Physiological responses during high intensity interval training and moderate intensity continuous training in patients attending early cardiac rehabilitation

OBJECTIVES: To compare the acute ventilatory, haemodynamic, metabolic, and perceptual responses during low-volume high-intensity interval training (HIIT) and time-matched moderate-intensity continuous training (MICT) in early outpatient cardiac rehabilitation (CR) patients with coronary artery disease (CAD). DESIGN: Single-centre, within-subject, time-matched exercise comparison SETTING: UK outpatient cardiac rehabilitation unit PARTICIPANTS: Eighteen clinically stable CAD patients (15 male, 60.1 ± 9.9 years) attending early outpatient CR; left ventricular ejection fraction > 35%. INTERVENTIONS: Two exercise sessions per participant: (1) low-volume HIIT (10 × 1-min intervals at 85-90% peak power output (PPO) with 1-min active recovery at 20-25% PPO; total 20-min) and (2) MICT (continuous cycling for 20 min at 40-70% heart rate reserve (HRR)). Sessions were preceded by a 15-minute warm-up and separated by ≥48 h. MAIN OUTCOME MEASURES: Gas exchange (VO₂, VCO₂, VE), heart rate (HR), blood pressure, rate pressure product (RPP), time >90% oxygen uptake reserve (VO₂R), time relative to ventilatory thresholds (VT1/VT2), blood lactate (BLa), energy expenditure and rating of perceived exertion (RPE). RESULTS: HIIT elicited greater mean VO₂, %VO₂R, time >90% VO₂R, %HRR, RPP and BLa than MICT (all P < 0.003). Participants spent more time above VT2 during HIIT (834 ± 387 vs. 393 ± 465 s; P < 0.0004). RPE was higher during HIIT (13 ± 1 vs. 11 ± 1; P < 0.003). CONCLUSIONS: Low-volume HIIT elicited greater ventilatory, haemodynamic and metabolic demand than time-matched MICT, alongside higher perceived exertion. These findings demonstrate that low-volume HIIT provides a greater individualised physiological stimulus and may inform personalised HIIT prescription in patients with CAD attending early CR.