Effects of head-down positioning on regional central nervous system perfusion in isoflurane-anesthetized horses

OBJECTIVE: To test the hypothesis that head-down positioning in anesthetized horses increases intracranial pressure (ICP) and decreases cerebral and spinal cord blood flows. ANIMALS: 6 adult horses. PROCEDURES: For each horse, anesthesia was induced with ketamine hydrochloride and xylazine hydrochloride and maintained with 1.57% isoflurane in oxygen. Once in right lateral recumbency, horses were ventilated to maintain normocapnia. An ICP transducer was placed in the subarachnoid space, and catheters were placed in the left cardiac ventricle and in multiple vessels. Blood flow measurements were made by use of a fluorescent microsphere technique while each horse was in horizontal and head-down positions. Inferential statistical analyses were performed via repeated-measures ANOVA and Dunn-Sidak comparisons. RESULTS: Because 1 horse developed extreme hypotension, data from 5 horses were analyzed. During head-down positioning, mean +/- SEM ICP increased to 55+/-2 mm Hg, compared with 31+/-2 mm Hg during horizontal positioning; cerebral perfusion pressure was unchanged. Compared with findings during horizontal positioning, blood flow to the cerebrum, cerebellum, and cranial portion of the brainstem decreased significantly by approximately 20% during head-down positioning; blood flows within the pons and medulla were mildly but not significantly decreased. Spinal cord blood flow was low (9 mL/min/100 g of tissue) and unaffected by position. CONCLUSIONS AND CLINICAL RELEVANCE: Head-down positioning increased heart-brain hydrostatic gradients in isoflurane-anesthetized horses, thereby decreasing cerebral blood flow and, to a greater extent, increasing ICP. During anesthesia, CNS regions with low blood flows in horses may be predisposed to ischemic injury induced by high ICP.     

Hemodynamic and respiratory responses to variable arterial partial pressure of oxygen in halothane-anesthetized horses during spontaneous and controlled ventilation.

Cardiovascular and respiratory responses to variable PaO2 were measured in 6 horses anesthetized only with halothane during spontaneous (SV) and controlled (CV) ventilation. The minimal alveolar concentration (MAC) for halothane in oxygen was determined in each spontaneously breathing horse prior to establishing PaO2 study conditions--mean +/- SEM, 0.95 +/- 0.03 vol%. The PaO2 conditions of > 250, 120, 80, and 50 mm of Hg were studied in each horse anesthetized at 1.2 MAC of halothane and positioned in left lateral recumbency. In response to a decrease in PaO2, total peripheral resistance and systolic and diastolic arterial blood pressure decreased (P < 0.05) during SV. Cardiac output tended to increase because heart rate increased (P < 0.05) during these same conditions. During CV, cardiovascular function was usually less than it was at comparable PaO2 during SV (P < 0.05). Heart rate, cardiac output, and left ventricular work increased (P < 0.05) in response to a decrease in PaO2, whereas total peripheral resistance decreased (P < 0.05). During SV, cardiac output and stroke volume increased and arterial blood pressure and total peripheral resistance decreased with duration of anesthesia at PaO2 > 250 mm of Hg. During SV, minute expired volume increased (P < 0.05) because respiratory frequency tended to increase as PaO2 decreased. Decrease in PaCO2 (P < 0.05) also accompanied these respiratory changes. Although oxygen utilization was nearly constant over all treatment periods, oxygen delivery decreased (P < 0.05) with decrease in PaO2, and was less (P < 0.05) during CV, compared with SV, for comparable PaO2 values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of ventilation and isoflurane end-tidal concentration on intracranial and cerebral perfusion pressures in horses

OBJECTIVE: To measure the effects of isoflurane end-tidal concentration and mode of ventilation (spontaneous vs controlled) on intracranial pressure (ICP) and cerebral perfusion pressure (CPP) in horses. ANIMAL: adult horses of various breeds. PROCEDURES: Anesthesia was induced and maintained with isoflurane in O2 in 6 healthy, unmedicated, adult horses. Using a subarachnoid strain gauge transducer, ICP was measured. Blood gas tensions and carotid artery pressures also were measured. Four isoflurane doses were studied, corresponding to the following multiples of the minimum alveolar concentration (MAC): 1.0 MAC, 1.2 MAC, 1.4 MAC, and 1.6 MAC. Data were collected during controlled ventilation and spontaneous ventilation at each dose. RESULTS: increasing isoflurane end-tidal concentration induced significant dose-dependent decreases in mean arterial pressure (MAP) and CPP but no change in ICR Hypercapnic spontaneous ventilation caused significant increases in MAP and ICR compared with normocapnic controlled ventilation; no change in CPP was observed. CONCLUSIONS AND CLINICAL RELEVANCE: Hypercapnia likely increases cerebral blood flow (CBF) by maintaining CPP in the face of presumed cerebral vasodilation in healthy anesthetized horses. The effect of isoflurane dose on CBF however, remains unresolved because it depends on the opposing influences of a decrease in CCP and cerebral vasodilation.     

The effect of changing the mode of ventilation on the arterial-to-end-tidal CO2 difference and physiological dead space in laterally and dorsally recumbent horses during halothane anesthesia

OBJECTIVE: To evaluate the effect of changing the mode of ventilation from spontaneous to controlled on the arterial-to-end-tidal CO2 difference [P(a-ET)CO2] and physiological dead space (VD(phys)/VT) in laterally and dorsally recumbent halothane-anesthetized horses. STUDY DESIGN; Prospective, experimental, nonrandomized trial. ANIMALS: Seven mixed breed adult horses (1 male and 6 female) weighing 320 +/- 11 kg. METHODS: Horses were anesthetized in 2 positions-right lateral and dorsal recumbency-with a minimum interval of 1 month. Anesthesia was maintained with halothane in oxygen for 180 minutes. Spontaneous ventilation (SV) was used for 90 minutes followed by 90 minutes of controlled ventilation (CV). The same ventilator settings were used for both laterally and dorsally recumbent horses. Arterial blood gas analysis was performed every 30 minutes during anesthesia. End-tidal CO2 (PETCO2) was measured continuously. P(a-ET)CO2 and VD(phys)NT were calculated. Statistical analysis included analysis of variance for repeated measures over time, followed by Student-Newman-Keuls test. Comparison between groups was performed using a paired t test; P < .05 was considered significant. RESULTS: P(a-ET)CO2 and VD(phys)/VT increased during SV, whereas CV reduced these variables. The variables did not change significantly throughout mechanical ventilation in either group. Dorsally recumbent horses showed greater P(a-ET)CO2 and VD(phys)/VT values throughout. PaCO2 was greater during CV in dorsally positioned horses. CONCLUSIONS AND CLINICAL RELEVANCE: Changing the mode of ventilation from spontaneous to controlled was effective in reducing P(a-ET)CO2 and physiological dead space in both laterally and dorsally recumbent halothane-anesthetized horses. Dorsal recumbency resulted in greater impairment of effective ventilation. Capnometry has a limited value for accurate estimation of PaCO2 in anesthetized horses, although it may be used to evaluate pulmonary function when paired with arterial blood gas analysis.     

Use of a laryngeal mask airway in rabbits during isoflurane anesthesia

The purpose of this study was to find out if an LMA (#1 LMA‐Classic) would provide a better airway than a face mask in spontaneously breathing anesthetized rabbits, and to test if it could be used for mechanically controlled ventilation. Sixteen rabbits (4.1 ± 0.8 kg, mean ± SD) were assigned randomly to three treatment groups; face mask with spontaneous ventilation (FM‐SV; n = 5), LMA with spontaneous ventilation (LMA‐SV; n = 5), and LMA with controlled ventilation (LMA‐CV; n = 6). Rabbits were anesthetized in dorsal recumbency using a circle circuit at constant ET isoflurane (2.3%, Datex airway gas monitor) and constant rectal temperature (38.85 °C) for 2 hours. PaCO₂, PaO₂, minute volume, tidal volume (Wright's respirometer), and Pe CO₂ were measured at 15 minute intervals. Two individuals in the FM‐SV group had PaCO₂ >100 mm Hg (>13.3 kPa). One rabbit in the FM‐SV had PaO₂ <80 mm Hg (<10.7 kPa). All FM‐SV rabbits showed signs of airway obstruction and two were withdrawn from the study at 45 and 90 minutes, respectively, because of cyanosis. Tidal volume could not be measured in the FM‐SV group. No signs of airway obstructions were observed in either of the LMA groups. Four rabbits in the LMA‐CV group developed gastric tympany, and one of these refluxed after 110 minutes. The significance of differences between the two spontaneously breathing groups and between the two LMA groups were measured using Wilcoxon's rank sum test (with significance assumed at p < 0.05). There were no statistical differences between FM‐SV and LMA‐SV in any variable tested. PaCO₂ and Pe ′CO₂ were less in the LMA‐CV group than in the LMA‐SV group, while PaO₂, tidal volume, and minute volume were all more. We conclude that biologically, the LMA provides a better airway than the face mask during spontaneous breathing and that it can be used for IPPV, but that gastric tympany is likely to occur during IPPV.     

Effects of duration of isoflurane anesthesia and mode of ventilation on intracranial and cerebral perfusion pressures in horses

OBJECTIVE: To test the hypothesis that isoflurane-anesthetized horses during controlled ventilation and spontaneous ventilation exhibit temporal changes in cerebral hemodynamics, as measured by intracranial pressure and cerebral perfusion pressure, that reflect temporal changes in systemic arterial pressure. ANIMALS: 6 healthy adult horses. PROCEDURE: Horses were anesthetized in left lateral recumbency with 1.57% isoflurane in O2 for 5 hours in 2 experiments by use of either controlled ventilation (with normocapnia) or spontaneous ventilation (with hypercapnia) in a randomized crossover design. Intracranial pressure was measured with a subarachnoid strain-gauge transducer. Carotid artery pressure, central venous pressure, airway pressures, blood gases, and minute ventilation also were measured. RESULTS: Intracranial pressure during controlled ventilation significantly increased during constant dose isoflurane anesthesia and thus contributed to decreasing cerebral perfusion pressure. Intracranial pressure was initially higher during spontaneous ventilation than during controlled ventilation, but this difference disappeared over time; no significant differences in cerebral perfusion pressures were observed between horses that had spontaneous or controlled ventilation. CONCLUSIONS AND CLINICAL RELEVANCE: Cerebral hemodynamics and their association with ventilation mode are altered over time in isoflurane-anesthetized horses and could contribute to decreased cerebral perfusion during prolonged anesthesia.     

Direct measurement of intracranial pressure in adult horses

OBJECTIVE: To develop a method for surgical placement of a commercial microsensor intracranial pressure (ICP) transducer and to characterize normal ICP and cerebral perfusion pressures (CPP) in conscious adult horses. ANIMALS: 6 healthy castrated male adult horses (1 Holsteiner, 1 Quarter Horse, and 4 Thoroughbreds). PROCEDURE: Anesthesia was induced and maintained by use of isoflurane as the sole agent. Catheters were inserted percutaneously into the jugular vein and carotid artery. A microsensor ICP transducer was inserted in the subarachnoid space by means of right parietal craniotomy. The burr hole was then sealed with bone wax, the surgical incision was sutured, and the transducer was secured in place. Measurements were collected 1 hour after horses were able to stand during recovery from anesthesia. RESULTS: Mean +/- SD values for ICP and CPP were 2 +/- 4 and 102 +/- 26 mm Hg, respectively. CONCLUSIONS AND CLINICAL RELEVANCE: This report describes a relatively facile technique for obtaining direct and accurate ICP measurements for adult horses. The ICP values obtained in this study are within reference ranges established for other species and provide a point of reference for the diagnosis of abnormal ICP in adult horses.     

Temporal effects of halothane and isoflurane in laterally recumbent ventilated male horses

Cardiopulmonary function was monitored in 6 non-medicated, healthy male horses, anesthetized with halothane or isoflurane in O2 at a constant dose (1.2 times the minimum alveolar concentration). Horses were exposed once to each anesthetic agent, and a minimum of 2 weeks separated anesthetic exposures. All horses were studied in left lateral recumbency, and ventilation was mechanically controlled to induce a PaCO2 of 35 to 45 mm of Hg and an inspiratory peak airway pressure of 18 to 22 cm of H2O. After 1 hour of horse preparation, constant conditions were begun. With duration of anesthesia, cardiac output increased (P less than 0.05) with both anesthetic agents, because of an increase in stroke volume (P less than 0.05). Heart rate did not change from initial values with either agent. Mean arterial blood pressure also increased (P less than 0.05) with both agents. With both anesthetics, respiratory rate (P less than 0.05) was increased progressively to maintain acceptable PaCO2 values. Arterial O2 tension did not change with time.     

Anesthetic potency and cardiopulmonary effects of sevoflurane in goats: comparison with isoflurane and halothane.

The anesthetic potency and cardiopulmonary effects of sevoflurane were compared with those of isoflurane and halothane in goats. The (mean +/- SD) minimal alveolar concentration (MAC) was 0.96 +/- 0.12% for halothane, 1.29 +/- 0.11% for isoflurane, and 2.33 +/- 0.15% for sevoflurane. Cardiopulmonary effects of sevoflurane, halothane and isoflurane were examined at end-tidal concentrations equivalent to 1, 1.5 and 2 MAC during either spontaneous or controlled ventilation (SV or CV). During SV, there were no significant differences in respiration rate, tidal volume and minute ventilation between anesthetics. Dose-dependent decreases in both tidal volume and minute ventilation induced by halothane were greater than those by either sevoflurane or isoflurane. Hypercapnia and acidosis induced by sevoflurane were not significantly different from those by either isoflurane or halothane at 1 and 1.5 MAC, but were less than those by halothane at 2 MAC. There was no significant difference in heart rate between anesthetics during SV and CV. During SV, all anesthetics induced dose-dependent decreases in arterial pressure, rate pressure product, systemic vascular resistance, left ventricular minute work index and left ventricular stroke work index. Systemic vascular resistance with isoflurane at 2 MAC was lower than that with sevoflurane. During CV, sevoflurane induced dose-dependent circulatory depression (decreases in arterial pressure, cardiac index, rate pressure product, systemic vascular resistance, left ventricular minute work index and right ventricular minute work index), similar to isoflurane. Halothane did not significantly alter systemic vascular resistance from 1 to 2 MAC.     

Effect of medetomidine on respiration and minimum alveolar concentration in halothane- and isoflurane-anesthetized dogs

OBJECTIVE: To evaluate the effect of medetomidine on minimum alveolar concentration (MAC), respiratory rate, tidal volume, minute volume (V(M)), and maximum inspiratory occlusion pressure (IOCP(max)) in halothane- and isoflurane-anesthetized dogs. ANIMALS: 6 healthy adult dogs (3 males and 3 females). PROCEDURE: The MAC of both inhalants was determined before and 5, 30, and 60 minutes after administration of medetomidine (5 microg/kg, IV). Dogs were subsequently anesthetized by administration of halothane or isoflurane and administered saline (0.9% NaCl) solution IV or medetomidine (5 microg/kg, IV). Respiratory variables and IOCP(max) were measured at specific MAC values 15 minutes before and 5, 30, and 60 minutes after IV administration of medetomidine while dogs breathed 0% and 10% fractional inspired carbon dioxide (FICO2). Slopes of the lines for VM/FICO2 and IOCP(max)/FICO2 were then calculated. RESULTS: Administration of medetomidine decreased MAC of both inhalants. Slope of V(M)/FICO2 increased in dogs anesthetized with halothane after administration of medetomidine, compared with corresponding values in dogs anesthetized with isoflurane. Administration of medetomidine with a simultaneous decrease in inhalant concentration significantly increased the slope for V(M)/FICO2, compared with values after administration of saline solution in dogs anesthetized with halothane but not isoflurane. Values for IOCP(max) did not differ significantly between groups. CONCLUSIONS AND CLINICAL RELEVANCE: Equipotent doses of halothane and isoflurane have differing effects on respiration that are most likely attributable to differences in drug effects on central respiratory centers. Relatively low doses of medetomidine decrease the MAC of halothane and isoflurane in dogs.     

Hemodynamic effects of ionized calcium in horses anesthetized with halothane or isoflurane

OBJECTIVES: To evaluate the effects of halothane and isoflurane on cardiovascular function and serum total and ionized calcium concentrations in horses, and to determine whether administration of calcium gluconate would attenuate these effects. ANIMALS: 6 clinically normal adult Thoroughbreds. PROCEDURE: Catheters were inserted for measurement of arterial blood pressures, pulmonary arterial blood pressures, right ventricular pressure (for determination of myocardial contractility), right atrial pressure, and cardiac output and for collection of arterial blood samples. Anesthesia was then induced with xylazine hydrochloride and ketamine hydrochloride and maintained with halothane or isoflurane. An i.v. infusion of calcium gluconate was begun 75 minutes after anesthetic induction; dosage of calcium gluconate was 0.1 mg/kg of body weight/min for the first 15 minutes, 0.2 mg/kg/min for the next 15 minutes, and 0.4 mg/kg/min for an additional 15 minutes. Data were collected before, during, and after administration of calcium gluconate. RESULTS: Halothane and isoflurane decreased myocardial contractility, cardiac index, and mean arterial pressure, but halothane caused greater depression than isoflurane. Calcium gluconate attenuated the anesthetic-induced depression in cardiac index, stroke index, and maximal rate of increase in right ventricular pressure when horses were anesthetized with isoflurane. When horses were anesthetized with halothane, a higher dosage of calcium gluconate was required to attenuate the depression in stroke index and maximal rate of increase in right ventricular pressure; cardiac index was not changed with calcium administration. CONCLUSIONS AND CLINICAL RELEVANCE: I.v. administration of calcium gluconate may support myocardial function in horses anesthetized with isoflurane.     

Circulatory, respiratory and behavioral responses in isoflurane anesthetized llamas

Objectives To evaluate the circulatory, respiratory and behavioral effects of isoflurane (ISO) anesthesia in llamas during mechanical ventilation and spontaneous breathing. Design Prospective randomised study. Animals Six adult, neutered male llamas (10 ± 1 years [mean ± SD], 179 ± 32 kg). Materials and methods Animals in which the minimum alveolar concentration (MAC) had been previously determined were anesthetized with ISO in oxygen. Inspired and end‐tidal (ET) ISO were sampled continuously. Arterial blood pH, respiratory and circulatory variables, and clinical signs of anesthesia were recorded at three doses (1.0, 1.5 and 2.0 times the individual animal's MAC; mean MAC value 1.13%) of ISO during spontaneous and controlled ventilation. A series of Latin squares was used to determine order of dose. Controlled ventilation (CV) (target PaCO₂ 38 ± 5 mm Hg [5.0 ± 0.6 kPa]) preceded spontaneous ventilation (SV) at each dose. Animals breathed spontaneously for approximately 10 minutes prior to data collection. Body temperature was maintained at 37 ± 0.6 °C. Circulatory and respiratory data were analysed with a mixed model, least squares analysis of variance, for repeated measures taken at equally spaced intervals. p < 0.05. Results Dose and mode of ventilation had significant influences on measured variables. For example, heart rate increased as dose increased; 67 ± 14 beats minute^?1 at 1.0 MAC‐CV versus 77 ± 6 beats minute^?1 at 2 MAC‐CV. Conversely, mean arterial pressure decreased with increasing dose; 82 ± 13 mm Hg at MAC‐CV versus 52 ± 15 mm Hg at 2 MAC‐CV. Arterial CO₂ increased with increasing dose during SV; 45 ± 5 mm Hg [6 ± 0.6 kPa] at MAC versus 53 ± 4 mmHg [7 ± 0.5 kPa] at 2 MAC. Reflex activity (e.g. palpebral reflex) and muscle tone (e.g. jaw tone) decreased while eyelid aperture increased with increasing anesthetic dose. Conclusions and Clinical Relevance The influence of ISO dose and mode of ventilation on circulatory and respiratory variables in llamas is qualitatively similar to that reported in other species. Changes in reflex activity and muscle tone may be used to guide appropriate anesthetic delivery in ISO‐induced llamas.     

Recovery From Sevoflurane Anesthesia in Horses: Comparison to Isoflurane and Effect of Postmedication With Xylazine

Objective—To compare recovery from sevoflurane or isoflurane anesthesia in horses. Study Design—Prospective, randomized cross-over design. Animals—Nine Arabian horses (3 mares, 3 geldings, and 3 stallions) weighing 318 to 409 kg, 4 to 20 years old. Methods—Horses were anesthetized on three occasions with xylazine (1.1 mg/kg), Diazepam (0.03 mg/kg intravenously [IV]), and ketamine (2.2 mg/kg IV). After intubation, they were maintained with isoflurane or sevoflurane for 90 minutes. On a third occasion, horses were maintained with sevoflurane and given xylazine (0.1 mg/kg IV) when the vaporizer was turned off. Horses were not assisted in recovery and all recoveries were videotaped. Time to extubation, first movement, sternal, and standing were recorded as was the number of attempts required to stand. Recoveries were scored on a 1 to 6 scoring system (1 = best, 6 = worst) by the investigators, and by three evaluators who were blinded to the treatments the horses received. These blinded evaluators assessed the degree of ataxia present at 10 minutes after each horse stood, and recorded the time at which they judged the horse to be ready to leave the recovery stall. Results—Mean times (± SD) to extubation, first movement, sternal, and standing were 4.1 (1.7), 6.7 (1.9), 12.6 (4.6), and 17.4 (7.2) minutes with isoflurane; 3.4 (0.8), 6.6 (3.1), 10.3 (3.1), and 13.9 (3.0) minutes with sevoflurane; and 4.0 (1.2), 9.1 (3.3), 13.8 (6.5), and 18.0 (7.1) with sevoflurane followed by xylazine. Horses required a mean number of 4 (2.3), 2 (0.9), and 2 (1.6) attempts to stand with isoflurane, sevoflurane, and sevoflurane followed by xylazine respectively. The mean recovery score (SD) for isoflurane was 2.9 (1.2) from investigators and 2.4 (1.1) from blinded evaluators. For sevoflurane, the mean recovery score was 1.7 (0.9) from investigators and 1.9 (1.1) from evaluators, whereas the recoveries from sevoflurane with xylazine treatment were scored as 1.7 (1.2) from investigators and 1.7 (1.0) from blinded evaluators. Conclusions—Recoveries appeared to vary widely from horse to horse, but were significantly shorter with sevoflurane than isoflurane, although sevoflurane followed by xylazine was no different from isoflurane. Under the conditions of the study, recoveries from sevoflurane and sevoflurane followed by xylazine were of better quality than those from isoflurane. Clinical Relevance—Sevoflurane anesthesia in horses may contribute to a shorter, safer recovery from anesthesia.     

The cardiopulmonary effects of severe blood loss in anesthetized horses

Objective To characterize the acute cardiopulmonary effects of severe hemorrhage in anesthetized horses. Study design Prospective experimental study. Animals Three geldings and six mares, aged 14.4 ± 2.7 years, weighing 486 ± 41 kg (range: 425–550 kg). Methods Horses were anesthetized using xylazine, guaifenesin, ketamine and halothane or isoflurane. Cardiovascular variables, hematocrit, total solids, capillary refill time (CRT) and color of mucous membranes were measured as blood was collected from the carotid artery into sterile plastic bags. Arterial blood gas analysis was also performed. Results The average amount of blood collected from these horses was (mean ± SD) 53 ± 4.8 mL kg^?1 bodyweight (range: 23–32 kg) over 39 ± 4 minutes. Hematocrit decreased from 38 ± 3 to 32 ± 2% after induction of anesthesia and did not change significantly over the period of blood loss. Total solids decreased significantly after induction of anesthesia, and over the period of blood loss. Systolic, mean, diastolic and pulse pressures decreased as blood was lost. Heart rate did not change significantly. Capillary refill time increased from 1.6 ± 0.4 seconds to 4.8 ± 1.3 seconds as blood loss increased. Mucous membrane color deteriorated progressively. Arterial PO₂ decreased significantly over the period of blood loss. Conclusions Hematocrit and heart rate do not change significantly during acute severe hemorrhage in the anesthetized horse. Arterial blood pressure, pulse pressure and PaO₂ decrease as blood loss increases. Changes in mucous membrane color and CRT also occur as blood loss increases. Clinical relevance During severe hemorrhage in the inhalant‐anesthetized horse, both heart rate and hematocrit remain unchanged. Blood pressure decreases and changes in arterial PO₂ correlate most strongly with volume of blood lost.     

Predictors of arterial oxygen tension in anesthetized horses: 1,610 cases (1992-1994).

OBJECTIVE: To identify factors associated with various arterial partial pressures of oxygen (Pao2) in anesthetized horses. DESIGN: Retrospective study. ANIMALS: 1,450 horses anesthetized a total of 1,610 times with isoflurane or halothane. PROCEDURE: Anesthesia records, particularly results of blood gas analyses, were reviewed, and horses were grouped on the basis of lowest Pao2. RESULTS: For horses with lowest Pao2 < 120 mm Hg, those with low pulse pressure, that underwent anesthesia on an emergency basis, or that were males were more likely to have Pao2 < 80 mm Hg. For horses with lowest Pao2 < 250 mm Hg, those that were positioned in dorsal recumbency, that underwent anesthesia on an emergency basis, or that had a shorter duration of anesthesia were more likely to have lowest Pao2 < 120 mm Hg. For horses with lowest Pao2 < 400 mm Hg, those that were positioned in dorsal recumbency, that underwent anesthesia on an emergency basis, that had a shorter duration of anesthesia, that were older, that were heavier, or that were being ventilated mechanically were more likely to have lowest Pao2 < 250 mm Hg. CONCLUSIONS AND CLINICAL RELEVANCE: Low pulse pressure, emergency case status, dorsal recumbency, and short duration of anesthesia were significantly related with lower Pao2 in anesthetized horses.     

Comparison between facemask and laryngeal mask airway in rabbits during isoflurane anesthesia

OBJECTIVE: To determine whether a laryngeal mask airway (LMA) provides a better airway than a facemask in spontaneously breathing anesthetized rabbits, and to test if it can be used for mechanically controlled ventilation. STUDY DESIGN: Randomized prospective experimental trial. ANIMALS: Sixteen young, healthy, specific pathogen-free Giant Flemish cross Chinchilla rabbits (10 females and 6 males) weighing 4.1 +/- 0.8 kg. METHODS: Rabbits were assigned randomly to one of three treatment groups: facemask with spontaneous ventilation (FM-SV; n = 5), LMA with spontaneous ventilation (LMA-SV; n = 5), and LMA with controlled ventilation (LMA-CV; n = 6). In dorsal recumbency, and at 2.3% end-tidal isoflurane concentration, Fé isoflurane, Fi isoflurane, partial pressure of expired isoflurane (PECO(2)), partial pressure of inspired carbon dioxide (PiCO(2)), heart rate, respiratory rate, minute volume, arterial oxygen tensions (PaO(2)), arterial carbon dioxide tensions (PaCO(2)), arterial pH (pH(a)), arterial standard base excess (SBE(a)) values were measured for 120 minutes. Results Two individuals in the FM-SV group had PaCO(2) > 100 mm Hg. One rabbit in the FM-SV had PaO(2) < 80 mm Hg. All FM-SV rabbits showed signs of airway obstruction, and two were withdrawn from the study at 45 and 90 minutes, respectively, because cyanosis was observed. No signs of airway obstruction were observed in either LMA group. Four rabbits in the LMA-CV group developed gastric tympanism, one of which refluxed gastric contents after 110 minutes. There were no differences between FM-SV and LMA-SV in any variable tested. PaCO(2) and PECO(2) were decreased, while PaO(2) and minute volume were increased in the LMA-CV group compared to the LMA-SV group. CONCLUSIONS: An LMA provided a better airway than a facemask during spontaneous breathing in rabbits, as the use of a facemask was associated with hypercapnia and low partial pressures of oxygen. Although an LMA can be used for intermittent positive pressure ventilation (IPPV), gastric tympanism may develop, especially at a peak inspiratory pressure of 14 cm H(2)O. CLINICAL RELEVANCE: The LMA can be used in rabbits but further work is needed before it is applied routinely.     

A comparison of equine recovery characteristics after isoflurane or isoflurane followed by a xylazine-ketamine infusion.

OBJECTIVE: To determine whether infusion of xylazine (XYL) and ketamine (KET) for 30 minutes after isoflurane administration in horses would result in improved quality of recovery from anesthesia, without detrimental cardiopulmonary changes. STUDY DESIGN: Randomized, blinded experimental trial. ANIMALS: Seven healthy adult horses aged 6.4 +/- 1.9 years and weighing 506 +/- 30 kg. METHODS: Horses were anesthetized twice, at least 1 week apart. On both occasions, anesthesia was induced by the administration of XYL, diazepam, and KET, and maintained with isoflurane for approximately 90 minutes, the last 60 minutes of which were under steady-state conditions (1.2 times the minimum alveolar concentration isoflurane). On one occasion, horses were allowed to recover from isoflurane anesthesia, while on the other, XYL and KET were infused for 30 minutes after termination of isoflurane administration. Heart rate, respiratory rate, arterial blood pressure, pH, and blood-gases were measured and recorded at set intervals during steady-state isoflurane anesthesia and XYL-KET infusion. Recovery events were timed and subjectively scored by one nonblinded and two blinded observers. Data were analyzed using a restricted maximum likelihood-based mixed effect model repeated measures analysis. RESULTS: Infusion of XYL and KET resulted in longer recovery times, but there was no significant improvement in recovery quality score. CONCLUSIONS: Under the conditions of this study, infusion of XYL and KET does not positively influence recovery from isoflurane anesthesia in horses. CLINICAL RELEVANCE: This study does not support the routine use of XYL and KET infusions in horses during the transition from isoflurane anesthesia to recovery.     

Arterial to end-tidal CO2 tension and alveolar dead space in halothane- or isoflurane-anesthetized ponies

The correlation between end-tidal partial pressure of CO2 (PETCO2) and arterial (PaCO2) was determined for spontaneously breathing ponies under halothane or isoflurane anesthesia. The PETCO2 was useful as a trend indicator of PaCO2 during the first 60 minutes of halothane or isoflurane anesthesia when PaCO2 values were less than 60 to 70 mm of Hg. Halothane anesthesia lasting greater than 90 minutes was associated with PaCO2 values in excess of 60 to 70 mm of Hg, a large arterial- to end-tidal PCO2 difference (PaCO2-PETCO2) and a significant increase in alveolar dead space. These effects were not seen during the same period of isoflurane anesthesia. Arterial blood gas analysis is therefore recommended during halothane anesthesia when the PETCO2 is greater than 60 to 70 mm of Hg. A decrease in alveolar capillary perfusion relative to alveolar ventilation is the most likely cause for the increase in alveolar dead space during halothane anesthesia. Based on these findings, isoflurane may be superior to halothane for prolonged anesthesia of spontaneously breathing horses.     

Effects of xylazine hydrochloride during isoflurane-induced anesthesia in horses

OBJECTIVE: To quantitate dose- and time-related anesthetic-sparing effects of xylazine hydrochloride (XYL) during isoflurane-induced anesthesia in horses and to characterize selected physiologic responses of anesthetized horses to administration of XYL. ANIMALS: 6 healthy adult horses. PROCEDURE: Horses were anesthetized 2 times to determine the minimum alveolar concentration (MAC) of isoflurane in O2 and to characterize the anesthetic-sparing effect (MAC reduction) after IV administration of XYL (0.5 and 1 mg/kg of body weight, random order). Selected measures of cardiopulmonary function, blood glucose concentrations, and urinary output also were measured during the anesthetic studies. RESULTS: Isoflurane MAC (mean +/- SEM) was reduced by 24.8 +/- 0.5 and 34.2 +/- 1.9% at 42 +/- 7 and 67 +/- 10 minutes, respectively, after administration of XYL at 0.5 and 1 mg/kg. Amount of MAC reduction by XYL was dose- and time-dependent. Overall, cardiovascular and respiratory values varied little among treatments. Administration of XYL increased blood glucose concentration; the magnitude of change was dose- and time-dependent. Urine volume increased but not significantly. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of XYL reduced the anesthetic requirement for isoflurane in horses. The magnitude of the decrease is dose- and time-dependent. Administration of XYL increases blood glucose concentration in anesthetized horses in a dose-related manner.     

Anesthesia of horses with a combination of detomidine, zolazepam, tiletamine, and isoflurane immediately after strenuous treadmill exercise.

OBJECTIVES: To evaluate effects of strenuous exercise in adult horses immediately before anesthesia and to determine whether prior exercise affects anesthesia induction, recovery, or both. ANIMALS: 6 healthy Thoroughbreds in good condition and trained to run on a treadmill, each horse serving as its own control. PROCEDURE: Horses ran on a treadmill until fatigued, then were sedated immediately with detomidine hydrochloride and anesthetized with a zolazepam hydrochloride-tiletamine combination. Anesthesia was maintained with isoflurane in oxygen for another 90 minutes. Blood samples were taken before, during, and after exercise and during anesthesia. RESULTS: During exercise, changes in heart rate, core body temperature, plasma lactate concentration, arterial pH, and PaCO2 were significant. Plasma ionized calcium concentration was lower after exercise, compared with baseline values, and remained lower at 30 minutes of isoflurane anesthesia. Compared with baseline values, plasma chloride concentration decreased significantly during anesthesia after exercise. Cardiac output during anesthesia was significantly lower than that during preexercise, but significant differences between experimental and control periods were not observed. Arterial blood pressure during anesthesia was significantly lower than that during preexercise and initially was maintained better during isoflurane anesthesia after exercise. Cardiac output and blood pressure values were clinically acceptable throughout anesthesia. CONCLUSION: Administration of detomidine hydrochloride followed by zolazepam hydrochloride-tiletamine appeared to be safe and effective for sedation and anesthesia of horses that had just completed strenuous exercise. CLINICAL RELEVANCE: Anesthetic given in accordance with this protocol can be used to anesthetize horses that are injured during athletic competition to assess injuries, facilitate first aid, and possibly allow salvage of injured horses.