CENTRAL HYPERVOLEMIA IN THE CONSCIOUS RAT - A MODEL OF CARDIOVASCULARDECONDITIONING

The aim of the present study was to investigate whether increased cent ral hypervolemia induced by tail suspension (TS) in the rat is an appr opriate model of cardiovascular deconditioning (CVD). First, the physi ological relationship between central venous pressure (CVP) and extrac ellular fluid volume (ECFV) was studied. TS (20 degrees) increased CVP (5.8 +/- 0.7 vs. 2.8 +/- 0.8 mmHg; P < 0.01). After 24 h of TS, CVP h ad returned to control range while ECFV eras reduced by 19%. CVP kinet ics during 24 h of TS was not affected by either reduction (-20%) or a ugmentation (+35%) of the ECFV. The normalization of CVP is Likely to be a consequence of ECFV reduction, which itself is reduced by increas ed urinary excretion of water and sodium. Second, recovery from TS was studied. Resumption of the horizontal position was shown to be associ ated with a significant increase of heart rate (HR) and a slight reduc tion of blood pressure (BP); there was an apparent delay between incre ased HR and reduced BP. This imbalance between KR and BP is compatible with CVD. A model of simulated orthostatism (SO) was developed to fur ther investigate the responses of HR and BP. Interestingly, SO (90 deg rees rotation) in the normal rat was associated with significant tachy cardia and a slight increase of BP. This pattern remained stable for a t least 3 h. In rats that were tail suspended for 48 h, episodes of hy potension and bradycardia (5 +/- 1 in 3 h) suggested a defect in adapt ation to increased hydrostatic pressure. In conclusion, TS appears to be an appropriate model of CVD. Reduction of ECFV is likely to play a major role in the deconditioning process. Return to horizontal positio n in TS rats induced a tachycardia with minimal effects on BP; this pa ttern is close to that observed in humans assuming upright posture. SO in previously TS rats disclosed episodes of hypotension and bradycard ia that deserve further investigation.