Gas exchange and metabolic transients in heart transplant recipients

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

Highly athletic terrestrial mammals: horses and dogs

Evolutionary forces drive beneficial adaptations in response to a complex array of environmental conditions. In contrast, over several millennia, humans have been so enamored by the running/athletic prowess of horses and dogs that they have sculpted their anatomy and physiology based solely upon running speed. Thus, through hundreds of generations, those structural and functional traits crucial for running fast have been optimized. Central among these traits is the capacity to uptake, transport and utilize oxygen at spectacular rates. Moreover, the coupling of the key systems--pulmonary-cardiovascular-muscular is so exquisitely tuned in horses and dogs that oxygen uptake response kinetics evidence little inertia as the animal transitions from rest to exercise. These fast oxygen uptake kinetics minimize Intramyocyte perturbations that can limit exercise tolerance. For the physiologist, study of horses and dogs allows investigation not only of a broader range of oxidative function than available in humans, but explores the very limits of mammalian biological adaptability. Specifically, the unparalleled equine cardiovascular and muscular systems can transport and utilize more oxygen than the lungs can supply. Two consequences of this situation, particularly in the horse, are profound exercise-induced arterial hypoxemia and hypercapnia as well as structural failure of the delicate blood-gas barrier causing pulmonary hemorrhage and, in the extreme, overt epistaxis. This chapter compares and contrasts horses and dogs with humans with respect to the structural and functional features that enable these extraordinary mammals to support their prodigious oxidative and therefore athletic capabilities.

V̇o2 on-kinetics in isolated canine muscle in situ during slowed convective O2 delivery

The purpose of this study was to examine O2 uptake (V̇o2) on-kinetics when the spontaneous blood flow (and therefore O2 delivery) on-response was slowed by 25 and 50 s. The isolated gastrocnemius muscle complex (GS) in situ was studied in six anesthetized dogs during transitions from rest to a submaximal metabolic rate (≈50–70% of peak V̇o2). Four trials were performed: 1) a pretrial in which resting and steady-state blood flows were established, 2) a control trial in which the blood flow on-kinetics mean response time (MRT) was set at 20 s (CT20), 3) an experimental trial in which the blood flow on-kinetics MRT was set at 45 s (EX45), and 4) an experimental trial in which the blood flow on-kinetics MRT was set at 70 s (EX70). Slowing O2 delivery via slowing blood flow on-kinetics resulted in a linear slowing of the V̇o2 on-kinetics response ( R = 0.96). Average MRT values for CT20, EX45, and EX70 V̇o2 on-kinetics were (means ± SD) 17 ± 2, 23 ± 4, and 26 ± 3 s, respectively ( P < 0.05 among all). During these transitions, slowing blood flow resulted in greater muscle deoxygenation (as indicated by near-infrared spectroscopy), suggesting that lower intracellular Po2 values were reached. In this oxidative muscle, V̇o2 and O2 delivery were closely matched during the transition period from rest to steady-state contractions. In conjunction with our previous work showing that speeding O2 delivery did not alter V̇o2 on-kinetics under similar conditions, it appears that spontaneously perfused skeletal muscle operates at the nexus of sufficient and insufficient O2 delivery in the transition from rest to contractions.

Cardiovascular responses to incremental and sustained submaximal exercise in heart transplant recipients

The cardiovascular response to exercise in heart transplant recipients (HTR) has been compared with that of healthy individuals matched to the recipient age (RM controls). However, no study has compared HTR with donor age-matched (DM) controls. Moreover, the cardiovascular response to sustained submaximal exercise in HTR requires further evaluation. We therefore examined cardiovascular responses during incremental exercise and sustained (1 h) submaximal aerobic exercise in 9 clinically stable HTR [63 +/- 10 yr of age, 24.2 +/- 10.9 ml x kg(-1) x min(-1) peak O(2) uptake (Vo(2peak))] and 11 healthy age-matched controls (60 +/- 11 yr of age and 36.3 +/- 10.7 ml.kg(-1) x min(-1) Vo(2peak) for 6 RM controls and 35 +/- 8 yr of age and 51.1 +/- 10.4 ml x kg(-1) x min(-1) Vo(2peak) for 5 DM controls). Heart rate (HR) and left ventricular systolic and diastolic volumes (2-dimensional echocardiography) indexed to body surface area [end-systolic and end-diastolic volume indexes (EDVI and ESVI)], cardiac output (CI), ejection fraction (EF), systemic vascular resistance (SVRI), end-systolic elastance index, and arterial elastance index were determined. Although systolic function was maintained during incremental exercise, peak CI was significantly reduced (6.7 +/- 2.4 vs. 11.6 +/- 1.4 l x min(-1) x m(-2)), secondary to blunted HR, EDVI, and increased peak SVRI, in HTR compared with DM controls. The lower peak CI in HTR than in RM controls was due to blunted peak EDVI (54.1 +/- 13.2 vs. 68.6 +/- 5.7 ml/m(2)). During sustained submaximal exercise, HTR exhausted their preload reserve, a response for which changes in ESVI, HR, or EF did not fully compensate. Thus it appears that HTR are limited by impaired preload reserve, HR reserve, and vascular reserve during exercise conditions.

Impaired pulmonary oxygen uptake kinetics and reduced peak aerobic power during small muscle mass exercise in heart transplant recipients

We examined peak and reserve cardiovascular function and skeletal muscle oxygenation during unilateral knee extension (ULKE) exercise in five heart transplant recipients (HTR, mean +/- SE; age: 53 +/- 3 years; years posttransplant: 6 +/- 4) and five age- and body mass-matched healthy controls (CON). Pulmonary oxygen uptake (Vo(2)(p)), heart rate (HR), stroke volume (SV), cardiac output (Q), and skeletal muscle deoxygenation (HHb) kinetics were assessed during moderate-intensity ULKE exercise. Peak exercise and reserve Vo(2)(p), Q, and systemic arterial-venous oxygen difference (a-vO(2diff)) were 23-52% lower (P < 0.05) in HTR. The reduced Q and a-vO(2diff) reserves were associated with lower HR and HHb reserves, respectively. The phase II Vo(2)(p) time delay was greater (HTR: 38 +/- 2 vs. CON: 25 +/- 1 s, P < 0.05), while time constants for phase II Vo(2)(p) (HTR: 54 +/- 8 vs. CON: 31 +/- 3 s), Q (HTR: 66 +/- 8 vs. CON: 28 +/- 4 s), and HHb (HTR: 27 +/- 5 vs. CON: 13 +/- 3 s) were significantly slower in HTR. The HR half-time was slower in HTR (113 +/- 21 s) vs. CON (21 +/- 2 s, P < 0.05); however, no significant difference was found between groups for SV kinetics (HTR: 39 +/- 8 s vs. CON 31 +/- 6 s). The lower peak Vo(2)(p) and prolonged Vo(2)(p) kinetics in HTR were secondary to impairments in both cardiovascular and skeletal muscle function that result in reduced oxygen delivery and utilization by the active muscles.

Noninvasive Evaluation of Skeletal Muscle Oxidative Metabolism after Heart Transplant

PURPOSE

The main aim of the present study was to investigate skeletal muscle oxidative metabolism in heart transplant recipients (HTR) by noninvasive tools.

METHODS

Twenty male HTR (age 50.4 +/- 2.6 yr; mean +/- SE) and 17 healthy untrained age-matched controls (CTRL) performed an incremental exercise (IE) and a series of constant-load (CLE) moderate-intensity exercise tests on a cycloergometer. The following variables were determined: heart rate (HR); breath-by-breath pulmonary O2 uptake (VO2); and skeletal muscle (vastus lateralis) oxygenation indices by continuous-wave near-infrared spectroscopy. Changes in concentration of deoxygenated hemoglobin (Hb) and myoglobin (Mb) (Delta[deoxy(Hb + Mb)]), expressed as a fraction of values obtained during a transient limb ischemia, were taken as an index of skeletal muscle O2 extraction. "Peak" values were determined at exhaustion during IE. Kinetics of adjustment of variables were determined during CLE.

RESULTS

VO2peak, HRpeak, and Delta[deoxy(Hb + Mb)] peak were significantly lower in HTR than in CTRL (17.1 +/- 0.7 vs 34.0 +/- 1.9 mL.kg(-1).min(-1), 133.8 +/- 3.8 vs 173.0 +/- 4.8 bpm, and 0.42 +/- 0.03 vs 0.58 +/- 0.04, respectively). In HTR, Delta[deoxy(Hb + Mb)] increase at submaximal workloads was steeper than in CTRL, suggesting an impaired O2 delivery to skeletal muscles, whereas the lower Delta[deoxy(Hb + Mb)] peak values suggest an impaired capacity of O2 extraction at peak exercise. VO2 and HR kinetics during CLE were significantly slower in HTR than in CTRL, whereas, unexpectedly, no significant differences were found for Delta[deoxy(Hb+Mb)] kinetics (mean response time: 21.3 +/- 1.1 vs 20.2 +/- 1.2 s).

CONCLUSION

The findings confirm the presence of both "central" (cardiovascular) and "peripheral" (at the skeletal muscle level) impairments to oxidative metabolism in HTR. The noninvasiveness of the measurements will allow for serial evaluation of the patients, in the presence and/or absence of rehabilitation programs.

Oxygen uptake kinetics: old and recent lessons from experiments on isolated muscle in situ

The various mechanisms responsible for ATP resynthesis include phosphocreatine (PCr) hydrolysis, anaerobic glycolysis and oxidative phosphorylation. Among these, the latter represents the most important mechanism of energy provision. However, oxidative phosphorylation is characterized by a lower maximal power and a slow attainment of a steady state in response to increased metabolic demand. The rate of adjustment of oxidative metabolism during metabolic transitions, which can be evaluated on the basis of the analysis of O2 uptake (VO2) kinetics, has implications for exercise tolerance and muscle fatigue. Analysis of VO2 kinetics represents a valid tool for the functional evaluation of healthy subjects, athletes and patients. Over the last 35 years experiments conducted on isolated muscle preparations in situ have allowed us to gain insights into several key aspects of skeletal muscle VO2 kinetics. Their main limiting factor resides in an intrinsic slowness of intracellular oxidative metabolism when adjusting to augmented metabolic needs. The rate of adjustment of oxidative phosphorylation in mitochondria can be functionally related to PCr hydrolysis occurring in the cytoplasm.

Exercise after heart transplantation

Exercise intolerance in heart transplant recipients (HTR) has a multifactorial origin, involving complex interactions among cardiac, neurohormonal, vascular, skeletal muscle and pulmonary abnormalities. However, the role of these abnormalities may differ as a function of time after transplantation and of many other variables. The present review is aimed at evaluating the role of cardiac, pulmonary and muscular factors in limiting maximal aerobic performance of HTR, and the benefits of chronic exercise. Whereas pulmonary function does not seem to affect gas exchange until a critical value of diffusing lung capacity is attained, cardiac and skeletal muscle function deterioration may represent relevant factors limiting maximal and submaximal aerobic performance. Cardiac function is mainly limited by chronotropic incompetence and diastolic dysfunction, whereas muscle activity seems to be limited by impaired oxygen supply as a consequence of the reduced capillary network. The latter may be due to either immunosuppressive regimen or deconditioning. Endurance and strength training may greatly improve muscle function and maximal aerobic performance of HTR, and may also reduce side effects of immunosuppressive therapy and control risk factors for cardiac allograft vasculopathy. For the above reasons exercise should be considered an important therapeutic tool in the long-term treatment of heart transplant recipients.

Effects of an enhanced heart rate reserve on aerobic performance in patients with a heart transplant

OBJECTIVE

The aim of this study was to investigate whether a high-intensity warm-up at the start of a graded, symptom-limited exercise test would enhance heart rate reserve and thus improve the aerobic performance of orthotopic heart transplant patients.

DESIGN

Adrenal and cardiorespiratory responses were compared in 10 orthotopic heart transplant patients who performed two graded, symptom-limited exercise tests on an ergocycle.

RESULTS

At the start of the graded, symptom-limited exercise test, high intensity increased the norepinephrine level more than usual intensity between rest and the third minute of exercise. This higher norepinephrine level was followed by a higher heart rate response from the fourth minute of exercise. Heart rate reserve was enhanced during high-intensity exercise, without any significant change in peak oxygen uptake.

CONCLUSIONS

This specific warm-up enhanced heart rate reserve during a graded, symptom-limited exercise test on an ergocycle. Mechanisms more important than limited heart rate reserve are involved in the limitation of exercise tolerance in orthotopic heart transplant patients.

Exercise does not induce oxidative stress in trained heart transplant recipients

PURPOSE

The objectives of this study were twofold: 1) to determine the effect of incremental exercise to volitional fatigue on plasma levels of lipid peroxidation (malondialdehyde) in heart transplant recipients (HRT) and 2) to examine blood antioxidant capacity in HTR by assessment of antioxidant enzyme activities and vitamin E levels.

METHODS

Seven endurance-trained HTR (mean +/- SD; age 39.7 +/- 12.8 yr) and seven endurance-trained healthy, age-matched control subjects (HC) (mean age 40.6 +/- 10.7 yr) performed a symptom-limited incremental exercise test on a cycle ergometer. Venous blood samples were obtained at rest, exercise, and during recovery and analyzed for plasma levels of malondialdehyde (MDA) as well as markers of blood antioxidant capacity. After exercise and during recovery, all dependent measures were corrected for plasma volume changes induced by exercise. Significance was established at (P < 0.05).

RESULTS

No group differences existed in plasma levels of MDA at rest. Further, graded exercise did not alter plasma levels of MDA in either group. Resting erythrocyte glutathione peroxidase (GPX) activity was significantly lower and erythrocyte superoxide dismutase (SOD) activity was higher in HTR compared with HC. Finally, at rest, no group differences existed in plasma GPX activity or vitamin E levels.

CONCLUSIONS

Graded exercise to fatigue does not promote an increase in oxidative stress in blood of exercise trained HTR. Therefore, physical exercise does not appear to pose an oxidative-stress risk for these patients.

VO(2) kinetics reveal a central limitation at the onset of subthreshold exercise in heart transplant recipients

Because the cardiocirculatory response of heart transplant recipients (HTR) to exercise is delayed, we hypothesized that their O(2) uptake (VO(2)) kinetics at the onset of subthreshold exercise are slowed because of an impaired early "cardiodynamic" phase 1, rather than an abnormal subsequent "metabolic" phase 2. Thus we compared the VO(2) kinetics in 10 HTR submitted to six identical 10-min square-wave exercises set at 75% (36 +/- 5 W) of the load at their ventilatory threshold (VT) to those of 10 controls (C) similarly exercising at the same absolute (40 W; C40W group) and relative load (67 +/- 14 W; C67W group). Time-averaged heart rate, breath-by-breath VO(2), and O(2) pulse (O(2)p) data yielded monoexponential time constants of the VO(2) (s) and O(2)p increase. Separating phase 1 and 2 data permitted assessment of the phase 1 duration and phase 2 VO(2) time constant (). The VO(2) time constant was higher in HTR (38.4 +/- 7.5) than in C40W (22.9 +/- 9.6; P < or = 0. 002) or C67W (30.8 +/- 8.2; P < or = 0.05), as was the O(2)p time constant, resulting from a lower phase 1 VO(2) increase (287 +/- 59 vs. 349 +/- 66 ml/min; P < or = 0.05), O(2)p increase (2.8 +/- 0.6 vs. 3.6 +/- 1.0 ml/beat; P < or = 0.0001), and a longer phase 1 duration (36.7 +/- 12.3 vs. 26.8 +/- 6.0 s; P < or = 0.05), whereas the was similar in HTR and C (31.4 +/- 9.6 vs. 29.9 +/- 5.6 s; P = 0.85). Thus the HTR have slower subthreshold VO(2) kinetics due to an abnormal phase 1, suggesting that the heart is unable to increase its output abruptly when exercise begins. We expected a faster in HTR because of their prolonged phase 1 duration. Because this was not the case, their muscular metabolism may also be impaired at the onset of subthreshold exercise.

Chronotropic competence in endurance trained heart transplant recipients: heart rate is not a limiting factor for exercise capacity

OBJECTIVES

The purpose of this study was to show that the chronotropic potential of the well trained heart transplant recipient (HTR) does not limit exercise capacity.

BACKGROUND

Chronotropic incompetence is considered to be the main limiting factor of the functional capacity of heart transplant recipients. However, no systematic study had been published on patients who had spontaneously undergone heavy endurance training for several years.

METHODS

Heart rate (HR) and respiratory gas exchanges (VO2, VCO2, VE) were measured in 14 trained HTRs (T-HTRs) during exercise tests on a bicycle, on a treadmill and by Holter electrocardiography during a race.

RESULTS

Peak values observed in T-HTRs during the treadmill test were higher than those reached during the bicycle test (VO2peak: 39.8+/-6.9 vs. 32.5+/-7.8 ml x kg(-1) x min(-1), p < 0.001; HRpeak: 169+/-14 vs. 159+/-16 bpm, p < 0.01). During treadmill exercise VO2peak and HRpeak values observed were very close to the mean predicted VO2pmax and HRpmax. The maximum heart rate during the race (HRrace) was greater than HRpeak values during the treadmill test (179+/-14 vs 169+/-14 bpm, p < 0.01) and slightly above the mean predicted values (HRrace/HRpmax X 100 = 101+/-10%). The treadmill exercise test yields more reliable data than does the bicycle test.

CONCLUSIONS

Extensive endurance training enables heart transplant recipients to reach physical fitness levels similar to those of normal sedentary subjects; heart rate does not limit their exercise capacity.

Enhanced myocardial 18F-2-fluoro-2-deoxyglucose uptake after orthotopic heart transplantation assessed by positron emission tomography

OBJECTIVES

We sought to assess the relation between glucose metabolism, myocardial perfusion and cardiac work after orthotopic heart transplantation.

BACKGROUND

The metabolic profile of the transplanted cardiac muscle is affected by the lack of sympathetic innervation, impaired inotropic function, chronic vasculopathy, allograft rejection and immunosuppressive therapy. In relation to myocardial perfusion and cardiac work, glucose metabolism has not previously been studied in heart transplant recipients.

METHODS

Regional myocardial blood flow (ml.min-1.g-1) and 18F-2-fluoro-2-deoxyglucose (18FDG) uptake rate (ml.s-1.g-1) were measured after an overnight fast in 9 healthy male volunteers (mean age +/- SD 32 +/- 7 years) and in 10 male patients (mean age 50 +/- 10 years) who had a nonrejecting heart transplant, normal left ventricular function and no angiographic evidence of epicardial coronary sclerosis. Measurements were made by using dynamic positron emission tomography (PET) with 15O-labeled water and 18FDG, respectively. Heart rate and blood pressure were also measured for calculation of rate-pressure product.

RESULTS

18FDG uptake was similar in all heart regions in the patients and volunteers (intrasubject regional variably 12 +/- 8% and 16 +/- 12%, respectively, p = 0.51). Regional myocardial blood flow was similarly evenly distributed (intrasubject regional variability 14 +/- 10% and 12 +/- 8%, respectively, p = 0.67). Mean 18FDG uptake and myocardial blood flow values for the whole heart are given because no regional differences were identified. 18FDG uptake was on average 196% higher in the patients than in the volunteers (2.90 +/- 1.79 x 10(-4) vs. 0.98 +/- 0.38 x 10(-4) ml.s-1.g-1, p = 0.006). Regional myocardial blood flow and rate-pressure product were similarly increased in the patient group, but by only 41% (1.14 +/- 0.3 vs. 0.81 +/- 0.13 ml.min-1.g-1, p = 0.008) and 53% (11,740 +/- 2,830 vs. 7,689 +/- 1,488, p = 0.001), respectively.

CONCLUSIONS

18FDG uptake is homogeneously increased in normally functioning nonrejecting heart transplants. This finding suggests that glucose may be a preferred substrate in the transplanted heart. The magnitude of this observed increase is significantly greater than that observed for myocardial blood flow or cardiac work. In the patient group, the latter two variables were increased to a similar degree over values in control hearts, indicating a coupling between cardiac work load and myocardial blood flow. The disproportionate rise in 18FDG uptake may be accounted for by inefficient metabolic utilization of glucose by the transplanted myocardium or by the influence of circulating catecholamines, which may stimulate glucose uptake independently of changes in cardiac work load.

Electromyographic response to exercise in cardiac transplant patients: a new method for anaerobic threshold determination?

The purpose of this study was to investigate the possible use of integrated surface electromyography (iEMG) in cardiac transplant patients (CTPs) as a new noninvasive determinant of the metabolic response to exercise by studying the relationship between the iEMG threshold (iEMGT) and other more conventional methods for anaerobic threshold (AT) determination, such as the lactate threshold (LT) and the ventilatory threshold (VT). Thirteen patients (age: 57+/-7 years, mean+/-SD; height: 163+/-7 cm; body mass: 70.5+/-8.6 kg; posttransplant time: 87+/-49 weeks) were selected as subjects. Each of them performed a ramp protocol on a cycle ergometer (starting at 0 W, the workload was increased in 10 W/min). During the tests, gas exchange data, blood lactate levels, and iEMG of the vastus lateralis were collected to determine VT, LT, and iEMGT, respectively. The results evidenced no significant difference between mean values of VT, LT, or iEMGT, when expressed either as oxygen uptake (11.1+/-2.4, 11.7+/-2.3, and 11.0+/-2.8 mL/kg/min, respectively) or as percent maximum oxygen uptake (61.6+/-7.5, 62.2+/-7.7, and 59.6+/-8.2%, respectively). In conclusion, our findings suggest that iEMG might be used as a complementary, noninvasive method for AT determination in CTPs. In addition, since the aerobic impairment of these patients is largely due to peripheral limitation, determination of iEMGT could be used to assess the effectiveness of an exercise rehabilitation program to improve muscle aerobic capacity in CTPs.

Gas exchange and cardiovascular kinetics with different exercise protocols in heart transplant recipients

Metabolic and cardiovascular adjustments to various submaximal exercises were evaluated in 82 heart transplant recipients (HTR) and in 35 control subjects (C). HTR were tested 21.5 +/- 25.3 (SD) mo (range 1.0-137.1 mo) posttransplantation. Three protocols were used: protocol A consisted of 5 min of rectangular 50-W load repeated twice, 5 min apart [5 min rest, 5 min 50 W (Ex 1), 5 min recovery, 5 min 50 W (Ex 2)]; protocol B consisted of 5 min of rectangular load at 25, 50, or 75 W; protocol C consisted of 15 min of rectangular load at 25 W. Breath-by-breath pulmonary ventilation (VE), O2 uptake (VO2), and CO2 output (VCO2) were determined. During protocol A, beat-by-beat cardiac output (Q) was estimated by impedance cardiography. The half times (t1/2) of the on- and off-kinetics of the variables were calculated. In all protocols, t1/2 values for VO2 on-, VE on-, and VCO2 on-kinetics were higher (i.e., the kinetics were slower) in HTR than in C, independently of workload and of the time post-transplantation. Also, t1/2 Q on- was higher in HTR than in C. In protocol A, no significant difference of t1/2 VO2 on- was observed in HTR between Ex 1 (48 +/- 9 s) and Ex 2 (46 +/- 8 s), whereas t1/2 Q on- was higher during Ex 1 (55 +/- 24 s) than during Ex 2 (47 +/- 15 s). In all protocols and for all variables, the t1/2 off-values were higher in HTR than in C, In protocol C, no differences of steady-state VE, VO2, and VCO2 were observed in both groups between 5, 10, and 15 min of exercise. We conclude that 1) in HTR, a "priming" exercise, while effective in speeding up the adjustment of convective O2 flow to muscle fibers during a second on-transition, did not affect the VO2 on-kinetics, suggesting that the slower VO2 on- in HTR was attributable to peripheral (muscular) factors; 2) the dissociation between Q on- and VO2 on-kinetics in HTR indicates that an inertia of muscle metabolic machinery is the main factor dictating the VO2 on-kinetics; and 3) the VO2 off-kinetics was slower in HTR than in C, indicating a greater alactic O2 deficit in HTR and, therefore, a sluggish muscle VO2 adjustment.

Relationship between lactate and ammonia thresholds in heart transplant patients

The purpose of this investigation was to study the relationship between both blood ammonia thresholds (AmT) and lactate thresholds (LT) during dynamic exercise in cardiac transplant patients (CTPs). Eleven male patients who had undergone orthotopic cardiac transplantation (age: 54 +/- 11 years, mean +/- SD; height: 165.1 +/- 6.6 cm; body mass: 78.3 +/- 16.1 kg) participated in this study. Each of them performed a bicycle ergometer test (ramp protocol) until volitional fatigue. During each test, gas exchange parameters and ECG responses were determined continuously. In addition, blood lactate and ammonia concentrations were measured every 2 min for determination of both LT and AmT, respectively. Peak values of oxygen uptake (Vo2), respiratory exchange ratio, ventilation, and heart rate averaged 15.9 +/- 3.03 mL.Kg-1.min-1, 1.02 +/- 0.06, 46.69 +/- 5.69 L.min-1, and 124 +/- 16 beats per minute, respectively. However, blood concentrations of lactate and ammonia at peak exercise were 3.7 +/- 0.4 mmol.L-1 and 85.6 +/- 31.7 micrograms.dL-1, respectively. LT and AmT were detected in 8 (72.7% of total) and 9 (81.8% of total) of 11 subjects, respectively. No significant differences were found between mean values of LT and AmT, when both were expressed either as Vo2 (10.01 +/- 1.19 vs 10.5 +/- 2.38 mL.kg-1.min-1, respectively) or as percent Vo2 peak (64.62 +/- 11.362 vs 66.48 +/- 9.19%, respectively). In addition, LT and AmT were significantly correlated (p < 0.05) when both were expressed either as Vo2 (mL.kg-1.min-1) or as percent Vo2 peak (r = 0.70 and r = 0.68, respectively). Our findings suggest that in CTPs, both LT and AmT occur at similar workloads, probably as a result of skeletal muscle alterations associated with chronic deconditioning and immunosuppressive therapy.

Short endurance training improves lactate removal ability in patients with heart transplants

Eight male patients with heart transplants at least a year after the operation were submitted to a 6-wk endurance training program and explored for their blood lactate kinetics before and after exercise. The tests consisted of a bicycle exercise upgraded by 20 W every 2 min until volitional fatigue. Training induced a significant (P < 0.025) decrease in lactate concentrations from the 40-W to the 120-W exercise step and a significant increase (P < 0.025) in the time into exercise (9.87 +/- 0.87 min vs 7.17 +/- 0.90 min) at which a lactate concentration of 2 mmol.l-1 was reached. Lactate recovery curves were significantly lower (P < 0.036) after training than before training, except at minutes 1, 2, 8, and 60. The fits of a biexponential mathematical model to the lactate recovery curves reveal a significant (P < 0.036) training-induced increase (+71%) in the slow-velocity constant gamma 2v of the model. In view of the functional meaning given to this parameter, namely the ability to remove lactate, it is concluded that training lowers blood lactate concentrations during exercise and recovery in patients with heart transplants at least in part by raising the efficiency with which lactate is removed, and that the ability to remove lactate can be a valuable criterion to evaluate physical fitness.

Influence of post-surgery time after cardiac transplantation on exercise responses

Influence of post-surgery time after cardiac transplantation on exercise responses. Med. Sci. Sports Exerc., Vol. 28, No. 2, pp. 171-175, 1996. To test the hypothesis that exercise response changes with time after cardiac transplantation, we investigated the cardiorespiratory responses of nine orthotopic heart transplant patients (52.4 +/- 2 yr) during graded exercise tests (30 W.3 min-1) done at 1, 3, 6, 9 and 12 months post-surgery. At peak exercise, 1) oxygen uptake per kg of body weight (VO2), minute ventilation (VE) and oxygen pulse (O2 pulse) did not change significantly between 1 and 12 months postsurgery; 2) transplanted heart rate (HRt) and delta heart rate (peak exercise heart rate--resting heart rate) increased significantly over time (P < 0.01; P < 0.05) with a marked increase between 1 and 3 months (P < 0.05); and (3) a significant negative correlation existed between O2 pulse and HRt (r = -0.36, P < 0.05), whereas no correlation was found between delta heart rate and delta VO2 (peak exercise VO2- resting VO2, l.min-1). During submaximal exercise, HRt increased significantly over time (P < 0.001); VO2, VE, and O2 pulse showed no significant change; and the VO2-HRt relationship shifted toward higher values of HRt. We conclude that, in the absence of formal physical training, the exercise response of denervated transplanted heart increases in relation to post-surgery time but does not affect oxygen uptake at submaximal and peak levels of exercise.

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

BACKGROUND

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

METHODS AND RESULTS

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

CONCLUSIONS

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

Blood gas dynamics at the onset of exercise in heart transplant recipients

One hypothesis to explain the rapid neural component of exercise hyperpnea contends that afferent stimuli originating in the ventricles of the heart act reflexly on the respiratory center at the onset of exercise, ie, "cardiodynamic hyperpnea." Orthotopic cardiac transplantation (Tx) results in the loss of afferent information from the ventricles. Thus, Tx possibly results in transient hypercapnia and hypoxemia in deafferented heart transplant recipients (HTR) at the onset of exercise due to hypoventilation. To examine the cardiodynamic hypothesis, we collected serial arterial blood gas (ABG) samples during both the transient and the steady-state responses to moderate cycle exercise in 5 HTRs (55 +/- 7 years) 14 +/- 7 months post-Tx and 5 control subjects matched with respect to gender, age, and body composition. Forced vital capacity, forced expiratory volume in 1 s, total lung capacity, and diffusion capacity did not differ (p > or = 0.05) between groups. Resting arterial PO2, PCO2, and pH did not differ between groups (p > or = 0.05). The ABGs were drawn every 30 s during the first 5 min and at 6, 8, and 10 min of constant load square wave cycle exercise at 40 percent of the peak power output (watts). Absolute and relative changes in arterial PO2, PCO2, and pH were similar (p > or = 0.05) between HTR and the control group at all measurement periods during exercise. Heart rate (%HRmax reserve), rating of perceived exertion, and reductions in plasma volume (% delta from baseline) did not differ between HTR and control during exercise at 40 percent of peak power output (p > or = 0.05). Our results demonstrate that there is no discernible abnormality in ABG dynamics during the transient response to exercise at 40 percent of peak power output in patients with known cardiac denervation. These data do not support the cardiodynamic hyperpnea hypothesis of ventilatory control in humans. The absence of hypercapnia in HTRs is further evidence for the existence of redundant mechanisms capable of stimulating exercise hyperpnea.

Ventilatory response to exercise after heart and lung denervation in humans

This study, aimed at investigating some aspects of breathing control at work, was conducted on 8 heart and lung transplant recipients (HLTR) (age 33 +/- 13 years, mean +/- SD; 10 +/- 6 months post-transplantation) and on two control groups, i.e. 11 heart transplant recipients (HTR) and 11 healthy untrained subjects (C). The patients performed a series of 2 to 6 1-min exercise bouts (at 25 or 50 W, corresponding to about 50% of their VO2max) on a bicycle ergometer, followed by a 5 min 25 or 50 W constant load. C exercised both at 50 W (C1) and at 50% of their VO2max (C2). Inspiratory (VI) and expiratory (VE) ventilation, tidal volume (VT), respiratory frequency (fR), end-tidal O2 and CO2 partial pressures (PETO2 and PETCO2 and gas exchange (VO2 and VCO) were measured breath-by-breath. "Phase I" ventilatory response (ph I) was determined as the mean changes of VI, VE, VT, fR, PETO2 and PETCO2, compared to rest, during the first two respiratory cycles following exercise onset. In HLTR ph I did not significantly differ from that of C1 and C2, whereas the response was lower in HTR. VE, VO2 and VCO2 responses during "phase II" (t 1/2 on-) and "phase III" (steady state exercise) were similar in HLTR and in HTR. t 1/2 on- were longer in HLTR and in HTR compared to C1. In 3 HLTR the ventilatory pattern during the 5 min constant loads was similar to that of HTR and C, whereas 4 HLTR presented higher VT and lower fR values. It is concluded that: 1) The ventilatory response to exercise, in all its phases, is substantially preserved despite lung denervation. When slight alterations are found (i.e. the slower phase II), they are presumably of peripheral origin. 2) The normal ph I in HLTR indicates that cardiac and/or pulmonary inputs to the respiratory centers are not involved in its regulation, or that their role can be subserved by other ventilatory control mechanisms.

Kinetics of heart rate and catecholamines during exercise in humans. The effect of heart denervation

To elucidate the role of factors other than the nervous system in heart rate (fc) control during exercise, the kinetics of fc and plasma catecholamine concentrations were studied in ten heart transplant recipients during and after 10-min cycle ergometer exercise at 50 W. The fc did not increase at the beginning of the exercise for about 60 s. Then in the eight subjects who completed the exercise it increased following an exponential kinetic with a mean time constant of 210 (SEM 22) s. The two other subjects were exhausted after 5 and 8 min of exercise during which fc increased linearly. At the cessation of the exercise, fc remained unchanged for about 50 s and then decreased exponentially with a time constant which was unchanged from that at the beginning of exercise. In the group of eight subjects plasma noradrenaline concentration ([NA]) increased after 30 s to a mean value above resting of 547 (SEM 124) pg.ml-1, showing a tendency to a plateau, while adrenaline concentration ([A]) did not increase significantly. In the two subjects who became exhausted an almost linear increase in [NA] occurred up to about 1,300 pg.ml-1 coupled with a significant increase in [A]. During recovery an immediate decrease in [NA] was observed towards resting values. The values of the fc increase above resting levels determined at the time of blood collection were linearly related with [NA] increments both at the beginning and end of exercise with a similar slope, i.e. about 2.5 beats.min-1 per 100 pg.ml-1 of [NA] change.(ABSTRACT TRUNCATED AT 250 WORDS)

Gas exchange kinetics in heart transplant recipients

Submaximal and/or peak levels of oxygen consumption (VO2), pulmonary ventilation (VE), heart rate (HR), stroke volume of heart (SV), cardiac output (Q), muscle blood flow (qm), as well as the kinetics of readjustment of gas exchange, Q and qm (t1/2 VO2on-, t1/2 Qon-, t1/2 qmon-) were determined in 44 heart transplant recipients (HTRs) and in a group of age-, sex-, and physical activity-matched control subjects (CTL) when carrying out rectangular loads (25 to 125 W) on a bicycle ergometer. The increase of SV occurring at work onset appears to compensate in HTRs for the sluggish readjustment of HR so that the rate of readjustment of Q is kept within normal limits. As a consequence, qm, t1/2 q mon-, and t1/2 VO2on- appear to be similar or only moderately delayed in HTRs compared with CTLs. It is concluded that in HTRs, because of constrained maximum HR, only work loads up to 60% of the VO2max of CTLs may be attained; also, owing to the fast readjustment of Q, up to work loads of 75 to 100 W, the rest to work transition phase is not impaired.

Myocardial lactate dehydrogenase and its isoenzyme activities in transplanted human hearts

Lactate dehydrogenase and its heart (H) and muscle (M) subunit activities were studied in right ventricular endomyocardial biopsies from eight transplanted human hearts and compared with five chronically failing hearts and six normal human hearts from brain-dead liver/kidney donors. Of the 17 transplant biopsies (taken 5-95 weeks postoperatively), only two showed histologic signs of chronic rejection: They were excluded from the group analysis. A higher proportion of the M subunit of lactate dehydrogenase (M%) was found in the transplanted and the chronically failing hearts than in the normal hearts, presumably reflecting increased myocardial anaerobic glycolytic stress. In the early post-transplantation period, M% was higher in the transplanted than in the chronically failing hearts. Thereafter M% gradually fell, but had not reached normal levels 1-2 years after transplantation. During that time it was similar to the values in the chronic-failure hearts. In the two biopsies with chronic rejection, M% was nearly twice as high as in contemporaneous biopsies showing mild or no rejection. Monitoring of enzymatic adaptation from endomyocardial biopsies may be of clinical interest.