Comparison of Oxygen Uptake Kinetics Between Kidney Transplant Recipients and Healthy Subjects

Abstract

Aerobic exercise capacity is reduced in kidney transplant recipients (KTRs), with a progressive improvement after transplantation. KTRs show central exercise limiting factors such as chronotropic incompetence, anemia, heart or vascular diseases. Moreover, peripheral alterations at a muscular level are present in this population. It has been reported that slower oxygen uptake (VO2)-kinetics during a moderate constant load exercise, expressed as an increased time constant (τ), may reflect an impaired muscular oxidative metabolism. PURPOSE: To analyze the VO2-kinetics in a population of KTRs. METHODS: two groups of KTRs enrolled 3 and 12 months (n=21 and 14, respectively) after transplantation and a control group of healthy young adults (n=16) underwent cardiopulmonary exercise testing at cycle-ergometer. The protocol consisted in two subsequent constant, moderate-load exercise phases with a final incremental test until exhaustion. RESULTS: The τ was increased in KTRs compared to controls (50.4±13.11 s at 3 and 43.84±11.57 s at 12 months vs 28.91±8.37 s in controls; both P<0.01) while VO2peak was reduced, but significantly higher in the group evaluated at 12 months (21.30±4.34 vs 26.36±7.96 ml/kg/min (P=0.04), vs 41.7±7.82 ml/kg/min in controls (both P<0.01)). Consistently with this result, an increased hemoglobin (Hb) concentration was found 12 months after transplantation (12.77±1.67 vs 14.55±1.74 g/dL (P<0.01)). Also, lower peak heart rate (HR) might affect KTRs’ exercise capacity (79.67±12.16% of predicted at 3 and 84.29±11.49% at 12 months vs. 93.38±5.21% in controls; both P≤0.01). Among KTRs, the τ showed a moderate negative correlation with VO2peak and oxygen uptake efficiency slope (R=-0.51 and R=-0.57, respectively), less depending on Hb and particularly on peak HR (R=-0.33 and R=-0.13, respectively). CONCLUSION: KTRs show slower VO2-kinetics, reduced peak VO2 and HR when compared to a population of healthy young adults. VO2peak and Hb seem to improve during the first year after transplantation. The reduced aerobic capacity of KTRs was associated with slower VO2-kinetics, which seem to be less affected by the oxygen transport. These findings suggest that an impaired oxidative muscle metabolism could be a peripheral limiting factor contributing to decreased exercise capacity in KTRs.