Haemodynamic responses to hypotensive haemorrhage in conscious sheep with emphasis on renal and femoral blood flow

Abstract

SUMMARYHaemodynamic responses to slow (0 7 ml kg-1 min-') haemorrhage were investigated in eight adult conscious sheep, fitted with permanent ultrasonic flow probes around the renal and femoral artery. The haemorrhage was continued until the mean systemic arterial pressure (MSAP) suddenly dropped to about 50 mmHg. The spontaneous recovery was followed for 60 min and then the blood was retransfused. A distinct fall in MSAP was obtained after 14-5 + 09 ml blood (kg body weight)-' (24% of the estimated blood volume) had been removed, but the interindividual differences were rather large (range, 102-18-1 ml kg-1).Therefore, the haemodynamic responses during haemorrhage were related to the total bleeding volume and not to time or volume per kilogram body weight. Before 90% of the total haemorrhage volume had been removed, the MSAP decreased gradually, reaching statistical significance at 80 % (P < 0 05). At end of bleeding the cardiac output (CO) had fallen from a pre-haemorrhage level of 4 7 + 0-2 to 2 9 + 0-2 1 min-'. Although the femoral blood flow (FBF) was largely unaffected by the haemorrhage, and thus increased as a fraction of CO, the total systemic vascular resistance was markedly increased during the post-haemorrhage period, indicating a longer lasting elevation of vascular resistance in other vascular beds. In spite of a concomitant marginal decrease in renal blood flow (RBF), the renal vascular resistance (RVR) was unchanged until 90 % total haemorrhage had occurred. At the same time as blood pressure fell, RBF decreased markedly, concomitant with a transient increase in RVR, which was followed by a decrease towards basal levels. As a result, the RBF as a percentage of CO decreased, indicating a significant redistribution of blood away from the kidneys; this lasted for about 15 min after the end of haemorrhage. While MSAP, central venous pressure and FBF were elevated above basal levels by the retransfusion, RBF was not fully restored. The results show that RBF is preserved during haemorrhage in conscious sheep, until marked systemic hypotension ensues. The redistribution of blood away from the kidneys appears rather transient after an acute hypotensive haemorrhage in this species; this is also true for the blood flow to the hindlimb, which was little affected by the blood loss.

Lower body negative pressure as a model to study progression to acute hemorrhagic shock in humans

“…Mean arterial pressure is well maintained during slow (0.7 ml⅐kg Ϫ1 ⅐min Ϫ1 ) hemorrhage in sheep but falls precipitously when total blood loss equals ϳ24% of total blood volume (50). An early study by Henry et al (56) documented a tight linear relationship between changes in blood volume between ϩ30 and Ϫ30% of total volume, and pulmonary arterial, left ventricular diastolic, and central venous pressures in dogs.…”

“…In monkeys, 30-50% withdrawal of total blood volume decreases fractional cardiac output to skin, kidney, and skeletal muscle and increases fractional cardiac output to heart, brain, and liver (41). Renal blood flow is well maintained in sheep during hemorrhage until systemic hypotension, at which time renal blood flow decreases and cardiac output is redistributed elsewhere (50). Renal blood flow rises during moderate hemorrhage but decreases during severe hemorrhage in dogs (117), and removal of 20% total blood volume decreases renal blood flow in conscious pigs by 30% (104).…”

Systemic responses to trauma

“…Barcroft et al found that when their volunteers became hypotensive and fainted, there was a sudden fall in SVR and a sudden increase in fore-arm blood Trauma flow, suggesting a sudden skeletal muscle vasodilatation 8 . A similar skeletal muscle vasodilatation occurred when sheep were haemorrhaged until an abrupt fall in blood pressure was seen 31 .…”