Yves Moens

Evaluation of the analgesic effect of lidocaine and bupivacaine used to provide a brachial plexus block for forelimb surgery in 10 dogs

Twenty adult dogs weighing between 1·4 and 53·5 kg and aged between six months and nine years were anaesthetised and the brachial plexus was localised with the aid of a nerve stimulator. In 10 of the dogs a brachial plexus block was induced with a mixture of lidocaine and bupivacaine and the other 10 each received 0·25 ml/kg saline as a control. The end-tidal isoflurane concentration was maintained between 1·3 and 1·4 per cent during surgery for carpal arthrodesis or a fracture of the radius or ulna. Acute heart rate or blood pressure increases of 20 per cent or more were treated with 1 µg/kg fentanyl intravenously. Postoperatively, signs of pain were scored by a single blinded observer at hourly intervals until eight hours after the block had been induced, on a scale from 0 to 18. Dogs with pain scores above 5 received 0·1 to 0·2 mg/kg methadone intravenously, repeated as necessary. During surgery the control dogs received significantly more fentanyl (median 0·05 µg/kg/minute, range 0·02 to 0·20 µg/kg/minute) than the group given local anaesthetic (median 0 µg/kg/minute, range 0 to 0·02 µg/kg/minute). Postoperatively, the control group required significantly more methadone (median 0·2 mg/kg, range 0·1 to 1 mg/kg) than the treated group (median 0 mg/kg, range 0 to 0·13 mg/kg).

Influence of drugs on the response characteristics of the LiDCO sensor: an in vitro study

BACKGROUND In a previous study, the authors found a large bias (50%) for lithium (LiDCO) compared with thermodilution cardiac output measurement methods in ponies receiving i.v. infusions of xylazine, ketamine, and midazolam. This prompted the authors to examine the effect of drugs on the LiDCO sensor. METHODS Drugs and lithium were dissolved in 0.9% saline to produce the following solutions: saline, saline-lithium, saline-drug, and saline-drug-lithium. The drug concentrations were overlapping the range of clinical interest as estimated from the published literature. These 38°C solutions were pumped through the LiDCO sensor in predetermined order. Sensor voltages were measured. Differences between lithium-induced voltage changes in the absence and presence of drugs indicated erroneous lithium detections that, if they occurred in vivo, may cause biases in LiDCO measurements. RESULTS Clonidine, detomidine, dexmedetomidine, medetomidine, romifidine, xylazine, ketamine, S-ketamine, lidocaine, and rocuronium caused concentration-dependent increases in sensor voltages and negative biases in lithium detection that were mathematically equivalent to greater than +10% biases in LiDCO. The drug-induced voltage changes correlated with calculated biases in LiDCO (r(2)=0.91). Atipamezole, acepromazine, butorphanol, diazepam, midazolam, and guaifenesin caused minimal or no interaction in this study. CONCLUSIONS A number of drugs influenced the accuracy of the LiDCO sensor in vitro but, based on published pharmacokinetic data, only xylazine, ketamine, lidocaine, and rocuronium may cause biases at clinically relevant concentrations. These findings need to be confirmed in vivo. Relevant (>3 mV) changes in sensor voltages due to the presence of drugs may indicate possible interactions with the LiDCO sensor.

Anaesthesia in horses using halothane and intravenous ketamine–guaiphenesin: a clinical study

OBJECTIVE The aim of this study was to define and evaluate a combined inhalation-intravenous anaesthetic protocol for use in equine anaesthesia. STUDY DESIGN Prospective, randomized clinical trial. ANIMALS Twenty-eight horses (body mass 522 ± 82; 330-700 kg [mean ± SD; range]) with a mean age of 6 ± 4 years (range: 2-18 years) presented to the university hospital for various surgical procedures requiring general anaesthesia. MATERIALS AND METHODS Animals were randomly allocated to one of two treatment groups. Anaesthesia was maintained in 14 horses with halothane alone (H group). The mean end-tidal halothane concentration was 1.24%. In the second group (n = 14) anaesthesia was maintained with both halothane (end-tidal concentration 0.61%) and a continuous infusion of a ketamine-guaiphenesin mixture (HKG group). The two techniques were compared in terms of qualitative differences and cardiopulmonary effects. RESULTS The stability of anaesthesia was significantly greater in group HKG and the need for dobutamine to maintain blood pressure was significantly less. Recovery times and quality were acceptable in all cases. There were no significant differences between the groups. CONCLUSIONS The infusion of ketamine and guaiphenesin in horses receiving low inspired concentrations of halothane provides suitable surgical anaesthesia and lowers the risk of hypotension. CLINICAL RELEVANCE The anaesthetic technique described in this study is a useful and practical alternative to inhalation anaesthesia using halothane alone.

Mechanical Ventilation and Respiratory Mechanics During Equine Anesthesia

The mechanical ventilation of horses during anesthesia remains a crucial option for optimal anesthetic management, if the possible negative cardiovascular side effects are managed, because this species is prone to hypercapnia and hypoxemia. The combined use of capnography and pitot-based spirometry provide complementary information on ventilation and respiratory mechanics, respectively. This facilitates management of mechanical ventilation in conditions of changing respiratory system compliance (ie, laparoscopy) and when investigating new ventilatory strategies including alveolar recruitment maneuvers and optimization of positive expiratory pressure.

Lithium dilution, pulse power analysis, and continuous thermodilution cardiac output measurements compared with bolus thermodilution in anaesthetized ponies

BACKGROUND This study compares cardiac output (CO) measurements obtained by lithium dilution (LiDCO), pulse power analysis (PulseCO), and continuous thermodilution (CTD) with bolus thermodilution (BTD) in ponies. METHODS Eight isoflurane-anaesthetized Shetland ponies received xylazine, ketamine, and midazolam infusions (0.3, 1.2, and 0.018 mg kg(-1) h(-1), respectively). CO was measured with BTD, CTD, LiDCO, and PulseCO. Lithium was injected into the jugular vein and blood was sampled from the facial artery for lithium detection and this artery was also used for PulseCO. Measurements were obtained during four stable haemodynamic conditions in the following order: isoflurane 1% (end-tidal concentration), isoflurane 2%, isoflurane 1%, and isoflurane 1%+dobutamine 5 µg kg(-1) min(-1). RESULTS The bias (2 sd) was 2.5 (2.1) and 0.5 (2.9) litre min(-1) for LiDCO-BTD and for CTD-BTD comparisons, respectively. The limits of agreement were wider than ±30%; therefore, interchangeability was rejected for both comparisons. A possible error in LiDCO might explain the bias observed because CTD-BTD comparison showed less bias. Changes in PulseCO did not correlate with those of BTD and a weak correlation (r(2)=0.23; P=0.018) and concordance (Pc=0.42) was found between CTD and BTD. CONCLUSIONS This is the first study to show a large bias for LiDCO-BTD comparison in animals receiving xylazine, ketamine, and midazolam infusions. The trending abilities of neither PulseCO nor CTD were reliable. Further studies are needed to elucidate possible influences of drugs on the accuracy of the LiDCOplus system.

Two cases of bradyarrhythmia and hypotension after extradural injections in dogs

OBSERVATIONS Two healthy obese, seven-year-old, female Rottweilers weighing 40 and 57 kg were submitted for cranial cruciate repair. They were premedicated with intravenous methadone (0.1 mg kg(-1)) and acepromazine (0.01 and 0.02 mg kg(-1)). Anesthesia was induced with propofol (3.6 and 2.5 mg kg(-1)) and maintained with isoflurane in oxygen using a circle breathing system. The dogs were placed in sternal recumbency and epidural injection of lidocaine/bupivacaine or lidocaine/bupivacaine/morphine (0.2 mL/kg, 8 and 11 mL) was carried out over 1.5 and 4 minutes. Epidural pressures were 79 and 72 mmHg at the end of the injections. The first dogs heart rate decreased from 80 to 65 beats minute(-1) with a second degree atrioventricular (AV) block. The arterial pressure decreased from 100 to 50 mmHg. These responded to atropine (0.01 mg kg(-1) IV). The second dogs heart rate decreased from 120 to 60 beats minute(-1) while arterial pressure decreased from 72 to 38 mmHg. No treatment was given and heart rate and arterial blood pressure returned to acceptable ranges. CONCLUSIONS These cases suggest that large increases in epidural pressure may cause significant cardiovascular effects. This may be avoided by using lower volumes and discontinuing injection if significant back pressure is detected.

A scavenging double mask to reduce workplace contamination during mask induction of inhalation anesthesia in dogs

BackgroundWorkplace contamination by the use of volatile anesthetic agents should be kept to a minimum if a potential health hazard is to be minimised. Mask induction of animals is a common procedure. The present study investigates the efficiency of a novel scavenging double mask in reducing waste gas concentrations in the breathing zone of the anesthetist performing this procedure.MethodsTwelve beagle dogs (ASA I) undergoing general anesthesia for a dental procedure were intravenously premedicated with medetomidine and butorphanol (10 μg/kg and 0.2 mg/kg). Anesthesia was induced via a custom-made scavenging mask using isoflurane in oxygen. In six dogs (group S), scavenging from the mask was performed whereas in six other dogs (group NS) the scavenging function was disabled. Isoflurane concentration was continuously measured with photoacoustic spectroscopy at the level of the shoulder of the anesthetist before and during mask induction and additionally during intubation. Statistical analysis was performed with a Student t- test and a Mann-Whitney U test (p < 0.05 for significance).ResultsThe mean isoflurane concentration during baseline (premedication) was 1.8 ± 0.8 ppm and 2.3 ± 0.6 ppm in group S and NS respectively. This increased during mask induction to 2.0 ± 0.8 ppm and 11.2 ± 6.0 ppm respectively (p < 0.01). The maximum isoflurane concentration ranged from 0.7 ppm to 2.8 ppm and from from 8.3 ppm to 43.7 ppm in group S and NS respectively.ConclusionThis double mask can be used to induce inhalation anesthesia in dogs. Scavenging from the mask significantly decreases the amount of waste anaesthetic gas concentrations in the breathing zone of the anesthetist. Therefore, such a system can be recommended whenever induction or maintenance of general anesthesia by mask is considered.

In vitro validation of a Pitot-based flow meter for the measurement of respiratory volume and flow in large animal anaesthesia

OBJECTIVE To remodel and validate commercially available monitors and their Pitot tube-based flow sensors for use in large animals, using in vitro techniques. STUDY DESIGN Prospective, in vitro experiment. METHODS Both the original and the remodelled sensor were studied with a reference flow generator. Measurements were taken of the static flow-pressure relationship and linearity of the flow signal. Sensor airway resistance was calculated. Following recalibration of the host monitor, volumes ranging from 1 to 7 L were generated by a calibration syringe, and bias and precision of spirometric volume was determined. Where manual recalibration was not available, a conversion factor for volume measurement was determined. The influence of gas composition mixture and peak flow on the conversion factor was studied. RESULTS Both the original and the remodelled sensor showed similar static flow-pressure relationships and linearity of the flow signal. Mean bias (%) of displayed values compared with the reference volume of 3, 5 and 7 L varied between -0.4% and +2.4%, and this was significantly smaller than that for 1 L (4.8% to +5.0%). Conversion factors for 3, 5 and 7 L were very similar (mean 6.00 +/- 0.2, range 5.91-6.06) and were not significantly influenced by the gas mixture used. Increasing peak flow caused a small decrease in the conversion factor. Volume measurement error and conversion factors for inspiration and expiration were close to identity. CONCLUSION The combination of the host monitor with the remodelled flow sensor allowed accurate in vitro measurement of flows and volumes in a range expected during large animal anaesthesia. CLINICAL RELEVANCE This combination has potential as a reliable spirometric monitor for use during large animal anaesthesia.

Evaluation of extradural pressure waves and the ‘lack of resistance’ test to confirm extradural needle placement in dogs ☆

This study aimed to evaluate the use of extradural pressure (EDP) waves to confirm extradural needle placement in clinical practice. Lumbosacral extradural anaesthesia was performed in 98 dogs, of which 85 were included for statistical analysis. The extradural space was identified using conventional methods and, after testing lack of resistance to injection of saline, a pressure transducer was connected to the needle. EDP and the occurrence of pressure waves were recorded before and following injection of local anaesthetic. Successful administration of the drug was confirmed by clinical assessment. Extradural anaesthesia was successful in 88% of the dogs. Pressure waves were present in 89% of the animals with successful extradural puncture, but in 35% of dogs the waves occurred following extradural injection but not before. In 11% of dogs no EDP waves were observed. EDP prior to administration of the local anaesthetic was 0.4+/-1.0 kPa but following the injection values were significantly higher (4.7+/-2.9 kPa) and there was no difference between pressures following successful and unsuccessful punctures. It was concluded that EDP waves can be used to confirm correct needle placement in dogs in clinical practice and measurement is most reliable following extradural injection.

Guidelines and criteria for the placement of laryngeal mask airways in dogs

OBJECTIVE To evaluate the criteria for the insertion and correct placement of the laryngeal mask airway (LMA) in dogs. Study design Prospective descriptive clinical study. Animals Thirty healthy dogs (ASA I or II) of different breeds, age 0.33-7.0 years (2.8 +/- 2.1; mean +/- SD), weight 2.2-59.0 kg (23.9 +/- 14.4), anaesthetized for elective surgery. MATERIALS AND METHODS The dogs were sedated with intravenous (IV) medetomidine (10 microg kg(-1)) and butorphanol (0.2 mg kg(-1)). If considered necessary, IV propofol (1 mg kg(-1) over 30 seconds) was administered until the LMA was inserted and positioned correctly. The position of the LMA was evaluated using predefined criteria for its insertion and inflation of the cuff, together with the ability to ventilate the dogs through the LMA. RESULTS The criteria for insertion, inflation and ventilation which indicated a clinically optimal position of the LMA and its seal around the larynx were met in 19 dogs (63.3%). The dogs could be manually ventilated with inspiratory peak pressures of 10 cm H(2)O without capnographic or audible evidence of leakage. In 11 dogs (36.7%), the LMA was positioned suboptimally with leakage during manual ventilation with inspiratory peak pressures not exceeding 10 cmH(2)O. There was no evidence of breed-related differences in LMA placement and position. CONCLUSIONS AND CLINICAL RELEVANCE The technique for the insertion of the LMA using predefined criteria to evaluate a correct positioning and a seal led to a successful placement in dogs of both brachycephalic and nonbrachycephalic breeds. The LMA, in most of the dogs, was easily placed, well tolerated and offered a useful less invasive means of securing the upper airway.