Improvement in arterial oxygen tension with change in posture in anaesthetised horses

Observations were made on horses spontaneously breathing oxygen, with halothane at a constant end tidal concentration. The horses were positioned in dorsal recumbency for the first 45 minutes of each anaesthetic episode during which the arterial oxygen tension (PaO2) was found to peak and then decline. The remaining 60 minutes of each anaesthesia was used to test the effect of various manoeuvres on PaO2. The PaO2 of horses decreased further both when remaining in dorsal recumbency and when repositioned in right or left recumbency. In contrast, placing the horses in sternal recumbency for these remaining 60 minutes caused the PaO2 to rise rapidly providing evidence for redistribution of ventilation. Replacing some inspired oxygen with less absorbable nitrogen did not improve PaO2 in dorsal recumbency. Thus there was no evidence that the low PaO2 of dorsal recumbency was associated with alveoli that had collapsed because of gas absorption.     

The effect of a 50% inspired mixture of nitrous oxide on arterial oxygen tension in spontaneously breathing horses anaesthetised with halothane

The effect of nitrous oxide (N₂O) on arterial partial pressure of oxygen (P_aO₂) was evaluated in 20 adult horses anaesthetised with halothane. A fresh gas flow rate of 20ml/kg/min, comprising a 1:1 N₂O/oxygen (O₂) mixture, was supplied via the rotameter flowmeters of an anaesthetic machine to a large animal breathing system. The horses breathed spontaneously from the circuit immediately after endotracheal intubation. Ten horses were subsequently positioned in lateral recumbency and ten in dorsal recumbency. A further twenty adult horses were anaesthetised with halothane and acted as controls; halothane in 20mls/kg/min of O₂ being supplied to the same breathing system. Fifty percent NO caused significant decreases in P_aO₂ for horses in lateral and dorsal recumbency. However when administered to horses in lateral recumbency it did not promote arterial hypoxaemia. There was a higher risk of intraopera- tive arterial hypoxaemia (P_aO₂ < 8.6kPa) associated with its use in spontaneously breathing horses in dorsal recumbency. Arterial hypoxaemia occurred in all horses during the first fifteen minutes of recovery but when N₂O was discontinued, halothane in oxygen supplied to the breathing circuit for five minutes at a flow rate of 20ml/kg/minute was sufficient to ensure that diffusion hypoxia did not occur. The magnitude of the hypoxaemia was not signficantly different between the groups. The time taken to adopt sternal recumbency was significantly shorter in the horses that had received N₂O.     

Cardiorespiratory effects of a 7° reverse Trendelenburg position in anaesthetized horses: a randomized clinical trial

Objective

To evaluate the cardiorespiratory effects of a 7° reverse Trendelenburg position (RTP) in anaesthetized horses.

Equine post anaesthetic myositis: muscular post ischaemic hyperaemia measured by laser Doppler flowmetry

Measurements of muscular microcirculation in horses anaesthetised with halothane were performed by laser Doppler flowmetry. Variations of microcirculation in the compressed and uncompressed triceps brachii were measured when horses were positioned in dorsal recumbency after a prolonged period in lateral recumbency. A significant post ischaemic hyperaemia was recorded in horses which developed a post anaesthetic myositis.     

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.

The effects of halothane and isoflurane on cardiovascular function in dorsally recumbent horses undergoing surgery

ObjectiveTo determine the haemodynamic effects of halothane and isoflurane with spontaneous and controlled ventilation in dorsally recumbent horses undergoing elective surgery.Study designProspective randomized clinical trial.AnimalsTwenty-five adult horses, body mass 487 kg (range: 267–690).MethodsHorses undergoing elective surgery in dorsal recumbency were randomly assigned to one of four treatment groups, isoflurane (I) or halothane (H) anaesthesia, each with spontaneous (SB) or controlled ventilation (IPPV). Indices of cardiac function and femoral arterial blood flow (ABF) and resistance were measured using transoesophageal and transcutaneous Doppler echocardiography, respectively. Arterial blood pressure was measured directly.ResultsFour horses assigned to receive isoflurane and spontaneous ventilation (SBI) required IPPV, leaving only three groups for analysis: SBH, IPPVH and IPPVI. Two horses were excluded from the halothane groups because dobutamine was infused to maintain arterial blood pressure. Cardiac index (CI) was significantly greater, and pre-ejection period (PEP) shorter, during isoflurane compared with halothane anaesthesia with both spontaneous (p = 0.04, p = 0.0006, respectively) or controlled ventilation (p = 0.04, p = 0.008, respectively). There was an association between CI and PaCO₂ (p = 0.04) such that CI increased by 0.45 L minute⁻¹m⁻² for every kPa increase in PaCO₂. Femoral ABF was only significantly higher during isoflurane compared with halothane anaesthesia during IPPV (p = 0.0006). There was a significant temporal decrease in CI, but not femoral arterial flow.ConclusionThe previously reported superior cardiovascular function during isoflurane compared with halothane anaesthesia was maintained in horses undergoing surgery. However, in these clinical subjects, a progressive decrease in CI, which was independent of ventilatory mode, was observed with both anaesthetic agents.Clinical relevanceCardiovascular function may deteriorate progressively in horses anaesthetized for brief (<2 hours) surgical procedures in dorsal recumbency. Although cardiovascular function is superior with isoflurane in dorsally recumbent horses, the need for IPPV may be greater.     

Atelectasis causes gas exchange impairment in the anaesthetised horse

The anatomical basis of gas exchange impairment in the anaesthetised horse was studied by computerised tomography (CT; three shetland ponies) and morphological analysis (one pony and three horses). By means of CT, densities were seen in dependent lung regions early during anaesthesia, both with spontaneous breathing and with mechanical ventilation. The densities remained for some time where they had initially been created when the animal was turned from dorsal to sternal recumbency. Deep insufflation of the lungs reduced the dense area. Gas exchange was impaired roughly in proportion to the dense area. On histological analysis, the densities were atelectatic and congested with blood. Gravimetry showed no more extravascular water per unit lung tissue in the atelectatic than in the 'normal' regions, and the blood content was increased only slightly. It is concluded that the horse develops atelectasis in dependent lung regions early during anaesthesia in dorsal recumbency, and that atelectasis is the most likely explanation for the large shunt and impaired arterial oxygenation regularly seen during anaesthesia.     

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.     

Anesthetic and cardiovascular effects of balanced anesthesia using constant rate infusion of midazolam-ketamine-medetomidine with inhalation of oxygen-sevoflurane (MKM-OS anesthesia) in horses

The anesthetic sparring and cardiovascular effects produced by midazolam 0.8 mg/ml-ketamine 40 mg/ml-medetomidine 0.05 mg/ml (0.025 ml/kg/hr) drug infusion during sevoflurane in oxygen (MKM-OS) anesthesia was determined in healthy horses. The anesthetic sparring effects of MKM-OS were assessed in 6 healthy thoroughbred horses in which the right carotid artery was surgically relocated to a subcutaneous position. All horses were intubated and ventilated with oxygen using intermittent positive pressure ventilation (IPPV). The end-tidal concentration of sevoflurane (ET(SEV)) required to maintain surgical anesthesia was approximately 1.7%. Heart rate and mean arterial blood pressure averaged 23-41 beats/min and 70-112 mmHg, respectively. All horses stood between 23-44 min after the cessation of all anesthetic drugs. The cardiovascular effects of MKM-OS anesthesia were evaluated in 5 healthy thoroughbred horses ventilated using IPPV. Anesthesia was maintained for 4 hr at an ET(SEV) of 1.7%. Each horse was studied during left lateral (LR) and dorsal recumbency (DR) with a minimum interval between evaluations of 1 month. Cardiac output and cardiac index were maintained between 70-80% of baseline values during LR and 65-70% of baseline values during DR. Stroke volume was maintained between 75-85% of baseline values during LR and 60-70% of baseline values during DR. Systemic vascular resistance was not different from baseline values regardless of position. MKM-OS anesthesia may be useful for prolonged equine surgery because of its minimal cardiovascular depression in both of lateral and dorsal recumbency.     

Hypoxaemia during anaesthesia in seven horses with colic

Anaesthetic records of horses with colic anaesthetised between June 1987 and May 1989 were reviewed. pH and blood gas analyses were performed during 157 operations from which the horses were allowed to recover. A PaO₂ of 8.0 kPa or less was measured during anaesthesia in seven of these horses. The horses were of different breeds, ages and sexes. Anaesthesia was induced with xylazine, guaifenesin and ketamine in four horses and with xylazine, guaifenesin and thiobarbiturate in three horses. Anaesthesia was maintained with inhalation anaesthetic agent and oxygen: isoflurane in five horses, halothane in one horse, and initially halothane but later isoflurane in one horse. Systolic arterial pressures during anaesthesia ranged from 80 to 150 mmHg, diastolic arterial pressures were between 60 and 128 mmHg, and heart rates were between 28 and 44 beats /min. Controlled ventilation was initiated at the start of anaesthesia. PaCO₂ exceeded 6.7 kPa in three horses but was subsequently decreased by adjustment of the ventilator. PaO₂ of 8.0 kPa or less was measured during early anaesthesia, with one exception, and persisted for the duration of anaesthesia. The horses' inspired air was supplemented with oxygen during recovery from anaesthesia, at which time measurement of blood gases in three horses revealed no increase in PaO₂. Recovery from anaesthesia was uneventful. The surgical problems involved primarily the large intestine in five horses and the small intestine in two horses. Six horses were discharged from the hospital alive; one horse was reanaesthetised later the same day and destroyed without regaining consciousness. We concluded that none of the objective values recorded during the pre-anaesthetic evaluation could have been used to predict the complication of intraoperative hypoxaemia. We observed that once hypoxaemia developed it persisted for the duration of anaesthesia and even into the recovery period when the horses were in lateral recumbency and regaining consciousness. We assume that the altered metabolism from anaesthetic agents and hypothermia combined with adequate peripheral perfusion contributed to the lack of adverse consequences in six of the horses. The contribution of hypoxaemia to the deteriorating condition of the seventh horse is speculative.     

The effects of spontaneous and mechanical ventilation on central cardiovascular function and peripheral perfusion during isoflurane anaesthesia in horses

OBJECTIVE: To compare the effects of spontaneous breathing and mechanical ventilation on haemodynamic variables, including muscle and skin perfusion measured with laser Doppler flowmetery, in horses anaesthetized with isoflurane. STUDY DESIGN: Prospective controlled study. ANIMALS: Ten warm-blood trotter horses (five males, five females). Mean mass was 492 kg (range 420-584 kg) and mean age was 5 years (range 4-8 years). MATERIALS AND METHODS: After pre-anaesthetic medication with detomidine (10 microg kg(-1)) anaesthesia was induced with intravenous (IV) guaifenesin and thiopental (4-5 mg kg(-1) IV) and maintained using isoflurane in oxygen. The horses were positioned in dorsal recumbency. In five animals breathing was initially spontaneous (SB) while the lungs of the other five were ventilated mechanically using intermittent positive pressure ventilation (IPPV). Total anaesthesia time was 4 hours with the ventilatory mode changed after 2 hours. During anaesthesia, heart rate (HR) cardiac output (Qt) stroke volume (SV) systemic arterial blood pressures (sAP), and pulmonary arterial pressure (pAP) were recorded. Peripheral perfusion was measured in the semimembranosus and gluteal muscles and on the tail skin using laser Doppler flowmetry. Arterial (a) and mixed venous (v) blood gases, pH, haemoglobin concentration [Hb], haematocrit (Hct), plasma lactate concentration and muscle temperature were measured. Oxygen content, venous admixture (s/Qt) oxygen delivery (DO(2)) and oxygen consumption (VO(2)) were calculated. RESULTS: During mechanical ventilation, HR, sAP, pAP, Qt, SV, Qs/Qt and PaCO(2) were lower and PaO(2) was higher compared with spontaneous breathing. There were no differences between the modes of ventilation in the level of perfusion, DO(2), VO(2), [Hb], (Hct), or plasma lactate concentration. After the change from IPPV to SB, left semimembranosus muscle and skin perfusion improved, while muscle perfusion tended to decrease when SB was changed to IPPV. Low-frequency flow motion was seen twice as frequently during IPPV compared with SB. CONCLUSIONS: Mechanical ventilation impaired cardiovascular function compared with SB in horses during isoflurane anaesthesia. Muscle and skin perfusion changes occurred with ventilation, although further studies are needed to elucidate the underlying mechanisms.     

A study of cardiovascular function under controlled and spontaneous ventilation in isoflurane–medetomidine anaesthetized horses

Objective To determine, in mildly hypercapnic horses under isoflurane–medetomidine balanced anaesthesia, whether there is a difference in cardiovascular function between spontaneous ventilation (SV) and intermittent positive pressure ventilation (IPPV). Study design Prospective randomized clinical study. Animals Sixty horses, undergoing elective surgical procedures under general anaesthesia: ASA classification I or II. Methods Horses were sedated with medetomidine and anaesthesia was induced with ketamine and diazepam. Anaesthesia was maintained with isoflurane and a constant rate infusion of medetomidine. Horses were assigned to either SV or IPPV for the duration of anaesthesia. Horses in group IPPV were maintained mildly hypercapnic (arterial partial pressure of carbon dioxide (PaCO₂) 50–60 mmHg, 6.7–8 kPa). Mean arterial blood pressure (MAP) was maintained above 70 mmHg by an infusion of dobutamine administered to effect. Heart rate (HR), respiratory rate (f_R), arterial blood pressure and inspiratory and expiratory gases were monitored continuously. A bolus of ketamine was administered when horses showed nystagmus. Cardiac output was measured using lithium dilution. Arterial blood‐gas analysis was performed regularly. Recovery time was noted and recovery quality scored. Results There were no differences between groups concerning age, weight, body position during anaesthesia and anaesthetic duration. Respiratory rate was significantly higher in group IPPV. Significantly more horses in group IPPV received supplemental ketamine. There were no other significant differences between groups. All horses recovered from anaesthesia without complications. Conclusions There was no difference in cardiovascular function in horses undergoing elective surgery during isoflurane–medetomidine anaesthesia with SV in comparison with IPPV, provided the horses are maintained slightly hypercapnic. Clinical relevance In horses with health status ASA I and II, cardiovascular function under general anaesthesia is equal with or without IPPV if the PaCO₂ is maintained at 50–60 mmHg.     

Prolonging dissociative anaesthesia in horses with a repeated bolus injection

The effects of prolonging romifidine/ketamine anaesthesia in horses with a second injection of ketamine alone or both romifidine/ketamine compared with only induction injection of romifidine and tiletamine/zolazepam were studied in 6 horses anaesthetised in lateral recumbency on 3 random occasions. All horses were sedated with romifidine 0.1 mg/kg bwt iv and, on 2 occasions, anaesthesia was induced by iv injection of ketamine 2.2 mg/kg bwt. To prolong the ketamine-induced anaesthesia, either ketamine (I.1 mg/kg bwt iv) or ketamine and romifidine (I.1 mg/kg bwt and 0.04 mg/kg bwt iv, respectively) were given 18–20 min after the start of the ketamine injection for induction. On the third occasion, anaesthesia was induced by iv injection of 1.4 mg/kg bwt Zoletil (0.7 mg/kg bwt tiletamhe + 0.7 mg/kg bwt zolazepam). No statistically significant differences in the measured cardiorespiratory function were found between the 3 groups. Heart rate was decreased significantly after sedation but increased during anaesthesia. Arterial blood pressure increased after sedation and remained high during anaesthesia. A significant decrease in arterial oxygen tension was observed in all groups during anaesthesia. The muscle relaxation induced by romifidine was, in most cases, not sufficient to abolish the catalepsy following a repeated injection of ketamine alone. Zoletil or a repeated injection of ketaminehornifidine resulted in smoother anaesthesia. When additional time is required to complete surgery during field anaesthesia, it is advisable to prolong romifidine/ketamine anaesthesia with an injection of both romifidine and ketamine in healthy horses. When a longer procedure is anticipated from the start Zoletil is an alternative for induction of anaesthesia. The mean time to response to noxious stimuli and mean time spent in lateral recumbency was 28 and 38 min for the anaesthesia prolonged with ketamine, 3.5 and 43 rnin for the anaesthesia prolonged with ketaminehornifidine and 33 and 45 min for the anaesthesia with Zoletil. All horses reached a standing position at the first attempt.     

Blood Gas Values During Intermittent Positive Pressure Ventilation and Spontaneous Ventilation in 160 Anesthetized Horses Positioned in Lateral or Dorsal Recumbency

One hundred sixty horses were anesthetized with xylazine, guaifenesin, thiamylal, and halothane for elective soft tissue and orthopedic procedures. Horses were randomly assigned to one of four groups. Group 1 (n = 40): Horses positioned in lateral (LR_G1,; n = 20) or dorsal (DR_G1,; n = 20) recumbency breathed spontaneously throughout anesthesia. Group 2 (n = 40): Intermittent positive pressure ventilation (IPPV) was instituted throughout anesthesia in horses positioned in lateral (LR_G2; n = 20) or dorsal (DR_G2; n = 20) recumbency. Group 3 (n = 40): Horses positioned in lateral (LR_G3; n = 20) or dorsal (DR_G3; n = 20) recumbency breathed spontaneously for the first half of anesthesia and intermittent positive pressure ventilation was instituted for the second half of anesthesia. Group 4 (n = 40): Intermittent positive pressure ventilation was instituted for the first half of anesthesia in horses positioned in lateral (LR_G4; n = 20) or dorsal (DR_G4; n = 20) recumbency. Spontaneous ventilation (SV) occured for the second half of anesthesia. The mean time of anesthesia was not significantly different within or between groups. The mean time of SV and IPPV was not significantly different in groups 3 and 4. Variables analyzed included pH, PaCO₂, PaO₂, and P(A-a)O₂ (calculated). Spontaneous ventilation resulted in significantly higher PaCO₂ and P(A-a)O₂ values and significantly lower PaO₂ values in LR_G1, and DR_G1, horses compared with LR_G2 and DR_G2 horses. Intermittent positive pressure ventilation resulted in normocarbia and significantly lower P(A-a)O₂ values in LR_G2 and DR_G2 horses. In LR_G2 the Pao₂ values significantly increased from 20 minutes after induction to the end of anesthesia. The PaO₂ and P(A-a)O₂ values were not significantly different from the beginning of anesthesia after IPPV in DR_G2 or DR_G3. The PaO₂ values significantly decreased and the P(A-a)O₂ values significantly increased after return to SV in horses in LR_G4, and DR_G4. The PaO₂ values were lowest and the P(A-a)O₂ values were highest in all horses positioned in dorsal recumbency compared with lateral recumbency and in SV horses compared with IPPV horses. The pH changes paralleled the changes in PaCO₂. Blood gas values during right versus left lateral recumbency in all groups were also evaluated. The PaO₂ values were significantly lower and the P(A-a)O₂ values were significantly higher during SV in horses positioned in left lateral (LR_LG1) compared with right lateral (LR_RG1) recumbency. No other significant changes were found comparing left and right lateral recumbency. Arterial hypoxemia (PaO₂ < 60 mm Hg) developed in 35% of DR_G1 horses and 20% of DR_G2 horses at the end of anesthesia. Arterial hypercarbia (PaCO₂= 50–60 mm Hg) developed in DRoi horses. Arterial hypoxemia that developed in 20% of DR_G3 horses was not improved with IPPV. Arterial hypoxemia developed in 55% of DR_G4 horses after return to SV. Some DR_G4 horses with hypoxemia also developed hypercarbia, whereas some had PaCO₂ values within normal limits. Arterial hypoxemia developed in one LR_G1, and two LR_G4, horses. Hypercarbia developed in onlv one LR_G4 horse.     

Ventilation-perfusion relationships in the anaesthetised horse

Ventilation-perfusion relationships were studied by the multiple inert gas elimination technique in seven horses while they were conscious and during inhalation anaesthesia with halothane. A generally good match between ventilation and perfusion was found in the conscious, standing horse. During anaesthesia a huge shunt developed, ie perfusion of completely unventilated lung regions, both in dorsal and left lateral recumbency and whether the horse was breathing spontaneously or mechanically ventilated. The shunt was significantly greater and the arterial oxygen tension (PaO2) significantly lower in dorsal than in left lateral recumbency. Little or no perfusion of low VA/Q regions was observed during anaesthesia, whether ventilation was spontaneous or mechanical. Positive end-expiratory pressure (PEEP) did not significantly improve PaO2 or reduce the shunt. Selective mechanical ventilation of dependent lung regions with PEEP reduced the shunt markedly, an effect that was not achieved by conventional mechanical ventilation with general PEEP. The findings seem compatible with alveolar collapse during anaesthesia, causing shunt, whereas the absence of clearly low VA/Q regions questions the role of airway closure as the major disturbance of gas exchange.     

Stress responses in ponies during halothane or isoflurane anaesthesia after induction with thiopentone or xylazine/ketamine

Endocrine and metabolic responses to anaesthesia with three different anaesthetic regimes were examined in six ponies. All animals were anaesthetised with each protocol: acepromazine-thiopentone-isoflurane, xylazine-ketamine- halothane and xylazine-ketamine-isoflurane. Anaesthesia was maintained for 2 h. Pulse rate, respiratory rate, arterial blood pressure, arterial blood gases and pharyngeal and skin temperature were measured and blood was withdrawn for glucose, lactate, cortisol, insulin, liver and muscle enzymes and total protein assay. Measurements were made before anaesthesia, at 20 min intervals during anaesthesia and at 20 mins and 2, 4, 6 and 24 h after anaesthesia. The effects of anaesthesia were similar in all groups. Arterial blood pressure decreased and oxygen tension and plasma cortisol concentration increased in all groups. Arterial carbon dioxide tension increased and respiratory rate and pH decreased in all ponies anaesthetised with isoflurane. There was a tendency for increased glucose and lactate concentrations and decreased insulin concentration and packed cell volume, particularly in the xylazine-ketamine groups. There was no change in pulse rate except for a transient increase at induction with thiopentone. The results were compared with data reported by Taylor (1989), which were collected from the same animals during acepromazine-thiopentone-halothane anaesthesia, and were found to be similar. It was concluded that these commonly used anaesthetic protocols themselves constitute a considerable insult or stressor in horses. However, the stress response to all the regimes investigated was similar and the precise stimulus to this response has yet to be elucidated.     

High inspired oxygen concentrations increase intrapulmonary shunt in anaesthetized horses

OBJECTIVES: To compare pulmonary function and gas exchange in anaesthetized horses during and after breathing either O2-rich gas mixtures or air. ANIMALS: Six healthy standard bred trotters (age range 3-12 years; mass range 423-520 kg), four geldings and two mares. Study design Randomized, cross-over experimental study. METHODS: Horses were anaesthetized on two occasions with tiletamine-zolazepam after pre-anaesthetic medication with acepromazine, romifidine and butorphanol. After endotracheal intubation and positioning in left lateral recumbency, animals were allowed to breathe spontaneously. One of two, randomly allocated inspired gas treatments was provided: either i) room air (fractional concentration of inspired O2 [FIO2] = 0.21) provided throughout anaesthesia; or ii) an O2-rich gas mixture (FIO2 = >0.95) for 15 minutes, followed by room air. The alternative treatment was delivered at the second anaesthetic. Respiratory and haemodynamic variables and the distribution of ventilation-perfusion (VA/Q) ratios (using the multiple inert gas elimination technique) were determined in the standing conscious horse (baseline) after sedation and during anaesthesia. RESULTS: Breathing O2-rich gas was associated with a decreased respiratory rate (p = 0.015) increased PaCO2 (p < 0.001) and increased PaO2 (p = 0.004) compared with breathing air. All horses developed intrapulmonary shunt during anaesthesia, but shunt was significantly greater (13 +/- 5%) when O2-rich gas was delivered compared with air breathing (5 +/- 2%; p = 0.013). Ten minutes after O2-rich gas was replaced by air, shunt remained larger in horses that had initially received oxygen compared with those breathing air (p = 0.042). Mixed venous oxygen tensions were significantly lower during sedation than at baseline (p < 0.001) and during anaesthesia (p < 0.001). CONCLUSIONS: During dissociative anaesthesia, arterial oxygenation was greater when horses breathed gas containing more than 95% oxygen, compared with when they breathed air. However, breathing O2-rich gas increased intrapulmonary shunt and caused hypoventilation. The intrapulmonary shunt created during anaesthesia by high inspired O2 concentrations remained larger when FIO2 was reduced to 0.21, indicating that absorption atelectasis produced during O2-rich gas breathing persisted throughout anaesthesia. CLINICAL RELEVANCE: In healthy horses undergoing short-term dissociative anaesthesia, air breathing ensures a level of oxygen delivery that meets tissue demand. There is no benefit to horses in breathing O2-rich gas after the gas supply is discontinued. On the contrary, the degree of shunt induced by breathing O2-rich gas persists. The clinical relevance of this during recovery requires investigation.     

Intermittent positive pressure ventilation with constant positive end-expiratory pressure and alveolar recruitment manoeuvre during inhalation anaesthesia in horses undergoing surgery for colic, and its influence on the early recovery period

Objective To compare, ventilation using intermittent positive pressure ventilation (IPPV) with constant positive end‐expiratory pressure (PEEP) and alveolar recruitment manoeuvres (RM) to classical IPPV without PEEP on gas exchange during anaesthesia and early recovery. Study design Prospective randomized study. Animals Twenty‐four warm‐blood horses, weight mean 548 ± SD 49 kg undergoing surgery for colic. Methods Premedication, induction and maintenance (isoflurane in oxygen) were identical in all horses. Group C (n = 12) was ventilated using conventional IPPV, inspiratory pressure (PIP) 35–45 cmH₂O; group RM (n = 12) using similar IPPV with constant PEEP (10 cmH₂O) and intermittent RMs (three consecutive breaths PIP 60, 80 then 60 cmH₂O, held for 10–12 seconds). RMs were applied as required to maintain arterial oxygen tension (PaO₂) at >400 mmHg (53.3 kPa). Physiological parameters were recorded intraoperatively. Arterial blood gases were measured intra‐ and postoperatively. Recovery times and quality of recovery were measured or scored. Results Statistically significant findings were that horses in group RM had an overall higher PaO₂ (432 ± 101 mmHg) than those in group C (187 ± 112 mmHg) at all time points including during the early recovery period. Recovery time to standing position was significantly shorter in group RM (49.6 ± 20.7 minutes) than group C (70.7 ± 24.9). Other measured parameters did not differ significantly. The median (range) of number of RMs required to maintain PaO₂ above 400 mmHg per anaesthetic was 3 (1–8). Conclusion Ventilation using IPPV with constant PEEP and RM improved arterial oxygenation lasting into the early recovery period in conjunction with faster recovery of similar quality. However this ventilation mode was not able to open up the lung completely and to keep it open without repeated recruitment. Clinical relevance This mode of ventilation may provide a clinically practicable method of improving oxygenation in anaesthetized horses.     

Body position and mode of ventilation influences arterial pH, oxygen, and carbon dioxide tensions in halothane-anesthetized horses.

Effects of body position and type of ventilation were determined on arterial blood gases (PaO2, PaCO2) and pH during and immediately following clinical halothane anesthesia in 36 young, physically conditioned horses. Horses in dorsal recumbency had a lower PaO2 than did similarly breathing horses in a lateral position. Predictably controlled positive-pressure ventilation inproved arterial oxygenation and permitted maintenance of a normal PaCO2. Most horses, regardless of type of ventilation and operative body positioning, were hypoxemic in the immediate postanesthetic period.     

Clinical experiences with isoflurane in dogs and horses

The inhalational anaesthetic agent isoflurane was administered to 22 dogs and 21 horses undergoing a variety of surgical procedures. Satisfactory anaesthesia was produced in all the animals. The cardiopulmonary changes were similar to those observed with halothane. Rapid changes in the depth of anaesthesia were achieved and recovery from anaesthesia was rapid in both dogs and horses, which was a reflection of the relative insolubility of isoflurane. Recovery from anaesthesia in the horses was particularly smooth and rapid with the animals spending a greater part of their recumbency in the sternal position, as opposed to lateral recumbency, before standing in a well coordinated manner.