Objective: The purpose of this study was to identify sources and strategies for the mitigation of bias in studies of spinal manipulation and heart rate variability.

Methods: A small-scale study compared the effects of a single session of sham and authentic cervical manipulation on heart rate variability as measured by power spectrum analysis. The participants were a sample of 31 healthy young students from the Canadian Memorial Chiropractic College, randomized into 2 study arms. The effectiveness of blinding was evaluated, and 2 alternative methods of data analysis were explored to mitigate risk of bias. Following execution of the study, the stages of implementation and data processing were scored against version 2 of the Cochrane risk-of-bias tool for randomized trials for risk of bias.

Results: The risk of bias arising from (1) the randomization process, (2) missing outcome data, and (3) selection of reported results was judged to be low. Risk of bias in (1) deviations from intended interventions (particularly due to the failure of masking) and (2) the measurement of the outcome, for example, through cleaning of the data, were judged to be high.

Conclusion: The use of power spectrum analysis of heart rate variability based on 5-minute recordings of echocardiogram pre-and post-intervention contained multiple sources of bias that were challenging to mitigate. Based upon these findings, power spectrum analysis of heart rate variability using these parameters may be ill-suited to the study of physiological effects of spinal manipulative therapy.

Author keywords: Heart Rate; Manipulation, Spinal; Bias; Electrocardiography; Autonomic Nervous System

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