Systemic and renal hemodynamic effects of hemorrhage in conscious lambs.

The purpose of the present study was to investigate the systemic and renal hemodynamic effects of hemorrhage in the newborn to determine whether the newborn is capable of restoring blood pressure in the face of blood loss at hemorrhage of up to 20% of vascular volume. Experiments were carried out in conscious, chronically instrumented lambs and consisted of measurements before, during, and after hemorrhage at 0 (n = 9), 10 (n = 8), 15 (n = 8), and 20% (n = 8) of blood volume. Right atrial pressure decreased but only after 20% hemorrhage. There was a transient decrease in blood pressure at 10-15% hemorrhage and a sustained decrease in blood pressure after 20% hemorrhage (from 82 +/- 7 to 66 +/- 9 mmHg). Heart rate increased transiently after 15% hemorrhage (from 173 +/- 32 to 204 +/- 66 beats/min); heart rate remained increased for 60 min after 20% hemorrhage from 171 +/- 17 to 214 +/- 31 beats/min. There were no changes in renal vascular resistance in response to hemorrhage of up to 20% of vascular volume. These observations provide evidence that the newborn is capable of buffering blood pressure in response to blood loss of up to 20% of vascular volume and that the renal bed does not appear to contribute to the restoration of blood pressure after blood loss early in life.