Letter by Lopez-Messa and Alonso-Fernandez Regarding Article, “Temporal Differences in Out-Of-Hospital Cardiac Arrest (OHCA): Incidence and Survival”
To the Editor:
We read with great interest the article by Bagai et al,1 “Temporal Differences in Out-Of-Hospital Cardiac Arrest (OHCA): Incidence and Survival,” which reports a study conducted in OHCA patients enrolled in CARES Registry. The authors conclude that there is significant temporal variability in the incidence of and survival after OHCA, with 2 peaks of incidence, in the morning and in early evening.
Although we agree with the authors about the use as reference time emergency system call, the appearance of a circadian rhythm with 2 peaks, morning and evening, and the importance of knowledge for establishing preventive strategies and optimizing care systems at certain times of day, days of the week, or times of year, we disagree with some of their conclusions because of differences in survival of some groups and the striking absence of both peaks among witnessed arrest with shockable first cardiac rhythm, with presence only among unwitnessed arrest with nonshockable first cardiac rhythm, considering as a novel finding for different reasons.
The grouping of the activation times of emergency services in 1-hour periods presents a number of problems to make statistical comparisons, which could be the consequence of the results. We suggest that the cosinor method should have been used, taking into account the exact times of all cases included. An oscillation is a cyclic change measurable, displaying a waveform and a relatively constant. The cosinor method2 has the advantage of using an objective test to establish the presence of a rhythmic phenomenon with a level of statistical significance. Furthermore, the method3,4 provides an approximation of accuracy of the test by estimating a confidence interval for the best fit cosine function. Another advantage over other methods is that the data obtained from the time series need not be equally spaced, and cases of shortages or loss of cases during the observation are well tolerated.
We thus believe that an analysis of temporal variations in the occurrence of cases of OHCA, using the exact times of activation of emergency services, without grouping them into time periods, and using the cosinor method for the analysis of the temporal variability could show different results that obtained, concerning the increased occurrence of cases in weekly periods between Friday and Monday, and confirm whether there are differences between the witnessed arrest with shockable first cardiac rhythm, and unwitnessed arrest with nonshockable first cardiac rhythm.
Juan B. Lopez-Messa, MD, PhD
Jose Alonso-Fernandez MD, PhD
Intensive Care Unit
Hospital “Río Carrión”
- © 2014 American Heart Association, Inc.