Effects of posture and intraruminal pressure on the bronchial calibre of cattle during xylazine/halothane anaesthesia.

The effects of changes in body position and intraruminal pressure on the bronchial calibre of anaesthetised, paralysed adult cattle were investigated using a computer-aided forced airflow oscillation technique which enabled the measurement of specific lower airways conductance (sGlaw), expiratory reserve volume (ERV) and residual resistance (RA). sGlaw and RA were highest in dorsal and lowest in sternal recumbency, but ERV was very much greater in sternal recumbency than in other positions. Turning the animals from right to left lateral recumbency resulted in a marked reduction in sGlaw and increase in ERV, providing further evidence of functional impairment of the dependent areas of the lungs of large animals. Changes in intraruminal pressure had little effect on bronchial calibre, suggesting that the respiratory dysfunction resulting from ruminal tympany is primarily caused by a reduction in compliance rather than an increase in airway resistance.

Radiographic evidence of pulmonary dysfunction in anaesthetised cattle.

The effects of fasting, sedation, recumbency and general anaesthesia on the radiographic appearance of the thoracic cavity of adult cattle were investigated. Fasting and sedation had little effect on the visible lung field but recumbency and general anaesthesia were followed by a reduction in lung field area. The reduction in area appeared to be greatest in the more dependent parts of the lung and was associated with the development of a persistent radiographic opacity. The changes were similar to those previously observed in horses.

Effects of xylazine and acepromazine on bronchomotor tone of anaesthetised ponies.

The effects of xylazine (an alpha 2-adrenoceptor agonist) and acepromazine (an alpha-adrenoceptor antagonist) on bronchomotor tone were investigated in seven anaesthetised, apnoeic ponies using a computer aided forced oscillation technique, which separates changes in bronchial calibre from changes in lung volume. Both agents produced bronchodilatation and a decrease in lung volume.

Median effective dosage of propofol for induction of anesthesia in dogs.

The median effective dosage (ED50) of propofol for induction of anesthesia was determined in 25 dogs premedicated with acepromazine, 0.05 mg/kg of body weight, and in 35 unpremedicated dogs. The ED50 was found to be 2.2 mg/kg in premedicated dogs and was 3.8 mg/kg in unpremedicated dogs. The mean +/- SD total dosage of propofol required to induce anesthesia in premedicated animals was 2.8 +/- 0.5 mg/kg and was 4.7 +/- 1.3 mg/kg in unpremedicated animals. Signs of excitement were observed in 5 of the unpremedicated dogs, but in none of those that were premedicated.

Accuracy of a reflectance pulse oximeter in anesthetized horses.

The accuracy of a reflectance pulse oximeter was determined in 22 anesthetized horses. Oximetric blood oxygen saturation was measured with the pulse oximeter probe attached to the mucosa of the mandible. Arterial blood oxygen saturation (SaO2) was calculated from arterial blood gas values and the equine blood oxygen dissociation curve. The mean +/- SD difference between oximetric blood oxygen saturation and SaO2 was -1.3 +/- 3.1% for values of SaO2 ranging from 80 to 100%. The difference between oximetric blood oxygen saturation and SaO2 was influenced by SaO2, the mean arterial blood pressure, and preanesthetic medication administered, but not by age, sex, or body weight of the horse, individual animal effect, anesthetic induction or maintenance agent, procedure performed, body position, mode of lung ventilation, time of sampling, arterial pH, or carbon dioxide tension.

Effect of halothane on bronchial calibre of anaesthetised cattle.

A computer-aided forced oscillation technique was used to examine the effects of halothane on bronchial calibre in three adult cows after anaesthesia had been induced with xylazine and thiopentone. The administration of halothane failed to produce bronchodilatation, possibly owing to low resting bronchomotor tone in the animals. However, an increase in expiratory reserve volume, associated with a small fall in airway resistance, was observed, suggesting that changes in elastic recoil may make a significant contribution to changes in airway resistance during anaesthesia. The results also emphasise the importance of relating airway resistance to lung volume.

Cardiac arrest during anaesthesia in two horses.

Unexpected cardiac arrest occurred in two horses during routine surgical anaesthesia. Both were successfully resuscitated. The aetiology of these occurrences and their possible relationship to second degree heart block is discussed.

Effects of a demand valve on pulmonary ventilation in spontaneously breathing, anaesthetised horses.

The pressure flow characteristics of a demand valve which has been suggested to be suitable for use in anaesthetised horses were determined at a range of flow rates commonly encountered in equine anaesthesia. The resistance of the valve was found to be very much greater than the resistance of normal large animal anaesthetic apparatus or the equine lower respiratory tract. The effects of the valve on pulmonary ventilation were investigated in seven anaesthetised, intubated horses. Respiratory rate and dynamic compliance were unaffected by connection of the valve but mean tidal and minute volumes and peak flow rates were substantially reduced. The change in transpulmonary pressure over the respiratory cycle was doubled and indices of work of breathing increased by a factor of three. It was concluded that the resistance offered by the valve was too great for its use in spontaneously breathing horses to be recommended.

A clinical trial of three anaesthetic regimens for the castration of ponies.

After premedication with intravenous xylazine 30 ponies were anaesthetised for castration under field conditions with intravenous ketamine, thiopentone or methohexitone. The duration of anaesthesia was adequate for surgery and the times taken to stand were similar in each group. The recovery of the ponies after ketamine was quieter than after either of the barbiturates and this may be an advantage when skilled assistance is lacking. However, all the anaesthetics appeared to be suitable for performing minor surgery in the field.

Effect of halothane, enflurane and isoflurane on bronchomotor tone in anaesthetized ponies.

The effects of halothane, enflurane and isoflurane on bronchial calibre were investigated in five anaesthetized ponies using a computer-aided forced airflow oscillation technique to derive specific lower airways conductance (s.Glaw) and expiratory reserve volume (ERV). All the agents tended to increase s.Glaw (indicating bronchodilatation), but ERV was reduced by halothane and enflurane, and increased by isoflurane. It was concluded that the effects of these agents on bronchomotor tone were similar to those which occur in man. However, the reasons for the differences in their effects on ERV compared with those found in man remain to be determined.

Influence of acute isocapnic hypoxia on bronchial calibre and "expiratory reserve" volume in dogs.

The effects of hypoxia on bronchial calibre were investigated in anaesthetized dogs using a computer-aided forced airflow oscillation technique which derived specific lower airways conductance (sGlaw) and "expiratory reserve volume" (ERV). Measurements were made with inspired oxygen concentrations varying from 40% to 10%. sGlaw tended to be slightly reduced (indicating bronchoconstriction) by decreasing the oxygen concentration from 40% to 15%, these changes being statistically significant after the administration of atropine. ERV was unaffected by changes in oxygen concentration from 40% to 15%, but was significantly reduced during ventilation with 10% oxygen. This effect was reversed by re-oxygenation and was not abolished by prior administration of atropine. It was concluded that both bronchoconstriction and changes in lung volume may be responsible for changes in airway resistance during hypoxia.

Pulse oximetry in horses.

The clinical usefulness of two pulse oximeters was evaluated at two probe sites in nine anesthetized horses. The hemoglobin saturation determined by the pulse oximeters (SaOx) was compared with the hemoglobin saturation calculated from the measured arterial oxygen tension (SaO2). The mean and standard deviation (SD) were calculated from the differences in saturation measurements, over the saturation range of 80% to 100%, for each oximeter used at the tongue probe site and for one oximeter used at the ear. The oximeter results tended to underestimate the SaO2 with mean differences of -3.7% on the tongue and -6.0% on the ear. The limits of agreement were defined as the mean difference +/- 2 SD. Each oximeter used at the tongue produced limits of agreement of +1% to -8%, which meant that 95% of the SaOx values were 1 percentage point above or 8 percentage points below the SaO2. The variability of the differences and limits of agreement were larger when the ear was used as the probe site and at saturations less than 80%. Although both oximeters tended to underestimate the SaO2, they appeared to be clinically useful in detecting changes in arterial hemoglobin saturation.