Monitoring changes in distribution of pulmonary ventilation by functional electrical impedance tomography in anaesthetized ponies.

OBJECTIVE: To assess changes in the distribution in pulmonary ventilation in anaesthetized ponies using electrical impedance tomography (EIT). STUDY DESIGN: Prospective experimental study. ANIMALS: A group of eight adult pony geldings. METHODS: General anaesthesia was induced and maintained using isoflurane in oxygen with volume-controlled ventilation [tidal volume (VT) (10 mL kg-1), respiratory rate (8-12 breaths minute-1)] to maintain end-tidal carbon dioxide between 35 and 40 mmHg (4.66-5.32 kPa). Regional distribution of ventilation was assessed with EIT (plane of fifth to sixth intercostal space) at 30, 60 and 90 minutes after intubation. The resulting functional images were divided into four regions of interest (ROI, A-D) to determine: 1) the ratio of VT distribution between dependent to nondependent lung (D/ND) and 2) changes in distribution within the lungs in the latero-lateral direction. The centre of ventilation (COV) was calculated. The dimension of the chest in the latero-lateral (W) and dorso-ventral (H) direction was measured at the height of the withers and the W/H ratio was calculated. The influence of time on the D/ND ratio, COV and ROI A-D were analysed with anova followed by post hoc Bonferroni tests. Pearson correlation coefficient and the coefficient of determination (r2) were calculated to evaluate the relationship between the W/H ratio and D/ND ratio, and COV. Statistical significance was set at p < 0.05. RESULTS: The median D/ND ratio (T30-T90) was 0.67 (0.40-1.01) and for COV 31.15% (11.20-45.20), confirming the expected uneven distribution of ventilation. The D/ND ratio had a moderate negative correlation with the W/H ratio (r = -0.68, r2 = 0.46, p < 0.001), while the COV did not correlate with the W/H ratio (r = -0.04). CONCLUSIONS AND CLINICAL RELEVANCE: Uneven ventilation in mechanically ventilated right laterally recumbent anaesthetized ponies occurs within 30 minutes and changed little over the following 60 minutes.

Vertebral Subluxation Repair in a Pet Goat.

OBJECTIVES: To describe the perioperative management, including surgery, anesthesia, metabolic derangements, and physiotherapy, in a goat referred for paraparesis secondary to a road traffic accident. STUDY DESIGN: Case report. ANIMALS: 2-year-old mixed breed dwarf 44 kg female pet goat. METHODS: Clinical examination showed symptoms of early compensatory stages of shock, paraparesis with hyperextension of the thoracic limbs, pain on palpation of the thoracolumbar spine, increased patellar reflexes of both pelvic limbs without superficial sensitivity, but preserved deep pain sensation. These signs suggested a spinal cord injury with upper motor neuron syndrome and an anatomic localization between the third thoracic and third lumbar vertebrae. Radiographic examination revealed a thoracolumbar vertebral subluxation. Vertebral stabilization was achieved with the application of pins in the vertebral bodies stabilized by an interconnecting bridge of polymethylmethacrylate, a technique commonly adopted in companion animals. RESULTS: Surgery and recovery from anesthesia were uneventful, but 3 days later ruminal atony and subsequent bloating occurred. This was associated with metabolic derangements (metabolic alkalosis), decreased mentation, and marked tachypnea that responded to medical treatment. From day 3 post-surgery, the goat underwent physiotherapy with manual and active exercises during the rehabilitation period of 21 days duration. CONCLUSION: The injury in this goat was successfully managed using vertebral stabilization similar to that used in dogs and cats. Extensive postoperative physiotherapeutic support contributed to the complete recovery of the animal.

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.

A novel flow partition device for spirometry during large animal anaesthesia.

OBJECTIVE: We describe and test a novel device for large animal anaesthesia monitoring that uses standard human medicine spirometry sensors. STUDY DESIGN: In-vitro study. METHODS: The device consists of two adapters that enable the flow to be split evenly into four tubes in parallel, each tube containing a D-lite sensor. The performance of this flow partitioning device (FPD) over a range of flows from 100 to 700 L minute⁻¹ was determined and the pressure versus flow relation, resistance and dead space was compared with a Horse-lite (Moens 2010). RESULTS: Equipped with four D-lite sensors, and a flow of 700 L minute⁻¹ the pressure drop of the FPD was 13.5 cm H2O, resistance 1.17 cm H2O second L⁻¹ and volume (potential dead space) 182 mL, compared to 2.8 cm H2O, 0.24 cm H2O second L⁻¹ and 54 mL respectively for the Horse-lite. The predicted value of the flow partition of » could be confirmed. Limits of agreement were found to be 4.2% in inspiratory direction and 7.1% in expiratory direction. CONCLUSIONS AND CLINICAL RELEVANCE: The FPD is an affordable device that extends the specification of any commercially available human spirometry sensors to large animal applications. However, an increase in total resistance and dead space has to be taken into account. Therefore, the new device could be useful during equine anaesthesia.

Measurement of tidal volume using respiratory ultrasonic plethysmography in anaesthetized, mechanically ventilated horses.

OBJECTIVE: To compare tidal volume estimations obtained from Respiratory Ultrasonic Plethysmography (RUP) with simultaneous spirometric measurements in anaesthetized, mechanically ventilated horses. STUDY DESIGN: Prospective randomized experimental study. ANIMALS: Five experimental horses. METHODS: Five horses were anaesthetized twice (1 week apart) in random order in lateral and in dorsal recumbency. Nine ventilation modes (treatments) were scheduled in random order (each lasting 4 minutes) applying combinations of different tidal volumes (8, 10, 12 mL kg(-1)) and positive end-expiratory pressures (PEEP) (0, 10, 20 cm H(2)O). Baseline ventilation mode (tidal volume=15 mL kg(-1), PEEP=0 cm H(2)O) was applied for 4 minutes between all treatments. Spirometry and RUP data were downloaded to personal computers. Linear regression analyses (RUP versus spirometric tidal volume) were performed using different subsets of data. Additonally RUP was calibrated against spirometry using a regression equation for all RUP signal values (thoracic, abdominal and combined) with all data collectively and also by an individually determined best regression equation (highest R(2)) for each experiment (horse versus recumbency) separately. Agreement between methods was assessed with Bland-Altman analyses. RESULTS: The highest correlation of RUP and spirometric tidal volume (R(2)=0.81) was found with the combined RUP signal in horses in lateral recumbency and ventilated without PEEP. The bias ±2 SD was 0±2.66 L when RUP was calibrated for collective data, but decreased to 0±0.87 L when RUP was calibrated with individual data. CONCLUSIONS AND CLINICAL RELEVANCE: A possible use of RUP for tidal volume measurement during IPPV needs individual calibration to obtain limits of agreement within ±20%.

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.

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 effect of local anaesthesia on anaesthetic requirements for feline ovariectomy.

A dose of supplementary ketamine was used to evaluate the anaesthetic sparing effect of adding local anaesthesia to general anaesthesia in cats undergoing ovariectomy. Fifty-six healthy cats were randomly assigned to receive lidocaine 2% (group L) as skin infiltration (1 mg kg(-1)), topical application (splash block) on both the ovaries (2 mg kg(-1), each) and on abdominal muscular layers (1 mg kg(-1)), or an equal volume of NaCl 0.9% at the same sites (group S). Anaesthesia was induced with a mixture of 20 microg kg(-1) medetomidine and 5 mg kg(-1) ketamine administered intramuscularly. Rectal temperature, ECG, heart rate and respiratory rate were measured continuously. Ketamine supplemental boli (1 mg kg(-1), intravenously) were administered in response to movements during surgery. Local lidocaine significantly reduced the need for supplementary ketamine. All animals were returned to their owners without complications. With this protocol, local anaesthetics reduced the need for injectable anaesthetic during feline ovariectomy.

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.

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.

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.

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.

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.