Hyperosmolality and transcapillary fluid and protein movement in skeletal muscle

Effects of hypertonic saline, mannitol, and urea with regard to absorption and rebound filtration in cat skeletal muscle

OBJECTIVE

To study the effects of the hypertonic solutions 15% mannitol, 3% and 7.5% saline, and 30% urea at clinically relevant plasma concentrations with regard to absorption and rebound effects on tissue volume in skeletal muscle.

DESIGN

A prospective, experimental study.

SETTING

University laboratory.

SUBJECTS

Twenty-eight anesthetized cats.

INTERVENTIONS

The study was performed on an autoperfused and denervated cat calf muscle placed in a fluid-filled plethysmograph. Muscle volume changes and capillary filtration coefficient (reflecting capillary fluid conductivity) were measured before, during, and after intra-arterial infusion (4 mL/hr) of the hypertonic solutions. Mannitol and 3% saline have the same osmolality and were compared specifically in an attempt to distinguish osmotic effects from those specific to the compound.

MEASUREMENTS AND MAIN RESULTS

All solutions reduced muscle volume during the infusion (p < .05). The maximum volume reduction persisted after 2 hrs of infusion for 3% and 7.5% saline, whereas there was a tendency for volume recovery during the urea infusion and a complete recovery back to control for mannitol. After discontinuation of the infusions, the muscle volume increased for all four solutions, stabilizing at the initial control for 3% and 7.5% saline, whereas it increased to levels above control for mannitol and urea (p < .05). Capillary filtration coefficient was increased by hypertonic saline (p < .05) but was unaffected by mannitol and urea.

CONCLUSIONS

The effectiveness of a hypertonic solution in reducing tissue volume and its tendency to cause a rebound volume increase depends not only on the osmolality of the solution. Hypertonic saline may in the long run be superior to mannitol and urea to increase plasma volume or decrease tissue volume of an organ, because it lacks rebound effects. Alterations in capillary filtration coefficient (fluid conductivity) may reflect volume changes of the capillary endothelial cell and thereby differences in cell membrane permeability for the hypertonic solutions, also consistent with the obtained differences in tissue volume effects.

Iatrogenic compartment syndrome from hypertonic saline injection in Bier block

A case study of an iatrogenic forearm compartment syndrome is presented. The patient underwent intravenous regional anesthesia (Bier's block) for an attempted closed reduction of a Bennett's fracture. Hypertonic saline (23.4%) was inadvertently used as a lidocaine diluent for the Bier's block, and after tourniquet release, the patient developed signs and symptoms of an acute compartment syndrome. An emergent fasciotomy was required. A review of the theoretical pathogenesis of compartment syndrome resulting from intravenous injection of hypertonic saline is presented. While vein sclerosis from i.v. hypertonic saline may play a role in the development of a compartment syndrome, we speculate that the major factor was an increase in the extracellular tissue fluid volume resulting from osmotic equilibrium after the tourniquet was released. The need for meticulous attention to details when performing this technique is emphasized.