Continuous blood densitometry: fluid shifts after graded hemorrhage in animals

To evaluate rapid fluid shifts after graded hemorrhage in splenectomized animals, four pigs and two dogs were bled 15-23 ml/kg body wt in steps of 2.2-6.0 ml/kg. Arterial blood density (BD), mean arterial pressure (MAP), and central venous pressure (CVP) were recorded continuously, and arterial plasma density (PD) and hematocrit (Hct) were determined from blood samples. Erythrocyte density was computed from PD, BD, and Hct. Starting with stable control conditions, MAP, CVP, and BD fell from the beginning of hemorrhage. Each blood withdrawal was followed by an immediate and rapid decrease in BD, even at the lowest (less than 3 ml/kg) initial blood losses. The time course of BD change mirrored that of the volume replacement, with time constants of 3.0-9.6 min and amplitudes depending on the magnitude of the relative volume loss. The PD decrease was significant (P less than 0.01) after 5.4 +/- 0.7 ml/kg hemorrhage. At 15 ml/kg blood loss the mean PD and BD had dropped by 0.99 +/- 0.15 and 2.42 +/- 0.26 g/l, respectively, and Hct had dropped by 2.40 +/- 0.47 units. Calculations suggest that either the inward-shifted fluid has a higher density than normal ultrafiltrate and/or there is a rise of the whole-body-to-large vessel Hct (F cell ratio). The rapid fluid replacement ranged from 5.8 +/- 0.8 to 10.6 +/- 2.0% of the initial plasma volume, or one-fifth to one-third of the lost volume with a 20% hemorrhage. Transvascular fluid shifts can be monitored with continuous high-precision blood densitometry.