Haemodynamic instability can be defined as perfusion failure, represented by clinical features of circulatory shock and advanced heart failure (Weil 2005). It may also be defined as 1 or more out-of-range vital sign measurements, such as low blood pressure. Impaired cardiac performance is a frequent cause of haemodynamic instability and circulatory failure in critically ill patients (Hütteman 2006). Other causes include: cardiac contusion, haemothorax, embolism (air or fat), spinal cord injury, cardiac tamponade, tension pneumothorax, rupture of the heart, aortic injury, uncorrected blood and fluid loss, myocardial ischaemia, arrhythmias, injury, adrenal insufficiency, anaphylaxis, acute severe brain injury, and metabolic causes (Ho 1998). People who have had major surgery, such as organ transplant, are also at risk of perioperative haemodynamic instability. Each of these causes of haemodynamic instability has its own incidence and survival rates. Consequently, there is no record of overall incidence of haemodynamic instability in the literature.
Cardiac output is the product of heart rate and stroke volume. Relevant haemodynamic measures of stroke volume include preload (delivery of adequate blood volume to the left ventricle), contractility and afterload. Preload depends on many factors, including volume and right ventricular function, which can be assessed by transoesophageal echocardiography (Hastings 2012).
In critical care, haemodynamic management options comprise fluid resuscitation to increase preload, administration of vasopressors to maintain systemic blood pressure, and administration of inotropes to increase contractility and cardiac output.