Read: H is for Hypoxaemia
The human body contains a significant volume of fluid. The typical 70 kilogram male has 42 litres of fluid, comprising 60 per cent of his body weight. The typical adult female body has slightly less - around 55 per cent - because females have a higher proportion of body fat (Brandis 2019).
Two-thirds of total body fluid is intracellular. The remainder is mostly extracellular (blood, plasma, interstitial fluid), with a small amount of transcellular fluid (cerebrospinal, synovial, peritoneal, and pleural) (Tobias & Mohiuddin 2020; Brandis 2019).
Absolute hypovolaemia is the term used to describe the loss of volume of fluid from the body (e.g. during haemorrhage). Relative hypovolaemia is the term used when there is shifting or inappropriate redistribution of body fluids within the body (e.g. major burn trauma). When there is significant acute volume loss or redistribution of body fluid, the patient is at risk of hypovolaemic shock (Modric 2018).
| Volume of fluid loss | Percentage of total volume* | |
|---|---|---|
| Mild hypovolaemia | 750ml | 15% |
| Moderate hypovolaemia | 750 – 1500ml | 15 – 30% |
| Severe hypovolaemia | >2L | >40% |
Note: The above values are based on a 70 kg adult.
(Clinical Quality and Patient Safety Unit QAS 2020)
A fluctuation in fluid volume of 5 to 10 per cent can have a serious adverse effect on the patient (Shepherd, 2011). As the hypovolaemic patient deteriorates, they will become tachycardic, hypotensive, oliguric and peripherally hypoperfused, constituting a medical emergency. Acute loss of consciousness occurs once volume loss becomes severe. Once haemodynamic compensatory mechanisms are overwhelmed, cardiac arrest may be imminent.
Symptoms of hypovolaemia do not always present until a significant volume change has occurred (see table), however, patients can become dehydrated with a negative fluid balance of as little as one per cent (Shepherd, 2011). Nurses should be aware of the early signs of dehydration, which include:
(Shepherd 2011)
If you suspect hypovolaemia, any source of volume loss should be identified and managed if possible. For example, apply direct pressure to bleeding wounds. Observe for any obvious fluid loss (e.g. vomiting, diarrhoea); review urine output; and review output from wound drains, nasogastric tube and other sources. Surgical intervention may be indicated. For example, an urgent laparotomy if internal bleeding is suspected, external fixation of open fractures or surgical closure of wounds.
Close attention should be given to fluid balance monitoring, including clinical assessment, review of fluid balance charts and reviewing hydration. If inappropriately managed, dehydration can lead to hypovolaemic shock (Shepherd 2011).
During cardiac arrest, hypovolaemia is managed by rapid fluid resuscitation to replace intravascular volume (ANZCOR 2017).
The current recommendation from the Australian and New Zealand Committee On Resuscitation (2016) is that fluids should be infused at a rate of at least 20mL/kg if hypovolaemia is suspected.
Caution is advised, however, as complications can occur when excessive volumes are administered, including organ failure (Frazee and Kashani 2016). Clinicians should therefore aim to achieve normovolaemia and not overload the patient (Soar et. al 2015).
Even in non-monitored settings, cardiac arrest is rarely a sudden, unpredictable event, as most patients demonstrate slow but progressive physiological deterioration. Clinical assessment, fluid balance and blood chemistry monitoring can all give early indicators of alterations in fluid status. Where these elements are unnoticed or poorly managed, poorer outcomes are inevitable.
Early, effective recognition and response to signs of hypovolaemia may prevent some cardiac arrests, deaths and unanticipated ICU admissions (Soar, Nolan, Böttiger et al., 2015).
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