Cardiopulmonary effects of anesthesia induced and maintained with isoflurane in cats.

OBJECTIVE: To determine the cardiopulmonary effects of anesthesia induced and maintained with isoflurane (ISO) in cats. ANIMALS: 8 healthy cats between 1 and 5 years old. PROCEDURE: Anesthesia was induced with ISO in oxygen. Two anesthetic depths were maintained in each cat; mean alveolar concentrations (MAC) were 1.3 and 2.0 times MAC. Ventilation was either spontaneous or controlled. Each cat received each treatment combination according to a Latin square design. Cardiopulmonary measurements were made after 20 minutes of constant conditions with each combination of anesthetic depth and ventilatory mode. RESULTS: Cardiac index was not different between ISO doses, but 2.0 MAC ISO reduced arterial blood pressure and total peripheral resistance. Cardiac index and systolic arterial blood pressure were reduced by controlled ventilation. The PaCO2 and pulmonary artery pressure were highest in association with 2.0 MAC ISO during spontaneous ventilation. Changes in pHa were attributable to changes in PaCO2. CONCLUSIONS: 2.0 MAC ISO causes hypotension and hypercapnia; however, cardiac index is maintained. Hypercapnia may be abolished with controlled ventilation, but at the expense of reduced cardiac index. 1.3 MAC ISO results in minimal cardiopulmonary depression, especially when healthy cats are allowed to breathe spontaneously. CLINICAL RELEVANCE: Hypoventilation associated with untoward physiologic responses to 2.0 MAC may be overcome with controlled ventilation, but results in marked reduction in cardiovascular performance; thus, use of 2.0 MAC ISO should be avoided in cats.

Sensory nerve conduction velocity of the caudal cutaneous sural and medial cutaneous antebrachial nerves of adult horses.

Maximal conduction velocities of compound action potentials evoked by stimuli of 2 times threshold in the caudal cutaneous sural (CCSN) and medial cutaneous antebrachial (MCAN) nerves were determined by averaging potentials evoked and recorded through percutaneous needle electrodes. Mean maximal conduction velocities of compound action potentials were: CCSN = 61.3 +/- 2.0 meters/second (m/s) and MCAN = 56.4 +/- 2.8 m/s. To confirm accuracy of our percutaneous recordings, compound action potentials were recorded through bipolar chlorided silver electrodes from the exposed surfaces of fascicles of the CCSN and the MCAN. The maximal conduction velocities of these potentials were in agreement with the conduction velocities of compound action potentials that were evoked and recorded through percutaneous needle electrodes. The specificity of stimulating and recording sites was verified by recording before and after section of the nerves. Stimuli from 3 to 5 times threshold evoked a second, longer latency, compound action potential that consisted of a variable number of components in the CCSN and MCAN. The configurations and conduction velocities of the shorter latency potentials were the same as those of the single compound action potentials evoked by stimuli of 2 times threshold. Mean conduction velocities of the longer latency potentials were: CCSN = 24.4 +/- 2.6 m/s and MCAN = 24.5 +/- 2.2 m/s. Needle electrode and direct stimulation of either the CCSN or the MCAN at 3 to 5 times threshold failed to evoke contractions of limb muscles. Therefore, action potentials that contributed to the evoked compound potentials recorded in these horses arose, most likely, from afferent nerve fibers.

Evaluation of the Doppler ultrasonic method of measuring systolic arterial blood pressure in cats.

The accuracy of the Doppler technique for indirect systolic blood pressure measurement was assessed in 16 anesthetized cats. Eight cats were anesthetized with isoflurane and 8 were anesthetized with halothane. Anesthetic depth and mode of ventilation were varied to obtain a wide range of arterial blood pressure. A Doppler transducer was placed on the palmer surface of the left forelimb over the common digital branch of the radial artery to detect blood flow, and a blood pressure monitoring cuff with a width 37% the limb circumference was placed half way between the elbow and the carpus. To enable direct arterial pressure measurements, the left femoral artery was catheterized and the blood pressure waveforms recorded simultaneously. Systolic blood pressure measured by use of the Doppler ultrasonic technique was significantly lower than that obtained from the femoral artery catheter. Using linear regression, we determined a clinically useful calibration adjustment for Doppler indirect blood pressure measurement in cats: femoral systolic pressure = Doppler systolic pressure + 14 mm of Hg.

Cardiopulmonary responses to experimentally induced gastric dilatation in isoflurane-anesthetized dogs.

Gastric dilatation was experimentally induced in 6 anesthetized dogs maintained with constant-dose isoflurane in oxygen. An intragastric balloon was used to distend the stomach with a constant 30 mm of Hg pressure for 3.5 hours. The PaCO2 was maintained between 35 and 45 mm of Hg, using intermittent positive-pressure ventilation. Cardiopulmonary measurements prior to stomach distension (baseline) were compared with measurements taken during 0.1, 0.5, 1.0, 1.5, 2.5, and 3.5 hours of stomach distension by analyzing the change from baseline in a randomized-block analysis with each dog as a block. After distending the stomach, cardiac index increased (P less than 0.01) from 1.5 to 3.5 hours. Stroke volume did not change, thus the increase in the cardiac index was attributable to an increase in heart rate. During inflation, increases were observed in systemic arterial, pulmonary arterial, and right atrial pressure. Respiratory frequency was unchanged; however, to maintain PaCO2 constant, it was necessary to progressively increase peak airway pressure. Although PaO2 tended to decrease during gastric dilation, the dogs were never hypoxemic. These results indicate that when our methods are used to maintain a constant anesthetic dose of isoflurane in oxygen, an observed increase in cardiovascular performance is expected. This differs from other studies in anesthetized dogs that have shown reduction in cardiovascular performance following gastric dilatation.

Thermodilution estimation of cardiac output at high flows in anesthetized horses.

The purpose of this study was to compare the thermodilution technique for estimation of cardiac output with the indocyanine green dye dilution technique at flows between 10 and 39 L/min in halothane-anesthetized horses. The estimation of area of dye dilution cardiac output curves was made by using the fore-'n-aft (FA) triangle method. This shorthand technique was compared with logarithmic exponential extrapolation and summation (extrapolated area), using 64 cardiac output curves. Then, 256 simultaneous thermodilution measurements were compared with dye dilution measurements calculated by use of the FA technique. Forty milliliters of iced 0.9% NaCl solution containing 15 mg of indocyanine green dye was used as the indicator. This was delivered in less than 1 second to the right atrium, using a power injector. A thermistor positioned in the pulmonary artery detected the thermal indicator. Blood was withdrawn from the carotid artery through a densitometer cuvette to measure the dye concentration. The FA estimations of area were higher than those determined by use of extrapolated area. A multiplicative adjustment of 0.837 was estimated. On average, thermodilution estimates of cardiac output exceeded the adjusted FA determinations. Using a weighted linear regression, we determined the following calibration adjustment: thermal dilution cardiac output/1.048 = indocyanine green dye dilution cardiac output.

Effect of body posture on cardiopulmonary function in horses during five hours of constant-dose halothane anesthesia.

Cardiovascular and respiratory functions were serially characterized in 7 healthy, spontaneously breathing, adult horses (from which food had been withheld) during 5 hours of constant 1.06% alveolar halothane (end-expired halothane concentration of 1.06%; equivalent to 1.2 times the minimal alveolar anesthetic concentration for horses). To enable comparison of temporal results in relation to 2 body postures, horses were studied in lateral recumbency (LR) and dorsal recumbency (DR) on separate occasions. Temporal changes in results of measures of circulation previously reported from this laboratory for horses in LR were confirmed (ie, a time-related increase in systemic arterial blood pressure, cardiac output, stroke volume, and PCV). During DR, systemic arterial blood pressure was initially significantly (P less than 0.05) greater and pulmonary artery pressure less than results at comparable periods during LR. Differences ceased to exist with duration of anesthesia. Except for a greater heart rate at hour 5 of DR, no other significant differences in circulation were found between LR and DR. In general, except for PaO2, measures of ventilation did not change with time in either LR or DR. The PaO2 was significantly greater during LR, compared with DR, but the average did not change significantly with time in either body posture.

Cardiopulmonary effects of ephedrine in halothane-anesthetized horses.

The cardiopulmonary effects of intravenous (i.v.) administration of the sympathomimetic drug ephedrine during two different levels of halothane anesthesia [end-tidal concentration of 1.37% (light anesthesia) and 2.1% (deep anesthesia)] were studied in eight horses. Anesthesia was induced and maintained using only halothane in O2. Ventilation was controlled to maintain a Paco2 of 38-42 mmHg. Following instrumentation and stabilization of the horse at the halothane concentration being studied, baseline measurements of cardiac output (Q), arterial blood pressure (AP), pulmonary artery pressure, heart rate, Pao2, Paco2 and pH were made. Ephedrine was then administered (0.06 mg/kg i.v.) and these measurements repeated at 10, 20, 30, 45 and 60 min after injection. At both doses of halothane there was a significant (P less than 0.05) increase in Q, stroke volume (SV), and systolic AP following ephedrine administration. In addition, at 2.1% halothane, ephedrine administration resulted in a significant (P less than 0.05) increase in mean AP and Pao2 and a decrease in total peripheral resistance. The increase in systolic AP, Q, and SV was significantly (P less than 0.05) greater at 2.1% halothane than at 1.37% halothane. Ephedrine administration to horses during both light and deep halothane anesthesia results in an increase in AP that is due to an increase in Q and SV.

Cardiopulmonary effects of halothane anesthesia in cats.

The cardiopulmonary effects of 2 planes of halothane anesthesia (halothane end-tidal concentrations of 1.78% [light anesthesia] and 2.75% [deep anesthesia]) and 2 ventilatory modes (spontaneous ventilation [SV] or mechanically controlled ventilation [CV]) were studied in 8 cats. Anesthesia was induced and maintained with halothane in O2 only, and each cat was administered each treatment according to a Latin square design. Cardiac output, arterial blood pressure, pulmonary arterial pressure, heart rate, respiratory frequency, and PaO2, PaCO2, and pH were measured during each treatment. Stroke volume, cardiac index, and total peripheral resistance were calculated. A probability value of less than 5% was accepted as significant. In the cats, cardiac output, cardiac index, and stroke volume were reduced by deep anesthesia and CV, although only the reduction attributable to CV was significant. Systemic arterial pressure was significantly reduced by use of deep anesthesia and CV. Respiratory frequency was significantly lower during CV than during SV. Arterial PO2 was significantly decreased at the deeper plan of anesthesia, compared with the lighter plane. At the deeper plane of anesthesia, arterial PCO2 and pulmonary arterial pressure were significantly lower during CV than during SV. The deeper plane of halothane anesthesia depressed cardiopulmonary function in these cats, resulting in hypotension and considerable hypercapnia. Compared with SV, CV significantly reduced circulatory variables and should be used with care in cats. Arterial blood pressure was judged to be more useful for assessing anesthetic depth than was heart rate or respiratory frequency.

Female pseudohermaphroditism in three sibling greyhounds.

Three 18-month-old Greyhound siblings were found to have female pseudohermaphroditism. The disorder in sexual differentiation likely developed because the dam received testosterone proprionate for estrus prevention. All 3 siblings had a prepuce (no penis), vagina, uterus, and ovaries.

Anesthetic considerations for emergency equine abdominal surgery.

The physiologic changes that take place in horses presented for emergency abdominal surgery and the effects of these changes on the horse's anesthetic management are discussed.

Cardiopulmonary and behavioral effects of fentanyl-droperidol in cats.

The combination of the narcotic fentanyl (0.4 mg/ml) and the tranquilizer droperidol (20 mg/ml) was injected into 12 healthy adult cats at a rate of 1 ml/9 kg of body weight, sc. Arterial blood pressure, heart rate, respiratory frequency, PaCO2, and PaO2, arterial pH (pHa), and rectal temperature were measured before and 30, 60, 90, and 120 minutes after injection of fentanyl-droperidol. Respiratory frequency was decreased significantly (P less than 0.05) and heart rate increased significantly (P less than 0.05) at all measurement intervals after drug administration. Observed decrements in arterial blood pressure were not significant. Arterial PO2, PCO2, and pH did not change significantly. A significant (P less than 0.05) decrease in body temperature was measured 90 minutes after drug administration. All cats were calm, tractable, and frequently assumed lateral recumbency after administration of fentanyl-droperidol. This maximal tranquil state was determined to be 30 to 60 minutes after drug administration.

Arterial blood PO2 and PCO2 in horses during early halothane-oxygen anaesthesia.

Arterial blood was collected from 25 clinically normal horses immediately before and serially throughout the first hour of halothane oxygen anaesthesia. Blood was analysed for oxygen and carbon dioxide partial pressure (PaO2, PaCO2). Measurements of inspired oxygen concentration during anaesthesia permitted direct correlation with blood gases. Horses were divided arbitrarily into two groups based on their age: two to seven years, n = 15; over seven years, n = 10. Average (+/- sd) PaO2 and PaCO2 was 14.1 +/- 1.5 kPa (106 +/- 11 mmHg) and 5.9 +/- 0.6 kPa (44.4 +/- 4.4 mmHg) respectively in conscious, young horses and 14.0 +/- 0.7 and 5.8 +/- 0.5 kPa (105 +/- 5 and 43.3 +/- 3.8 mmHg) respectively in conscious older horses. Arterial oxygen tension decreased to 9.3 +/- 1.0 and 8.5 +/- 1.4 kPa (69.6 +/- 7.8 and 63.7 +/- 10.4 mmHg) in young and older air breathing horses respectively immediately following intravenous anaesthetic induction, recumbency and orotracheal intubation. At this time, PaCO2 was 6.5 +/- 0.5 and 6.0 +/- 0.7 kPa (48.7 +/- 3.5 and 45.1 +/- 4.9 mmHg) respectively. By 30 mins after the start of halothane in oxygen (6 litres/min) anaesthesia PaO2 increased to a maximum in both study groups. Arterial PCO2 increased steadily during anaesthesia and 60 mins after induction PaCO2 was 10.5 +/- 2.4 kPa (78.5 +/- 17.8 mmHg) in the younger horses and 9.2 +/- 1.6 kPa (68.8 +/- 11.8 mmHg) in the older horses. During inhalation anaesthesia PaO2 tended to be greater at comparable time periods in the younger horses despite a slightly greater degree of hypoventilation.

Arterial hypotension and the development of postanesthetic myopathy in halothane-anesthetized horses.

The effect of halothane-induced hypotension on the development of postanesthetic myopathy was studied, using 6 healthy adult horses. Horses were anesthetized with halothane in oxygen for 3.5 hours on each of 2 occasions. Intermittent positive-pressure ventilation was used to maintain PaCO2 of 45 to 55 mm of Hg throughout both anesthetic exposures. By regulating the inspired halothane concentration, a mean arterial blood pressure of 85 to 95 mm of Hg (normotension) was maintained throughout the 1st anesthetic exposure, and a mean arterial blood pressure of 55 to 65 mm of Hg (hypotension) was maintained during the 2nd anesthetic exposure. All horses recovered uneventfully from normotensive anesthesia, but all had some muscle dysfunction after prolonged hypotensive anesthesia. Because of apparent animal discomfort and lameness involving more than 1 limb, 3 horses were euthanatized soon after they recovered from hypotensive anesthesia. The 3 other horses showed a degree of lameness. In addition, 1 horse had raised, swollen plaques over the hip, rib, and facial areas which were in contact with the surgical table, and another had evidence of facial nerve paralysis. One hour after the 6 horses stood after hypotensive anesthesia was completed, values obtained for aspartate transaminase and creatinine were significantly (P less than 0.05) greater than those obtained after normotensive anesthesia was completed. Aspartate transaminase, total bilirubin, and creatinine values were significantly (P less than 0.05) increased when compared with those obtained before horses were anesthetized. A large increase was measured in creatine kinase. Twenty-four hours after hypotensive anesthesia was completed, creatine kinase and lactate dehydrogenase in the 3 surviving horses were significantly (P less than 0.05) greater than those values after normotensive anesthesia was completed.

Effects of spontaneous, assisted, and controlled ventilatory modes in halothane-anesthetized geldings.

Cardiopulmonary effects of spontaneous, assisted, and controlled ventilatory modes were determined with 6 young, healthy geldings anesthetized with halothane at a constant dose (1.3 minimum alveolar concentration). All horses were in lateral recumbency, and all modes of ventilation were studied at least once during each anesthetic exposure. Cardiac output did not differ between spontaneous and assisted ventilation modes, but both modes were associated with significantly (P less than 0.05) higher cardiac output than that with controlled ventilation. The PaCO2 differed significantly (P less than 0.01) between all modes of ventilation. Although controlled ventilation maintained a normal PaCO2, assisted ventilation reduced PaCO2 as compared with spontaneous ventilation with less cardiovascular depression than that with controlled ventilation. Mixed venous O2 tensions were higher with spontaneous and assisted ventilation modes than with controlled ventilation. Except for shorter inspiratory time and smaller inspiratory/expiratory ratio associated with spontaneous ventilation, there were no ventilatory mode-related effects on ventilatory variables.

Postanesthetic hemorrhagic myelopathy in a horse.

Acute hemorrhagic myelopathy developed in the sixth cervical to the eighth thoracic spinal cord segments of a 1-year-old Quarter Horse colt that was castrated under general anesthesia while in dorsal recumbency. Clinical signs were consistent with severe transverse myelopathy caudal to the brachial enlargement and cranial to the lumbosacral enlargement of the spinal cord. Histologic examination of the spinal cord revealed hemorrhage in the gray matter, with multiple blood-filled clefts in otherwise normal neuropil. Hemodynamic changes in the spinal cord associated with anesthesia and dorsal recumbency may have led to hypoxic vessel damage, with massive hemorrhage after surgery, when the horse was returned to lateral recumbency. Postanesthetic hemorrhagic myelopathy is a possible complication of positioning in dorsal recumbency, during anesthesia, in rapidly growing, young horses.

Evaluation of concentrated solutions of guaifenesin for equine anesthesia.

The pH, osmolality, stability, and bacteriostatic characteristics of 5%, 10%, and 15% solutions of guaifenesin were studied. In vitro and in vivo experiments were done to determine the hemolytic potential of the more concentrated solutions on equine blood, as compared with the recommended 5% solution. The primary objective was to determine whether more concentrated solutions could be used clinically. The secondary objective was to determine the optimal diluent (water, 0.9% saline, or 5% dextrose). It was concluded that a 10% solution of guaifenesin made in sterile distilled water was most suitable for clinical equine anesthesia. Such a preparation has reasonable storage qualities and does not induce clinically significant hemolysis.