Managing Hypotension during Equine Anaesthesia

What is arterial blood pressure?
Everyone is familiar with the term blood pressure but do we really know what it means? Blood pressure is the pressure exerted by blood at right angles to the walls of blood vessels. When the left ventricle ejects blood into the aorta it causes a rise in blood pressure in the aorta (the peak rise is termed systolic pressure, the lowest aortic blood pressure in the cardiac cycle is termed diastolic pressure). As the aortic pressure wave travels down the aorta and into distributing arteries distinct changes in systolic and diastolic pressure occur. The systolic pressure tends to rise and the diastolic pressure falls, so that overall there is only a slight reduction in mean arterial blood pressure (MAP). Mean arterial blood pressure is the average arterial blood pressure during the cardiac cycle and is considered to represent the perfusion pressure of organs in the body. Across species a MAP greater than 60 mmHg is considered adequate to maintain organ perfusion under most conditions.

Why monitor blood ressure during equine anaesthesia?
Hypotension is common in horses during volatile agent anaesthesia. This is considered to result from the myocardial depressant effects of volatile agents in this species, coupled with the tendency to maintain anaesthesia with volatile agents alone rather than adopting more balanced anaesthetic techniques prevalent in other species. Hypotension during anaesthesia is a contributing factor to the development of post-anaesthetic myopathy (PAM), a significant cause of peri-operative mortality in anaesthetized horses. Although the precise etiology of PAM is unknown, it results from muscle ischaemia caused by decreased muscle perfusion during anaesthesia. There are two elements recognized to contribute to decreased muscle perfusion; a reduction in blood pressure per se (hypo- tension), and increased intracompartmental muscle pressure. Monitoring blood pressure during anaesthesia is essential in order to prevent or manage hypotension, which may reduce the risk of PAM in some patients. [...]