Question 16

Method

Advantages

Limitations

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

Body-weight

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.

CVP/ PCWP/Echo

Used to predict
intravascular status

Significant limitations

EVLW

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

Method

Advantages

Disadvantages

• Cheap
• Easily performed at the bedside

• Interpreter-dependent
• Serial assessments across a series of clinicians may yield variation purely due to technique
• The assessment is qualitative
• Individually, no clinical sign has a clinically useful predictive value.
• In combination, all clinical assessment methods still have poor sensitivity and specificity.

• 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.

• 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

• Invasive
• Measures pressure, not volume
• Outdated as a means of assessing intravascular volume
• Assumption that patients with a higher CVP are better hydrated isnot based on any robust evidence

• Solid physiological basis
• Reproducable measurements
• Use of PAC to guide fluid therapy may decrease the total fluid volume used

• Very invasive
• Mortality is either not affected or actually increased by this technique
• In comparison to the (known to be useless) CVP measurements, PAC is no better in terms of mortality, but is associated with more complications.

• Rapid
• Non-invasive
• Easily repeated serially
• Good sensitivity and specificity when compared to clinical examination by an expert

• 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.

• May decrease ICU stay and duration of ventilation
• Appears related to survival
• Differentiates oedema from ARDS

• Invasive
• Labour-intensive (thermodilution measurement)
• Association with cardiac function makes it difficult to use lung water to estimate whole-body fluid balance

• Non-invasive
• Potentially, easily reproduceable
• Potential benefits in patients with sepsis and multi-organ system failure

• 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

• Gold standard for total body water measurement

• Tritium is not exactly cheap
• The measurements require a mass spectrometer, which most ICUs do not have.