Mean arterial pressure( MAP)
Systemic vascular resistance( SVR) Cardiac output( CO)
Heart rate( HR) Stroke volume( SV)
Preload
Afterload
Contractility
Figure 1. Determinants of blood pressure.
CO is largely influenced by stroke volume( SV), which is the volume of blood ejected by the left ventricle with each heartbeat. SV depends on three key determinants: preload, afterload and myocardial contractility, all of which VNs should understand in order to anticipate and manage hypotensive events.
Preload refers to the extent to which the ventricular walls are stretched by venous return during diastolic filling immediately before contraction. Afterload describes the force or resistance that the ventricles must overcome to eject blood into the arterial circulation, which is primarily influenced by SVR and ABP.
Contractility is the intrinsic ability of the myocardium to contract, independent of preload and afterload, and reflects the strength of ventricular contraction for a given filling volume. Any reduction in preload or contractility, or a marked increase or decrease in afterload, can therefore diminish SV, reduce CO and ultimately contribute to hypotension.
Steagall [ 13 ] stated that hypotension is normally a result of a combination of factors such as bradycardia or vasodilation, which in turn will reduce SV and CO, as well as a decrease in cardiac contractility. Sturgess [ 14 ] categorised the causes of hypotension( Table 2).
Table 2. Causes of hypotension [ 14 ].
Cause
Factors that decrease preload
Conditions or irregularities that decrease cardiac output
Factors that decrease vascular tone
Examples Haemorrhage, trauma
Cardiomyopathy, bradyarrhythmia
Sepsis, administration of anaesthetic drugs
Many anaesthetic drugs can reduce ABP because they have a depressive action on body systems, including the cardiovascular system. One of the most commonly used drugs in anaesthesia is medetomidine, an alpha-2 adrenergic receptor agonist that acts as a profound sedative. Rankin [ 15 ] stated that medetomidine causes bradycardia, a decrease in CO and an increase in SVR. This will decrease cardiac contractility and increase vasodilation. This explains why anaesthesia is a common cause of hypotension in animals [ 7 ].
36 Veterinary Nursing Journal