Simplifying total intravenous anaesthesia in dogs
10 October 2023
Total intravenous anaesthesia (TIVA) has a number of potential benefits over intravenous induction followed by inhalational maintenance of anaesthesia. These include improved recovery quality and haemodynamic stability, the avoidance of occupational exposure to inhalational anaesthetics, and reduced environmental impact. Many veterinary surgeons have used TIVA in small animals at least once, but few use it in the majority of their cases. This disparity may be due to the perceived increased labour intensity of TIVA or a clinical tradition.
Target controlled infusion (TCI) is a TIVA technique which uses a microprocessor-controlled syringe driver to achieve a target plasma concentration of the anaesthetic agent based on population pharmacokinetics. The dose is set and adjusted based on plasma concentration independent of body mass, making it very similar to setting the concentration on a vaporiser when delivering inhalational agents, and improving ease of use. Several pharmacokinetic models for propofol TCI in dogs have been published and the optimal plasma target for induction determined. The use of propofol TCI in dogs has been extensively researched and there are several case reports from clinical cases in the literature. However, the widespread use of this technique in clinical practice has been limited by the lack of suitable equipment. For example, commercially available TCI pumps come programmed with human pharmacokinetic models which can not be readily modified. It is possible to control a syringe driver by connecting it to a computer, but this reduces practicality and increases the risk of technical failures.
The MADTCI® Project
I first became interested in TIVA, and TCI in particular, during my residency at the University of Glasgow, where much of the propofol TCI in dogs research was conducted. I regularly used propofol TCI in clinical cases, both where TIVA was specifically indicated and where it was not. When I moved on from Glasgow I missed the simplicity and ease of using TCI, and found my use of TIVA limited to only those cases where I could justify the additional effort. The start of the project was inspired by reading a paper from the human anaesthesia literature describing the development of a slide rule-based manual system for propofol TCI.
The objective of the project is to develop a system which can deliver a propofol TCI using a standard syringe driver. This will make TIVA easier to use and eliminate some of the barriers to its use by first opinion and referral clinicians in appropriate cases. In the long term if the reliance of veterinary practice on the use of inhalant anaesthetics can be reduced, it will enhance the sustainability of veterinary anaesthesia.
The early stages of the project involved the development of a manual TCI system which uses one of the published pharmacokinetic models for propofol in dogs. A spreadsheet-based version was tested using pharmacokinetic simulations to ensure it performed as expected using data from a range of typical clinical cases.
The current phase of the project began by transforming the system into a smartphone application which calculates the infusion rate which must be set on the syringe driver based on the dog’s body mass (entered on the initial screen) and the target plasma concentration, which can be altered to adjust the anaesthetic depth as required. The application also incorporates a timer and notifies the user when a reduction in the infusion rate is needed to maintain the target plasma concentration (every 15 minutes for the first hour and hourly thereafter).
The aim of the clinical trial is to demonstrate that the propofol MADTCI® performs as anticipated in clinical cases. The three hypotheses being tested are that when compared with a conventional propofol TIVA the MADTCI® will result in a shorter recovery, fewer adverse events and lower total dose of propofol administered.
The trial will recruit 40 dogs undergoing elective ovariohysterectomy surgery in a first opinion practice setting, which will be randomised into two groups. The MADTCI group will have propofol TIVA with infusion rate calculations made using the MADTCI® smartphone application. The variable rate infusion (VRI) group will have infusion rate calculations made manually from the body mass in the traditional way.
Participants all receive the same premedication, analgesic treatment and a standardised approach to adjusting propofol dose based on clinical signs of anaesthetic depth, to ensure that there is consistency between them. The study design also includes a standard approach to the management of adverse events during anaesthesia (e.g. hypotension, bradycardia, and hypoventilation) to ensure parity between the groups. Physiological parameters, e.g. heart rate, respiratory frequency, non-invasive blood pressure, haemoglobin oxygen saturation, end-tidal carbon dioxide and temperature, measured with a multiparameter monitor are recorded every five minutes. The time to extubation, head lift and sternal recumbency are recorded relative to the termination of the propofol infusion. The duration of anaesthesia and surgery are also recorded.
Progress and Future Developments
The study got off to a slow start, which was to be expected whilst the software developers were working on the smartphone application. Around a third of the cases have been recruited and there is a clear plan for completion within the expected timescale. It is too early to draw any conclusion from the data. Subjectively it seems that the MADTCI® is doing what it is supposed to and there have been no unforeseen issues during anaesthesia. On a personal level, the highlight so far has been seeing something that started as an intellectual exercise, then cutting and gluing paper to build a slide rule, turn into a clinical reality.
The next step for the project will be the creation and distribution of a final version of the application based on the findings of the clinical trial. Although the trial patients underwent elective surgery, the MADTCI® could be used to facilitate propofol infusion in any situation where it would be appropriate. This could include anaesthesia for other types of surgery or diagnostics (e.g. bronchoscopy), procedural sedation and the management of status epilepticus. In making TIVA simpler and bringing TCI to anyone with a syringe driver, the MADTCI® will have benefits for those working in all types of small animal practice. A crude free version of the application could be available soon after completion of the clinical trial and it is hoped that the final version of the MADTCI® will be available in a couple of years depending on the funding model.
The clinical trial has been funded by a BSAVA PetSavers Clinical Research Project grant. I would like to thank the staff and volunteers at the Blue Cross Merton Animal Hospital for hosting the trial, recruiting the cases and generally being awesome. Thanks are also due to Clare Gotto and Ieuan French at the University of Surrey and Georgios Michalakidis at Blue Tea Ltd.
About the author
Sean graduated from the Royal Veterinary College in 2006, then worked in first opinion mixed practice before starting residency training in anaesthesia at the University of Glasgow in 2009. After finishing his residency, Sean held clinical positions at Glasgow and Cambridge vet schools. He is currently a Senior Lecturer at the University of Surrey where his role mixes teaching in the classroom and clinic. Sean achieved the diploma of the European College of Veterinary Anaesthesia and Analgesia in 2013 and fellowship of the Academy of Medical Educators in 2019. He is a European and RCVS Specialist in Anaesthesia and Analgesia.