Impact of four different recumbencies on the distribution of ventilation in conscious or anaesthetized spontaneously breathing beagle dogs: An electrical impedance tomography study.

The aim was to examine the effects of recumbency and anaesthesia on distribution of ventilation in beagle dogs using Electrical Impedance Tomography (EIT). Nine healthy beagle dogs, aging 3.7±1.7 (mean±SD) years and weighing 16.3±1.6 kg, received a series of treatments in a fixed order on a single occasion. Conscious dogs were positioned in right lateral recumbency (RLR) and equipped with 32 EIT electrodes around the thorax. Following five minutes of equilibration, two minutes of EIT recordings were made in each recumbency in the following order: RLR, dorsal (DR), left (LLR) and sternal (SR). The dogs were then positioned in RLR, premedicated (medetomidine 0.01, midazolam 0.1, butorphanol 0.1 mg kg-1 iv) and pre-oxygenated. Fifteen minutes later anaesthesia was induced with 1 mg kg-1 propofol iv and maintained with propofol infusion (0.1-0.2 mg kg-1 minute-1 iv). After induction, the animals were intubated and allowed to breathe spontaneously (FIO2 = 1). Recordings of EIT were performed again in four recumbencies similarly to conscious state. Centre of ventilation (COV) and global inhomogeneity (GI) index were calculated from the functional EIT images. Repeated-measures ANOVA and Bonferroni tests were used for statistical analysis (p < 0.05). None of the variables changed in the conscious state. During anaesthesia left-to-right COV increased from 46.8±2.8% in DR to 49.8±2.9% in SR indicating a right shift, and ventral-to-dorsal COV increased from 49.8±1.7% in DR to 51.8±1.1% in LLR indicating a dorsal shift in distribution of ventilation. Recumbency affected distribution of ventilation in anaesthetized but not in conscious dogs. This can be related to loss of respiratory muscle tone (e.g. diaphragm) and changes in thoracic shape. Changing position of thoraco-abdominal organs under the EIT belt should be considered as alternative explanation of these findings.

Comparison of an infrared anaesthetic agent analyser (Datex-Ohmeda) with refractometry for measurement of isoflurane, sevoflurane and desflurane concentrations.

OBJECTIVE: To assess agreement between infrared (IR) analysers and a refractometer for measurements of isoflurane, sevoflurane and desflurane concentrations and to demonstrate the effect of customized calibration of IR analysers. STUDY DESIGN: In vitro experiment. SUBJECTS: Six IR anaesthetic monitors (Datex-Ohmeda) and a single portable refractometer (Riken). METHODS: Both devices were calibrated following the manufacturer's recommendations. Gas samples were collected at common gas outlets of anaesthesia machines. A range of agent concentrations was produced by stepwise changes in dial settings: isoflurane (0-5% in 0.5% increments), sevoflurane (0-8% in 1% increments), or desflurane (0-18% in 2% increments). Oxygen flow was 2 L minute(-1) . The orders of testing IR analysers, agents and dial settings were randomized. Duplicate measurements were performed at each setting. The entire procedure was repeated 24 hours later. Bland-Altman analysis was performed. Measurements on day-1 were used to yield calibration equations (IR measurements as dependent and refractometry measurements as independent variables), which were used to modify the IR measurements on day-2. RESULTS: Bias ± limits of agreement for isoflurane, sevoflurane and desflurane were 0.2 ± 0.3, 0.1 ± 0.4 and 0.7 ± 0.9 volume%, respectively. There were significant linear relationships between differences and means for all agents. The IR analysers became less accurate at higher gas concentrations. After customized calibration, the bias became almost zero and the limits of agreement became narrower. CONCLUSIONS AND CLINICAL RELEVANCE: If similar IR analysers are used in research studies, they need to be calibrated against a reference method using the agent in question at multiple calibration points overlapping the range of interest.

The effect of four anaesthetic protocols for maintenance of anaesthesia on trans-diaphragmatic pressure in dogs.

The diaphragm is the main inspiratory muscle and the main indicator of diaphragmatic contractility is the trans-diaphragmatic pressure (Pdi). The aim of this clinical study was to determine the effect of four different anaesthetic protocols on Pdi in anaesthetized healthy dogs. Eighty client-owned dogs were recruited in this clinical study. All the animals received dexmedetomidine and morphine as premedication and propofol for induction. Anaesthesia was maintained with one of four protocols: isoflurane (I), isoflurane with CRI of propofol (IP), isoflurane with CRI of fentanyl (IF), and isoflurane with CRI of ketamine (IK). When the surgical plane of anaesthesia was achieved, two balloon catheters were inserted, one into the stomach and one into the mid-third of the oesophagus for Pdi measurement. Pdi value was the highest in groups I (14.9±4.7 mmHg) and IK (15.2±3.5 mmHg) and the lowest in groups IP (12.2±3.2 mmHg) and IF (12.0±5.9 mmHg). There was a statistically significant difference (p = 0.029) between groups IK and IF. PE'CO2 was statistically significantly higher (p<0.0005) in group IF (7.7±0.8 kPa) than in group IK (6.5±0.7 kPa). Isoflurane alone or isoflurane with ketamine for the maintenance of anaesthesia seem to better preserve the respiratory function and the diaphragmatic contractility than isoflurane with either propofol or fentanyl in dogs. Therefore, the use of isoflurane or isoflurane with ketamine may be of benefit when animals with respiratory problems have to be anaesthetized.

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.

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.

Metabolic changes associated with anaesthesia and cherry poisoning in a pony.

OBSERVATIONS: A case of a pony with severe cyanide intoxication as a result of cherry ingestion is presented. General anaesthesia was performed for colic surgery. Severe metabolic lactate acidosis in combination with a high arterial oxygen partial pressure and clinically good peripheral perfusion parameters were the remarkable signs during anaesthesia. Severe hypothermia was obvious during recovery. Ten hours post-surgery the pony was euthanized as a result of neurological signs. The diagnosis of cyanide intoxication was made post-mortem. CONCLUSION: Cherry ingestion can lead to lethal cyanide intoxication in horses indicated by severe nonhypoxic lactic acidosis during anaesthesia.

The extradural pressure profile in goats following extradural injection.

OBJECTIVE: To measure the extradural pressures in goats before and after extradural injection, and to investigate the occurrence of extradural pressure waves. STUDY DESIGN: Prospective experimental trial. ANIMALS: Nine healthy adult goats weighing 59.4 +/- 6.4 kg, scheduled for stifle arthroscopy. METHODS: The goats were pre-medicated with midazolam and anaesthesia was induced with propofol and maintained with sevoflurane. The goats were placed in lateral recumbency and extradural puncture was performed via the lumbosacral space. Correct placement of the needle was assessed by lack of resistance to the injection of saline. The needle was connected to an electronic pressure transducer to record extradural pressure. Measurements were taken before and after extradural injection of methadone (0.1 mg kg(-1), diluted to a total volume of 0.2 mL kg(-1)) and 10 minutes later. Contrast medium was injected and correct extradural needle placement confirmed by radiography. RESULTS: Lack of resistance to injection of saline occurred in all goats, but there were no pressure waves observed before injection in any animal. Radiography indicated incorrect needle placement in four animals and one had pressure waves synchronous with the arterial pulse after methadone injection. Correct needle placement was confirmed in the remaining five animals which exhibited pressure waves after extradural methadone injection. In the five goats with successful needle placement the baseline extradural pressure ranged from 0.4 to 2.5 kPa (3-19 mmHg), increasing to 4.4-39.9 kPa (33-300 mmHg) after injection. Ten minutes after injection, extradural pressure remained elevated and ranged from 2.5 to 17.3 kPa (19-130 mmHg). CONCLUSIONS AND CLINICAL RELEVANCE: Extradural pressure waves were not useful to confirm correct extradural needle placement in laterally recumbent goats. The presence of such waves after injection of 0.2 mL kg(-1) may be indicative of correct placement but even here we saw one of nine animals with extradural pressure waves where we failed to confirm correct needle placement. Extradural pressure increases after extradural injection.

Anaesthetic sparing effect of local anaesthesia of the ovarian pedicle during ovariohysterectomy in dogs.

OBJECTIVE: To evaluate the effect of local anaesthesia of the mesovarium on end-tidal isoflurane (Fe'(iso)) concentration and vital parameters during canine ovariohysterectomy. STUDY DESIGN: Prospective, randomized, blinded study. ANIMALS: Twenty client-owned dogs undergoing elective ovariohysterectomy. Mean age 1.7 (+/-0.53, SD) years and mean body weight 21 kg (+/-5.9, SD). METHODS: Pre-medication was with intravenous acepromazine (0.02 mg kg(-1)) and methadone (0.1 mg kg(-1)). Anaesthesia was induced with propofol and maintained with isoflurane in oxygen. One group (n = 10) received local infiltration of the mesovarium with 0.5 mL lidocaine 2% and one group (n = 10) with 0.5 mL NaCl 0.9%. Heart (HR) and respiratory rates (fr), invasive mean arterial blood pressure (MAP) and Fe'(iso)were recorded. The Fe'(iso) was adjusted according to changes in HR, RR and MAP. Time points used for comparison were T1 (after induction of anaesthesia before surgery), T2 (after lidocaine infiltration of the mesovarium) and T3 (surgical manipulation of the ovaries). Data were analysed using a mixed model for repeated measurement anova and the Tukey adjustment. Results are presented as mean +/- SD; p < 0.05 was considered significant. RESULTS: In both groups, HR and fr remained stable at the three time points. Mean values ranged from 84 to 94 beats minute(-1) and from 10 to 14 breaths minute(-1). The Fe'(iso) was significantly lower at T3 compared to T1 and mean values ranged from 0.95% to 1.24%. The mean arterial blood pressure was significantly higher at T3 compared to T1 and mean values ranged from 58 to 96 mm Hg. At none of the time points were there significant differences between the two groups for HR, fr, MAP or Fe'(iso). CONCLUSION: Neither an isoflurane sparing effect nor a difference in autonomic response to surgery was demonstrated following local anaesthesia of the mesovarium. CLINICAL RELEVANCE: There appeared to be minimal benefit from local anaesthesia of the mesovarium during this study.

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.

The effect of low dose rocuronium on globe position, muscle relaxation and ventilation in dogs: a clinical study.

OBJECTIVE: The purpose of this study was to evaluate globe position, muscle relaxation and changes in ventilatory parameters after intravenous administration of 0.1 mg/kg rocuronium. STUDY DESIGN: Prospective clinical study. ANIMAL STUDIED: Sixteen dogs of different breeds, with a body weight of 22.1 +/- 13 kg and age of 5.6 +/- 2.8 years (mean +/- SD), were anesthetized for a short ophthalmic examination requiring central position of the globe. PROCEDURES: All dogs were premedicated with 0.005 mg/kg medetomidine and 0.1 mg/kg methadone IV. Anesthesia was induced with propofol to effect and maintained with 10 mg/kg/h propofol by continuous rate infusion. Following endotracheal intubation all dogs breathed 100% oxygen via an anesthetic circle system. Neuromuscular function was assessed with an acceleromyograph (TOF-Guard, Organon Teknika NV, Turnhout, Belgium) and by stimulation of the nervus peroneus superficialis. The ventilation parameters were measured using spirometry and capnography. After baseline measurements 0.1 mg/kg rocuronium was administered IV. Minute volume (MV), tidal volume (Vt), respiratory rate (RR), end expiratory carbon dioxide concentration (PE'CO(2)) and maximal depression of the response of the first twitch (T1) of train-of-four (TOF) stimulation and train-of-four ratio (TOFR) was measured. The change in the position of the globe was recorded. RESULTS: T1 decreased to 61 +/- 18% and the TOF ratio to 45 +/- 21% of baseline values. Both parameters returned to baseline after 9 min. There was no significant reduction in MV, TV and RR and no increase in PE'CO(2). The globe rotated to a central position of 45 +/- 7.7 s after administration of rocuronium and remained there for 23 +/- 10.8 min in all dogs. CONCLUSION: Rocuronium administered intravenously at a dose of 0.1 mg/kg to dogs causes a central position of the globe but minimal impairment of ventilation parameters.