Where do administered fluids go?

Referring back to the main fluid compartments (page 2), there are the ICF and ECF . The ICF is separated from the ECF by the cell membrane (figure 2). The cell membrane is freely permeable to water and small molecules. However the sodium potassium pump pushes Na out and K into the cell. In addition the intracellular proteins can not pass the cell membrane and their negative charge has to be balanced by the movement of Cl out of the cell.

The ECF is divided into the interstitial space and the intravascular space by the capillary endothelium. This barrier is freely permeable to water and small molecules and impermeable to proteins. Movement of fluids across the capillary is described by Starling's law of the capillary.

Colloids will stay mainly in the intravascular space, permitting rapid expansion of the circulating blood volume in patients with hypovolaemic shock.

The distribution of crystalloids depends upon their make up. Dextrose 5% is essentially water and is distributed throughout the total body water. It is infused into a vein, it crosses the capillary barrier and then the cell membrane barrier. Normal saline and Hartman's solution will remain in the ECF . Following infusion into a vein they pass freely across the capillary barrier but there is no incentive for them to move across the cell membrane into the ICF . Solution 18 (or one fifth normal saline in 4% dextrose) will be distributed throughout the body water but less so than dextrose 5%.

For example, haemorrhagic shock will usually occur if the blood volume is acutely depleted by 20%. This loss is about 1 Litres in your typical 70 Kg adult male. An immediate check on the patients haemoglobin will show no change as haemo-dilution has not had a chance to occur. Full haemo-dilution takes about 36 hours, although the majority has occurred within 8 hours. Starling's law of the capillary means that the loss in circulating blood volume will result in a shift of fluid from the interstitial space to the intra vascular space. This fluid shift results in restoration of the blood volume and a dilution of the remaining haemoglobin which falls as a consequence.

Faced with a patient with acute haemorrhagic shock, the treatment is simple (conceptually at least);

1. Stop the bleeding
2. Restore the circulating blood volume

The best fluid to restore the circulating blood volume would logically be fresh whole blood. However, due its lack of availability it is usually not available. In any case, most blood products require cross matching with the patients own blood and this takes time. Colloids will stay in the intra vascular space and rapidly correct the deficit in circulating blood volume. You will need to give 1 volume of colloid for each volume of blood that has been lost. Now what about crystalloid?

Q

In our patient with shock, if we only have dextrose 5% to give then how much will we have to give to restore the circulating blood volume in someone with an estimated 20% haemorrhage?

A

The estimated total blood volume in a 70 Kg man is about 75 ml per Kg, which is equivalent to 5 Litres. In a 20% bleed then we need to expand the intra vascular space by about 1 Litres. As the dextrose is distributed through out all of the body water, quite a lot of dextrose would need to be administered. 60% of the body weight is water and 5% of the body weight is water in plasma. So about one twelfth of water given to the patient will stay in the blood volume. To replace 1 Litres in the intra vascular space we would have to give about 12 Litres of dextrose 5%. Of course the patient would not do well with this treatment regimen.

Q

What about saline or Hartman's?

A

These are distributed in the ECF compartment. 20% of the body weight is water in the ECF , of this 15% is interstitial and 5% again is intra vascular. About one quarter of the fluid given into the ECF will remain in the intra vascular space. We would therefore need to administer about four Litres of saline or Hartman's for each Litre of blood volume lost, or in this patients situation 4 Litres. This inequality regarding the amount of saline or Hartman's that is needed to replace a volume of blood is known as the three to one rule (or four to one if you followed the argument above).

Q

Which crystalloid would you use, in a patient with haemorrhagic hypovolaemic shock?

A

Hartman's or Saline.