Effects of 12 and 17 cmH2O positive end-expiratory pressure applied after alveolar recruitment maneuver on pulmonary gas exchange and compliance in isoflurane-anesthetized horses

Objective

To compare static compliance (C_st) and alveolar–arterial oxygen tension difference [P(a–a)O₂] between positive end-expiratory pressures (PEEP) of 7, 12 and 17 cmH₂O applied after an alveolar recruitment maneuver (RM) in isoflurane-anesthetized horses.

Oxygenation and plasma endothelin‐1 concentrations in healthy horses recovering from isoflurane anaesthesia administered with or without pulse‐delivered inhaled nitric oxide

ObjectiveTo assess oxygenation, ventilation‐perfusion (V/Q) matching and plasma endothelin (ET‐1) concentrations in healthy horses recovering from isoflurane anaesthesia administered with or without pulse‐delivered inhaled nitric oxide (iNO).Study DesignProspective experimental trial.AnimalsHealthy adult Standardbred horses.MethodsHorses were anaesthetized with isoflurane in oxygen and placed in lateral recumbency. Six control (C group) horses were anaesthetized without iNO delivery and six horses received pulse‐delivered iNO (NO group). After 2.5 hours of anaesthesia isoflurane and iNO were abruptly discontinued, inhaled oxygen was reduced from 100% to approximately 30%, and the horses were moved to the recovery stall. At intervals during a 30‐minute period following the discontinuation of anaesthesia, arterial and mixed venous blood gas values, shunt fraction (Qs/Qt), plasma ET‐1 concentration, pulse rate and respiratory rate were measured or calculated. Repeated measures anova and a Bonferroni post hoc test was used to analyze data with significance set at p <0.05.ResultsAt all time points in the recovery period, NO horses maintained better arterial oxygenation (oxygen partial pressure: NO 13.2 ± 2.7–11.1 ± 2.7 versus C 6.7 ± 1.1–7.1 ± 1.1 kPa) and better V/Q matching (Qs/Qt NO 0.23 ± 0.05–0.14 ± 0.06 versus C 0.48 ± 0.03–0.32 ± 0.08%) than C horses. Mixed venous oxygenation was higher in NO for 25 minutes following the discontinuation of anaesthesia (NO 6.3 ± 0.2–4.5 ± 0.07 versus C 4.7 ± 0.6–3.7 ± 0.3 kPa). In both groups of horses arterial oxygenation remained fairly stable; venous oxygenation declined over this time period in the NO group but still remained higher than venous oxygen in the C group. ET‐1 concentrations were higher at most time points in C than NO. Changes in other parameters were either minor or absent.Conclusions and Clinical RelevanceDelivery of iNO to healthy horses during anaesthesia results in better arterial and venous oxygenation and V/Q matching (as determined by lower Qs/Qt) and lower ET‐1 concentrations throughout a 30‐minute anaesthetic recovery period.     

The influence of body mass and thoracic dimensions on arterial oxygenation in anaesthetized horses and ponies

ObjectiveTo examine the relationship between body mass and thoracic dimensions on arterial oxygen tensions (PaO₂) in anaesthetized horses and ponies positioned in dorsal recumbency.Study designProspective clinical study.AnimalsThirty six client-owned horses and ponies, mean [±SD (range)] age 8.1 ± 4.8 (1.5–20) years and mean body mass 467 ± 115 (203–656) kg.MethodsBefore general anaesthesia, food and water were withheld for 12 and 1 hours respectively. Body mass (kg), height at the withers (H), thoracic circumference (C), thoracic depth (length between dorsal spinous process and sternum; D), thoracic width (between point of shoulders; W), and thoracic diagonal length (point of shoulder to last rib; L) were measured. Pre-anaesthetic medication was with intravenous (IV) romifidine (0.1 mg kg⁻¹). Anaesthesia was induced with an IV ketamine (2.2 mg kg⁻¹) and diazepam (0.05 mg kg⁻¹) combination and maintained with halothane in 1:1 oxygen:nitrous oxide (N₂O) mixture. Animals were positioned in dorsal recumbency and allowed to breathe spontaneously. Nitrous oxide was discontinued after 10 minutes, and arterial blood samples obtained and analysed for gas tensions at 15, 30 and 60 minutes after connection to the anaesthetic breathing circuit. Data were analysed using anova and Pearson's correlation co-efficient.ResultsThe height per unit body mass (H kg⁻¹) and thoracic circumference per unit body mass (C kg⁻¹) correlated strongly (r = 0.85, p < 0.001 and r = 0.82, p < 0.001 respectively) with arterial oxygen tensions (PaO₂) at 15 minutes.ConclusionsThere is a strong positive correlation between H kg⁻¹ and C kg⁻¹ and PaO₂ after 15 minutes of anaesthesia in halothane-anaesthetized horses positioned in dorsal recumbency.Clinical relevanceReadily obtained linear measurements (height and thoracic circumference) and body mass may be used to predict the ability of horses to oxygenate during anaesthesia.     

Controlled mechanical ventilation with constant positive end-expiratory pressure and alveolar recruitment manoeuvres during anaesthesia in laterally or dorsally recumbent horses

Objective

To compare the effects of controlled mechanical ventilation (CMV) and constant positive end-expiratory pressure (PEEP) and interposed recruitment manoeuvres (RMs) with those of CMV without PEEP on gas exchange during general anaesthesia and the early recovery period.

Evaluation of intramuscular anesthetic protocols in healthy domestic horses

ObjectiveTo assess anesthetic induction, recovery quality and cardiopulmonary variables after intramuscular (IM) injection of three drug combinations for immobilization of horses.Study designRandomized, blinded, three-way crossover prospective design.AnimalsA total of eight healthy adult horses weighing 470–575 kg.MethodsHorses were administered three treatments IM separated by ≥1 week. Combinations were tiletamine–zolazepam (1.2 mg kg⁻¹), ketamine (1 mg kg⁻¹) and detomidine (0.04 mg kg⁻¹) (treatment TKD); ketamine (3 mg kg⁻¹) and detomidine (0.04 mg kg⁻¹) (treatment KD); and tiletamine–zolazepam (2.4 mg kg⁻¹) and detomidine (0.04 mg kg⁻¹) (treatment TD). Parametric data were analyzed using mixed model linear regression. Nonparametric data were compared using Skillings–Mack test. A p value <0.05 was considered statistically significant.ResultsAll horses in treatment TD became recumbent. In treatments KD and TKD, one horse remained standing. PaO₂ 15 minutes after recumbency was significantly lower in treatments TD (p < 0.0005) and TKD (p = 0.001) than in treatment KD. Times to first movement (25 ± 15 minutes) and sternal recumbency (55 ± 11 minutes) in treatment KD were faster than in treatments TD (57 ± 17 and 76 ± 19 minutes; p < 0.0005, p = 0.001) and TKD (45 ± 18 and 73 ± 31 minutes; p = 0.005, p = 0.021). There were no differences in induction quality, muscle relaxation score, number of attempts to stand or recovery quality.Conclusions and clinical relevanceIn domestic horses, IM injections of tiletamine–zolazepam–detomidine resulted in more reliable recumbency with a longer duration when compared with ketamine–detomidine and tiletamine–zolazepam–ketamine–detomidine. Recoveries were comparable among protocols.     

Ventilation distribution assessed with electrical impedance tomography and the influence of tidal volume,recruitment and positive end-expiratory pressure in isoflurane-anesthetized dogs

Objective

To examine the intrapulmonary gas distribution of low and high tidal volumes (V_T) and to investigate whether this is altered by an alveolar recruitment maneuver (ARM) and 5 cmH₂O positive end-expiratory pressure (PEEP) during anesthesia.

Endothelin receptor subtype A blockade does not affect the haemodynamic recovery from haemorrhage during xenon/remifentanil or isoflurane/remifentanil anaesthesia in dogs

ObjectiveTo test the compensatory role of endothelin-1 when acute blood loss is superimposed on anaesthesia, by characterizing the effect of systemic endothelin receptor subtype A (ET_A) blockade on the haemodynamic and hormonal responses to haemorrhage in dogs anaesthetized with xenon/remifentanil (X/R) or isoflurane/remifentanil (I/R).Study designProspective experimental randomized controlled study.AnimalsSix female Beagle dogs, 13.4 ± 1.3 kg.MethodsAnimals were anaesthetized with remifentanil 0.5 μg kg^?1 minute^?1 plus either 0.8% isoflurane (I/R) or 63% xenon (X/R), with and without (Control) the systemic intravenous endothelin receptor subtype A antagonist atrasentan (four groups, n = 6 each). After 60 minutes of baseline anaesthesia, the dogs were bled (20 mL kg^?1) over 5 minutes and hypovolemia was maintained for 1 hour. Continuous haemodynamic monitoring was performed via femoral and pulmonary artery catheters; vasoactive hormones were measured before and after haemorrhage.ResultsIn Controls, systemic vascular resistance (SVR), vasopressin and catecholamine plasma concentrations were higher with X/R than with I/R anaesthesia at pre-haemorrhage baseline. The peak increase after haemorrhage was higher during X/R than during I/R anaesthesia (SVR 7420 ± 867 versus 5423 ± 547 dyne seconds cm^?5; vasopressin 104 ± 23 versus 44 ± 6 pg mL^?1; epinephrine 2956 ± 310 versus 177 ± 99 pg mL^?1; norepinephrine 862 ± 117 versus 195 ± 33 pg mL^?1, p < 0.05). Haemorrhage reduced central venous pressure from 3 ± 1 to 1 ± 1 cmH₂O (I/R, ns) and from 8 ± 1 to 5 ± 1 cmH₂O (X/R, p < 0.05), but did not reduce mean arterial pressure, nor cardiac output. Atrasentan did not alter the haemodynamic and hormonal response to haemorrhage during either anaesthetic protocol.Conclusions and clinical relevanceSelective ET_A receptor blockade with atrasentan did not impair the haemodynamic and hormonal compensation of acute haemorrhage during X/R or I/R anaesthesia in dogs.     

A comparative study of xylazine-induced mechanical hypoalgesia in donkeys and horses

ObjectiveTo compare the effects of xylazine on mechanical nociceptive thresholds in donkeys and horses.Study designRandomized, controlled, crossover, Latin-square, operator-blinded design.AnimalsSix 3.1 ± 0.89 year old standard donkeys weighing 145.0 ± 30.5 kg and six 9.6 ± 4.4 year old Thoroughbred horses weighing 456.0 ± 69.0 kg.MethodsEach animal received one of four doses of xylazine (0.5, 0.7, 0.9, and 1.1 mg kg^?1), or acepromazine (0.05 mg kg^?1) or saline solution (0.9%) intravenously and mechanical nociceptive thresholds were assessed over 90 minutes. The areas under the threshold change versus time curve values for 60 minutes (AUC_0-60) post-drug administration were used to compare the effect of treatment. A 1-week interval was allowed between successive trials on each animal.ResultsAll doses of xylazine, but not acepromazine or saline, increased mechanical thresholds for up to 60 minutes. Xylazine-induced hypoalgesia was dose-dependent and corresponding AUC_0-60 values for each treatment were not significantly different between donkeys and horses (p≥ 0.0697).ConclusionThe hypoalgesic effects of xylazine at four different doses were not different between donkeys and horses.Clinical relevanceXylazine induced a similar degree of mechanical hypoalgesia in donkeys and horses suggesting that similar doses are needed for both species with regard to analgesia.     

Effect of end-inspiratory pause on airway and physiological dead space in anesthetized horses

ObjectiveTo evaluate the impact of a 30% end-inspiratory pause (EIP) on alveolar tidal volume (V_Talv), airway (V_Daw) and physiological (V_Dphys) dead spaces in mechanically ventilated horses using volumetric capnography, and to evaluate the effect of EIP on carbon dioxide (CO₂) elimination per breath (Vco₂br^–1), PaCO_2, and the ratio of PaO₂-to-fractional inspired oxygen (PaO₂:FiO₂).Study designProspective research study.AnimalsA group of eight healthy research horses undergoing laparotomy.MethodsAnesthetized horses were mechanically ventilated as follows: 6 breaths minute^–1, tidal volume (V_T) 13 mL kg^–1, inspiratory-to-expiratory time ratio 1:2, positive end-expiratory pressure 5 cmH₂O and EIP 0%. Vco₂br^–1 and expired tidal volume (V_TE) of 10 consecutive breaths were recorded 30 minutes after induction, after adding 30% EIP and upon EIP removal to construct volumetric capnograms. A stabilization period of 15 minutes was allowed between phases. Data were analyzed using a mixed-effect linear model. Significance was set at p < 0.05.ResultsThe EIP decreased V_Daw from 6.6 (6.1–6.7) to 5.5 (5.3–6.1) mL kg^–1 (p < 0.001) and increased V_Talv from 7.7 ± 0.7 to 8.6 ± 0.6 mL kg^–1 (p = 0.002) without changing the V_TE. The V_Dphys to V_TE ratio decreased from 51.0% to 45.5% (p < 0.001) with EIP. The EIP also increased PaO₂:FiO₂ from 393.3 ± 160.7 to 450.5 ± 182.5 mmHg (52.5 ± 21.4 to 60.0 ± 24.3 kPa; p < 0.001) and Vco₂br^–1 from 0.49 (0.45–0.50) to 0.59 (0.45–0.61) mL kg^–1 (p = 0.008) without reducing PaCO₂.Conclusions and clinical relevanceThe EIP improved oxygenation and reduced V_Daw and V_Dphys, without reductions in PaCO₂. Future studies should evaluate the impact of different EIP in healthy and pathological equine populations under anesthesia.     

Effect of different inspired fractions of oxygen on F-shunt and arterial partial pressure of oxygen in isoflurane-anaesthetized and mechanically ventilated Shetland ponies

ObjectiveTo determine the effect of fraction of inspired oxygen (FiO₂) on intrapulmonary shunt fraction as measured by F-shunt in ponies during isoflurane anaesthesia.Study designProspective, randomized clinical study.AnimalsA group of 23 adult Shetland ponies undergoing a total of 32 anaesthetic procedures.MethodsPonies were premedicated intravenously (IV) with detomidine (0.01 mg kg^–1) and either morphine (0.1 mg kg^–1) or butorphanol (0.02 mg kg^–1). Anaesthesia was induced with ketamine (2.2 mg kg^–1) and midazolam (0.07 mg kg^–1) administered IV. Ponies were randomly allocated to maintenance of anaesthesia with isoflurane in oxygen (group TH; FiO₂ = 0.95) or a mixture of oxygen and medical air (group TL; FiO₂ = 0.65); all ponies were given a constant rate of infusion of detomidine. Animals were mechanically ventilated to maintain PaCO₂ between 40 and 50 mmHg. Arterial blood gas analysis was performed every 30 minutes. The F-shunt equation was calculated for each time point T0, T30, T60 and T90. Data were analysed using linear mixed model analysis and presented as mean ± standard deviation (p < 0.05).ResultsPaO₂ was greater in group TH than in group TL (TH: 406 ± 90, 438 ± 83, 441 ± 69 and 464 ± 53 mmHg versus TL: 202 ± 90, 186 ± 84, 172 ± 85 and 191 ± 98 mmHg at T0, T30, T60 and T90, respectively; p < 0.0001). In TH, F-shunt was < TL. Significant differences were found at T60 (TH: 13.2% ± 4.3 versus TL: 19.4% ± 8.3; p = 0.016) and T90 (TH: 11.7% ± 3.5 versus TL: 18.6% ± 9.5; p = 0.036).Conclusions and clinical relevanceOur findings do not support a beneficial effect of using a reduced FiO₂ to improve oxygenation in anaesthetized and mechanically ventilated Shetland ponies.     

The pharmacokinetics and pharmacodynamics of the injectable anaesthetic alfaxalone in the horse

ObjectiveTo determine the pharmacokinetics and pharmacodynamics of the neurosteroidal anaesthetic, alfaxalone, in horses after a single intravenous (IV) injection of alfaxalone, following premedication with acepromazine, xylazine and guaiphenesin.Study designProspective experimental study.AnimalsTen (five male and five female), adult, healthy, Standardbred horses.MethodsHorses were premedicated with acepromazine (0.03 mg kg^?1 IV). Twenty minutes later they received xylazine (1 mg kg^?1 IV), then after 5 minutes, guaiphenesin (35 mg kg^?1 IV) followed immediately by IV induction of anaesthesia with alfaxalone (1 mg kg^?1). Cardiorespiratory variables (pulse rate, respiratory rate, pulse oximetry) and clinical signs of anaesthetic depth were evaluated throughout anaesthesia. Venous blood samples were collected at strategic time points and plasma concentrations of alfaxalone were assayed using liquid chromatography-mass spectrometry (LC/MS) and analysed by noncompartmental pharmacokinetic analysis. The quality of anaesthetic induction and recovery was scored on a scale of 1–5 (1 very poor, 5 excellent).ResultsThe median (range) induction and recovery scores were 4 (3–5) (good: horse slowly and moderately gently attained recumbency with minimal or no rigidity or paddling) and 4 (1–5) (good: horse stood on first attempt with some knuckling and ataxia) respectively. The monitored cardiopulmonary variables were within the range expected for clinical equine anaesthesia. The mean ± SD durations of anaesthesia from induction to sternal recumbency and from induction to standing were 42.7 ± 8.4 and 47 ± 9.6 minutes, respectively. The mean ± SD plasma elimination half life (t_1/2), plasma clearance (Clp) and volume of distribution (V_d) for alfaxalone were 33.4 minutes, 37.1 ± 11.1 mL minute^?1 kg^?1 and 1.6 ± 0.4 L kg^?1, respectively.Conclusions and clinical relevanceAlfaxalone, in a 2-hydroxypropyl-beta-cyclodextrin formulation, provides anaesthesia with a short duration of recumbency that is characterised by a smooth induction and satisfactory recovery in the horse. As in other species, alfaxalone is rapidly cleared from the plasma in the horse.     

The bispectral index during recovery from halothane and sevoflurane anaesthesia in horses

ObjectiveTo record the bispectral index (BIS) when horses moved during either halothane or sevoflurane anaesthesia and when they made volitional movements during recovery from these anaesthetics.Study designRandomized prospective clinical study.AnimalsTwenty-five client-owned horses undergoing surgery aged 8.8 (± 5.3; 1–19) years (mean ± SD; range).MethodsBaseline BIS values were recorded before pre-anaesthetic medication (BIS_B) and during anaesthesia (BIS_A) maintained with halothane (group H; n = 12) or sevoflurane (group S; n =13) at approximately 0.8–0.9 × minimum alveolar concentrations (MAC). Bispectral indices were recorded during the surgery when unexpected movement occurred (BIS_MA), during recovery when the first movement convincingly associated with consciousness was observed (BIS_M1) and once sternal recumbency was achieved (BIS_ST).ResultsNo significant difference in BIS_M1 was found between halothane- (85 ± 7; 75–93) and sevoflurane- (87 ± 10; 70–98) anaesthetized horses although BIS_A was significantly (p = 0.0002) lower in group S (62 ± 7; 53–72) than group H (74 ± 7; 60–84). Differences between BIS_M1 and BIS_A were significant in sevoflurane (p = 0.00001) and halothane recipients (p = 0.002) but were greater in group S (25 ± 9; 4–38) compared with group H (12 ± 10; ?9–25). In six of eight horses, BIS_MA values ranged between those recorded during anaesthesia and at first movement.Conclusions and clinical relevanceBispectral indices appear to approximate levels of unconsciousness, suggesting that monitoring the BIS may assist equine anaesthesia. However, it does not predict intra-operative movement.     

Comparison of respiratory function during TIVA and isoflurane anaesthesia in ponies Part II: breathing patterns and transdiaphragmatic pressure

ObjectiveTo compare breathing patterns and transdiaphragmatic pressure during total intravenous (TIVA) and isoflurane anaesthesia in ponies.Study designExperimental, cross–over study.AnimalsSix healthy ponies weighing 286 (233–388) ± 61 kg, age 13 (9–16) ± 3 years.MethodsFollowing premedication with romifidine [80 μg kg^?1 intravenously (IV)], general anaesthesia was induced with midazolam (0.06 mg kg^?1 IV) and ketamine (2.5 mg kg^?1 IV) and maintained with either isoflurane (Fe’Iso = 1.1%) (T-ISO) or an IV combination of romifidine (120 μg kg^?1 per hour), midazolam (0.09 mg kg^?1 hour^?1) and ketamine (3.3 mg kg^?1 hour^?1) (T-TIVA), while breathing 60% oxygen (FIO₂). The circumference changes of the rib cage (RC) and abdominal compartment (ABD) were recorded using respiratory ultrasonic plethysmography (RUP). Balloon tipped catheters were placed in the distal oesophagus and the stomach and maximal transdiaphragmatic pressure (P_{di max)} was calculated during Mueller's manoeuvre.ResultsThe breathing pattern T-ISO was more regular and respiratory rate significantly lower compared with T-TIVA. Ponies in T-TIVA showed regularly appearing sighs, which were never observed in T-ISO. Different contribution of the RC and ABD compartments to the breathing pattern was observed with a smaller participation of the RC to the total volume change during T-ISO. Transdiaphragmatic pressures (mean 13.7 ± SD 8.61 versus 23.4 ± 7.27 cmH₂O, p < 0.0001) were higher in T-TIVA compared to T-ISO. The sum of the RC and ABD circumferential changes was lower during T-TIVA compared to T-ISO (6.32 ± 4.42 versus 11.72 ± 4.38 units, p < 0.0001).Conclusion and clinical relevanceMarked differences in breathing pattern and transdiaphragmatic pressure exist during inhalation- and TIVA and these should be taken into account for clinical estimation of anaesthetic depth.     

Cardiovascular function,pulmonary gas exchange and tissue oxygenation in isoflurane-anesthetized,mechanically ventilated Beagle dogs with four levels of positive end-expiratory pressure

ObjectivesTo compare pulmonary gas exchange, tissue oxygenation and cardiovascular effects of four levels of end-expiratory pressure: no positive end-expiratory pressure (ZEEP), positive end-expiratory pressure (PEEP) of maximal respiratory system compliance (PEEPmaxC_rs), PEEPmaxC_rs + 2 cmH₂O (PEEPmaxC_rs+2), PEEPmaxC_rs + 4 cmH₂O (PEEPmaxC_rs+4), in isoflurane-anesthetized dogs.Study designProspective randomized crossover study.AnimalsA total of seven healthy male Beagle dogs, aged 1 year and weighing 10.2 ± 0.7 kg (mean ± standard deviation).MethodsThe dogs were administered acepromazine and anesthesia was induced with propofol and maintained with isoflurane. Ventilation was controlled for 4 hours with ZEEP, PEEPmaxC_rs, PEEPmaxC_rs+2 or PEEPmaxC_rs+4. Cardiovascular, pulmonary gas exchange and tissue oxygenation data were evaluated at 5, 60, 120, 180 and 240 minutes of ventilation and compared using a mixed-model anova followed by Bonferroni test. p < 0.05 was considered significant.ResultsCardiac index (CI) and mean arterial pressure (MAP) were lower in all PEEP treatments at 5 minutes when compared with ZEEP. CI persisted lower throughout the 4 hours only in PEEPmaxC_rs+4 with the lowest CI at 5 minutes (2.15 ± 0.70 versus 3.45 ± 0.94 L minute^–1 m^–2). At 180 and 240 minutes, MAP was lower in PEEPmaxC_rs+4 than in PEEPmaxC_rs, with the lowest value at 180 minutes (58 ± 7 versus 67 ± 7 mmHg). Oxygen delivery index (DO₂I) was lower in PEEPmaxC_rs+4 than in ZEEP at 5, 60, 120 and 180 minutes. Venous admixture was not different among treatments.Conclusion and clinical relevanceThe use of PEEP caused a transient decrease in MAP and CI in lung-healthy dogs anesthetized with isoflurane, which improved after 60 minutes of ventilation in all levels of PEEP except PEEPmaxC_rs+4. A clinically significant improvement in arterial oxygenation and DO₂I was not observed with PEEPmaxC_rs and PEEPmaxC_rs+2 in comparison with ZEEP, whereas PEEPmaxC_rs+4 decreased DO₂I.     

Arterial blood gas tensions during recovery in horses anesthetized with apneustic anesthesia ventilation compared with conventional mechanical ventilation

ObjectiveTo compare PaO₂ and PaCO₂ in horses recovering from general anesthesia maintained with either apneustic anesthesia ventilation (AAV) or conventional mechanical ventilation (CMV).Study designRandomized, crossover design.AnimalsA total of 10 healthy adult horses from a university-owned herd.MethodsDorsally recumbent horses were anesthetized with isoflurane in oxygen [inspired oxygen fraction = 0.3 initially, with subsequent titration to maintain PaO₂ ≥ 85 mmHg (11.3 kPa)] and ventilated with AAV or CMV according to predefined criteria [10 mL kg^–1 tidal volume, PaCO₂ 40–45 mmHg (5.3–6.0 kPa) during CMV and < 60 mmHg (8.0 kPa) during AAV]. Horses were weaned from ventilation using a predefined protocol and transferred to a stall for unassisted recovery. Arterial blood samples were collected and analyzed at predefined time points. Tracheal oxygen insufflation at 15 L minute^–1 was provided if PaO₂ < 60 mmHg (8.0 kPa) on any analysis. Time to oxygen insufflation, first movement, sternal recumbency and standing were recorded. Data were analyzed using repeated measures anova, paired t tests and Fisher’s exact test with significance defined as p < 0.05.ResultsData from 10 horses were analyzed. Between modes, PaO₂ was significantly higher immediately after weaning from ventilation and lower at sternal recumbency for AAV than for CMV. No PaCO₂ differences were noted between ventilation modes. All horses ventilated with CMV required supplemental oxygen, whereas three horses ventilated with AAV did not. Time to first movement was shorter with AAV. Time to oxygen insufflation was not different between ventilation modes.ConclusionsAlthough horses ventilated with AAV entered the recovery period with higher PaO₂, this advantage was not sustained during recovery. Whereas fewer horses required supplemental oxygen after AAV, the use of AAV does not preclude the need for routine supplemental oxygen administration in horses recovering from general anesthesia.     

Antinociceptive effects of levomethadone in standing horses sedated with romifidine

ObjectiveTo evaluate the antinociceptive effect of a bolus of intravenous levomethadone administered to horses during romifidine constant rate infusion (CRI).Study designProspective, randomized, masked, crossover experimental study.AnimalsA group of eight adult Warmblood horses (seven geldings, one mare) aged 6.6 ± 4.4 years, weighing 548 ± 52 kg [mean ± standard deviation (SD)].MethodsLevomethadone 0.1 mg kg^–1 or an equivalent volume of saline (control) was administered intravenously to standing horses 60 minutes after starting a romifidine CRI. Blood samples to quantify romifidine and levomethadone plasma concentrations by capillary electrophoresis were collected up to 150 minutes after levomethadone administration. The nociceptive withdrawal reflex threshold (NWRT) was determined continuously using an automated threshold tracking device. Sedation and cardiopulmonary variables were assessed at regular intervals. A pharmacokinetic-pharmacodynamic (PK-PD) model was elaborated. Data are presented as mean ± SD or median (interquartile range, 25%–75%) where appropriate. Differences between groups were considered statistically significant for p < 0.05.ResultsHorses exhibited higher NWRTs after levomethadone administration than after saline (123 ± 9% versus 101 ± 9% relative to baseline, p < 0.05). The PK-PD model identified a contribution of levomethadone to the NWRT increase. Effect size was variable among individuals. No adverse reactions to levomethadone administration were observed. A slight effect of levomethadone on sedation scores was evident for the 60 minutes following its administration.Conclusions and Clinical RelevanceA single injection of levomethadone has the potential to increase the NWRT during romifidine CRI in horses and can be administered in combination with α₂-adrencoceptor agonists to enhance antinociception in horses. However, individual variation is marked.     

Hyoscine‐N‐butylbromide premedication on cardiovascular variables of horses sedated with medetomidine

ObjectiveTo evaluate the effects of intravenous (IV) or intramuscular (IM) hyoscine premedication on physiologic variables following IV administration of medetomidine in horses.Study designRandomized, crossover experimental study.AnimalsEight healthy crossbred horses weighing 330 ± 39 kg and aged 7 ± 4 years.MethodsBaseline measurements of heart rate (HR), cardiac index (CI), respiratory rate, systemic vascular resistance (SVR), percentage of patients with second degree atrioventricular (2^oAV) block, mean arterial pressure (MAP), pH, and arterial partial pressures of carbon dioxide (PaCO₂) and oxygen (PaO₂) were obtained 5 minutes before administration of IV hyoscine (0.14 mg kg^?1; group HIV), IM hyoscine (0.3 mg kg^?1; group HIM), or an equal volume of physiologic saline IV (group C). Five minutes later, medetomidine (7.5 μg kg^?1) was administered IV and measurements were recorded at various time points for 130 minutes.ResultsMedetomidine induced bradycardia, 2^oAV blocks and increased SVR immediately after administration, without significant changes in CI or MAP in C. Hyoscine administration induced tachycardia and hypertension, and decreased the percentage of 2^oAV blocks induced by medetomidine. Peak HR and MAP were higher in HIV than HIM at 88 ± 18 beats minute^?1 and 241 ± 37 mmHg versus 65 ± 16 beats minute^?1 and 192 ± 38 mmHg, respectively. CI was increased significantly in HIV (p ≤ 0.05). Respiratory rate decreased significantly in all groups during the recording period. pH, PaCO₂ and PaO₂ were not significantly changed by administration of medetomidine with or without hyoscine.Conclusion and clinical relevanceHyoscine administered IV or IM before medetomidine in horses resulted in tachycardia and hypertension under the conditions of this study. The significance of these changes, and responses to other dose rates, requires further investigation.     

Efficacy of a portable oxygen concentrator with pulsed delivery for treatment of hypoxemia during equine field anesthesia

ObjectiveHypoxemia is common during equine field anesthesia. Our hypothesis was that oxygen therapy from a portable oxygen concentrator would increase PaO₂ during field anesthesia compared with the breathing of ambient air.Study designProspective clinical study.AnimalsFifteen yearling (250 – 400 kg) horses during field castration.MethodsHorses were maintained in dorsal recumbency during anesthesia with an intravenous infusion of 2000 mg ketamine and 500 mg xylazine in 1 L of 5% guaifenesin. Arterial samples for blood gas analysis were collected immediately post-induction (PI), and at 15 and 30 minutes PI. The control group (n = 6) breathed ambient air. The treatment group (n = 9) were administered pulsed-flow oxygen (192 mL per bolus) by nasal insufflation during inspiration for 15 minutes PI, then breathed ambient air. The study was performed at 1300 m above sea level. One-way and two-way repeated-measures anova with post-hoc Bonferroni tests were used for within and between-group comparisons, respectively. Significance was set at p ≤ 0.05.ResultsMean ± SD PaO₂ in controls at 0, 15 and 30 minutes PI were 46 ± 7 mmHg (6.1 ± 0.9 kPa), 42 ± 9 mmHg (5.6 ± 1.1 kPa), and 48 ± 7 mmHg (6.4 ± 0.1 kPa), respectively (p = 0.4). In treatment animals, oxygen administration significantly increased PaO₂ at 15 minutes PI to 60 ± 13 mmHg (8.0 ± 1.7 kPa), compared with baseline values of 46 ± 8 mmHg (6.1 ± 1 kPa) (p = 0.007), and 30 minute PI values of 48 ± 7 mmHg (6.5 ± 0.9 kPa) (p = 0.003).ConclusionsThese data show that a pulsed-flow delivery of oxygen can increase PaO₂ in dorsally recumbent horses during field anesthesia with ketamine-xylazine-guaifenesin.Clinical relevanceThe portable oxygen concentrator may help combat hypoxemia during field anesthesia in horses.     

Comparison of sedative and some cardiopulmonary effects of intramuscular medetomidine or medetomidine–tramadol in dogs

ObjectiveTo evaluate the clinical and physiologic effects of intramuscular (IM) administration of medetomidine with and without tramadol in dogs.Study designProspective experimental study.AnimalsA group of eight mixed breed dogs of both sexes, aged 1–2 years, weighing 16.0 ± 0.6 kg.MethodsEach dog was studied twice at ≥1 week interval. Medetomidine (5 μg kg^–1; treatment M) was administered IM alone or with tramadol (4 mg kg^–1; treatment MT). Sedation was scored by a system that included vocalization, posture, appearance, interactive behaviors, resistance to restraint and response to noise. Times from drug administration to ataxia, impaired walking, head drop, sternal and lateral position and standing were recorded. Sedation score, heart rate, respiratory rate, rectal temperature, end-tidal carbon dioxide (Pe′CO₂), hemoglobin oxygen saturation and mean noninvasive blood pressure were recorded and compared 15 minutes before and 15, 30 and 45 minutes after drug administration.ResultsDogs administered MT had higher sedation scores than dogs administered M at 30 and 45 minutes after drug administration (p < 0.05). Times to ataxia, impaired walking, head drop and sternal recumbency were not different between the treatments. Time to lateral recumbency was longer in M than in MT (21.1 ± 1.0 versus 17.6 ± 0.7 minutes, respectively; p < 0.05). Time to standing was longer in MT than in M (67.9 ± 1.4 versus 54.5 ± 1.9 minutes, respectively; p < 0.001). Measured physiological variables did not differ between the treatments, with the exception of Pe′CO₂, which was higher in MT than in M at all post-treatment evaluation times (p < 0.001).Conclusions and clinical relevanceTramadol combined with medetomidine resulted in greater sedation scores (deeper sedation) than medetomidine alone in dogs, and minimal adverse changes in the physiologic variables were measured.