An interesting abstract came across Google Reader just now about the prehospital use of hypertonic fluids in patients with traumatic brain injury. Before discussing the results themselves, I’d like to point out one aspect of particular note: The study authors looked at 6 month outcomes. Compare and contrast this to most of the available cardiac arrest research which is only looking at return of spontaneous circulation (ROSC). The authors in this study are clearly thinking further down the road. ROSC in and of itself does not equate to improved patient outcomes. Perhaps we could start looking at resuscitation outcomes 6 month after arrest when publishing new cardiac arrest guidelines, just a thought.
Time for a little review. If you remember, there are three types of solutions used in medicine: Hypotonic, hypertonic and isotonic. Isotonic solutions like 0.9% “normal” saline have the same concentration of solutes (stuff dissolved in them) as the body. Hypotonic solutions have a lower concentration of solute and hypertonic solutions have a greater concentration. What does this mean in the body? Water (the solvent) tends to move to areas which have a higher concentration of solute (solids). This tendency of water to “even out” concentration creates a force called osmotic pressure. When blood cells (as an example) are exposed to solutions with different concentrations of solute the following results are typical:
When the cells are placed in an isotonic solution, nothing changes. The flow of water into the cells is matched by the flow of water out of the cells. When the cells are placed in the hypotonic solution, water flows into the cells to offset the higher concentration inside, causing the cell to swell and break open. And finally, when the cells are placed in the hypertonic solution, water flows out of the cells in an attempt to normalize the solution outside of the cell.
APPLICATION IN MEDICINE
So what does this all mean to medicine? We carry isotonic fluids on our vehicles and routinely use them for fluid resuscitation because we want to increase blood volume without placing unneeded stress on the body’s cells. This study looked at giving a single 250cc bolus of a hypertonic solution to patient with traumatic brain injury (TBI). One of the effects of TBI is swelling of the brain or cerebral edema. The idea of giving a hypertonic solution to a TBI patient makes sense from a chemistry sense; it will keep the fluid from being taken up by the brain tissue because osmotic pressure is keeping the fluid in the blood stream. Basically a hypertonic solution has a tendency to pull water into the blood stream rather than allowing water to leave the blood stream into surrounding tissues. Giving a hypotonic solution to a TBI patient would likely increase cerebral edema.
The authors of the study planned to enroll 2122 subjects who would be given a 250cc bolus of 7.5%saline/6%dextran, 7.5%saline or 0.9% “normal” saline by prehospital providers. The study was terminated after 1331 patients when the study had met “predefined futility criteria”. Not having access to the full article, I’m unaware of what those criteria were. The results, however, show that there was not a statistically significant change in patient outcomes with regard to which fluid bolus was given.
So there you have it. Prehospital administration of hypertonic fluids does not change six month outcomes in TBI patients. Hopefully the chemistry review was worthwhile. If there’s any interest in continuing these types of reviews, let me know and I’d be happy to make it a regular feature. I also think that the study design, and focusing on longer term outcomes is a beneficial approach to prehospital research. At the end of the day, getting pulses back on a cardiac arrest patient doesn’t matter much if they don’t leave the hospital and go on to live a healthy life just like an improvement for a period of hours or days for TBI patient doesn’t mean much if their long term outcome doesn’t improve. What do you think? Are there any other areas of EMS treatment which could benefit from the study of long term outcomes?