Fluid Therapy - Patient Assessment

Fluid therapy is arguably one of the most commonly used therapies in small animal emergency practice. There are a wide variety of fluid therapy products and they should be treated much like any other product in our pharmaceutical armoury. Many patients presenting as emergencies will benefit from the application of fluid therapy and in some it will be life saving.

Patient Assessment

When assessing patients for fluid therapy requirements it is beneficial to consider their acute requirements and chronic requirements separately.
Acute requirements include volume replacement and should be restored quickly to ensure rapid restoration of tissue perfusion. Chronic requirements include replacement of dehydration, ongoing losses and everyday requirements. Dehydration should be replaced more slowly as attempting to replace this too quickly will be less effective.
Assessment of patients presenting as emergencies should therefore be aimed at identifying for the presence of volume deficits (hypoperfusion) primarily as this is the most life threatening problem. Many patients will have some degree of hypoperfusion, dehydration may or may not be present depending on the pathophysiology of fluid loss. For example patients presenting after road traffic accidents will be hypovolaemic without dehydration, whereas those presenting with severe vomiting and diarrhoea will typically have some degree of both.

Hypoperfusion

By far the most important aspect of assessment in the emergency situation is the identification of hypoperfusion.
Identification of hypoperfusion caused by hypovolaemia is based upon assessment of the cardiovascular system. The response to hypovolaemia includes tachycardia, alterations in mucous membrane colour and capillary refill time and changes in pulse quality. Dogs tend to follow a typical pattern to worsening hypovolaemia, with initial compensatory changes (faster capillary refill time, bounding pulses) through to decompensation (absent capillary refill times and white mucous membranes and barely palpable femoral pulses). With administration of fluid therapy there is reversal of these cardiovascular changes. Assessment is fairly accurate with practice and allows categorisation into mild, moderate and severe hypovolaemia and therefore decisions on the amount of resuscitation to provide (between 20 and 60ml/kg of isotonic crystalloid fluid depending on severity).

Dehydration

Assessment for dehydration is less accurate, but provides a rough estimate of amount of fluid to replace. Typically the aim is to replace this deficit over the first 24 to 36 hours after hypoperfusion has been corrected. Assessment of dehydration includes evaluation of skin turgour, mucous membrane moistness, eye position, laboratory measurements such as packed cell volume and total solids, urea and urine specific gravity. None of these changes are absolutely specific to dehydration and there are differences in assessment between operators. Dehydration is estimated as a percentage deficit which is calculated from body weight. As day to day weight loss and gain is mainly due to water losses body weight is a useful assessment of hydration status but can only be used on patients that are hospitalised.
Ongoing requirements should take into account the presence of increased losses such as vomiting, diarrhoea, polyuria, abdominal effusions etc. Daily requirements is based upon body surface area and is thus higher in smaller animals (such as puppies and cats) and lower in large breed dogs. These should be considered when calculating a fluid therapy plan for patients.
When providing fluid therapy for any reason it is vital to reassess patients regularly to ensure that the patient is responding and adjusted as required according to the required end points.