SINUS ARRHYTHMIA

Though there are many comments on respiratory sinus arrhythmia in articles and textbooks, there have been surprisingly few data collected. We have presented data from 4,096 electrocardiograms of 199 healthy individuals on pulse rate, range (max. difference in cycle length), and frequency index (range/mean cycle lengthx100). Pulse rates for this group, though not as low as those taken under basal conditions,32 show generally lower values with a greater variation than those often reported.30,31 The girls have higher pulse rates than the boys as a group after the age of 6 years. Sinus arrhythmia defined as a range of 0.16 seconds or more is present under a year of age in both sexes. The range increases to a maximum between 8 and 12 years and only the 75th and 90th percentiles for boys show a definite decrease after this. Of 81 individuals from whom we have records from the ages of at least 8 years through 17 years, only one boy and one girl do not at some time show a range of 0.16 seconds or more. The values for the frequency index (defined in text) we obtained were higher than those of Landtman.19 The girls show higher values than the boys after the age of 12 years for the 75th and 90th percentiles because of their higher range and higher pulse rate. From a study of individuals over a number of years, we find that there is a strong negative association between range and pulse rate and a definite though less strong negative association between range and P-R interval. Siblings show marked similarities in their group position for pulse rate, sinus arrhythmia (range), and P-R interval. The influence of variation in tidal volume and respiratory rate on sinus arrhythmia (range) was investigated. The results show that there is usually an increase in range with increase in tidal volume but a variable response to respiratory rate changes. Rapid respiratory rates decrease sinus arrhythmia (range) probably by the increase in pulse rate from the greater work involved, though changes in CO2 tension of the blood might also be important. From studying the effect of breath-holding on sinus arrhythmia we find that slowing usually occurs 3 to 6 heart beats after the increase in rate which is associated with inspiration, and occurs with expiration only by coincidence. The amount of slowing is not related to the time of expiration.