두부하위(-6°)의 체위변동에 따른 심폐기능의 변화

Abstract

Head-down tilt(HDT) at-6° has been commonly used as the experimental model of weightlessness in both man and animals. Inconsisteant responses in cardiopulmonary parameters under different protocols have been shown among studies by various authors in the simulated weightlessness. Although many investigations have been carried out, little is known of the transient physiological responses upon the cardiovascular regulatory system and microcirculation of peripheral blood flow, or the pulmonary mechanics to the -6˚ HDT within a relatively early period. Therefore, the purpose of this study was to observe the transient minute by minute changes of cardiopulmonary responses to the -6° HDT for 3- minutes and to evaluate early regulatory mechanism in the simulated weightlessness. Present study was done on15 healthy young adult males and females. The postural changes were performed from the sitting to the supine control position. And the -6° HDT was followed by the control supine position and lasted for 30 minutes. Then, subjects were returned to the supine recovery position and remained for 10 minutes. Functional residual capacity(FRC), ventilation($$\dot V_E$$), tidal volume(TV), respiratory rate(f), vital capacity(VC), expiratory reserve volume(ERV)and inspiratory capacity(IC)were determined by the computerized spirometer, N₂ swash-out test with 100% oxygen was employed for FRC measurement. Cardiac output($$\dot Q$$), O₂ uptake($$\dot V$$o₂) and CO₂ output($$\dot V$$co₂) were determined by computerized cardiopulmonary-exercise system. The $$\dot Q$$ was measured by O₂-rebreathing and equilibrium method by indirect Fick's principle. Blood pressure(BP) and heart rate(HR) were also measured. Total peripheral resistance(TPR) was calculated by dividing respective mean BP(MBP) values by $$\dot Q$$. In addition, photoelectric pulse amplitudes(PEPA) to evaluate the ear lobe and the finger blood flow, Galvanic skin resistance(GSR) of the finger that is influenced by the activity of autonomic nervous system and the vascular resistance, and also skin temperature(Ts) of the finger were recorded simultaneously. The results obtained are as follows: $$\dot V_E$$ and TV were decreased at 30 minutes of HDT, then recovered in the supine recovery position(SRP). VC was decreased at the control supine position comparing to that of the sitting, and it was further decreased in HDT insignificantly. was increased transiently at 10 minutes in HDT. FRC was decreased about 28% at the control supine than at the sitting and further declined about 8% in HDT than at the control supine. $$\dot V$$O₂ was significantly increased in SRP after HDT. $$\dot Q$$ was elevated during HDT but was not significant. TPR was diminished continuously through the control supine and HDT. MBP was maintained, but systolic BP(SBP) was slightly declined in HDT. HR was decreased at the supine control position and was increased transiently at 5 and 10 minutes of HDT. Blood flows in the ear lobe and the finger were increased during HDT. Through the time course of various postural changes, GSR and Ts showed rapid fluctuational changes. However these changes were not statistically significant. From the above results, it may be suggested obviously that decrease of the FRC in the lung volumes was caused by consistant body fluid shift into the pulmonary vascular bed and the ascent of the diaphragm toward the thoracic cavity. Furthermore, elevated blood Pco₂ to the diminished lung volumes and the increased venous pooling, might give rise to increase respiratory rate in early period of HDT. The elevated VO₂ in SRP may be due to the increased ventilation as well as the increased metabolism. Present study provides evidence that the changes in peripheral blood flow to the ear lobe and the finger are proportional to $$\dot Q$$ and inversely proportional to TPR during HDT. In the view points of intrathoracic sequstration of fluid volume during HDT, it seems likely that cardiopulmonary baroreceptors mainly act through the vasomotor center in the brainstem to regulate peripheral vascular tone in order to maintain the constant blood pressure and possibly constant blood flow in central portion of the body i.e. the heart and the brain.

본 실험은 두부 및 체중심부로의 혈액이동을 야기시키는 -6˚도립위로 체위변화시 30분 이내에 나타나는 심폐기능의 변화와 조절기전을 관찰하고, 또한 미세순환동태를 관찰함으로써 무중력에 노출시 조기의 생리적 적응기전을 규명해 보고자 하였다. 평소 건강한 남녀 대학생을 대상으로 체위변화는 좌위, 대조 앙와위, -6˚ 도립위 및 도립후 앙와위의 순으로 하였다. 호흡반응을 보기위하여 폐용적들과 환기량을 측정하였으며, 기능적 잔기량의 측정은 폐내질소제거법으로 시행하였다. 순환 반응으로는 혈압, 심박수, 전 말초저항, 심박출량 및 미세 혈류량 등을 관찰하였으며, 심박출량은 간접 Fick 원리에 의한 CO₂재호흡법의 평형법을 측정하였다. 이상의 실험에서 분시 환기량 및 일회 호흡량은 도립위 30분에 감소하였고, 호흡수는 도립위 10분에 일시적으로 증가하였다. 폐활량은 대조 앙와위에서 감소하였고, 도립위시는 유의한 변화는 없었으나 감소하는 경향이었다. 기능적 잔기량은 대조 앙와위 및 도립위시 점진적으로 감소하였다. 심박출량은 도립위시 증가하는 양상이나 유의한 변화는 아니었으며, 산소 섭취량은 도립후 앙와위시에 유의한 증가를 보였다. 귓불 및 수지의 혈류량은 도립위시 전반적으로 증가하였으며, 피부전기저항이나 피부온도는 유의한 변화를 보이지 않았다. 좌위에서 시작하여 대조 앙와위, 그리고 -6˚ 도립위로 체위변화시 폐용적의 감소는 폐장내의 점진적인 혈액량 증가와 횡격막의 흉곽내부로의 이동으로 초래된 것 같다. 또한 폐용적의 감소와 함께 정맥환류량의 증가가 일시적인 Pco₂ 증가를 초래하여 도립초기에 호흡수의 증가를 유발한 것으로 사료된다. 도립후 앙와위시의 산소 섭취량 증가는 일회 호흡량의 증가와, 이에 따른 산소 소비량의 증가 및 분시 환기량의 증가와, 교감신경계의 흥분에 의한 2차적인 대사증가에 기인한 것으로 사료된다. 도립초기에 평균 혈압의 일시적인 감소경향은 압력수용체를 통한 심박수의 증가를 유발한 것으로 추측되며, 도립초기의 일시적인 정신적 긴장이 심박수 증가에 영향을 미친것으로 보인다. 귓불 및 수지의 혈류량은 심박출량에 비례적으로, 전 말포 저항에 반비례적으로 변화하는 것 같으며, 심혈관계의 동태를 반영하고 있다고 사료된다. 피부전기 저항의 변화로 보아 체위변화 과정이나, -6˚ 도립위가 초래한 정신적 긴장은 평균적으로 미약했던 것으로 추측된다. -6˚ 도립위시에 심박출량은 체위변화의 각도가 미미하고, 하지로 부터 온 혈액이 폐장내의 혈관으로 재분포 하므로 유의한 증가는 없었던 것으로 보인다. 또한, 심폐 압력수용체들은 주로 말초혈관긴장을 감소시켜, 혈액의 재분포와 함께, 심장 및 뇌를 포함하는 체중심부의 혈압과 혈류를 일정하게 하기 위해 작용하는 것으로 사료 된다.