Heart rate thresholds to limit activity in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome patients (Pacing): Comparison of heart rate formulae and measurements of the heart rate at the lactic acidosis threshold during Cardiopulmonary Exercise Testing, by C (Linda) MC van Campen, Peter C Rowe, Frans C Visser in Advances in Physical Education Vol. 10 No.2, May 2020 [DOI: 10.4236/ape.2020.102013]
Based on the hypothesis that oxidative metabolism is impaired in ME/CFS, a previous study recommended a pacing self-management strategy to prevent post-exertional malaise. This strategy involved a prescription to maintain a heart rate below the anaerobic threshold during physical activities. In the absence of lactate sampling or a cardiopulmonary exercise test (CPET), the pacing self-management formula defines 55% of the age-specific predicted maximal heart rate as the heart rate at the anaerobic threshold. Thus far there has been no empiric evidence to test this self-pacing method of predicting heart rate at anaerobic threshold.
The aim of this study was to compare published formula-derived heart rates at the anaerobic threshold with the actual heart rate at the lactic acidosis threshold as determined by CPET.
Methods and Results:
Adults with ME/CFS who had undergone a symptom-limited CPET were eligible for this study (30 males, 60 females). We analysed males and females separately because of sex-based differences in peak oxygen consumption. From a review paper, formulae to calculate maximal predicted heart rate were used for healthy subjects. We compared the actual heart rate at the lactic acid threshold during CPET to the predicted heart rates determined by formulae. Using Bland-Altman plots, calculated bias: the mean difference between the actual CPET heart rate at the anaerobic threshold and the formula predicted heart rate across several formulae varied between -28 and 19 bpm in male ME/CFS patients. Even in formulae with a clinically acceptable bias, the limits of agreement (mean bias ± 2SD) were unacceptably high for all formulae. For female ME/CFS patients, bias varied between 6 and 23 bpm, but the limits of agreement were also unacceptably high for all formulae.
Formulae generated in an attempt to help those with ME/CFS exercise below the anaerobic threshold do not reliably predict actual heart rates at the lactic acidosis threshold as measured by a cardiopulmonary exercise test. Formulae based on age-dependent predicted peak heart rate multiplied by 55% have a wide age-specific variability and therefore have a limited application in clinical practice.
Todd Davenport of the Workwell Foundation responded to the conclusions of this study on twitter:
Todd Davenport @sunsopeningband May 23
I think it’s important to point out the equation we proposed was never intended to accurately predict heart rate at VAT. It was intended to slightly under-estimate heart rate at ventilatory anaerobic threshold. The thought was this underestimation would provide a safety margin.
The authors of the present study, very helpfully, provide us with some important subject-level data in Figure 1. These data can help us get an idea of whether the formula we proposed in 2010 actually underestimates the heart rate at lactic acid threshold, as originally intended…
“All models are wrong. Some models are useful.” Formula based estimation for metabolic output using heart rate is fraught. We know there are additional problems with it from our work on chronotropic intolerance in #MECFS.
However, because (repeat) CPET measurements is still not a common evaluative assessment for people with #MECFS, patients and their physical therapists are left with few objective tools to monitor activity in our collective attempt to reduce post-exertional neuroimmune exhaustion.
Calculated values, for all their acknowledged problems with picking accurate values in single patients, still can be useful to help patients and clinicians select appropriate activity thresholds for pacing self management. The data from this study seems to support this premise.