Who knows?
There are two main groups of fluid replacement solution–one crystalloid and the other are the colloids. I mean there are two schools of thought here.
Apart from a use in fluid replacement in burns’ patients and a select other indications, the resuscitation in all but critical cases with colloid now seems frowned upon, as colloid resuscitation has been associated with a higher mortality in comparison to crystalloid infusion.1 This is the current state of play, but the future may bring newfound colloid (and, who knows, crystalloid) solution onto the market that alter the dynamics. After all, this is about dynamics.
It is the dynamics of fluid replacement and compartmental shifts in fluid that determine the type and rate of fluid resuscitation. I mean, who really knows when it comes to fluid compartments and fluid shifts: but we do have some idea what happens when a litre of crystalloid is rapidly infused into the vein of a patient. Except. … Except!
Except that the pathophysiological state of each patient is different, and changes, with time depending on the circumstance and (their physiological) reserve. For well easy might you say that 1 L of crystalloid will rapidly redistribute, pound for pound, into the respective compartments; well easy, but all that assumes a healthy capillary bed in all tissues. But of course a trauma patient, a patient with anaphylaxis, a patient in the recovery bay after an anaesthetic and surgery will, to an extent, some more and some less, present to the clinician a leaky capillary-bed which alters the fluid shift dynamics; pathophysiology alters those haemodynamics.
And each person’s pathophysiology is different.
So we extrapolate or interpolate from some average understanding. I prefer to work from the mean. That is, I prefer (and perhaps most clinicians; and most surgeons certainly do) to see resuscitation and all patient care as a reversion to the mean–the patient’s physiological mean.
That is why in a resuscitation we like to speak of the mean arterial pressure. Because that’s the goal we aim for—all things being equal—in the fluid resuscitation of a person. Now although everyone is different, everyone is equal. Equal in the eyes of the law and equal in the eyes of fluid haemodynamics, with respect to the MAP. So the mean arterial pressure is a good guide for optimal fluid resuscitative care–at least in the acute setting: there is a minimum MAP the patient needs in order to perfuse vital organs.
The MAP is perchance a measure of the potential energy stored as impedance resistance in the arterial circulation, waiting to “pulse through their veins.” Waiting, I say, to “pulse” through their capillary beds. By which, of course, I refer to tissue perfusion or blood flow.
Arteries pulsate, capillaries float, and veins gravitate under that weight.
The mean arterial pressure (MAP) is your clinical surrogate for tissue perfusion. The other’s are skin colour and skin temperature to the touch, urine output, and, of course, mentation. If a person’s flat, they’re flat. And if because of poor tissue perfusion, poor oxygen and glucose perfusing the brain, then they are flat aren’t they–flat, that is, in their MAP.
Fix the MAP.
Works cited:
Watts, J. “Fluid resuscitation with colloid or crystalloid solutions. Comparing different studies is difficult.” BMJ (Clinical research ed.) vol. 317,7153 (1998): 277; author reply 279.
