am J Cardiol 1992;69:1336 – 1339. 4. Kappagoda CT, Haennel RG, Serrano-Fiz S, Davies DH. The hemodynamic responses during upright exercise after orthotopic cardiac transplant. Arch Physical Med Rehabil 1993;14:484 – 489. 5. Kao AC, Van Trigt P, Schaeffer-Mccall GS, Shaw JP, Kuzil BB, Page RD, Higginbotham MB. Central and peripheral limitations to upright exercise in untrained cardiac transplant recipients. Circulation 1994;89:2605–2615. 6. Kao AC, Vantrigt P, Shaeffer-Mccall GS, Shaw JP, Kuzil BB, Page RD, Higginbotham MB. Allograft diastolic dysfunction and chronotropic incompetence limit cardiac output response to exercise two to six years after heart transplantation. J Heart Lung Transpl 1995;14:11–22. 7. Pope SE, Stinson EB, Daughters GT, Schroeder JS, Ingels NB, Alderman EL. Exercise response of the denervated heart in long-term cardiac transplant recipients. Am J Cardiol 1980;46:213–218. 8. Pflugfelder PW, Mckenzie N, Kostuk WJ. Hemodynamic profiles at rest and
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during supine exercise after orthotopic cardiac transplantation. Am J Cardiol 1988;61:1328 –1333. 9. Keteyian SJ, Charles RC, Marks AB, Levine TK, Frank F, Berry TL. Cardiovascular responses to submaximal arm and leg exercise in cardiac transplant patients. Med Sci Sports Exerc 1994;26:420 – 424. 10. Shephard RJ, Kavanagh T, Mertens D, Qureshi S. Kinetics of the transplanted heart. Implications for the choice of field-test exercise protocol. J Cardiopulm Rehabil 1995;15:288 –296. 11. Degre S, Niset G, DeSmet J, Ibrahim T, Stoupel E, LeClerc J, Primo G. Cardiorespiratory responses to early exercise testing after orthotopic cardiac transplantation. Am J Cardiol 1987;60:926 –928. 12. Yusef S, Phil D, Theodoropoulos S, Mathias CJ, Dhalla N, Wittes J, Mitchell A, Yacoub M. Increased sensitivity to the denervated transplanted human heart to isoprenaline both before and after b-adrenergic blockade. Circulation 1987;75: 696 –704. 13. Gilbert EM, Eiswirth CC, Mealey PC, Larrabee P, Herrick CM, Bristow MR. B-adrenergic supersensitivity of the transplanted human heart is presynaptic in origin. Circulation 1989;79:344 –349. 14. Braith RW, Mills RM, Wilcox CS, Convertino VA, Davis GL, Limacher MC, Wood CE. Fluid homeostasis after heart transplantation: the role of cardiac denervation. J Heart Lung Transpl 1996;15:872– 880. 15. Bellet M, Cabrol C, Sassano P, Leger P, Corvol P, Menard J. Systemic hypertension after cardiac transplantation: effect of cyclosporine on the reninangiotensin-aldosterone system. Am J Cardiol 1985;56:927–931. 16. Pepke-Zaba J, Higenbottam TW, Dinh Xuan AT, Scott JP, English TAH, Wallwork J. Validation of impedance cardiography measurements of cardiac output during limited exercise in heart transplant recipients. Transplant Int 1990;3:108 –112. 17. Greenleaf JE, Convertino VA, Mangseth GR. Plasma volume during stress testing in man: osmolality and red cell volume. J Appl Physiol 1979;47:1031– 1038. 18. Chaplin H, Mollison Pl, Vetter H. The body/venous hematocrit ratio: its constance over a wide hematocrit range. J Clin Invest 1953;32:1309 –1316. 19. Kavanagh T, Yacoub MH, Mertens DJ, Kennedy J, Campbell RB, Sawyer P. Cardiorespiratory responses to exercise training after orthotopic cardiac transplantation. Circulation 1988;77:162–171. 20. Braith RW, Wood CE, Limacher MC, Pollock ML, Lowenthal DT, Phillips MI, Staples ED. Abnormal neuroendocrine responses during exercise in heart transplant recipients. Circulation 1992;86:1453–1463.
MAY 1, 1998
PhD,
Mary B. Plunkett,
MS,
and Roger M. Mills, Jr.,
MD
The mechanisms responsible for immediate adjustments in cardiac output at onset of exercise, in the absence of neural drive, are not well defined in heart transplant (HT) recipients. Seven male HT recipients (mean 6 SD 57 6 6 years) and 7 age-matched sedentary normal control subjects (mean age 57 6 5 years) performed constant load cycle exercise at 40% of peak power output (Watts). Cardiac output and plasma norepinephrine were determined at rest and every 30 seconds during the first 5 minutes of exercise and at minutes 6, 8, and 10. All subjects were admitted to the General Clinical Research Center for determination of plasma volume. After 3 days of equilibration to a controlled and standardized diet, plasma volume was measured using a modified Evans Blue Dye (T-1824) dilution technique. Heart rate at rest was higher in the HT group (105 6 12 vs 74 6 6 beats/min), but during submaximum exercise, heart rates in the control group increased more rapidly (p <0.05) and to a greater magnitude (54 6 7% vs 17 6 4% above rest). Stroke volume at rest was lower in HT recipients (45 6 4 vs 68 6 9 ml) but was signifi-
cantly augmented immediately after onset of exercise (30 seconds) and the relative increase was greater than controls at peak exercise (61% vs 38% greater than baseline). Cardiac output at rest was within the normal range in both groups (4.58 6 0.27 vs 4.94 6 0.40 L/min). Relative increases in cardiac output were similar (p >0.05) for the HT (106 6 12%) and control groups (97 6 10%). Plasma norepinephrine did not become significantly greater than resting values until approximately 4 minutes after onset of exercise in both groups. Blood volume, normalized for body weight, was 12% greater in the HT group. Thus, HT recipients with expanded blood volume (12%) augment stroke volume immediately after the onset of exercise. Plasma norepinephrine levels contribute negligibly to the rapid adjustment in cardiac output. Rather, we speculate that abrupt on-transit increases in stroke volume are due to augmented venous return, secondary to expanded blood volume. Q1998 by Excerpta Medica, Inc. (Am J Cardiol 1998;81:1152–1156)
troke volume augmentation is known to occur during upright exercise in heart transplant (HT) S recipients, despite the absence of cardiac innerva-
METHODS
tion.1–10 It has been suggested that increased stroke volume could be due to the direct effect of enhanced plasma catecholamine concentration,11 or increased catecholamine sensitivity.12,13 However, abrupt increases in stroke volume could also be due to augmented venous return in HT recipients who have expanded extracellular fluid volume.14,15 This study examines the temporal pattern of stroke volume and cardiac output augmentation at the onset of exercise in HT recipients late after transplantation. In an attempt to clarify the mechanisms responsible for adjustments in stroke volume, we measured plasma norepinephrine (NE) levels during the transition from rest to exercise and extracellular fluid volume under controlled and standardized conditions.
From the Department of Exercise and Sport Sciences, and the Department of Medicine, University of Florida, Gainesville, Florida; and the Division of Cardiovascular Medicine, University of Kentucky, Lexington, Kentucky. This work was supported by National Institutes of Health Clinical Research Center Grant RR00082 and Dr. Braith was supported by a NIH National Research Service Award (HL08777), Bethesda, Maryland. Manuscript received June 4, 1997; revised manuscript received and accepted January 28, 1998. Address for reprints: Randy W. Braith, PhD, P.O. Box 118206, Center for Exercise Science, University of Florida, Gainesville, Florida 32611.
1152
©1998 by Excerpta Medica, Inc. All rights reserved.
Subjects: Descriptive characteristics of the HT recipients (n 5 7) and normal control subjects (n 5 7) are listed in Table I. The 2 groups did not differ (p $0.05) with respect to age, height, weight, and body composition. All of the HT recipients had bi-atrial anastomosis at transplantation and received immunosuppressive therapy with cyclosporine, prednisone, and azathioprine. The maintenance prednisone dose was 5 to 10 mg/day at the time of the study and no HT patient required enhanced corticosteroid doses within 6 months of the study. No a or b blockers or other cardiac medications were used by the HT recipients. All HT recipients received antihypertensive agents for management of hypertension. The HT recipients underwent endomyocardial biopsy within 2 months of the study: 6 of 7 patients were biopsied within 1 month of the study and 1 was biopsied within 2 months of the study. There was no evidence of rejection or allograft vascular disease at the time of biopsy in any of the HT recipients. All HT recipients participated in self-monitored walking programs. Healthy, age-matched control subjects were sedentary and had no cardiac or pulmonary disease as determined by clinical examination and graded exercise testing, and none were receiving prescription medication. The protocol was approved by the Institutional Review Board for the protection of human subjects at the University of Florida and all subjects 0002-9149/98/$19.00 PII S0002-9149(98)00113-1
TABLE I Physical Characteristics of the Normal Control and Heart Transplant (HT) Groups Variable Age (yrs) Height (cm) Weight (kg) Body fat (%) VO2† (ml/kg/min) Months post-Tx
Controls (n 5 7) 57 6 179 6 90 6 27 6 30 6 —
5 4 11 4 6
HT (n 5 7) 57 179 90 27 18 15
6 6 6 6 6 6
6 5 10 5 4.1* 5
Values are mean 6 SD. *p #0.5 HT group versus control group. † Peak systemic oxygen consumption. Tx 5 transplantation.
provided written informed consent for participation in the study. Maximum graded exercise test: During the first visit to the laboratory, subjects underwent a physical examination and a 12-lead electrocardiogram at rest. Following screening, they completed a graded exercise test on an electronically braked cycle ergometer. Initial power output was 20 W and power output was increased every 2 minutes by 20 or 40 W for the HT or control subjects, respectively, until the subject reached voluntary maximum exertion and could not maintain 60 pedal revolutions per minute. Peak oxygen consumption was determined by analysis of expired air using a metabolic cart (Medical Graphics, St. Paul, Minnesota). Submaximum exercise test: After 1 week, subjects returned to the laboratory for submaximum exercise testing on the cycle ergometer used for the graded exercise test. Submaximum tests were performed at the same time of day (12:00 to 3:00 P.M.) as the graded exercise test to control for a possible effect of circadian rhythms. To study stroke volume, cardiac output, heart rate, and plasma NE responses during the onset of exercise, each subject performed 10 minutes of cycle exercise at 40% of the peak power output (Watts) achieved during the graded exercise test. Before the test, subjects rested for 10 minutes while seated on the cycle. After determination of baseline heart rate, stroke volume, and cardiac output, subjects immediately began pedaling at 60 revolutions per minute at 40% of peak power output. Cardiac output: Cardiac output was estimated in duplicate by impedance cardiography using a Minnesota Impedance Cardiograph (Surcom Inc., Minneapolis, Minnesota) at rest and every 30 seconds during the first 5 minutes of exercise and at minutes 6, 8, and 10. Impedance cardiography measurement of cardiac output in HT recipients during submaximum bicycle exercise has previously been validated using the thermodilution technique as the reference method of measurement, with close agreement between the 2 techniques at rest and during exercise.16 Blood samples were drawn from a forearm vein for determination of plasma NE levels at rest and at 30-second intervals during the first 5 minutes of exercise and at minutes 6, 8, and 10.
Norepinephrine analysis: Plasma NE was measured by high performance liquid chromatography (ESA Coulochem, Bedford, Massachusetts). NE was extracted by adsorbing plasma samples onto alumina. After washing the adsorbed alumina with a buffer solution, NE was eluted from the alumina by treating it with an acid solution. 3, 4-Dihydroxybenzylamine was used as an internal standard, and extraction efficiency of NE was based on the extraction of known standards. All samples were corrected for percent recovery. After extraction, the samples were injected onto a reverse-phase C18 column. An ESA 16-channel Coulochem multielectrode array detector was used to determine the concentration of NE in the samples. The within-assay coefficient of variation (CV) was 1.3% and the between assay CV was 3.0%. Plasma volume measurement: After completion of the exercise protocol, HT recipients discontinued diuretics and all antihypertensive agents for 8 days. Subjects were then admitted to the Clinical Research Center at Shands Hospital, at the University of Florida, for a period of 4 days to measure plasma volume under controlled and standardized conditions. The first 3 days in the Center were an equilibration to the controlled diet that consisted of sufficient calories to maintain their current weight and provided 87 mEq/24 hours of Na1 and 80 mEq/24 hours of K1. Water intake was standardized to 1,000 ml on day 3 but provided ad libitum on all other days of the study. All subjects were 610% of Na1 balance before the plasma volume measurements. Plasma volume was determined on the morning of day 4 using a modified Evans Blue Dye (T-1824, New World Trading Corp., DeBary, Florida) dilution technique.17 Plasma samples were drawn at 10, 20, and 30 minutes following injection of blue dye and the data were extrapolated to 0-time in calculating the plasma volume. The dye from the plasma samples was extracted onto a wood-cellulose powder (Solka Floc SW 40A, Sigma Chemical, St Louis, Missouri) chromatographic column after it had been separated from albumin by the action of a detergent (Teepol 610 in 2% Na2HPO4, Sigma Chemical, St. Louis, Missouri). Interfering substances such as pigments, proteins, and chylomicrons were washed from the column with 2% Na2HPO4. The dye was then eluted from the column with a 1:1 acetone-water mixture. The addition of KH2PO4 buffered the pH of the eluate to 7.0; absorbance of the eluate was read at 615 nm. Duplicate microhematocrit determinations were made using a microhematocrit centrifuge and a Micro-Capillary Tube Reader (Fisher Scientific, Pittsburgh, Pennsylvania). Raw microhematocrit values were corrected (30.91) for trapped plasma and for whole body microhematocrit.18 Blood volume was calculated as PV/ (1 2 corrected microhematocrit). Statistical analysis: Descriptive characteristics and plasma volume values were compared between groups using analysis of variance. Analysis of covariance (ANCOVA) with repeated measures was used to analyze all response variables during submaximum exercise. When a significant group by time interaction MISCELLANEOUS/HEART TRANSPLANT CARDIAC OUTPUT
1153
group (145 6 10 vs 169 6 13 beats/ min). Heart rate at rest was higher in the HT recipients than in the control group (105 6 12 vs 74 6 6 beats/min). During submaximum exercise at 40% of peak power, heart rate at all measurement periods was significantly greater than rest for both groups. However, heart rate in the control group increased more rapidly (p #0.05) and to a greater magnitude than heart rate responses in the HT recipients (54 6 7% vs 17 6 4% above rest). The temporal pattern of relative changes in heart rate during submaximum exercise is presented in Figure 1. Stroke volume: Stroke volume at rest was lower in the HT recipients (45 6 4 vs 68 6 9 ml). During submaximum exercise, the stroke volume in HT recipients was augmented immediately after onset of exercise, becoming significantly greater than baseline values after 30 seconds of exercise and reached peak values that were 61% greater than baseline. In contrast, stroke volume in the normal control group did not become significantly greater than baseline until 2 minutes after the onset of exercise and reached peak values that were 38% above baseline. The temporal pattern of relative changes in stroke volume during submaximum exercise is presented in Figure 1. Cardiac output: Cardiac output at rest was within the normal range in the HT recipients (4.58 6 0.27 L/min) but FIGURE 1. Temporal pattern of relative changes (percent change from rest) in carsignificantly less than the control group diac output, heart rate, and stroke volume during 10 minutes of constant load cy(4.94 6 0.40 L/min). Absolute cardiac cle exercise at 40% of peak power output in the HT and normal control groups. Values are expressed as mean value 6 SEM. *p <0.05 rest versus exercise. †p output in the control group during ex<0.05 HT group versus control group. ercise was greater (p #0.05) than the HT recipients at all measurement periods. However, 40% of peak power outwas observed, within-group comparisons and be- put represented a greater absolute work rate (Watts) in tween-group comparisons were performed using AN- the control group and required a greater absolute carCOVA with contrast analysis for obtaining appropri- diac output. In contrast, relative increases in cardiac ate post-hoc custom hypothesis tests. All statistical output (percent change from rest) were similar (p analyses were performed using the SAS statistical $0.05) for HT recipients (106 6 12%) and the control program (SAS Institute Inc., Cary, North Carolina). group (97 6 10%) during 10 minutes of cycle exercise An a level of p #0.05 was required for statistical at 40% of peak power output. The temporal pattern of relative changes in cardiac output is presented in Figsignificance. ure 1. Norepinephrine: Absolute changes in plasma NE RESULTS Peak oxygen uptake: Peak oxygen uptake achieved are presented in Figure 2. NE levels at rest were by the HT recipients during the graded exercise test similar (p $0.05) in the HT (0.467 6 0.058 ng/ml) was 60% of that attained by the control group (18.2 6 and control groups (0.387 6 0.037 ng/ml). NE levels 4.1 vs 30.2 6 5.6 ml/kg21/min21) (p #0.05). Peak increased slowly and did not become significantly power output (Watts) in the HT recipients was 57% of greater than resting values until 3.5 and 4 minutes the value achieved by the control group (115 6 15 vs after the onset of exercise in the HT and control groups, respectively. The magnitude of NE increase 200 6 30 W) (p #0.05). Heart rate: Peak heart rate achieved by the HT during exercise was not significantly different in the recipients was 85% of that attained by the control HT and control groups. 1154 THE AMERICAN JOURNAL OF CARDIOLOGYT
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study also confirm our previous finding that blood volume is expanded (12%) in clinically stable HT recipients, when compared with age-matched controls.14 Stroke volume: Factors that mediate stroke volume in HT recipients include preload, afterload, compliance of the ventricles, and the inotropic state. The heart rate at rest is elevated in HT recipients and the intrinsic rate of the allograft sinoatrial node is approximately 100 beats/min in the absence of parasympathetic innervation.4,7,10 The increased rate and decreased diastolic period contribute to reduced stroke volume values at rest. Additionally, absence of sympathetic innervation appears to alter the inotropic state and decreases cardiac compliance, resulting in further diastolic dysfunction FIGURE 2. Temporal pattern of absolute changes in plasma NE concentrations dur(leftward shift in the pressure–volume ing 10 minutes of constant load cycle exercise at 40% of peak power output in curve).2,6 the HT and normal control groups. Values are expressed as mean value 6 SEM. *p <0.05 rest versus exercise. Blood volume: Although an altered inotropic state and decreased cardiac compliance contribute to an attenuated stroke volume response and diminished cardiac output TABLE II Fluid Volume and Plasma Electrolytes in Normal reserve in HT recipients,1– 6 our data indicate that Control and Heart Transplant Groups these patients are able to augment stroke volume imControls HT mediately after the onset of cycle exercise. One partial Variable (n 5 7) (n 5 7) explanation for the stroke volume response in HT recipients versus normal controls is that the lower Plasma volume (ml) 2,805 6 321 3,698 6 399* Plasma volume (ml/kg) 38 6 5 43 6 6* stroke volume at rest permits greater relative increases Blood volume (ml) 4,613 6 732 5,703 6 990* in stroke volume during exercise. However, our data Blood volume (ml/kg) 60 6 8 67 6 11* also suggest that an increase in venous return, facili1 Plasma Na (mEq/L) 140 6 2 142 6 2 tated by the “skeletal muscle pump,” during the restPlasma osmolality (mOsm/L) 300 6 25 302 6 20 to-exercise transition in volume-expanded HT recipiValues are mean 6 SD. ents may offset the altered inotropic state and diastolic *p #0.05 HT group versus control group. dysfunction of the denervated heart and permit acute augmentation of stroke volume. Our finding of a 12% expansion in blood volume in this and previous studPlasma volume: Blood volume, plasma volume, and ies,14 raises the possibility that volume expansion in electrolyte data are presented in Table II. Blood vol- these patients is a compensatory adaptation to cardiac ume and plasma volume, normalized for body weight denervation and that HT recipients may require an (milliliters per kilogram), were significantly greater in expanded blood volume to maintain cardiac output the HT recipients. The 12% blood volume expansion and blood pressure in the absence of efferent cardiac in the HT recipients was not associated with signifi- autonomic nerves. Norepinephrine: We previously reported that plasma cantly different concentrations in plasma Na1 or NE levels in HT recipients are significantly elevated at plasma osmolality. exercise intensities .70% of peak power output when compared with age-matched controls.20 Others have DISCUSSION The main finding of this study was that circulating speculated that high humoral levels of NE are responNE in HT recipients contributes negligibly to the rapid sible for the progressive increases in stroke volume adjustment in cardiac output during the rest to exercise observed in HT recipients (but not normals) at work transition. HT recipients augmented stroke volume intensities .50%.7,9,11 Certainly, high NE levels immediately at the onset of exercise (30 seconds) and would enhance venous tone (venous return) and inachieved stroke volume values that were 61% greater crease cardiac contractility. than baseline values at rest. However, this adjustment The present study was the first investigation to in cardiac output was too rapid to be ascribed to serially measure NE levels during the transition from elevated plasma NE concentrations. Indeed, plasma rest to steady-state exercise in HT recipients. We NE levels did not increase significantly above baseline observed no difference between groups in NE concenvalues in either group until approximately 4 minutes trations at rest or the absolute NE response to exercise after the onset of cycle exercise. The results of this at 40% of peak power output. Plasma NE in both MISCELLANEOUS/HEART TRANSPLANT CARDIAC OUTPUT
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groups did not increase significantly above resting values until approximately the fourth minute of constant load exercise. Therefore, we doubt that immediate augmentation of stroke volume in HT recipients at the onset of exercise is due to the effect of humoral catecholamines. Rather, we speculate that abrupt increases in stroke volume are likely due to augmented venous return, secondary to expanded blood volume. 1. Younis LT, Melin JA, Schoevaerdts JC, Van Dyck M, Detry JM, Robert A, Chalant C, Goenen M. Left ventricular systolic function and diastolic filling at rest and during upright exercise after orthotopic heart transplantation: comparison with young and aged normal subjects. J Heart Transpl 1990;9:683– 692. 2. Rudas L, Pflugfelder PW, Kostuk WJ. Comparison of hemodynamic responses during dynamic exercise in the upright and supine postures after orthotopic cardiac transplantation. J Am Coll Cardiol 1990;16:1367–1373. 3. Rudas L, Pflugfelder PW, Mckenzie FN, Menkis AH, Novick RJ, Kostuk WJ. Normalization of upright exercise hemodynamics and improved exercise capacity one year after orthotopic cardiac transplantation. Am J Cardiol 1992;69:1336 – 1339. 4. Kappagoda CT, Haennel RG, Serrano-Fiz S, Davies DH. The hemodynamic responses during upright exercise after orthotopic cardiac transplant. Arch Physical Med Rehabil 1993;14:484 – 489. 5. Kao AC, Van Trigt P, Schaeffer-Mccall GS, Shaw JP, Kuzil BB, Page RD, Higginbotham MB. Central and peripheral limitations to upright exercise in untrained cardiac transplant recipients. Circulation 1994;89:2605–2615. 6. Kao AC, Vantrigt P, Shaeffer-Mccall GS, Shaw JP, Kuzil BB, Page RD, Higginbotham MB. Allograft diastolic dysfunction and chronotropic incompetence limit cardiac output response to exercise two to six years after heart transplantation. J Heart Lung Transpl 1995;14:11–22. 7. Pope SE, Stinson EB, Daughters GT, Schroeder JS, Ingels NB, Alderman EL. Exercise response of the denervated heart in long-term cardiac transplant recipients. Am J Cardiol 1980;46:213–218. 8. Pflugfelder PW, Mckenzie N, Kostuk WJ. Hemodynamic profiles at rest and
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during supine exercise after orthotopic cardiac transplantation. Am J Cardiol 1988;61:1328 –1333. 9. Keteyian SJ, Charles RC, Marks AB, Levine TK, Frank F, Berry TL. Cardiovascular responses to submaximal arm and leg exercise in cardiac transplant patients. Med Sci Sports Exerc 1994;26:420 – 424. 10. Shephard RJ, Kavanagh T, Mertens D, Qureshi S. Kinetics of the transplanted heart. Implications for the choice of field-test exercise protocol. J Cardiopulm Rehabil 1995;15:288 –296. 11. Degre S, Niset G, DeSmet J, Ibrahim T, Stoupel E, LeClerc J, Primo G. Cardiorespiratory responses to early exercise testing after orthotopic cardiac transplantation. Am J Cardiol 1987;60:926 –928. 12. Yusef S, Phil D, Theodoropoulos S, Mathias CJ, Dhalla N, Wittes J, Mitchell A, Yacoub M. Increased sensitivity to the denervated transplanted human heart to isoprenaline both before and after b-adrenergic blockade. Circulation 1987;75: 696 –704. 13. Gilbert EM, Eiswirth CC, Mealey PC, Larrabee P, Herrick CM, Bristow MR. B-adrenergic supersensitivity of the transplanted human heart is presynaptic in origin. Circulation 1989;79:344 –349. 14. Braith RW, Mills RM, Wilcox CS, Convertino VA, Davis GL, Limacher MC, Wood CE. Fluid homeostasis after heart transplantation: the role of cardiac denervation. J Heart Lung Transpl 1996;15:872– 880. 15. Bellet M, Cabrol C, Sassano P, Leger P, Corvol P, Menard J. Systemic hypertension after cardiac transplantation: effect of cyclosporine on the reninangiotensin-aldosterone system. Am J Cardiol 1985;56:927–931. 16. Pepke-Zaba J, Higenbottam TW, Dinh Xuan AT, Scott JP, English TAH, Wallwork J. Validation of impedance cardiography measurements of cardiac output during limited exercise in heart transplant recipients. Transplant Int 1990;3:108 –112. 17. Greenleaf JE, Convertino VA, Mangseth GR. Plasma volume during stress testing in man: osmolality and red cell volume. J Appl Physiol 1979;47:1031– 1038. 18. Chaplin H, Mollison Pl, Vetter H. The body/venous hematocrit ratio: its constance over a wide hematocrit range. J Clin Invest 1953;32:1309 –1316. 19. Kavanagh T, Yacoub MH, Mertens DJ, Kennedy J, Campbell RB, Sawyer P. Cardiorespiratory responses to exercise training after orthotopic cardiac transplantation. Circulation 1988;77:162–171. 20. Braith RW, Wood CE, Limacher MC, Pollock ML, Lowenthal DT, Phillips MI, Staples ED. Abnormal neuroendocrine responses during exercise in heart transplant recipients. Circulation 1992;86:1453–1463.
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