Effect of heat stress shown in APG signals

Heat-related disorders can be life threatening; however, people experiencing these disorders often do not experience subjective symptoms until they become seriously ill. This makes it difficult to investigate signs of heat-related disorders in the early stages. Heat stress affects the cardiovascular system and its effects differ greatly due to the variations in heat tolerance depending on general health. Acceleration plethysmogram (APG) may have the potential to examine and measure the effects of heat stress quantitatively. The APG is a measure of the mechanical and fluid dynamic properties of cardiovascular systems and has been proposed for examining general health and well-being. APG signals for subjects experiencing heat stress were examined in this study. The shapes of APG waveform of all subjects at rest emulated to Type A and B which are said to be indicative of healthy subjects. During the heat stress test, the shapes of APG waveform changed as the heart rate and body core temperature of the subjects increased. However, these altered APG waveforms still matched Type A and B. The general characteristic of the change in the APG waveforms was a reduction of the depth formed between those three feature points of the APG waveform, the c, d and e points (d point depression). The depth was measured as a distance from d point to a tangential line that passes through both c and d points. The depth of the depression showed a correlation with both the heart rate and body core temperature.