Partitioning of small and large saline loads in supine and head-up tilted dogs

A previous study from our laboratory has demonstrated that acute saline loading in supine anaesthetized dogs (7% body weight) is associated with significant protein efflux from the intravascular compartment, and a smaller retention of infusate than would be predicted from theoretical considerations. The advent of postural stresses (quadruped and 45 degrees head-up tilt positions) aggravate this phenomenon. In the present study 'small' (3.5% BW) and 'large' (15% BW) saline loads were administered to supine and tilted anaesthetized dogs. In supine dogs, the 3.5% load was associated with a 22.8% reduction in circulating protein (TIP) and 14.6% retention of the infusate (increased from 10.9% in 7% BW loading), while 15% BW loading was associated with only a 4.3% retention of infusate and 47% reduction in TIP (p less than 0.05). When dogs were tilted, the 3.5% group retained zero infusate and the reduction in TIP rose to 43.6%. In the 15% group, 2.6% of the infusate was retained even though a 56.8% reduction of TIP was observed. We conclude that retention of isotonic infusate is most efficient when transcapillary protein efflux is limited. With very large degrees of volume loading, tilted animals will retain more infusate than dogs receiving a small load, presumably because the increased rate of infusion matches the vascular fluid losses.