The ACKM has been reported to be of great utility for detection of arrhythmias in symptomatic pediatric patients [5,6]; however, the overall accuracy of the ACKM device for QTc measurement did not meet cutoffs for clinical utility. The mean differences between ACKM and standard ECG measurements were very small at -0.6 ms for QTc and -1.3 ms for QRS. However, the ranges of confidence were large. The margin of error of the QTc range of 47.5 ms was too large to be considered clinically useful. In fact, only 64% of the patients had a QTc within 20 ms difference and the ACKM missed 6 of the 13 patients (46%) with QT prolongation. Garabelli et al. were able to show better accuracy of the single-lead ACKM in healthy adult patients with an average difference of 4 ms and a standard deviation of 11 ms (margin of error of 22 ms) . Chung et al. also found that all single-lead ACKM QTc measurements were within 20 ms of a standard ECG, though this was with an adult population of only 5 patients . Studies with pediatric and young adult patients have demonstrated less accurate ACKM results, more similar to our study. In a larger study on pediatric patients with the single-lead ACKM, Karacan et al. reported a slightly larger mean difference in QTc (4 ms) measurements than our study (0.6 ms), with a Pearson’s correlation of only 0.57 . Meanwhile a smaller study of 30 elite athletes with a mean age of 18.9 years by Orchard et al. using a six-lead ACKM showed an average difference of QTc of -10 ms with a standard deviation of 18 ms (margin of error 35 ms). The QRS results were slightly better than in our study, with a mean difference of -3 ms and a standard deviation of 7 ms (margin of error 14 ms) . Our study’s agreement rate of ACKM QTc measurements with 20 ms of the standard ECG was only slightly lower (64%) than those reported by Garabelli et al. and Beers et al (72% each) [9,13].
We postulated that sinus arrhythmia, variations in patient heart rate between the paired ECG readings, and artifact in the ACKM may have made significant contributions to the inaccuracy of the ACKM. The margin of error of the QTc difference improved slightly when excluding ECGs with significant heart rate differences between the ACKM and standard ECG (margin of error 43.0 ms) and artifact in the ACKM (margin of error 42.7 ms) but not when excluding sinus arrhythmia (margin of error 46.9 ms). However, none of the narrowed populations approached the cutoff for clinical usefulness.
A significant portion of the study population had a history of heart disease, cardiac surgery, or cardiac transplant. The rates are much higher than in similar studies such as the one performed by Karacan et al., in which 7% of patients had congenital heart disease . We hypothesized that these conditions may affect the accuracy of the ECG readings, given that Garabelli et al. found standard deviation of differences in QTc measurements to be much higher in hospitalized patients than healthy volunteers . However, confidence ranges for QTc and QRS measurement were not different in patient populations with and without heart disease, cardiac surgery, and cardiac transplant. This indicates that the device retains its level of accuracy in patients with heart disease and cardiac surgery. The transplant population of 14 patients was too small to draw conclusions.
Of the 144 ACKM ECG readings, only one (0.7%) was excluded for an ECG for which intervals could not be measured. This was for one with flattened T waves, when the corresponding standard ECG did not have flattened T waves. This is better than the 7% rate reported by Karacan et al., which they attributed to artifacts and unclear T wave termination . While accuracy of the ACKM device remained stable with later uses, we encountered some durability issues with the device. Subjectively, it appeared that the device had more difficulty detecting good bare-skin contact with all three electrodes for patients enrolled later in the study. This was particularly true for the left ankle/knee electrode, which lead to the last patients recruited often having to reposition the electrode on their leg multiple times. Signal noise also appeared to be increased in later patient readings, though the filter applied by the device software removed most of the noise. There were no battery issues with the device.
One limitation to this study is that it does not reflect the general pediatric population. Given that it was performed in a pediatric cardiology clinic at a tertiary academic pediatric hospital, large proportions of the study participants had a history of heart disease, cardiac surgery, and cardiac transplant. While the accuracy of the device appeared to be similar between patients with these conditions and those without, the population is still one that has more ECG abnormalities than the general pediatric population. Additionally, there were two limitations in how the ECGs were measured. First, the ACKM ECG was measured after the standard ECG as opposed to concurrently. This was done because it was deemed too difficult for many participants to do both at the same time and we did not want to compromise the standard ECG for the patient’s clinic visit. Additionally, all patients were supine for the standard ECG, while the vast majority of patients were sitting upright for the ACKM ECG. This occurred because most patients had difficulty remaining still and comfortable while trying to record the ACKM ECG supine. This could account for some of the difference in heart rates between corresponding ECGs.