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Estimation of the fluid balance in the ICU is generally held to be important because it offers some idea of whether or not the patient is "fluid overloaded", and by how much. The clinical significance of fluid status changes are not debated in the college papers; the past paper SAQs focus on the methodology ("how to measure", rather than "why would you bother") - the clinical utility of fluid balance data being an unquestioned truth. However, as it turns out the mindless worship of water content has little foundation in the published evidence.
With appropriate attention to this topic, the college have invited the candidates to tabulate the the advantages and limitations of the various ways of estimating fluid balance in Question 16 from the second paper of 2009 and Question 16 from the second paper of 2006. The typical methods are by fluid balance record, by daily weights, and by clinical examiantion. More exotic methods range from PA catheter wedge pressures though PiCCO and CVP and to weird stuff like bioimpedance and tritium dilution. The tabulated response offered as the college "model answer" is difficult to improve upon, as it strikes a balance between detail and brevity. As such, the time-poor exam candidate may be well advised to limit their revision to the college answer.
If, however, brevity were sacrificed to detail, an answer to Question 16 would resemble the following table.
Method |
Advantages |
Disadvantage |
Clinical estimates |
|
|
Fluid balance chart |
|
|
Daily weights |
|
|
CVP |
|
|
PAWP |
|
|
TTE |
|
|
PAC or PiCCO EVLW |
|
|
Bioimpedance |
|
|
Tritium indicator dilution |
|
|
The overall merit of fluid balance calculation (nevermind their accuracy) is questionable. After all, what precisely are we measuring, and does it matter? Consider. The normal healthy person, whose physiology is supposedly well understood, is still subject to fluid balance changes which may be measured in the litres, and which may not be accounted for by the usual cylinder diagrams.
There he is, the smug Healhy Person. Arrogantly sucking down an irresponsibly unmeasured volume of water while exercising with his iPod earphones. Probably listening to some sort of abominable Nu-Metal. He clearly has no idea of how much moisture he is losing per breath, nor of how much he is drinking, or how long he has been exercising for. Moreover, the surface of his body will produce evaporative losses which are even harder to estimate, because they are contingent on body surface covering, surface temperature, sun exposure, ambient air humidity and temperature, convection by way of wind, and what if it starts raining? In short, fluid balance estimates are difficult.
Worse yet, they may be pointless. Consider a situation where one has precise control of all the abovementioned variables, and is able to measure every droplet of intake and output very accurately. In such an implausible scenario, at the end of a day one ends up generating a number (eg. "negative balance 500ml") which represents the fluid balance difference between this morning and this afternoon.
What significance does this balance have in the healthy person?
One may posit that it has none.
Consider: the smug jogger has changed their body fluid volume by perhaps 1% of the total. At this level, neither the tonicity of the body fluid nor the pressure of the circulating compartment are affected, which means that the normal autoregulation mechanisms have not had to exert any homeostatic effect. In short, the man's own pitutary doesn't care about this fluid shift. So why should we?
Yes, there may be situations where fluid balance has clinical significance. It would be silly to completely ignore the value of input and output measurement, because ultimately a certain fluid input must be maintained for normal health, and in the intensive care unit the doctor has complete control over this variable. In a reductio ad abdsurdum scenario such as a man dehydrating to death in the desert the importance of input and output volumes becomes readily apparent. However, the question is not whether inputs and outputs are important, but whether there is a clinical significance in the fastidious attention to their measurement.
Consider now the critically ill patient.
Let's say you are measuring some of those inputs and some of those outputs.
Let's say youre measurements are accurate; all of the fluid boluses are signed and accounted for. Still there will be some fluid losses and gains - potentially, vast ones - which remain impossibl to measure. Within the context of the situation, with severe trauma, organ system failure, rampant infection, missing limbs and potentially weeks of ICU stay, the relevance of the daily fluid balance becomes questionable, even if it is accurate. It is hard to discern its effect on survival, that signal being lost in the deafening noise of critical illness.
So, if a fluid balance assessment question ever comes up again, it may come in the form of a miny-essay. "Critically evaluate the use of the cumulative fluid balance in estimating fluid gains and losses in critically ill patients", they might ask. In preparation for such a question, one should have a plan.
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