Do Clinical Exercise Tests Permit Exercise Threshold Identification in Patients Referred to Cardiac Rehabilitation?

BACKGROUND: To evaluate the feasibility of "threshold-based" aerobic exercise prescription in cardiovascular disease, we aimed to quantify the proportion of patients whose clinical cardiopulmonary exercise test (CPET) permit identification of estimated lactate threshold (θ_LT) and respiratory compensation point (RCP) and to characterize the variability at which these thresholds occur. METHODS: Breath-by-breath CPET data of 1102 patients (65 ± 12 years) referred to cardiac rehabilitation were analyzed to identify peak O₂ uptake (V˙O_2peak; mL·min^-1 and mL·kg^-1·min^-1) and θ_LT and RCP (reported as V˙O₂, %V˙O_2peak, and %peak heart rate [%HR_peak]). Patients were grouped by the presence or absence of thresholds: group 0: neither θ_LT nor RCP; group 1: θ_LT only; and group 2: both θ_LT and RCP. RESULTS: Mean V˙O_2peak was 1523 ± 627 mL·min^-1 (range: 315-3789 mL·min^-1) or 18.0 ± 6.5 mL·kg^-1·min^-1 (5.2-46.5 mL·kg^-1·min^-1) and HR_peak was 123 ± 24 beats per minute (bpm) (52 bpm-207 bpm). There were 556 patients (50%) in group 0, 196 (18%) in group 1, and 350 (32%) in group 2. In group 1, mean θ_LT was 1240 ± 410 mL·min^-1 (580-2560 mL·min^-1), 75% ± 8%V˙O_2peak (52%-92%V˙O_2peak), or 84% ± 6%HR_peak (64%-96%HR_peak). In group 2, θ_LT was 1390 ± 360 mL·min^-1 (640-2430 mL·min^-1), 70% ± 8%V˙O_2peak (41%-88%V˙O_2peak), or 78% ± 7%HR_peak (52%-96%HR_peak), and RCP was 1680 ± 440 mL·min^-1 (730-3090 mL·min^-1), 84% ± 7%V˙O_2peak (54%-99%V˙O_2peak), or 87% ± 6%HR_peak (59%-99%HR_peak). Compared with group 1, θ_LT in group 2 occurred at a higher V˙O₂ but lower %V˙O_2peak and %HR_peak (P < 0.05). CONCLUSIONS: Only 32% of CPETs exhibited both θ_LT and RCP despite flexibility in protocol options. Commonly used step-based protocols are suboptimal for "threshold-based" exercise prescription.