Background Resuscitation of the shocked patient presents a dilemma for the prehospital resuscitation team. Selection of the appropriate type and quantity of resuscitation fluid is not supported by evidence based consensus and discussions such as the ‘crystalloid colloid debate’ have been prolonged and unhelpful. We are seeking a straightforward way of guiding resuscitation of shocked patients.

The Prior Hypothesis suggests that capillary blood pressure and membrane osmotic pressure (MOP) must be equal and opposite to achieve fluid balance in the microcirculation. Membrane osmotic pressure is critically dependant on the pore size of small vessels – larger pores allowing the egress of large molecules from capillaries and therefore lowering capillary MOP. Our studies suggest that shocked patients fall into two groups: a ‘dry shock’ group with elevated MOP, and a ‘leaky shock’ group with a low MOP. The ability to identify these different groups has implications for treatment– both choice of resuscitation fluid, and the use of other vasoactive pharmacological agents which influence capillary pore size.

Methods Concurrent blood pressure and MOP were measured in 57 shocked patients on presentation to the Emergency Department. MOPs were measured using a process taking less than 5 min with a standalone osmometer requiring as little as 50 µl of serum.

Results In the patients tested two distinct patterns of abnormal MOP were distinguishable. 20 patients fell into the ‘leaky’ shock group (MOP 19–24 mmHg), 9 in the ‘dry’ shock group (MOP 27–30 mmHg) with a further 16 having MOP in the normal range (24–27 mmHg).

Conclusion Our findings are consistent with the use of a rapid MOP test to categorise acutely presenting patients into ‘dry’ and ‘leaky’ shock depending on capillary pore size. Further work is required to test the utility of near patient MOP testing as a guide to effective therapy during emergency resuscitation.