Hemodynamic Responses of Systemic and Pulmonary Vessels to Head-down Tilt (-6 degrees) and Effect of Anticholinergic Agent

ABSTRACT

Head-down tilt (HDT) has been commonly used as the simulation of a microgravity to evaluate the hemodynamic and the hormonal responses to the central blood shift observed during the spaceflight. But there were some differences in the systemic responses according to species or angle of tilt, and there are few reports in the hemodynamic response and the regulation on polmonary vascular bed during the early stage of HDT. This study was attempted to clarify the regulatory mechanism on the systemic and pulmonary vascular responses through the low pressure receptor, and the hormonal changes to the -6 degrees HDT, and to investigate the effect of anticholinergic agent, glycopyrrolate, on the hemodynamic and the hormonal responses. In 28 anesthetized dogs(8~14 kg), 15 dogs(TILT group) were tilted from the supine position to the -6 degrees head-down position(HDP ) for 30 minutes, then back to the supine recovery position for the next 30 minutes, and the same tilt procedure was taken at 20 minutes after the intra venous administration of 0.2g glycopyrrolate. The rest 13 dogs were kept in the supine position without tilt as the control group(CONT group ) for the same time course of the TILT group. Cardiac output(Q), heart rate(HR), stroke volume(SV), index of contractility(IC) and thoracic fluid volume(TFV) were deterkuned by the impedance cardiograph. Systemic arterial pressure (SAP) and pulmonary arterial pressure(PAP) were measured at the brachial and pulmonary arteries respectively, and central venous pressure(CVP) also at the right atrium. And systemic and pulmonary vascular resistances (SVR and PVR) were calculated. The concentration of plusma ANP and ADH in the arterial blood were measured by the radioimmunoassay in 8 dogs in each group at 5 minutes before, and 25 and 55 minutes after begining of the tilt. During the HDP, Q, SV and TFV increased to 9%,10.6% and 12.6ml respectively in the TILT group, where as they decreased by 10%, 13.8% and 7.5ml respectively in the CONT group. SAP, PAP and HR did not change significantly in the TILT group, but SAP and HR increased in the CONT group. At the same time, SVR and PVR reduced significantly by 4.5% and 8.9% respectively at 15 minutes of HDP in the TILT group, where as they elevated gradually to 21.8% and 13.4% respectively at the first 30 runutes In the CONT group. There were no significant differences in the changes of HR, PAP and CVP between the TILT and the CONT group. And the plasma ANP and ADH did not change significantly in the both groups. After the administration of glycopyrrolate, SAP and SVR Increased to 3.7mmHg and 5.1% respectively, and PAP, PVR, Q, HR, and SV did not change significantly during the HDP in the TILT group. There was no significant difference in the change of each hemodynamic parameter between the TILT and the CONT group, except in TFV, which showed the same change as that before the administration of glycopyrrolate in the both groups. The plsma ANP showed nonslgnificant increased trend at the HDP in the TILT group but did not change In the CONT group, and the plasma ADH showed nonsignificant decreased trend in the both groups. In summary, the cardiopulmonary baroreceptors induced the vasodilations of both systemic and pulmonary vessels by the expansion of thoracic fluid volume at the head-down tilt, and the anticholinergic agent(glycopyrrolate) tended to attenuate the hemodynamic responses to the tilt. The concentration of the plasma ANP and ADH did not show significant changes in this study. It is suggested that the regulatory mechanism inducing the systeruc and pulmonary vats odilation during the early phase of the head-down tilt is mediated by the cholinergic vasodilator fiber in the dog.