Colloids vs. Crystalloids - hypovolaemic shock?!


Question: I have always been told that with hypovolaemia if a patient is already hypotensive that colloids are best to bulk up blood volume quickly, but recently I was told that normal saline or Hartman's are the first port of call - is it the case if hypovolaemia is spotted BEFORE bp drops that crystalloids are more effective? Confused


Answers: I have always been told that with hypovolaemia if a patient is already hypotensive that colloids are best to bulk up blood volume quickly, but recently I was told that normal saline or Hartman's are the first port of call - is it the case if hypovolaemia is spotted BEFORE bp drops that crystalloids are more effective? Confused

a bit about hypovolaemia first (im going to use medical terms because you obviously have some kind of background if you are familar with the difference between colloids and crystaloids!). its a bit like tennis, remember how they score the games 15, 30, 40. hypovolaemia is the same, except in terms of percentage fluid loss. ( you should be able to calculate how much of the total body water is in the patient as well by using the theory 70% of body weight is water of which approximately 30% is in the blood). the added category is >40%. in an average individual, up to about 30% fluid loss can be compensated for i.e. there are physiological changes that occur but these changes facilitate dealing with the situation successfully. tachypnoea is a very good sign of hypovolaemia and may often initially be the only change which occurs. at about 30% loss, there would also be some tachycardia as well. note that because these are successfully compensating, the BP is NORMAL!! so in a situation where hypovolaemia is suspected, it would be wise to prescribe some crystaloids.

if you prescribe too much colloid you risk fluid overloading the patient, which also isnt a great situation to be in

at this point i am going to explain the difference between the two fluids just to make sure we are on the same page. because fluids such as hartmans are similar in composition to blood, they are distributed accross all the fluid compartments. therefore only approxiamtely 300mls out a litre of crystaloids stays in the vasculature. colloids on the other hand contain long chain sugar molecules that cannot pass through membranes and therefore exert an osmotic potential in the plasma so that water and electrolytes have a greater tendancy to stay in the vasculature.

back to the problem. because at 30% an individual is usually compensating, a few litres of crystalloids will be fine because they are not going to need that much fluid in the vasculature and in any case they are compensating. when someone is not compensating i.e. BP begins to drop, you already know that there is going to be greater than 30% volume loss so you want to get the colloids in fast to maintain some circulating volume.

having said all this it really depends on the situation. I have been in theatres observing some major operations, for which even though the patient shows no signs, the anaesthetist still wrote up some colloids.

if a patient comes in with an arterial bleed from a major vessell, then your not going to look at his BP and think hes ok, your going to want to get the colloids on board because he or she sure as hell wont be in a few minutes if all they are getting is crystalloids!

I think the important thing to remember is where the fluids are going to be distributed, how hypovolaemic the patient is, and how quickly you think the patient will deteriorate, to determine the fluid management. one thing I would say though, is that it is always advisable to write up a colloid with some crystalloid so that fluid of a similar osmolarity is also being replaced in the patient

apologies for the long answer!





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