Plus-Hex CLINICAL
Introduction
The provision of general anaesthesia is a complex, multifaceted procedure, and inherent risks to patients exist [ 1 ]. Therefore, it is vital that anaesthetics are made as safe as possible and associated risks are minimised. Arterial blood pressure( ABP) is an important measure of cardiovascular function during anaesthesia [ 2 ], but often it is not monitored as consistently as other parameters [ 3 ].
During anaesthesia, a veterinary nurse( VN) typically monitors the patient ' s physiological parameters, including blood pressure( BP), and is responsible for notifying the veterinary surgeon( VS) of any changes, while implementing corrective interventions under the VS ' s direction [ 4 ]. It is important for VNs to understand the risks associated with BP changes and the importance of monitoring BP and maintaining normotension in the anaesthetised patient.
This article will focus predominantly on hypotension rather than hypertension, as hypotension is more commonly seen in cats and dogs under anaesthesia.
Blood pressure
Scislowicz [ 5 ] stated that BP drives perfusion, which is the delivery of blood and oxygen to the organs and tissue beds within the body. BP is the pressure that the blood exerts on a vessel wall, and the difference in intravascular pressure at any two points in the vascular network is the driving force for blood circulation [ 6 ]. Skelding and Valverde [ 7 ] explained that ABP is the product of two mechanisms, systemic vascular resistance( SVR) and cardiac output( CO)( Table 1). An estimate of BP can be calculated by multiplying the SVR by the CO.
Table 1. Factors contributing to arterial blood pressure.
Factor
Systemic vascular resistance( SVR)
Cardiac output( CO)
Definition
The resistance experienced by blood travelling through the circulatory system, which reflects the peripheral resistance and dilation of the blood vessels.
The amount of blood pumped by the heart in the course of a minute, also called minute volume( MV). CO is determined by the heart rate( HR) and stroke volume( SV).
As explained by Sierra and Savino [ 8 ], ABP is measured as the mean arterial pressure( MAP). When the left ventricle of the heart contracts, blood is pushed into the aorta, which produces systolic arterial pressure( SAP); then, the left ventricle empties, relaxes and begins to fill again, and aortic pressure falls to create diastolic arterial pressure( DAP).
Murrell and Ford-Fennah [ 9 ] stated that normotension in conscious cats and dogs is normally a MAP of 60 – 85 mmHg. Additionally, Duke-Novakovski et al. [ 2 ] explained that a MAP of at least 60 mmHg is necessary for adequate perfusion of vital organs, and a MAP of < 60 mmHg or SAP of < 80 mmHg is defined as hypotensive. Therefore, BP should be maintained above these levels during anaesthesia.
BP can be monitored either directly or indirectly using invasive or non-invasive methods, respectively [ 10 ]. Latimer Jones [ 6 ] stated that the most common intravascular pressure measured in small animals is the peripheral ABP; this can be measured indirectly by placing a pressure cuff around a limb or the tail base and using flow-detection methods such as oscillometric or Doppler ultrasound to obtain a measurement of SAP [ 11 ]. Invasive ABP measurements require the placement of an arterial catheter into a peripheral artery. The catheter is attached to a monitor, which provides information on parameters such as SAP, MAP, DAP and heart rate( HR) [ 6 ].
This literature review explores evidence from research papers and other literature concerning hypotension and its consequences, particularly during anaesthesia, as well as the methods for obtaining ABP measurements and how they compare. The aim is to highlight the importance of maintaining normotension in anaesthetised veterinary patients.
Hypertension versus hypotension
Haskins [ 12 ] identified some potential causes of vasoconstriction resulting in hypertension during anaesthesia; these included a light level of anaesthesia, administration of sympathomimetic drugs, renal failure, Cushing ' s disease, hyperthyroidism, phaeochromocytoma and elevated intracranial pressure.
There are limited resources in the literature on the causes and implications of hypertension during anaesthesia in cats and dogs; therefore, the primary focus of this article is hypotension.
When assessing hypotension, it is important to identify both the factors that reduce BP and the potential systemic consequences of this reduction, such as impaired tissue perfusion and organ dysfunction. As mentioned earlier, BP is determined by two primary variables, CO and SVR, so a change in either will influence the measured BP( Figure 1, page 36 [ 9 ]).
Volume 41( 2) • April 2026
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