Attenuation of cardiac frequency startle acceleration by prior work or heat load elevation

Sensitivity of heart rate to attentional drive is ofconcern when field evaluating metabolic load by ECG telemetry. With six groups of women at varying levels of heart rate achieved by exercise or heating, an unexpected, explosive noise elicited a mean cardiac startle elevation of 9 beats min-1 in resting subjects with linear attenuation under increasing heart rate. With group mean values of 125 beats min-1 or greater, no significant acceleration from startle remained. Pre-startle, beat by beat variability decreased with elevation of heart rate; startle increased this variability. The attenuation of mean startle responses supports Wilder's Law of Initial Values and indicates that field metabolic evaluations which use telemetered heart rates from work of moderate or greater intensity are not confounded by the intrusion of such attention commanding stimuli.