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Hypotension in healthy dogs undergoing elective desexing

Sampaio Costa, Renata (2014) Hypotension in healthy dogs undergoing elective desexing. Masters by Research thesis, Murdoch University.

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Hypotension is the most common complication during anaesthesia of dogs and contributes to anaesthetic-related morbidity. The frequency of hypotension reported in anaesthetised dogs is quite variable due to the lack of a standardised definition of hypotension and the number of different factors present in each study that could influence the results. In addition, there is no study in the veterinary literature that has attempted to identify animal factors that may influence perioperative mean arterial blood pressure (MAP).

The aims of this thesis were to document the proportion of healthy dogs developing hypotension during elective desexing at Murdoch University Veterinary Hospital (MUVH) and investigate patient factors influencing perioperative MAP during a surgical plane of anaesthesia. To achieve these aims, a historical cohort study and two prospective studies were performed. These studies were approved by the Murdoch University Animal Ethics Committee (AEC R239611).

The historical cohort study reviewed anaesthetic records from dogs desexed in general practice (GP) between 2007 and 2011. The aim was to determine the frequency of hypotension and explore associations between gender, age, body mass, heart rate and anaesthetic drugs with MAP. Hypotension was defined as MAP <60 mmHg for ≥10 minutes. Records from 188 dogs were included, 87/188 developed hypotension and the frequency of hypotension was higher in younger dogs. However, this study had limitations such as the use of a non-invasive technique for measuring MAP and various anaesthetic protocols were utilised. Prospective studies were thus performed to clarify the previous findings. These studies used invasive blood pressure monitoring (the most accurate method of measuring blood pressure) and a standardised anaesthetic protocol.

A prospective study was performed in dogs undergoing elective desexing in student neutering clinics between 2011 and 2012. To determine the proportion of hypotensive dogs, the average of 10 consecutive MAP measurements were recorded every five minutes. Hypotension was defined as above. To investigate factors that influenced MAP, the area under the MAP*time curve (AUC) from 10 minutes before to 40 minutes after the start of surgery was calculated using the trapezoidal method. Association of explanatory variables including gender, age, body mass, urine specific gravity (USG), packed cell volume and total solids with the AUC were explored using regression models. Thirty five of 71 dogs developed hypotension. The combination of age and USG best explained the MAP with age being positively and USG being negatively associated with MAP.

A second prospective study was performed to determine if the findings of the previous study could be corroborated in dogs undergoing desexing in GP, where dogs were hospitalised for a shorter period and surgery was performed by experienced veterinarians. As duration of anaesthesia was shorter, the AUC was calculated from 5 minutes before to 30 minutes after the start of surgery. Association of explanatory variables with AUC were explored. The proportion of hypotensive dogs was higher than in student neutering clinics with 17 of 24 dogs developing hypotension. Urine specific gravity was also found to be negatively associated with MAP, which was consistent with the previous study.

The observed proportions of hypotensive dogs support the recommendation for blood pressure monitoring during anaesthesia in healthy young dogs and the presence of subclinical dehydration suggested by increases in USG support the administration of intravenous fluids.

Item Type: Thesis (Masters by Research)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): Raisis, Anthea, Hosgood, Giselle and Musk, Gabrielle
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