ECG and APG signal analysis during exercise in a hot environment

摘要

The body temperature is normally maintained within a narrow range. In a hot environment, however, heat gain can exceed heat loss and heat related disorders may occur. Heat-related disorders range in severity from a minor discomfort to life threatening. An excess of 14,800 deaths was reported during the 2003 heat wave in France. In this study, the effect of heat stress on the electrocardiogram (ECG) and the acceleration plethysmogram (APG) was investigated. The main peaks of the ECG and APG signals had to be detected in order to extract the features. Nine peak detection algorithms, described in literature, were examined but none of the nine these performed satisfactory when applied to the heat stress ECG or APG signals. A new peak detection technique achieved a comprehensive detection of more than 98 percent for both the ECG and APG signals without adjusting thresholds. A method to detect abnormalities using bior5.5 wavelet was developed and tested using the MT-BIH Arrhythmia Database. The method achieved successful separation of normal and abnormal beats. However, the technique did not detect significant changes in the ECG signals during heat stress. Results of ECG interval analysis were consistent with exercise tests in cooler conditions. An unexpected finding was the fact that the heart rate continued to increase, even though the exercise intensity and the environment remained the same. The heart rate variability indices seem to follow the change of the heart rate in the early stage of exercise, but not during the later stage of the test. This may be due to the fact that the heart is functioning close to its maximum capacity during the later stage of the test. The APG showed clear shape changes during heat stress for most subjects. Despites the large increase in the heart rate and the shape changes of the APG, the APG waveform could not be classified as one of the abnormal APG types described in literature. This indicates that the effect of heat stress is different from the previously described abnormalities. A decreasing depth at the d point of the APG was seen resulting in a smooth signal. An index to quantify this effect, the d point depression, was shown to correlate with both the heart rate and body core temperature.