Disabling atrial fibrillation (AF)-related symptoms and different testing settings may influence day-to-day cardiopulmonary exercise testing (CPET) measurements, which can affect exercise prescription for high-intensity interval training (HIIT) and moderate-to-vigorous intensity continuous training (M-VICT) and their outcomes. This study examined the reliability of CPET in patients with AF and assessed the proportion of participants achieving minimal detectable changes (MDC) in peak oxygen consumption (VO2peak) following HIIT and M-VICT. Participants were randomized into HIIT or M-VICT after completing two baseline CPETs: one with cardiac stress technologists (CPETdiag) and the other with a research team of exercise specialists (CPETresearch). Additional CPET was completed following 12 weeks of twice-weekly training. The reliability of CPETdiag and CPETresearch was assessed by intraclass correlation coefficient (ICC) and dependent t tests. The MDC score was calculated for VO2peak using a reliable change index. The proportion of participants achieving MDC was compared between HIIT and M-VICT using chi-square analysis. Eighteen participants (69 +/- 7 years, 33% females) completed two baseline CPETs. The ICCs were significant for all measured variables. However, peak power output (POpeak 124 +/- 40 versus 148 +/- 40 watts, p < 0.001) and HR (HRpeak 136 +/- 22 versus 148 +/- 30 bpm, p = 0.023) were significantly greater in CPETresearch than CPETdiag. Few participants achieved MDC in VO2peak (5.6 mL/kg/min) with no difference between HIIT (0%) and M-VICT (10.0%, p = 0.244). POpeak and HRpeak differed significantly in patients with AF when CPETs were repeated under different settings. Caution must be practised when prescribing exercise intensity based on these measures as under-prescription may increase the number of exercise non-responders.

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