Influencia del sistema nervioso autónomo en la respuesta inicial de la frecuencia cardiaca al ortostatismo activo y pasivo

Influence of the autonomic nervous system on the heart rate response to active and passive orthostatism. The purposes of this study were twofold. First, to compare heart rate responses as measured by R-R interval under two conditions of orthostatic stress, i.e. a change from supine to an active free standing-up position (active orthostatism, AO), and from supine to a passive 70 head-up tilt posture (passive orthostatism, PO) second, to utilize a standard pharmacological model to study the participation of the autonomic nervous system upon heart rate responses evoked by AO. In the first part of the research, eight healthy subjects (seven men, one woman) were evaluated for AO and PO. In both occasions, subjects were supine for 5 minutes and then adopted an upright or a tilted position in 3-5 seconds and remained motionless during 5 minutes. In the second part, eight men participated twice in the pharmacological studies. In day one, they stood up for control (AOC), after IV administration of atropine sulfate (0.04 mg/kg) (AO+atro) and after IV administration of propranolol hydrochloride (0.16 mg/kg) (AO+ATRO+PROPRA). In day two, subjects repeated the control AO (AOCII) and after the administration for propranolol hydrochloride alone. In the first study, AO was characterized by a fast shortening of R-R interval, which was maximal at beat 15th (relative tachycardia), followed by a rebound lengthening of R-R interval, reaching a plateau at beat 30 (relative bradycardia), demonstrating a biphasic response. PO was characterized by a small and gradual shortening of R-R interval without the biphasic responses of AO.(ABSTRACT TRUNCATED AT 250 WORDS)