The results of the studies [ 18 – 20 ] are further validated by Haskins [ 12 ], who stated that hypotension occurs in 38 % of dogs under anaesthesia and that it can jeopardise the perfusion of all organs, adding that it can be treated by reducing anaesthetic depth and administering intravenous fluid therapy( IVFT). Sturgess [ 14 ] further stated that IVFT is the recognised treatment for hypotension, as it increases vascular volume.
Valverde et al. [ 21 ] investigated the effects of rapid IVFT on cardiovascular function and haematological values during isoflurane-induced hypotension. The study followed six healthy dogs undergoing anaesthesia and investigated initial readings of BP, among other variables, and compared them with measurements made following 10, 30 and 60 min of high-volume, rapid IVFT. BP was measured using invasive BP measurement( IBPM). The authors discovered that the provision of 1 hour of rapid IVFT to patients in deep anaesthesia, which is the desired surgical plane for most surgical procedures, increased MAP from 47 mmHg to 69 mmHg. This was a significant increase and could be used to demonstrate the efficacy of IVFT in reversing hypotension in anaesthetised patients.
IVFT as a treatment for hypotension is also recognised in other literature, as Hughston [ 22 ] suggested that perfusion can be restored by providing rapid IVFT in patients that are unable to adequately perfuse their tissues, such as hypotensive patients. Therefore, VNs would benefit from understanding the effect of IVFT on maintaining normotension so they can act, under the VS ' s instruction, to reverse hypotension by administering IVFT. By increasing the blood volume, IVFT will increase preload and CO, causing a rise in BP.
Blood pressure monitoring and anaesthesia
There are several methods of monitoring BP in anaesthetised patients, and there is debate surrounding the efficacy of some methods compared with others.
A study by MacFarlane et al. [ 23 ] compared the use of IBPM and non-invasive BP monitoring( NIBPM) methods during anaesthesia in 19 dogs in a prospective, blinded, clinical study. The dogs were randomised into one of three groups, and each was given different combinations and doses of premedication. Indirect BP measurements were made oscillometrically, with the cuff applied just distal to the tarsus with the arrow placed over the dorsal metatarsal artery, and the same anaesthetist applied the cuff in each case to ensure similar tension. For IBPM, after aseptic preparation, a catheter was placed into the dorsal pedal artery of the hindlimb and was secured in place. This catheter was connected via non-distensible heparinised saline-filled tubing to a disposable BP transducer and results were displayed on a multiparameter monitor.
A study by Rysnick et al. [ 24 ] included 50 dogs in a prospective clinical trial, which compared the accuracy of NIBPM using high-definition oscillometry and arterial IBPM in anaesthetised dogs.
Both studies [ 23, 24 ] agreed that IBPM is more suitable for measuring BP in anaesthetised patients. This is further highlighted in current literature, as Latimer Jones [ 6 ] stated that IBPM provides‘ real time’ information on variables such as SAP, MAP, DAP and HR, and also provides a waveform.
Furthermore, NIBPM does not provide continuous measurement and may give inaccurate values in patients with severe hypotension and those with arrhythmias [ 25 ]. Therefore, direct monitoring through IBPM would be considered the ideal technique for VNs to monitor BP during anaesthesia, as it can overcome measurement inaccuracies in patients with underlying cardiac artefacts [ 6 ]. However, limitations such as lack of equipment or training in the use of IBPM means it is less likely to be used in practice than NIBPM.
Conclusions
This literature review highlights the nursing considerations and interventions for maintaining and supporting normotension in anaesthetised patients.
Anaesthetic agents have the potential to depress cardiovascular function, leading to a reduction in BP and the development of hypotension, which was the principal focus of the literature review.
Research findings and anecdotal reports highlight the occurrence of hypotension in anaesthetised patients, reinforcing the importance of ABP monitoring, as it facilitates the detection of hypotension.
Through a comparison of NIBPM and IBPM, MacFarlane et al. [ 23 ] and Rysnick et al. [ 24 ] recognised IBPM as the best method for VNs to monitor BP during anaesthesia. The authors concluded that IBPM provided greater accuracy and continuous measurement, as well as also providing measurements of MAP as opposed to just SAP.
Research concerning perioperative BP and hypotension in feline patients was difficult to source, and so the importance of maintaining normotension in feline patients is less clear.
To decide whether patients are at elevated risk of hypotension, VNs should also consider factors such as age, bodyweight and hydration status indicated by USG, which may predispose patients to hypotension during anaesthesia.
Nonetheless, BP should be routinely measured and monitored by VNs during anaesthesia to help maintain normotension in anaesthetised patients.
38 Veterinary Nursing Journal