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OBJECTIVE: To evaluate the efficacy of inhaled nitric oxide (NO) in anesthetized healthy newborn foals with experimentally induced pulmonary hypertension. ANIMALS: Five 1- to 3-day-old foals. PROCEDURE: Anesthesia was induced and maintained with propofol, and foals were intubated and mechanically ventilated. Systemic pressure and pulmonary arterial pressure (P(PA)) were recorded every 30 seconds. Hypertension was induced via a hypoxic gas mixture or chemical vasoconstriction, using the thromboxane mimetic U46619. Nitric oxide was added at a concentration of 80 parts per million (ppm) for 6 minutes under baseline conditions and during pulmonary hypertension-induced alveolar hypoxia (inspired oxygen concentration = 0.08). Nitric oxide (20, 40, 80, and 160 ppm) was evaluated during U46619-induced hypertension. Samples for determination of arterial blood gas tensions were collected before and after each NO treatment. RESULTS: Inhaled NO (approx 80 ppm) did not have an effect on baseline variables. Infusion of U46619 (0.35 +/- 0.04 microg/kg of body weight/min) or alveolar hypoxia resulted in increased P(PA) and decreased arterial oxygenation (PaO2) and hemoglobin saturation (HbSat). The increase in P(PA) was attenuated, in a dose-dependent manner, by NO during U46619 infusion and reversed by NO during induced hypoxemia. The PaO2 and HbSat were significantly improved at all NO doses during U44619 infusion but not during alveolar hypoxia. For all inhaled NO concentrations, nitrogen dioxide and methoglobin values were < 5 ppm and 3%, respectively. CONCLUSIONS AND CLINICAL RELEVANCE: Nitric oxide is a potent, selective vasodilator of the pulmonary circulation in healthy newborn foals. Inhaled NO may have value as a therapeutic agent in foals with pulmonary hypertension.

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OBJECTIVE: To evaluate changes in the pharmacokinetic disposition of diazepam in foals from 4 to 84 days of age. SAMPLE POPULATION: 4 male and 2 female full-term mixed-breed foals. PROCEDURE: Diazepam terminal half-life, volume of distribution, clearance, free fraction, unbound volume of distribution, free clearance, peak desmethyldiazepam concentration, and area under the desmethyldiazepam concentration-time curve were determined after i.v. administration of 0.25 mg of diazepam/kg of body weight to foals at 4, 21, 42, and 84 days of age. RESULTS: Disposition of diazepam was best described using a two-compartment model. Clearance and free fraction values (mean +/- SEM) determined at 4 days (5.06 +/- 0.79 and 51 +/- 8 ml/kg/min, respectively) were significantly less than those obtained at 21 (8.64 +/- 0.95 and 87 +/- 11 ml/kg/min), 42 (7.31 +/- 0.82 and 83 +/- 10 ml/kg/min), and 84 (8.41 +/- 0.56 and 100 +/- 12 ml/kg/ min) days. Volume of distribution and unbound volume of distribution values determined at 4 days (1.57 +/- 0.11 and 16.0 +/- 1.7 L/kg, respectively) were significantly less than those found at 21 (2.66 +/- 0.33 and 26.8 +/- 3.9 L/kg), 42 (3.00 +/- 0.42 and 33.9 +/- 5.0 L/kg), and 84 (2.55 +/- 0.35 and 30.2 +/- 5.3 L/kg) days. Peak plasma desmethyldiazepam concentration obtained at 4 days (22.7 +/- 2.4 ng/ml) was significantly lower than that obtained at 21 (36.1 +/- 4.5 ng/ml), 42 (38.3 +/- 4.8 ng/ml), and 84 (34.6 +/- 2.1 ng/ml) days. CONCLUSIONS: Factors likely to affect the pharmacokinetic disposition of diazepam in foals, such as body composition and hepatic enzyme activity, are in transition during the first 21 days of life. These have opposing effects on diazepam clearance and volume of distribution so that terminal half-life remains unchanged. However, clearance determines whether diazepam will accumulate with repeated doses, and care should be taken when administering repeated doses to foals < 21 days old.

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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.

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Muscle surface capillary blood flow was measured in the biceps femoris and lateral head of the triceps brachii muscles in six horses before and during halothane anesthesia by using laser Doppler flowmetry. During 90 minutes of anesthesia, muscle surface capillary blood flow was reduced to 20% to 40% of preanesthetic values. Muscle surface capillary blood flow tended to be lower in dependent muscles than in nondependent muscles, and this disparity was greater in the forelimbs than in the hind limbs.

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The incidence of post-anesthetic lameness in 655 horses undergoing 733 anesthetic episodes over a 3 year period was 6.4%. Nineteen factors previously reported or proposed to play a role in the development of post-anesthetic lameness were evaluated statistically. Only hypotension and the duration of the anesthetic period were significant factors.

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After surgery, high intramuscular pressures were measured in a horse with clinical signs of postanesthetic myopathy that developed following a 2.25-hour surgical procedure. In the postoperative period, tests for muscular and neurologic integrity were made, including measurement of intracompartmental muscle pressure.

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Interstitial pressure and pH in the dependent biceps femoris muscle were measured in anesthetized, laterally recumbent horses. The mean (+/- standard deviation) interstitial pressure in 10 horses was 19.70 +/- 0.15 mmHg in the 30 to 180 minute interval after induction of anesthesia. Pressures of this order have been associated with reduction in muscle perfusion. Mean (+/- standard deviation) interstitial pH in six horses decreased from 7.07 +/- 0.30 to 6.73 +/- 0.21 between 45 and 150 minutes of anesthesia. These results indicated the presence of circulatory compromise to intracompartmental structures.

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Five horses that underwent prolonged anesthesia (greater than 3 hours) in dorsal recumbency for a surgical procedure were unable to stand after recovery and were euthanatized. A provisional diagnosis of postanesthetic myopathy was confirmed at necropsy in all 5 horses. However, distribution of affected muscles in these horses was atypical, because there was bilateral hind limb adductor muscle involvement.

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