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 O ₂ uptake (V˙O _2peak ; mL·min ^-1 and mL·kg ^-1 ·min ^-1 ) and θ _LT and RCP (reported as V˙O ₂ , %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˙O ₂ 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.
(Copyright © 2023 Canadian Cardiovascular Society. Published by Elsevier Inc. All rights reserved.)