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Artificial colloids
· Examples: Voluven, volulyte, hetastarch, dextran, gelafusine
An Urban Experience
The two main ways we use intravenous fluid therapy is in the treatment of shock, in which fluids are administered as a bolus; or as a continuous infusion, in which it is used as maintenance therapy and for rehydration.
Shock
The definition of shock is inadequate oxygen delivery resulting in anaerobic metabolism and mitochondrial dysfunction, eventually resulting in organ dysfunction. There are 4 different types of shock. 1) Hypovolaemic; 2) Obstructive; 3) Cardiogenic; and 4) Vasodilatory. The first three types of shock is categorised as “vasoconstrictive shock”, where peripheral vasoconstriction occurs in order to maintain perfusion. In vasodilatory shock vessels remain vasodilated as vasoconstriction is impaired. Table 1 describes common physical examination parameters of vasoconstrictive and vasodilatory shock, which severity worsens from mild, moderate, to decompensated shock.
Table 1. Clinical findings of Shock
Vasodilatory shock occurs due to systemic inflammatory response syndrome (SIRS), sepsis, anaphylaxis or general anaesthesia. Treatment involves addressing
the underlying cause; administering intravenous fluids; +/- vasopressors (noradrenaline or dopamine) if the patient remains hypotensive. During vasodilatory shock, capillaries are more permeable, and patients are often hypoalbuminaemic resulting in increased risk of fluid extravascation and fluid overload.
The aim of treating shock with intravenous fluid therapy is to rapidly fill the intravascular space with intravenous fluids at doses which are described above. Intravenous fluids can be administered by various methods: Pressure bags are ideal in medium to large breed dogs, as it can deliver 1 litre of fluid within 8 minutes. If intravenous fluid is delivered too slowly, it can redistribute and will not treat shock. In small to medium sized dogs, a fluid pump can be used. In cats or small dogs prone to fluid overload you must be very careful about bolusing fluids. Safer methods to prevent fluid overload include drawing fluid up in a 50 mL syringe and delivering by hand; using a syringe driver; or using a paediatric burette (which can be used with or without a fluid pump).
· These are hyperosmolar solutions which increases oncotic pressure in the intravascular space. Unlike hypertonic saline, it doesn’t draw fluid in, but instead minimises redistribution of fluid into the interstitial space, hence helps maintain intravascular blood volume.
· Indications: severe hypoproteinaemia; large breed dogs; shock which is non-responsive to isotonic crystalloids
· Adverse effects/contraindications: Acute kidney injury, coagulopathy, increased risk of fluid overload
· Shock dose: Dogs 5 – 10 mL/kg; cats 2.5 – 5 mL/ kg delivered over 15 minutes IV. The maximum dose depends on the type of colloids.
  Mild
 Moderate
 Decompensated
 Heart rate
 ↑ Cats ↓ or ↑
 ↑↑ Cats ↓ or ↑
 ↓ Cats ↓
 Pulse quality
  ↓ or ↑ ↑ (vasodil)
  ↓↓ ↑ (vasodil)
  ↓↓↓ ↑ or ↓ (vasodil)
  Mucus membrane colour
  Pale pink Injected (vasodil)
Pale Injected (vasodil)
  Very pale
 Capillary refill time (seconds)
 1.5 – 2 < 1 (vasodil)
 2 – 3 < 1 (vasodil)
 >3
 Extremity/body temperature
 ↓ ↑ (vasodil)
 ↓↓ Cats ↓ body temp ↑ (vasodil)
 ↓ body temperature
 Mentation
  Mild obtundation
  Moderate obtundation
  Stuperous
 Hypovolaemic shock is the most common type of shock which occurs due to severe dehydration (> 8%), vomiting, diarrhoea, haemorrhage, third space loss of fluids, or polyuria. Treatment of hypovolaemic shock involves fluid resuscitation and correcting the underlying cause.
Obstructive shock is defined by the obstruction of great vessels or the heart. This can occur due to gastric dilation and volvulus, pericardial effusion causing
cardiac tamponade, and pneumothorax. Treatment of obstructive shock involves very quickly relieving the obstruction. Gastric dilation and volvulus will also require fluid resuscitation, however use of fluid in pericardial tamponade may not be required and can be detrimental.
Cardiogenic shock occurs when there is inadequate circulation of blood due to poor left ventricular function. This can occur due to mitral valve disease, myocardial disease, or arrhythmias. This is the only type of shock
in which fluid therapy is contraindicated. Cardiogenic shock can be identified by obtaining a good history, auscultating the heart for murmurs or arrhythmias while palpating pulses for pulse deficits, performing an ECG, and/or performing a bedside echocardiogram to assess the left atrium to aortic ratio (LA:Ao) which is usually less than 1.3 – 1.5 and subjectively assessing the contractility of the heart.
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