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Haemodynamic instability

Related theory

Haemodynamic instability is most commonly associated with an abnormal or unstable blood pressure, especially hypotension (Minokadeh and Pinsky [124]). A reduction in systolic blood pressure following surgery can indicate hypovolaemic shock, a condition in which the blood vessels do not contain sufficient blood (Hatfield and Tronson [73]). Bleeding is the most common cause but other causes can arise when tissue fluid is lost from the circulation, for example through bowel obstruction or nausea and vomiting. Ho et al. ([80]) outline three stages of hypovolaemic shock:
  • Compensated shock: blood flow to the brain and heart is preserved at the expense of the kidneys, gastrointestinal system, skin and muscles.
  • Decompensated shock: the body's compensatory mechanisms begin to fail and organ perfusion is severely reduced.
  • Irreversible shock: tissues become so deprived of oxygen that multiorgan failure occurs.

Evidence‐based approaches

Principles of care

During compensated shock, some patients can lose up to 30% of their circulatory volume before the effects of hypovolaemia are reflected in their systolic blood pressure measurements or heart rate (Ho et al. [80]). Therefore, when assessing post‐operative patients, it is also useful to consider the early signs of reduced tissue perfusion in detecting signs of hypovolaemic shock. These include:
  • restlessness, anxiety or confusion (as a result of cerebral hypoperfusion or hypoxia)
  • increased respiratory rate, becoming shallow (frequently occurring before signs of tachycardia and hypotension)
  • rising pulse rate (tachycardia as the heart attempts to compensate for the low circulatory blood volume)
  • low urine output of below 0.5 mL/kg/hour (as the kidneys experience a reduction in perfusion and pressure, which activates the renin–angiotensin system in an attempt to conserve fluid and increase circulatory blood volume)
  • pallor (pale, cyanotic skin) and later sweating
  • cool peripheries (pale, cyanotic lips and nailbed), resulting in a poor signal on the pulse oximeter
  • visible bleeding and haematoma from drains and wounds (Anderson [16], Hatfield and Tronson [73], Jameson [83], Jevon and Ewens [84]).
In most cases, if impending hypovolaemic shock is recognized and treated promptly, its progression through the aforementioned stages of shock can be circumvented (Kalkwarf and Cotton [86]). Irrespective of the cause of hypovolaemic shock, the aim of treatment is to restore adequate tissue perfusion (Ho et al. [80]). Excessive blood loss may require blood transfusion and occasionally surgical intervention. However, if signs indicate that the patient is in the compensated phase, fluid resuscitation with crystalloids or colloids and increased oxygenation to maintain saturation above 95% will often be sufficient to promote recovery.
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