Reduced oncotic pressure after shock: a proposed mechanism

Hemorrhagic shock plus resuscitation leads to expansion of both interstitial fluid space (IFS) and intracellular fluid (ICF). THe IFS expansion is thought to reflect reduced serum colloid oncotic pressure (COP) from capillary leak of serum albumin (SA). This hypothesis was analyzed in 138 injured patients who received an average of 13.6 units of blood and 10.7 L of saline solution for correction of shock. Plasma volume (PV), extracellular fluid (ECF) by inulin space technique, SA level, percent albumin leak (PAL), total intravascular albumin content (TIAC), COP, IFS, weight gain, and time of injury were analyzed. Patients studied during the postoperative fluid sequestration phase had a low to normal PV (2.9 L) with a high IFS (15.2 L) and a low PV-ECF ratio (0.15). Although the SA level (2.8 g/dL), TIAC (81 g), and COP (11.9 mOsm/L) were low, the PAL was also low (6.0%/hr; normal, 7%/hr). Patients studied during the fluid mobilization phase had a high PV (3.2 L) and normal PV-ECF ratio (0.21), whereas the SA level (3.0 g/dL), TIAC (103 g), and COP (14.1 mOsm/L) remained low. Despite these findings, the PAL was high (8.0%/hr). The fall in the PV-ECF ratio during the postresuscitation fluid sequestration phas is associated with but not due to reduced TIAC and COP, both of which remained low after the PV-ECF ratio has normalized during the mobilization phase. Contrary to prior reports, albumin leak is not responsible for reduced COP, TIAC, and postresuscitation weight gain. Presumably, the low COP results from decreased reentry of albumin into the plasma volume due to entrapment within the IFS matrix.