Question 16

Outline the methods available to estimate fluid balance in the critically ill patient  and briefly discuss their advantages and limitations. (You may tabulate your answer).

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College Answer




Clinical – oedema, JVP,
skin turgor, hydration of tongue

Simple, easily done by the
bedside, not time consuming

Lack specificity

Intake–output chart

Simple method, reasonably
accurate in most patients

Labour intensive,
Insensible water losses not factored in, Losses through leaks in bed, drain disconnections and in the case of burns patients, severe evaporative water losses not taken inot account


May be useful in
uncomplicated critically ill patients

Not routinely used in all
ICUs, time consuming, labour intensive, difficult in the ventilated patient, (zeroing has to be done properly with sheets and pillows), addition of moisture from perspiration and spills can change baseline weight
Correlation with I-O charts not high.


Used to predict
intravascular status

Significant limitations


Shown to be of value in a trial comparing it with PAC

Invasive technique

Research methods include
Bioelectrical impedance, determination of total body water and plasma volume using radionuclide techniques

Research tools, do not lend
themselves to serial measurements


This tabulated reponse is difficult to improve upon. It is reasonably comprehensive, and it remains within the realms of the achievable for a crazed exam candidate.

The college answer seems to ask about the estimation of total body water, rather than any other sort of fluid-related assessment. The candidate who carried on about estimation of fluid responsiveness would have been penalised.

The table mentioned above could be treated in a slightly more granular fashion, and I will attempt to do this with some references, expanding on some omitted details (for instance, it is perhaps insufficiently enlightening to simply say that the pulmonary artery catheter and CVP have "significant limitations").

Estimation of Fluid Balance in the Critically Ill Patient




Clinical estimates
  • Cheap
  • Easily performed at the bedside
Fluid balance chart
  • Cheap
  • Easily performed at the bedside
  • Accuracy depends on accuracy or recording
  • Usually, cumulative balance records are inaccurate and tend to disagree with body weight measurements
  • This technique fails to estimate losses into incontinence pads, spilled secretions, sweat, evaporative losses from wounds, and losses via the lungs; in short "insensate" losses are forgotten.
Daily weights
  • Easily performed in the presence of specialised bed equipment
  • Accuracy depends on accuracy of recording
  • Requires expensive bed equipment
  • Requires attention to detail - one must ensure the same amount of bedding and on-bed equipment is with the patient each time, otherwise fluctuations in weight may occur. Usually, there is about 3.5kg of nonremovable hardware in the bed together with the patient.
  • Correlates poorly with bedside charts
  • Lack of evidence for cost-effectiveness
  • Easily performed at the bedside
  • Constant monitoring is possible
  • Interpreter-dependent
  • Serial assessments across a series of clinicians may yield variation purely due to technique
  • Yields information regarding chamber filling volumes rather than total body fluid volume - and then you infer the fluid balance from this.
  • Not universally accepted as a method to assess intravascular volume
  • Only accurate when compared to the (known to be useless) clinical examination by an expert.
  • Invasive
  • Labour-intensive (thermodilution measurement)
  • Association with cardiac function makes it difficult to use lung water to estimate whole-body fluid balance
  • Experimental technique, yet to be validated
  • Does not agree with thermodilution measurements
  • Most of the available methods measure transthoracic bioimpedance, which relies on the absence of pleural effusion, and is usually useless in cardiac surgery or thoracic trauma
Tritium indicator dilution


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