Physiological significance of the defense response to intense auditory stimulation: a pharmacological blockade study

Abstract This paper examines through pharmacological blockade some questions related to the physiological significance of the defense response to intense auditory stimulation. Nine subjects received intravenous metoprolol (10–15 mg i.v.), intravenous atropine (0.03 mg/kg i.v.), or a saline solution as placebo condition before undertaking a test of the defense response to a distorted sound of 400 Hz frequency, 109 dB intensity, 0.5 sec duration and virtually instantaneous risetime. Dependent variables were continuous (beat-to-beat) heart rate, stroke volume and blood pressure. The results suggest: (1) a vagal origen of the first acceleration and first deceleration and a sympathetic- parasympathetic interaction during the second acceleration and second deceleration of the heart rate response; (2) a blood pressure response pattern characterized by an increase during the first heart rate deceleration (4–11 sec), a posterior decrease coinciding with the second heart rate acceleration (from 12 to 37 sec), and a lighter increase during the second heart rate deceleration (from 38 to 63 sec); and (3) an implication of the baroreceptor reflex, including a baroreceptor mediated inhibition of the parasympathetic cardiac activity during the second accelerative component of the cardiac response.