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

Introduction: Based on the hypothesis that oxidative metabolism is impaired in ME/CFS,\r\na previous study recommended a pacing self-management strategy to prevent\r\npost-exertional malaise. This strategy involved a prescription to maintain a\r\nheart rate below the anaerobic threshold during physical activities. In the\r\nabsence of lactate sampling or a cardiopulmonary exercise test (CPET), the\r\npacing self-management formula defines 55% of the age-specific predicted\r\nmaximal heart rate as the heart rate at the anaerobic threshold. Thus far there\r\nhas been no empiric evidence to test this self-pacing method of predicting\r\nheart rate at anaerobic threshold. The aim of this study was to compare\r\npublished formula-derived heart rates at the anaerobic threshold with the\r\nactual heart rate at the lactic acidosis threshold as determined by CPET. Methods\r\nand Results: Adults with ME/CFS who had undergone a symptom-limited CPET\r\nwere eligible for this study (30 males, 60 females). We analysed males and\r\nfemales separately because of sex-based differences in peak oxygen consumption.\r\nFrom a review paper, formulae to calculate maximal predicted heart rate were\r\nused for healthy subjects. We compared the actual heart rate at the lactic acid\r\nthreshold during CPET to the predicted heart rates determined by formulae.\r\nUsing Bland-Altman plots, calculated bias: the mean difference between the\r\nactual CPET heart rate at the anaerobic threshold and the formula predicted\r\nheart rate across several formulae varied between -28 and\r\n19 bpm in male ME/CFS patients. Even in formulae with a clinically acceptable\r\nbias, the limits of agreement (mean bias ± 2SD) were unacceptably high for all formulae. For female ME/CFS\r\npatients, bias varied between 6 and 23 bpm, but the limits of agreement were\r\nalso unacceptably high for all formulae. Conclusion: Formulae generated\r\nin an attempt to help those with ME/CFS exercise below the anaerobic threshold\r\ndo not reliably predict actual heart rates at the lactic acidosis threshold as\r\nmeasured by a cardiopulmonary exercise test. Formulae based on age-dependent\r\npredicted peak heart rate multiplied by 55% have a wide age-specific\r\nvariability and therefore have a limited application in clinical practice.