Paul D Macfarlane

Use of continuous positive airway pressure (CPAP) to optimise oxygenation in anaesthetised horses – a clinical study

REASONS FOR PERFORMING STUDY Hypoxaemia is a common problem during equine anaesthesia. Continuous positive airway pressure (CPAP) is a ventilation mode routinely employed in man to overcome hypoxaemia but has not been objectively assessed in horses. OBJECTIVES To test the effects of CPAP on oxygenation and its indices in anaesthetised horses in a clinical setting. METHODS Twenty-four healthy horses requiring anaesthesia in dorsal recumbency were anaesthetised using a standard protocol. Following orotracheal intubation and connection to an anaesthetic machine capable of applying CPAP, horses were randomly allocated to ventilate at physiological airway pressure measured at the airway opening (Group PAP) or to receive CPAP of 8 cmH2O (Group CPAP). Arterial blood gas analysis was performed as soon as arterial cannulation was achieved and 30, 60 and 90 min after induction. If PaCO2 increased above 9.31 kPa controlled ventilation was initiated. Groups were compared using a general linear model. RESULTS Horses receiving CPAP had significantly higher PaO2 and calculated oxygen indices than horses receiving PAP. No significant differences in ventilation indices were observed between the 2 groups. Eight horses receiving PAP and 5 receiving CPAP required controlled ventilation. No differences in dobutamine requirements or mean arterial pressures were recorded. CONCLUSIONS Continuous positive airway pressure of 8 cmH2O improved oxygenation indices in dorsally recumbent horses without significantly influencing ventilation. POTENTIAL RELEVANCE Continuous positive airway pressure reduces the incidence of hypoxaemia in anaesthetised horses. Further research is warranted to elucidate the effects of CPAP on the cardiovascular system.

Oxygenation and ventilation characteristics in obese sedated dogs before and after weight loss: a clinical trial.

This prospective clinical study examined the effect of obesity and subsequent weight loss on oxygenation and ventilation during deep sedation in pet dogs. Data from nine dogs completing a formalised weight loss programme were evaluated. Dual-energy X-ray absorptiometry (DEXA) was used to quantify body fat mass prior to and after weight loss. Dogs were deeply sedated and positioned in dorsal recumbency. Sedation was scored using a semi-objective scheme. As part of the monitoring of sedation, arterial oxygen partial pressure (PaO2) and arterial carbon dioxide partial pressure (PaCO2) were measured after 10 min in dorsal recumbency. Oxygen saturation of haemoglobin (SpO2) was monitored continuously using pulse oximetry, starting oxygen supplementation where indicated (SpO2<90%) via a face mask. Morphometric measurements were taken from DEXA images and compared before and after weight loss. Several oxygen indices were calculated and correlated with body fat variables evaluated by DEXA. All body fat variables improved significantly after weight loss. PaO2 increased from 27.9±19.2 kPa to 34.8±24.4 kPa, while FiO2 decreased from 0.74±0.31 to 0.66±0.35. Morphometric measurements improved significantly after weight loss. PaO2/FiO2 (inspired oxygen fraction) and Pa/AO2 (ratio of PaO2 to alveolar PO2) also improved significantly, but there was no change in f-shunt and PaCO2 after weight loss. On multiple linear regression analysis, all oxygen indices were negatively associated with thoracic fat percentage. In conclusion, obesity decreases oxygenation in dogs during deep sedation. Oxygenation status improves with successful weight loss, but ventilation is not influenced by obesity.

Preliminary investigation comparing a detomidine continuous rate infusion combined with either morphine or buprenorphine for standing sedation in horses

OBJECTIVE To compare sedative and analgesic properties of buprenorphine or morphine for standing procedures combined with a detomidine continuous rate infusion (CRI). STUDY DESIGN Blinded, prospective, randomized clinical pilot study. ANIMALS Ten horses presented for dental or sinus procedures. METHODS Horses received 0.02 mg kg(-1) acepromazine intravenously (IV), followed 30 minutes later by detomidine 10 μg kg(-1) IV. Five minutes later, buprenorphine 0.01 mg kg(-1) (n = 6) or morphine 0.1 mg kg(-1) (n = 4) was administered IV. Detomidine was administered by CRI (0.2 μg kg(-1) minute(-1)) and adjusted to maintain appropriate sedation. Heart rate, respiratory frequency, gastrointestinal motility and rectal temperature were measured; pain, ataxia and sedation were scored. Sedation, pain scores and ataxia scores were analysed using a mixed linear model. Detomidine dose and procedure success scores were compared using Wilcoxons rank sum test. Complications between groups were analysed using Fishers exact test. RESULTS Two horses had incomplete data. Weights and ages were not different between groups (p = 0.15 and p = 0.42, respectively). The dose rate for detomidine was not different between groups (0.33 ± 0.02 μg kg(-1) minute(-1) in the buprenorphine group and 0.33 ± 0.05 μg kg(-1) minute(-1), in the morphine group p = 0.89). Intraoperative visual analogue scale scores were greater after buprenorphine than morphine (mean ± SD, buprenorphine 48 ± 4, morphine 40 ± 5, p = 0.0497). Procedure duration was not different between groups (buprenorphine 142 ± 33, morphine 140 ± 12 minutes). All horses treated with buprenorphine experienced complications compared with none in the morphine group (p = 0.0286). CONCLUSIONS AND CLINICAL RELEVANCE At the doses used, buprenorphine produced greater sedation but more post-operative complications than morphine. However, Type I or Type II errors cannot be excluded and larger studies are required to confirm these findings.

Discospondylitis in a dog after attempted extradural injection

We wish to report a case of discospondylitis in a dog after attempted extradural injection for the perioperative management of a thoracotomy. A 6-year-old neutered female German Shepherd dog, weight 33.7 kg, with severe pulmonary dysfunction of unknown aetiology causing hypoxaemia presented for thoracotomy in order to biopsy lung tissue and ligate a patent ductus arteriosus. After induction of anaesthesia the dog was placed in sternal recumbency with the hind legs drawn forward, the skin over the lumbo-sacral junction was clipped and the skin prepared aseptically with chlorhexidine. A 22-gauge 3.5¢¢ spinal needle (Spinal needle, Quincke type point; Becton Dickinson, Spain) was advanced through the skin over the lumbosacral junction. A drop of sterile saline was placed into the hub in order to use the ‘hanging drop’ technique to identify the extradural space. As the needle was advanced the anaesthetist perceived an area of increased resistance with a ‘crunching’ sensation rather than the usual ‘pop’. The needle had not hit bone, neither had the drop of saline been displaced. After negative aspiration a test injection of saline was attempted, the saline was difficult to inject and the needle was withdrawn. After a second and a third attempt by the primary and a senior anaesthetist respectively, the extradural route was abandoned. Even though a clear change in tissue resistance was present, tissue resistance was different from that normally encountered during extradural injection as judged by an experienced anaesthetist. Surgery proceeded as expected for the dog’s physical status and surgery performed. During anaesthesia clavulonate potentiated amoxicillin (Augmentin; GlaxoSmithKline, UK) 20 mg kg was administered intravenously (IV) every 90 minutes. Post-operatively the dog was managed in the intensive care unit for the first 2 days with routine post-thoracotomy care. After discharge to the general ward 3 days after thoracotomy, lumbar and pelvic pain were detected. An orthopaedic examination suggested that pain localised to the hips and was presumed to be osteoarthritic in origin; the analgesic protocol was modified resulting in improvement of the symptoms sufficient for discharge to home care. Results of the lung biopsy revealed a sterile pyogranulomatous pneumonia and 14 days after surgery treatment with oral prednisolone [Prednisolone tablets BP (vet); Millpledge Veterinary, UK] 1 mg kg twice daily was commenced. Two days later the dog presented as an emergency to the hospital with severe pain localised to the hindquarters. Clinical examination revealed muscle atrophy of both pelvic limbs. The dog was unwilling to stand and walk but no neurological deficits were identified. Her body temperature was not elevated. Magnetic resonance imaging of the lumbar spine revealed marked changes consistent with discospondylitis at the L7-S1 space and the presence of empyema. Disc material was aspirated. Subsequent culture of this and a cystocentesis urine culture both revealed a pure E. coli infection. Together with appropriate medical therapy, constant rate intravenous infusions of morphine 0.06– 0.12 mg kg hour (Morphine Sulphate injection BP; Martindale Pharmaceuticals, UK), lidocaine 0.8–1.6 mg kg hour (Lidocaine injection 2% wv; B. Braun Melsungen AG, Germany) and ketamine 0.1–0.2 mg kg hour (Ketaset; Fort Dodge Animal Health, UK) were started and acetominophen 10 mg kg (Perfalgan; Bristol Meyers-Squibb Pharmaceuticals Ltd, UK) was administered every 8 hours IV. Gabapentin 3 mg kg (Gabapentin 100 mg capsules, Sandoz Ltd., UK) was administered orally. Over the next 2 weeks the discospondylitis symptoms gradually improved; however the respiratory disease worsened and the dog was euthanased.

Therapeutic options for the treatment of chronic pain in dogs.

Chronic pain is a widely recognised problem in humans and is being increasingly recognised as a significant problem in dogs. Whilst a large number of therapies are described and utilised to treat chronic pain in dogs, there is a severe shortage of evidence to guide practitioners in selection of treatments. Until more evidence becomes available, practitioners should adopt a cautious approach, utilising licensed treatments first when possible. Non-pharmacological therapies should be incorporated into the chronic pain management plan whenever possible. Given the probable prevalence of chronic pain in dogs there is an urgent need for research to identify effective treatments.

Horses Auto-Recruit Their Lungs by Inspiratory Breath Holding Following Recovery from General Anaesthesia

This study evaluated the breathing pattern and distribution of ventilation in horses prior to and following recovery from general anaesthesia using electrical impedance tomography (EIT). Six horses were anaesthetised for 6 hours in dorsal recumbency. Arterial blood gas and EIT measurements were performed 24 hours before (baseline) and 1, 2, 3, 4, 5 and 6 hours after horses stood following anaesthesia. At each time point 4 representative spontaneous breaths were analysed. The percentage of the total breath length during which impedance remained greater than 50% of the maximum inspiratory impedance change (breath holding), the fraction of total tidal ventilation within each of four stacked regions of interest (ROI) (distribution of ventilation) and the filling time and inflation period of seven ROI evenly distributed over the dorso-ventral height of the lungs were calculated. Mixed effects multi-linear regression and linear regression were used and significance was set at p<0.05. All horses demonstrated inspiratory breath holding until 5 hours after standing. No change from baseline was seen for the distribution of ventilation during inspiration. Filling time and inflation period were more rapid and shorter in ventral and slower and longer in most dorsal ROI compared to baseline, respectively. In a mixed effects multi-linear regression, breath holding was significantly correlated with PaCO2 in both the univariate and multivariate regression. Following recovery from anaesthesia, horses showed inspiratory breath holding during which gas redistributed from ventral into dorsal regions of the lungs. This suggests auto-recruitment of lung tissue which would have been dependent and likely atelectic during anaesthesia.

Perioperative trends in plasma colloid osmotic pressure in horses undergoing surgery

OBJECTIVE To compare perioperative trends in plasma colloid osmotic pressure (COP) between horses undergoing orthopedic and colic surgery. DESIGN Prospective clinical study September 2009-January 2011. SETTING Veterinary university teaching hospital. ANIMALS Thirty-three healthy, client-owned horses presenting for orthopedic surgery (non-GI) and 85 client-owned horses presenting for emergency exploratory celiotomy (GI, gastrointestinal). INTERVENTIONS None. MEASUREMENTS Data relating to the horses parameters on presentation, surgical lesion, post-operative management and survival were extracted from computerized clinical records. Heparinized blood samples were taken on presentation (PreOp, pre-operative), on recovery from anesthesia (T0), at 12 (T12) and 24 (T24) hours post recovery. COP was measured within 4 hours of collection. RESULTS There was no significant difference in PreOp or T0 COP between groups. Both groups had a significant decrease in COP during anesthesia. When compared to their respective pre-operative values, horses in the non-GI group had significantly increased COP at T12, whereas those in the GI group had significantly reduced COP. This trend was continued at T24. Horses in the GI group placed on intravenous crystalloid isotonic fluids post-operatively had a significantly lower COP at T12 and T24. Horses in the GI group that did not survive had significantly lower post-operative COP values at T24. CONCLUSIONS Horses undergoing exploratory celiotomy had significantly lower COP post-operatively than those horses undergoing orthopedic surgery. This difference was more marked in those horses receiving isotonic crystalloid intravenous fluid therapy post-operatively and in those that did not survive to discharge. In the non-GI group an increase in COP post-operatively was common.

Comparison of two passive warming devices for prevention of perioperative hypothermia in dogs

OBJECTIVE To compare effects of two passive warming methods combined with a resistive heating mat on perioperative hypothermia in dogs. MATERIALS AND METHODS Fifty-two dogs were enrolled and randomly allocated to receive a reflective blanket (Blizzard Blanket) or a fabric blanket (VetBed). In addition, in the operating room all dogs were placed onto a table with a resistive heating mat covered with a fabric blanket. Rectal temperature measurements were taken at defined points. Statistical analysis was performed comparing all Blizzard Blanket-treated to all VetBed-treated dogs, and VetBed versus Blizzard Blanket dogs within spay and castrate groups, spay versus castrate groups and within groups less than 10 kg or more than 10 kg bodyweight. RESULTS Data from 39 dogs were used for analysis. All dogs showed a reduction in perioperative rectal temperature. There were no detected statistical differences between treatments or between the different groups. CLINICAL SIGNIFICANCE This study supports previous data on prevalence of hypothermia during surgery. The combination of active and passive warming methods used in this study prevented the development of severe hypothermia, but there were no differences between treatment groups.

Laparoscopic ovariohysterectomy in dogs

SIR, — Laparoscopic or ‘keyhole’ surgery is commonly perceived to be less invasive and hence less painful than conventional surgery. Some authors ([Devitt and others 2005][1], [Hancock and others 2005][2]) have demonstrated decreased pain in the perioperative period when laparoscopic

Repeated hyperkalaemia during two separate episodes of general anaesthesia in a nine-year-old, female neutered greyhound

There is an increasing awareness of the development of hyperkalaemia during anaesthesia in otherwise healthy veterinary patients. In the human literature 63 per cent of in-hospital hyperkalaemic episodes are associated with drug administration. Anecdotal veterinary reports have suggested that a genetic component may also play a role, with greyhounds seemingly more susceptible to the development of hyperkalaemia under anaesthesia. This case report identifies the repeated development of hyperkalaemia, and its treatment, during two separate episodes of general anaesthesia in a nine-year-old, female neutered greyhound. The first episode of hyperkalaemia (7.89 mmol/l) was identified due to bradycardia and second-degree atrioventricular block on electrocardiogram. Treatment was with intravenous calcium gluconate, insulin, glucose and fluid therapy. The second episode (6.60 mmol/l) was associated with spiked T-waves, and treatment was with insulin and glucose infusions to allow completion of the anaesthetic and surgery. Possible causes and treatments are discussed, and the need for reporting of such cases is highlighted.