Derek Flaherty

Target-controlled infusion of propofol in dogs - evaluation of four targets for induction of anaesthesia

Four groups of 20 dogs were anaesthetised by means of target-controlled infusions of propofol designed to achieve 2·5 µg/ml, 3·0 µg/ml, 3·5 µg/ml or 4·0 µg/ml of propofol in blood. The dogs’ pulse rate and respiratory rate were recorded before premedication and induction, immediately after endotracheal intubation and three and five minutes later (times 0, 3 and 5, respectively), and their arterial blood pressure was recorded oscillometrically just before induction and at times 0, 3 and 5. The targets of 2·5, 3·0, 3·5 and 4·0 µg/ml resulted in the successful induction of anaesthesia in 13 (65 per cent), 16 (80 per cent), 20 (100 per cent) and 20 (100 per cent) of the dogs, respectively. The incidence of postinduction apnoea was 0 (0 per cent), one (5 per cent), two (10 per cent) and eight (40 per cent) at time 5 for groups 2·5, 3·0, 3·5 and 4·0 µg/ml, respectively, and its incidence at time 5 was significantly higher in the 4·0 µg/ml group (P<0·05) than in the other groups. In all the groups there was a significant (P<0·05) decrease in blood pressure between just before induction and the later measurements. Although there were no statistically significant differences between the groups in terms of inducing anaesthesia at a specific target, a target of 3·5 µg/ml appears to ensure a successful induction of anaesthesia without a significant increase in the incidence of apnoea.

An evaluation of anaesthetic induction in healthy dogs using rapid intravenous injection of propofol or alfaxalone

OBJECTIVE To evaluate quality of anaesthetic induction and cardiorespiratory effects following rapid intravenous (IV) injection of propofol or alfaxalone. STUDY DESIGN Prospective, randomised, blinded clinical study. ANIMALS Sixty healthy dogs (ASA I/II) anaesthetized for elective surgery or diagnostic procedures. METHODS Premedication was intramuscular acepromazine (0.03 mg kg(-1) ) and meperidine (pethidine) (3 mg kg(-1) ). For anaesthetic induction dogs received either 3 mg kg(-1) propofol (Group P) or 1.5 mg kg(-1) alfaxalone (Group A) by rapid IV injection. Heart rate (HR), respiratory rate (f(R) ) and oscillometric arterial pressures were recorded prior to induction, at endotracheal intubation and at 3 and 5 minutes post-intubation. The occurrence of post-induction apnoea or hypotension was recorded. Pre-induction sedation and aspects of induction quality were scored using 4 point scales. Data were analysed using Chi-squared tests, two sample t-tests and general linear model mixed effect anova (p < 0.05). RESULTS There were no significant differences between groups with respect to sex, age, body weight, f(R) , post-induction apnoea, arterial pressures, hypotension, SpO(2) , sedation score or quality of induction scores. Groups behaved differently over time with respect to HR. On induction HR decreased in Group P (-2 ± 28 beats minute(-1) ) but increased in Group A (14 ± 33 beats minute(-1) ) the difference being significant (p = 0.047). However HR change following premedication also differed between groups (p = 0.006). Arterial pressures decreased significantly over time in both groups and transient hypotension occurred in eight dogs (five in Group P, three in Group A). Post-induction apnoea occurred in 31 dogs (17 in Group P, 14 in Group A). Additional drug was required to achieve endotracheal intubation in two dogs. CONCLUSIONS AND CLINICAL RELEVANCE Rapid IV injection of propofol or alfaxalone provided suitable conditions for endotracheal intubation in healthy dogs but post-induction apnoea was observed commonly.

Two doses of dexmedetomidine in combination with buprenorphine for premedication in dogs; a comparison with acepromazine and buprenorphine.

OBJECTIVE To assess as premedicants, the sedative, cardiorespiratory and propofol-sparing effects in dogs of dexmedetomidine and buprenorphine compared to acepromazine and buprenorphine. STUDY DESIGN Prospective, randomised, blinded clinical study. ANIMALS Sixty healthy dogs (ASA grades I/II). Mean (SD) body mass 28.0 ± 9.1 kg, and mean age 3.4 ± 2.3 years. METHODS   Dogs were allocated randomly to receive 15 μg kg(-1) buprenorphine combined with either 30 μg kg(-1) acepromazine (group 1), 62.5 μg m(-2) dexmedetomidine (group 2), or 125 μg m(-2) dexmedetomidine (group 3) intramuscularly. After 30 minutes, anaesthesia was induced using a propofol target controlled infusion. Heart rate, respiratory rate, and oscillometric arterial blood pressure were recorded prior to induction, at endotracheal intubation and at 3 and 5 minutes post-intubation. Induction quality and pre-induction sedation were scored on 4 point scales. Propofol target required for endotracheal intubation was recorded. Data were analysed using Chi-squared tests, Kruskal-Wallis, one way and general linear model ANOVA (p<0.05). RESULTS Age was significantly lower in group 1 (1.0 (1.0-3.8) years) than group 2 (5.0 (2.0-7.0) years), (median, (IQR)). There were no significant differences in sedation or quality of induction between groups. After premedication, heart rate was significantly lower and arterial blood pressures higher in groups 2 and 3 than group 1, but there was no significant difference between groups 2 and 3. Propofol targets were significantly lower in group 3 (1.5 (1.0-2.5) μg mL(-1) ) than group 1 (2.5 (2.0-3.0) μg mL(-1) ); no significant differences existed between group 2 (2.0 (1.5-2.5) μg mL(-1) ) and the other groups (median, (interquartile range)). CONCLUSIONS AND CLINICAL RELEVANCE When administered with buprenorphine, at these doses, dexmedetomidine had no advantages in terms of sedation and induction quality over acepromazine. Both doses of dexmedetomidine produced characteristic cardiovascular and respiratory effects of a similar magnitude.

A comparison of the effects of two different doses of ketamine used for co-induction of anaesthesia with a target-controlled infusion of propofol in dogs

OBJECTIVE To assess the cardiorespiratory and hypnotic-sparing effects of ketamine co-induction with target-controlled infusion of propofol in dogs. STUDY DESIGN Prospective, randomized, blinded clinical study. ANIMALS Ninety healthy dogs (ASA grades I/II). Mean body mass 30.5 +/- SD 8.6 kg and mean age 4.2 +/- 2.6 years. METHODS All dogs received pre-anaesthetic medication with acepromazine (0.03 mg kg(-1)) and morphine (0.2 mg kg(-1)) administered intramuscularly 30 minutes prior to induction of anaesthesia. Heart rate and respiratory rate were recorded prior to pre-medication. Animals were allocated into three different groups: Group 1 (control) received 0.9% NaCl, group 2, 0.25 mg kg(-1) ketamine and group 3, 0.5 mg kg(-1) ketamine, intravenously 1 minute prior to induction of anaesthesia, which was accomplished using a propofol target-controlled infusion system. The target propofol concentration was gradually increased until endotracheal intubation was possible and the target concentration at intubation was recorded. Heart rate, respiratory rate and noninvasive blood pressure were recorded immediately prior to induction, at successful intubation and at 3 and 5 minutes post-intubation. The quality of induction was graded according to the amount of muscle twitching and paddling observed. Data were analysed using a combination of chi-squared tests, Fishers exact tests, Kruskal-Wallis, and anova with significance assumed at p < 0.05. RESULTS There were no significant differences between groups in the blood propofol targets required to achieve endotracheal intubation, nor with respect to heart rate, noninvasive blood pressure or quality of induction. Compared with the other groups, the incidence of post-induction apnoea was significantly higher in group 3, but despite this dogs in this group had higher respiratory rates overall. CONCLUSIONS AND CLINICAL RELEVANCE Under the conditions of this study, ketamine does not seem to be a useful agent for co-induction of anaesthesia with propofol in dogs.

A comparison of induction of anaesthesia using a target-controlled infusion device in dogs with propofol or a propofol and alfentanil admixture

OBJECTIVE To compare induction targets, and the haemodynamic and respiratory effects, of propofol, or as an admixture with two different concentrations of alfentanil, delivered via a propofol target-controlled infusion (TCI) system. STUDY DESIGN Prospective blinded randomized clinical study. ANIMALS Sixty client-owned dogs scheduled for elective surgery under general anaesthesia. Mean body mass (SD) 28.5 kg (8.7) and mean age (SD) 3.5 years (2.4). METHODS Dogs received pre-anaesthetic medication of acepromazine (0.03 mg kg(-1)) and morphine (0.2 mg kg(-1)) administered intramuscularly. Animals were randomly assigned to receive one of three induction protocols: propofol alone (group 1), a propofol/alfentanil (11.9 microg mL(-1)) admixture (group 2), or a propofol/alfentanil (23.8 microg mL(-1)) admixture (group 3), via a TCI system. Blood target concentrations were increased until endotracheal intubation was achieved, and induction targets were recorded. Heart rate (HR), respiratory rate (f(r)) and non-invasive arterial blood pressure were recorded pre-induction, at endotracheal intubation (time 0) and at 3 and 5 minutes post-intubation (times 3 and 5, respectively). Data were analysed using anova for normally distributed data or Kruskal-Wallis test, with significance assumed at p < 0.05. RESULTS There were no significant differences between groups with respect to age, body mass, HR, f(r), systolic and diastolic blood pressure. The blood propofol targets to achieve endotracheal intubation were significantly higher in group 1 compared with groups 2 and 3. Mean arterial blood pressure (MAP) was significantly higher in group 1 at time 0 when compared with groups 2 and 3. CONCLUSIONS AND CLINICAL RELEVANCE Induction of anaesthesia with a TCI system can be achieved at lower blood propofol targets when using a propofol/alfentanil admixture compared with using propofol alone. However, despite reduced targets with both propofol/alfentanil admixture groups, MAP was lower immediately following endotracheal intubation than when using propofol alone.

Comparison of the effects of thiopentone and propofol on the electrocardiogram of dogs

Sixty-four dogs were randomly assigned to receive either thiopentone or propofol and their electrocardiograms were recorded immediately before and shortly after they were anaesthetised. Thiopentone caused a marked increase in qt and jt intervals, a flattening of the T-wave and an increase in precordial qt dispersion. Propofol induced a less marked increase in qt and jt intervals, corrected for heart rate. Both agents induced an increase in heart rate and a decrease in heart rate variability, consistent with reduced vagal tone. Shortly after anaesthesia was induced, thiopentone affected ventricular repolarisation to a far greater extent than propofol, changes which suggest that it may be more likely to induce re-entrant ventricular arrhythmogenesis and could be associated with an increase in sympathetic tone. Propofol may therefore be more suitable than thiopentone for dogs with a susceptibility to ventricular arrhythmias or a long qt interval.

Avian reflex and electroencephalogram responses in different states of consciousness.

Defining states of clinical consciousness in animals is important in veterinary anaesthesia and in studies of euthanasia and welfare assessment at slaughter. The aim of this study was to validate readily observable reflex responses in relation to different conscious states, as confirmed by EEG analysis, in two species of birds under laboratory conditions (35-week-old layer hens (n=12) and 11-week-old turkeys (n=10)). We evaluated clinical reflexes and characterised electroencephalograph (EEG) activity (as a measure of brain function) using spectral analyses in four different clinical states of consciousness: conscious (fully awake), semi-conscious (sedated), unconscious-optimal (general anaesthesia), unconscious-sub optimal (deep hypnotic state), as well as assessment immediately following euthanasia. Jaw or neck muscle tone was the most reliable reflex measure distinguishing between conscious and unconscious states. Pupillary reflex was consistently observed until respiratory arrest. Nictitating membrane reflex persisted for a short time (<1 min) after respiratory arrest and brain death (isoelectric EEG). The results confirm that the nictitating membrane reflex is a conservative measure of death in poultry. Using spectral analyses of the EEG waveforms it was possible to readily distinguish between the different states of clinical consciousness. In all cases, when birds progressed from a conscious to unconscious state; total spectral power (PTOT) significantly increased, whereas median (F50) and spectral edge (F95) frequencies significantly decreased. This study demonstrates that EEG analysis can differentiate between clinical states (and loss of brain function at death) in birds and provides a unique integration of reflex responses and EEG activity.

Alpha2-adrenoceptor agonists in small animal practice 1. Why they do what they do

Alpha2-adrenoceptor agonists are widely used for premedication, sedation and, more recently, provision of analgesia in small animal practice. However, although these drugs may offer significant benefits over alternative agents, they also have a number of side effects that must be appreciated. This, the first in a series of two articles, explains how the pharmacology of these drugs leads to both the desirable and adverse effects observed with these agents. The second article, which will be published in a forthcoming issue of In Practice, will describe the most appropriate way to use alpha2-adrenoceptor agonists to optimise their clinical effects.

Statistics: more than pictures

This, the third of our series of articles on statistics in veterinary medicine, moves onto the more complex concepts of hypothesis testing and confidence intervals. As these two areas are widely discussed in many clinical research publications, an awareness of the underlying methodology behind their use is essential to appreciate the information they convey.

Statistics: dealing with categorical data

This, the fifth of our series of articles on statistics in veterinary medicine, moves onto modelling categorical data, in particular assessing associations between variables. Some of the questions we shall consider are widely discussed in many clinical research publications, and we will use the ideas of hypothesis tests and confidence intervals to answer those questions.