Dear Editor, We read with interest the report of Wallen et al. (2012) concerning the validity of the Polar RS800 heart rate monitor (HRM) as compared to a 5-min supine electrocardiogram (ECG) with respect to the calculation of various indices of heart rate variability (HRV). The Polar system consists of an HRM with bundled software (Polar Pro Trainer 5; PPT) which is used to derive HRV values. Wallen et al. (2012) compare the results from hand-corrected ECG data to this system, and conclude that traditional ECGs should be preferred as ‘‘...the Polar system did not identify errors satisfactorily, or return valid values of HRV for certain groups...’’. It should be noted first that within a research context, inaccuracy of a bundled hardware/software system is somewhat moot. Research groups overwhelmingly utilize the Polar system as a source of RR intervals, which are exported from the PPT software, and corrected if necessary to a normal-to-normal approximation then analysed using separate software. Recent work within this journal, for example, bears this out (e.g., Mateo et al. 2012; Mendonca et al. 2011; Vieira et al. 2012). As Wallen et al. (2012) note, the accuracy of this method is not in question, as previous research (e.g., Weippert et al. 2010) compared the accuracy of ECG-derived and Polar-derived RR intervals and concluded that the RR intervals are sufficiently interchangeable when analysed through identical methods. Wallen et al. (2012) note that recorded RR intervals (as provided from the Polar HRM) are less than ideal for the identification of cardiac dysrhythmia, as a normative ECG waveform is not available. This is undoubtedly the case, but as might be expected from a system designed primarily for tracking HR in a sporting context, the Polar system makes no systematic claims about its ability to identify and eliminate ectopy in groups that may display persistent ectopy at baseline (e.g., the elderly). Thus, the appropriate question is whether Polar-recorded data can optimally produce ECG-comparable measures of HRV, not whether the HRM system is able to identify and deal with cardiac dysrhythmia. Several recent implementations of correction methods applicable to RR series exist (e.g. Barbieri and Brown 2006; Clifford and Tarassenko 2005) but were not attempted by Wallen et al. (2012). The presence and correction of cardiac dysrhythmia (e.g., ectopy, premature atrial contraction) is a well-known source of error in the calculation for HRV, especially in the frequency domain as even one ectopic beat can bias the analysis of a short-term recording (Berntson and Stowell 1998). However, with regard to analytical methods, Wallen et al. (2012) did not provide an explicit methodology for the identification of ectopic beats in their ECG recording beyond visual inspection. The Task Force (1996) paper cited does not specify a methodology beyond stating that interpolation or regression methods may improve the bias conferred by ectopy. The lack of a uniform methodology for the detection of ectopy has the potential for unintended bias in the comparative ECG recording against which the HRM intervals were compared, especially if the assessors are not blind to the overall research question (which was the case in the report by Wallen et al. 2012). Furthermore, Communicated by Susan A. Ward.
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May 5, 2015
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