The relationship between respiratory exchange ratio, plasma lactate and muscle lactate concentrations in exercising horses using a valved gas collection system.

A valved gas collection system for horses was validated, then used to examine the relationship between the respiratory exchange ratio (RER), and plasma and muscle lactate in exercising horses. Four healthy Standardbred horses were trained to breathe through the apparatus while exercising on a treadmill. Comparisons of arterial blood gas tensions were made at 3 work levels for each horse, without (control), and with the gas collection system present. At the highest work level, the arterial oxygen tension (PaO2) was significantly lower (P < 0.05), and the arterial carbon dioxide tension (PaCO2) was significantly higher (P < 0.05), than control levels when the apparatus was present; however arterial oxygen content remained unchanged. The horses completed a standardized incremental treadmill test on 4 occasions to determine the repeatability of measurements of oxygen consumption (VO2), carbon dioxide production (VCO2), inspired minute ventilation (VI), respiratory exchange ratio (RER), ventilatory equivalent for oxygen (VI/VO2), tidal volume (VT), and ventilatory frequency (VF). All gas exchange and respiratory measurements showed good reproducibility with the mean coefficient of variation of the 4 horses ranging from 3.8 to 12%. We examined the relationship between 3 indices of energy metabolism in horses performing treadmill exercise: respiratory exchange ratio (RER), central venous plasma and muscle lactate concentrations. A relationship between RER and plasma lactate concentration was established. To compare muscle and plasma lactate concentrations, the horses completed a discontinuous exercise test without the gas collection apparatus present. Significant relationships (P < 0.05), between plasma lactate concentration and RER, and between plasma and muscle lactate concentration, were described for each horse. The valved gas collection system produced a measurable but tolerable degree of interference to respiration, and provided reproducible measurements of gas exchange and ventilatory measurements. It was concluded that measurements of both gas exchange and blood lactate may be used to indicate increased glycolytic activity within exercising skeletal muscle.     

Effect of postural changes on certain circulatory and respiratory values in the horse

Certain circulatory and respiratory values were measured in horses in the standing, lateral, and supine positions. Twelve adult horses were kept in the standing position under the influence of glycerol guiacolate. Alterations in position to lateral and dorsal recumbencies were achieved without any further drug administration. The changes from the standing to the lateral position decreased the arterial oxygen tension, but left the arterial carbon dioxide tension unchanged. There was no statistically significant effect of body position on respiratory flow rates or volumes. The shift from the lateral position to the supine position had no statistically significant effect on any of the measured values. It is assumed that the change from the standing to the lateral position results in an increase in ventilation of the uppermost parts of the lung. The perfusion inequalities are due mainly to the effect of gravity of the pulmonary circulation which may result in a decrease in arterial oxygen tension.     

Surface oximetry of healthy and ischemic equine intestine

Measurements of jejunal, ileal, and large colon (pelvic flexure) surface O2 tension (PSO2) were made in halothane-anesthetized horses with a nonheated miniature oxygen polarographic electrode. Assisted ventilation with 100% O2 was used to maintain PaCO2 tension at 50 +/- 8 mm of Hg while mean arterial blood pressure was maintained greater than or equal to 70 mm of Hg. Mean +/- SD PSO2 for the intestinal segments were: jejunum (horses 1 to 4), 71 +/- 20 mm of Hg; ileum (horses 1 to 4), 61 +/- 8 mm of Hg; and pelvic flexure of the large colon (horses 1 to 10), 55 +/- 13 mm of Hg. The response of the sensor to intestinal ischemia was studied in the large colon of an additional 12 halothane-anesthetized horses, using 4 types of vascular occlusion: venous (4 horses); arterial and venous (4 horses); venous and intramural vascular obstruction (2 horses); and arterial, venous, and intramural obstruction (2 horses). Venous and arterial occlusions were maintained for 30, 60, 90, and 120 minutes, whereas intramural obstruction combined with either type of vascular obstruction was studied for 60 to 120 minutes. After vascular occlusion, PSO2 decreased to 8 +/- 7 mm of Hg for venous obstruction, 4 +/- 3 mm of Hg for arterial and venous obstruction, 6 +/- 0 mm of Hg for intramural and venous obstruction, and 3 +/- 0 mm of Hg after intramural and arterial and venous obstruction. Thirty minutes after release of the clamps, the PSO2 increased to greater than or equal to 50% of the preoccluded large colon value.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Ventilation-perfusion relationships in the standing horse: an inert gas elimination study

The multiple inert gas elimination technique was adapted for use in the conscious standing horse. The modifications included increased infusion rate of the inert gases (30 ml/min), extended infusion time (60 mins) in order to reach steady state, and construction of a nose mask mixing box system for collection of expired gas. Eight adult horses with a mean weight of 454 kg and a mean age of 6.1 years were studied while standing under resting conditions. Ventilation was 65.2 liters/min and cardiac output measured by thermodilution 40.2 liters/min. Systemic and pulmonary artery mean pressures were 114 and 26 mmHg, respectively. Three horses showed a unimodal VA/Q distribution centred upon a mean VA/Q of one. Five horses had a bimodal distribution with an additional high VA/Q mode comprising 2 to 18 per cent of ventilation. The mean logarithmic standard deviation of perfusion was 0.41. There was no perfusion of 'low' VA/Q regions, but minor shunting on an average of 1 per cent was noted. Inert gas dead space (minus apparatus dead space) averaged 38 per cent of total ventilation. Arterial oxygen tension varied from 11 to 14 kPa and the mean arterial CO2 tension was 5.7 kPa. It is concluded that the standing horse in general has a good match between ventilation and perfusion but that some individuals display high VA/Q regions, possibly explained by poor perfusion of upper lung regions.     

Cardiovascular effects of halothane in the horse

Cardiovascular effects of venous alveolar concentrations of halothane in oxygen were studied in 8 young, healthy horses under conditions of constant arterial carbon dioxide tension. The alveolar concentration of halothane was expressed as a multiple of the minimal alveolar concentration (MAC) which was known for each animal. Increasing alveolar halothane concentrations to MAC 2.0 resulted in a progressive and significant (P less than 0.05) decline in systemic arterial pressure and left ventricular work. Cardiac output decreased between MAC 1.0 and MAC 2.0 as a result of a significant (P less than 0.05) decrease in stroke volume. Heart rate, total peripheral resistance, pulmonary artery pressure, hematocrit, plasma protein concentration, arterial oxygen tension, and arterial pH remained constant over the same range of anesthetic dosages. Continuation of anesthesia, spontaneous ventilation, and the accompanying rise in arterial carbon dioxide tension and electrical stimulation of the horse's oral mucous membranes produced varying degrees of stimulation of cardiovascular function at MAC 1.5.     

Response to nasopharyngeal oxygen administration in horses with lung disease

REASONS FOR PERFORMING STUDY: Guidelines for administration of oxygen to standing horses are unavailable because previous investigations of the efficacy of oxygen administration to increase arterial oxygenation in standing horses have produced equivocal results. OBJECTIVE: To determine the effect of nasal oxygen supplementation on inspired and arterial blood gas tensions in control horses and those with moderate to severe recurrent airway obstruction (RAO). METHODS: Normal horses (n = 6) and horses during an attack of RAO induced by stabling (n = 6) were studied. Oxygen was administered through either one or 2 cannulae, passed via the nares into the nasopharynx to the level of the medial canthus of each eye. Intratracheal inspired oxygen and carbon dioxide concentration and arterial blood gas tensions were measured at baseline and during delivery of 5, 10, 15, 20 and 30 l/min oxygen. RESULTS: Nasal cannulae and all but the highest oxygen flow rates were well tolerated. Fractional inspired oxygen concentration (F(I)O2) increased with flow but was significantly lower at all flow rates in horses with RAO compared with controls. Arterial oxygen tension (PaO2) was significantly increased (P < 0.001) by all flow rates, but was always lower in RAO-affected animals. At 30 l/min, PaO2 increased to 319 +/- 31 mmHg in control horses and 264 +/- 69 mmHg in horses with RAO. Additionally, a large arterial to end-tidal gradient for CO2 in RAO-affected horses was observed, indicating increased alveolar deadspace ventilation in these animals. CONCLUSIONS: The use of nasal cannulae to deliver oxygen effectively increases both F(I)O2 and PaO2 in horses with moderate to severe RAO. Oxygen flow rates up to 20 l/min are well tolerated, but flow rates of 30 l/min produce occasional coughing or gagging. POTENTIAL RELEVANCE: Oxygen therapy delivered by means of an intranasal cannula is a highly effective means of increasing arterial oxygen tension in horses with respiratory disease. Generally, flows of 10-20 l/min should be effective. If higher flows (20-30 l/min) are necessary, they should be delivered by means of 2 cannulae.     

Aerosolized salbutamol (albuterol) improves PaO2 in hypoxaemic anaesthetized horses – a prospective clinical trial in 81 horses

Objective To compare the arterial pH and blood gas values, heart rate and mean arterial blood pressure, in hypoxaemic anaesthetized horses, before and after treatment, with a salbutamol (albuterol) aerosol. Animal population Eighty‐one client‐owned horses weighing between 114 and 925 kg. Fifty‐seven underwent emergency abdominal surgery and 24 were anaesthetized for elective procedures. Materials and methods Pre‐anaesthetic medication included xylazine, detomidine, butorphanol and morphine, alone or in various combinations. Induction of anaesthesia was achieved with guaifenesin and ketamine, diazepam and ketamine, or guaifenesin and thiopental. The trachea of all animals was intubated and anaesthesia maintained with either halothane (33 horses) or isoflurane (48 horses) in oxygen. Heart rate and rhythm were monitored continuously. Arterial blood pressure was monitored directly, and arterial blood collected for pH and blood gas analyses. When arterial PaO₂ fell below 9.3 kPa (70 mm Hg) and failed to respond to corrective measures including positive pressure ventilation and treatment of hypotension (mean arterial blood pressures <70 mm Hg), a salbutamol aerosol (2 µg kg^?1) was delivered via the endotracheal tube. Twenty minutes later, a second arterial blood sample was analysed. Results There were no significant differences in mean arterial blood pressure, heart rate, arterial pH, base excess and bicarbonate before and after treatment. Arterial O₂ tension increased significantly from a mean ± SD of 8.3 ± 1.7 kPa (62.4 ± 13.1 mm Hg) before administration to 15.9 ± 9.8 kPa (119.4 ± 57.7 mm Hg) after treatment. There was a small but significant decrease in PaCO₂ from 7.4 ± 1.5 kPa (55.2 ± 11.2 mm Hg) to 7.0 ± 1.3 kPa (52.9 ± 9.8 mm Hg) between sample times. No changes in heart rhythm were observed. A high percentage (approximately 70%) of animals sweated following treatment. Conclusions Salbutamol administered at a dose of 2 µg kg^?1 via the endotracheal tube of anaesthetized horses with PaO₂ values less than 9.3 kPa (70 mm Hg) resulted in an almost two‐fold increase in PaO₂ values within 20 minutes of treatment. No changes in heart rate or mean arterial blood pressure were associated with the use of salbutamol in this study. The improvement in PaO₂ may be a result of bronchodilatation and improved ventilation, increased perfusion secondary to an increase in cardiac output, or a combination of these two factors. Cardiac output and ventilation–perfusion distribution were not measured in this study; therefore, the reason for the increase in PaO₂ values cannot be conclusively determined. Clinical relevance Administration of a salbutamol aerosol is a simple but effective technique that can be used to improve PaO₂ values in hypoxaemic horses during inhalant anaesthesia with no apparent detrimental side effects.     

Mixed venous oxygen tension as an estimate of cardiac output in anesthetized horses

The relationship between mixed venous O2 tension and cardiac output was studied in six anesthetized horses breathing 100% O2. Cardiac output, O2 consumption, mean arterial pressure, heart rate, and arterial and venous blood gases were measured after administration of xylazine or dobutamine to horses in lateral, sternal, and dorsal recumbencies. After approximately 3 hours, Escherichia coli endotoxin was administered while horses were in dorsal recumbency, and all measurements were repeated. Relationships between cardiac index (CI) and PVO2, heart rate, mean arterial pressure, jugular PVO2, and PVO2 of blood from a superficial limb vein were evaluated by linear regression analysis. Mean arterial pressure was significantly (P less than 0.05) correlated with CI in horses in all positions and after endotoxin administration. However, data points were poorly grouped. Heart rate and CI were significantly correlated in horses in all positions, but not after endotoxin administration. Correlations between jugular PVO2 and PVO2 of blood from a superficial limb vein were not significant in horses in sternal recumbency, and PVO2 of blood from a superficial limb vein was not significantly correlated with CI in horses in lateral recumbency. There was a significant and tight correlation between PVO2 and CI in horses in all positions and after endotoxin administration.     

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.     

Severe hypercarbia resulting from inspiratory valve malfunction in two anesthetized horses

Severe hypercarbia was documented by arterial blood gas analysis in 2 adult horses anesthetized for exploratory laparotomy. Both horses appeared to be adequately anesthetized, but continued to breathe against the ventilator. In both cases, the inspiratory valve on the anesthesia machine was found to be stuck open, permitting expired CO2 to return to the inspiratory limb of the anesthetic circuit and to be inhaled with the next breath. Correction of the malfunctioning valve alleviated the hypercarbia. Problems with the flow valves of the anesthesia machine should be suspected when anesthetized horses breathe against the ventilator and develop severe hypercarbia.     

Cardiopulmonary effects of positive end-expiratory pressure in anesthetized horses.

The cardiopulmonary effects of 0, 5, 10, and 15 cm of H2O positive end-expiratory pressures (PEEP) were determined in anesthetized, spontaneously breathing horses, using a 4 by 4 Latin-square design with one repetition. Cardiac output, alveolar-arterial oxygen tension difference, alveolar ventilation, dead space/tidal volume ratio, and carbon dioxide elimination were not significantly altered by the procedure. As PEEP was increased, alveolar and arterial oxygen tensions, respiratory exchange ratio, and pH decreased, whereas arterial carbon dioxide tension and oxygen consumption increased. These results indicate PEEP is contraindicated in laterally recumbent spontaneously ventilating anesthetized horses breathing air, because it causes alveolar hypoventilation and does not improve pulmonary gas exchange.     

Hemodynamic and respiratory responses in halothane-anesthetized horses exposed to positive end-expiratory pressure alone and with dobutamine

The influence of positive end-expiratory pressure (PEEP) on the alveolar-arterial O2 tension difference [P(A-a)O2], physiologic right-to-left shunt fraction, physiologic dead space-to-tidal volume ratio, and hemodynamic variables was studied in halothane-anesthetized horses maintained in dorsal recumbency during controlled ventilation. Dobutamine was used to minimize the adverse cardiovascular consequences of PEEP. Six adult horses were anesthetized, using xylazine (2.2 mg/kg of body weight, IM), guaifenesin (50 mg/kg, IV), thiamylal Na (4.4 mg/kg, IV), and halothane (1.5 to 2% inspired) in 100% O2. Mechanical ventilation was controlled to maintain arterial eucapnia for at least 45 minutes during base-line measurements. Hemodynamic and respiratory variables were determined every 15 minutes during equilibration. Each horse was subjected to 4 randomized treatments: 5 cm of H2O PEEP, 10 cm of H2O PEEP, 5 cm of H2O PEEP plus dobutamine (1 microgram/kg/min), and 10 cm of H2O PEEP plus dobutamine (1 microgram/kg/min). Each treatment lasted 15 minutes and immediately followed its predecessor. Although the magnitude of PEEP was randomized with and without dobutamine, PEEP without dobutamine always preceded PEEP with dobutamine. Differences in hemodynamic or respiratory variables among base-line measurements, 5 cm of H2O PEEP, or 10 cm of H2O PEEP were not significant (P greater than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Early experience with continuous positive airway pressure (CPAP) in 5 horses �� A case series

This case series is the first report of the use of CPAP (continuous positive airway pressure) ventilation in adult horses. Two horses and 3 ponies anesthetized for orthopedic procedures in lateral recumbency received 10 cm H₂O CPAP. During anesthesia, arterial oxygen partial pressure tended to increase and arterial carbon dioxide pressure tended to increase despite increased minute ventilation index. The measured cardiovascular parameters were within physiologic limits.     

Exercise-induced pulmonary haemorrhage in the horses: results of a detailed clinical, post mortem and imaging study. VII. Ventilation/perfusion scintigraphy in horses with EIPH

Detailed post mortem examination of the lungs of horses with exercise-induced pulmonary haemorrhage (EIPH) has demonstrated significant small airway disease and intense bronchial arterial proliferation in the dorsocaudal lungfields. The purpose of this study was to investigate ventilation and perfusion distribution in the lungs of a similar group of horses to compare changes in the live animal with the previously reported post mortem findings. Thoracic radiography and ventilation/perfusion (V/Q) scintigraphy were performed on five racing Thoroughbreds with recent histories of EIPH. Parametric images of V/Q ratios for left and right lungfields were also generated from the scan images. In all horses, ventilation and perfusion deficits were demonstrated in the dorsocaudal areas of the lung corresponding closely to the observed radiographic lesions. In particular, the perfusion images and V/Q ratio displays indicated that, in affected areas of lung, pulmonary arterial perfusion was the more seriously impaired. This finding appears to confirm the post mortem evidence of reduced pulmonary arterial perfusion and bronchial arterial dominance in these areas. Ventilation deficits in the same areas also confirmed the likelihood of partial airway obstruction consistent with the small airway disease noted in previous post mortem observations. These results suggest that the vascular and airway lesions demonstrated in detailed post mortems of horses with EIPH are also functionally important in affected horses, even at rest. As a consequence of the apparent persistent, insidious and progressive nature of the lesions associated with EIPH there are serious long term implications for management of the condition.     

Clinical Tolerance and Bronchoscopic Changes Associated With Transtracheal High-Frequency jet Ventilation in Dogs and Cats

Eleven awake dogs and two cats received high-frequency jet ventilation (HFJV) via a transtracheal catheter for 6 hours to evaluate their clinical tolerance to the technique. A bronchoscopic examination was performed in all animals prior to and the morning of the day after the procedure to determine the gross effects of the technique on the tracheal epithelium.
All animals tolerated the technique well, exhibiting no discomfort and only a minimal amount of coughing. Only one dog exhibited coughing on the day following the procedure. No bronchoscopic changes were noted after HFJV in one dog. In one dog and one cat, the only observed change was an increase in the prominence of the vascularity compared to that observed prior to HFJV. The remaining animals exhibited more severe tracheal changes that included: an accumulation of mucus (seven dogs, one cat), focal spots of hemorrhage (two dogs), linear stretches of epithelial denuding (two dogs), and diffuse reddening and epithelial denuding (four dogs).
High-frequency jet ventilation by a transtracheal intravenous catheter is well tolerated for short-term ventilatory support in dogs and cats, but the magnitude of the tracheal damage observed in the present report may preclude long-term ventilatory support by this tecnique.     

Effects of tension of the girth strap on respiratory system mechanics in horses at rest and during hyperpnea induced by administration of lobeline hydrochloride

OBJECTIVE: To determine whether tension of the girth strap of a saddle would sufficiently affect rib motion and reduce lung volume to alter pulmonary resistance in horses. ANIMALS: 10 healthy adult horses. PROCEDURE: We used classical techniques to measure the effects of tightening a girth strap (15 kg of tension) on pulmonary dynamics during eupnea and hyperpnea in horses. Respiratory impedance was evaluated by use of oscillometry, and resistance and reactance data were partitioned into lung and chest wall components. Rib cage and abdominal contributions to tidal volume and minute ventilation were measured by use of respiratory inductance plethysmography. Effects of strap tension on functional residual capacity (FRC) were measured during eupnea by use of a helium-dilution technique. In a subgroup of 6 horses, we also measured transdiaphragmatic pressures during eupnea and hyperpnea induced by administration of lobeline hydrochloride (0.2 mg/kg, i.v.). RESULTS: Pulmonary resistance measured by use of oscillometry but not by use of classical methods was significantly increased by the tension of the girth strap. However, the increase in pulmonary resistance could not be explained by a decrease in FRC. Motion of the rib cage was significantly reduced during eupnea and hyperpnea. However, ventilatory variables (tidal volume, minute ventilation, and peak flows), FRC, and transdiaphragmatic pressures were unaltered by strap tension. CONCLUSIONS AND CLINICAL RELEVANCE: Although tension of the girth strap caused measurable changes in respiratory mechanics (loss of rib motion and increased pulmonary resistance), there was no evidence that ventilation was limited.     

Cardiopulmonary effects of dexmedetomidine and ketamine infusions with either propofol infusion or isoflurane for anesthesia in horses

ObjectiveTo examine the cardiopulmonary effects of two anesthetic protocols for dorsally recumbent horses undergoing carpal arthroscopy.Study designProspective, randomized, crossover study.AnimalsSix horses weighing 488.3 ± 29.1 kg.MethodsHorses were sedated with intravenous (IV) xylazine and pulmonary artery balloon and right atrial catheters inserted. More xylazine was administered prior to anesthetic induction with ketamine and propofol IV. Anesthesia was maintained for 60 minutes (or until surgery was complete) using either propofol IV infusion or isoflurane to effect. All horses were administered dexmedetomidine and ketamine infusions IV, and IV butorphanol. The endotracheal tube was attached to a large animal circle system and the lungs were ventilated with oxygen to maintain end-tidal CO₂ 40 ± 5 mmHg. Measurements of cardiac output, heart rate, pulmonary arterial and right atrial pressures, and body temperature were made under xylazine sedation. These, arterial and venous blood gas analyses were repeated 10, 30 and 60 minutes after induction. Systemic arterial blood pressures, expired and inspired gas concentrations were measured at 10, 20, 30, 40, 50 and 60 minutes after induction. Horses were recovered from anesthesia with IV romifidine. Times to extubation, sternal recumbency and standing were recorded. Data were analyzed using one and two-way anovas for repeated measures and paired t-tests. Significance was taken at p=0.05.ResultsPulmonary arterial and right atrial pressures, and body temperature decreased from pre-induction values in both groups. PaO₂ and arterial pH were lower in propofol-anesthetized horses compared to isoflurane-anesthetized horses. The lowest PaO₂ values (70–80 mmHg) occurred 10 minutes after induction in two propofol-anesthetized horses. Cardiac output decreased in isoflurane-anesthetized horses 10 minutes after induction. End-tidal isoflurane concentration ranged 0.5%–1.3%.Conclusion and clinical relevanceBoth anesthetic protocols were suitable for arthroscopy. Administration of oxygen and ability to ventilate lungs is necessary for propofol-based anesthesia.     

Cardiovascular and Pulmonary Effects of Sevoflurane Anesthesia in Horses

The effects of 1.0, 1.5, and 2.0 minimum alveolar concentration (MAC) of sevoflurane on hemodynamic, pulmonary and blood chemistry variables were measured during spontaneous and controlled ventilation in healthy horses. Sevoflurane was the only anesthetic drug administered to the horses. In a dose-dependent manner, sevoflurane significantly decreased ( P <.05) mean arterial blood pressure, cardiac output, and stroke volume. There was a progressive decrease in peripheral vascular resistance and an increase in heart rate as the concentration of sevoflurane was increased, but the differences were not significant. During spontaneous ventilation there was a dose-dependent decrease in respiratory rate that caused a decrease in the minute volume. As the dose of sevoflurane increased, the arterial carbon dioxide tension also increased ( P <.05). All blood chemistries remained within normal limits. Recovery from anesthesia was without incident. In conclusion, sevoflurane induces a dose-dependent decrease in hemodynamic variables and pulmonary function in horses that is not greatly different from that of other approved inhalant anesthetics.     

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.