Alternatives to the six-minute walk test in pulmonary arterial hypertension

Abstract

Introduction: The physiological response during the endurance shuttle walk test (ESWT), the cycle endurance test (CET) and the incremental shuttle walk test (ISWT) remains unknown in PAH. We tested the hypothesis that endurance tests induce a near-maximal physiological demand comparable to incremental tests. We also hypothesized that differences in respiratory response during exercise would be related to the characteristics of the exercise tests.

Methods: Within two weeks, twenty-one PAH patients (mean age: 54(15) years; mean pulmonary arterial pressure: 42(12) mmHg) completed two cycling exercise tests (incremental cardiopulmonary cycling exercise test (CPET) and CET) and three field tests (ISWT, ESWT and six-minute walk test (6MWT)). Physiological parameters were continuously monitored using the same portable telemetric device.

Results: Peak oxygen consumption (VO(2peak)) was similar amongst the five exercise tests (p = 0.90 by ANOVA). Walking distance correlated markedly with the VO(2peak) reached during field tests, especially when weight was taken into account. At 100% exercise, most physiological parameters were similar between incremental and endurance tests. However, the trends overtime differed. In the incremental tests, slopes for these parameters rose steadily over the entire duration of the tests, whereas in the endurance tests, slopes rose sharply from baseline to 25% of maximum exercise at which point they appeared far less steep until test end. Moreover, cycling exercise tests induced higher respiratory exchange ratio, ventilatory demand and enhanced leg fatigue measured subjectively and objectively.

Conclusion: Endurance tests induce a maximal physiological demand in PAH. Differences in peak respiratory response during exercise are related to the modality (cycling vs. walking) rather than the progression (endurance vs. incremental) of the exercise tests.

Figure 1. Comparison of the physiological parameters between incremental and constant work rate exercise tests.

At 100% exercise, Oxygen consumption (VO₂), heart rate (HR), minute ventilation (V_E) and respiratory exchange ratio (RER) were similar between incremental and endurance tests. However, slopes for these parameters rose steadily over the entire duration of the incremental cardiopulmonary exercise test (CPET) and the incremental shuttle walk test (ISWT), whereas their slopes rose sharply from baseline to 25% of maximum exercise at which point they appeared far less steep until test end for the cycle endurance test (CET) and the endurance shuttle walk test (ESWT). Mean (SE) values of physiological parameters are expressed at the same relative time (e.g. 0%, 25%, 50%, 75% and 100%) from the maximal exercise duration. The shaded zone represents the initial warm-up period of the CPET, the CET and the ESWT. *p<0.05; **p<0.01; ***p<0.001 for the comparison of the slopes of each parameter from baseline to 25% of exercise duration, and from 25% to 100% (test end).

Figure 2. Comparison of the physiological parameters between cycling and walking incremental exercise tests.

Physiological response during the cardiopulmonary exercise test (CPET) and the incremental shuttle walk test (ISWT) at the same relative time (e.g. 0%, 25%, 50%, 75% and 100%) from the maximal exercise duration. The CET was characterized by increased respiratory exchange ratio (RER) compared to the ISWT. The shaded zone represents the initial warm-up period of the CPET. Values are means (SE). *p<0.05; **p<0.01; ***p<0.001 for the comparison of the slopes of each parameter from baseline to 25% of exercise duration, and from 25% to 100% (test end). ^#p<0.05 for end-exercise value compared by ANOVA. Figure legend: VO₂, Oxygen consumption; VCO₂, Carbon dioxide output; RER, Respiratory exchange ratio; V_E, Minute ventilation; HR, Heart rate; VO₂ /HR, Oxygen pulse; CPET, Cardiopulmonary exercise test; ISWT, Incremental shuttle walk test.

Figure 3. Comparison of the physiological parameters between cycling and walking constant work rate exercise tests.

Physiological response during the cycle endurance test (CET) and the endurance shuttle walk test (ESWT) at the same relative time (e.g. 0%, 25%, 50%, 75% and 100%) from maximal exercise duration. The ESWT was characterized by a higher oxygen consumption (VO₂) and a lower respiratory exchange ratio (RER) throughout the exercise. Conversely, carbon dioxide output (VCO₂), minute ventilation (V_E) and oxygen pulse (VO₂/HR) slopes were slightly steeper in the early phase of the CET (from baseline to 25% of exercise duration), to end up with similar end-exercise values. Similarly, heart rate (HR) slopes, although statistically significant, were virtually the same during the CET and the ESWT. The shaded zone represents the initial warm-up period of the ESWT. ^#p≤0.01 between CET and ESWT. Values are means (SE). *p<0.05; **p<0.01; ***p<0.001 for the comparison of the slopes of each parameter from baseline to 25% of exercise duration, and from 25% to 100% (test end).

Figure 4. Quadriceps muscle fatigue induced by the constant work rate exercise tests.

Relative voluntary and non-volitional quadriceps strengths lost from baseline as assessed by maximal voluntary contraction (MVC) and potentiated twitch (TWq) following the cycle endurance test (CET), the endurance shuttle walk test (ESWT) and the six-minute walk test (6MWT). The CET induced significant quadriceps muscle fatigue defined as a 15% decrease in TWq following exercise (dotted line). Indeed, 12 (57%), 2 (10%) and 3 (14%) patients developed significant quadriceps fatigue following CET, ESWT and 6MWT, respectively. ^*p≤0.01; ^# p≤0.05. Values are mean (SE). Figure legend: CET, Cycle endurance test; ESWT, Endurance shuttle walk test; 6MWT, Six-minute walk test; MVC, Maximal voluntary contraction; TWq: Potentiated twitches.