Effects of blast exposure on exercise performance in sheep.

BACKGROUND: The effects of blast on maximal exercise performance were investigated in sheep that were trained to perform maximal exercise. METHODS AND RESULTS: Sheep were fully instrumented for determination of pulmonary and systemic hemodynamics. Blast exposure was administered by using a compressed air driven shock tube that was positioned to primarily produce cardiopulmonary injury. Four levels of exposure were used that were known to produce sublethal injury ranging from little or no grossly observable cardiopulmonary injury (level 1) to confluent ecchymosis of the heart, lung, or both (level 4). We evaluated maximal exercise performance 1 hour after exposure to level 1, level 2, and level 3 and 24 hours after level 3 and level 4. VO2max was not significantly decreased 1 hour after exposure to level 1 but was decreased after exposure to level 2 (29.9%) and level 3 (49.3%). Significant improvement in exercise performance was observed in 24 hours, as VO2max was not significantly decreased 24 hour after level 3. VO2max was decreased 24 hour after level 4 injury (30.8%). CONCLUSION: Cardiovascular data collected during exercise suggested that acute cardiopulmonary injury is responsible for the exercise performance decrement observed 1 hour after exposure and that significant recovery of function is observed 24 hours after blast injury.

Cardiopulmonary effects of high-impulse noise exposure.

In high-energy impulse noise environments, the biomechanical coupling process between the external forces and the pathophysiology of cardiopulmonary injury is not well understood. A 12-in-diameter compressed air-driven shock tube with reflector plate was used to induce three levels of pulmonary contusion injury in a large animal model. Twenty-one anesthetized sheep were exposed to the various levels of impulse noise generated by the shock tube, with six additional sheep serving as a control group. Pathologic evaluations, performed 3 hours after exposure, showed pulmonary contusion ranging from minor petechial changes on the surface of the lung parenchyma to diffuse ecchymoses affecting as much as 60% of the lung. The gross pathologic observations of injury produced by exposure to the impulse noise produced by the shock tube were similar to those reported for blunt impact trauma or exposure to chemical or grain-dust explosions. The extent of lung injury (lung injury index) was quantitatively assessed. A semilogarithmic relationship between the lung injury index and the measured peak pressure was demonstrated. A significant linear correlation was demonstrated between lung injury index and lung weight-to-body weight ratio. Significant cardiopulmonary changes were also observed as a result of exposure to high-impulse noise. Although in most cases the degree of change was related to the severity of the injury, significant cardiopulmonary function changes were also observed in the absence of significant grossly observable pulmonary injury. Cardiac injury was indicated by decreased cardiac output and hypotension at all levels of injury and might be the result of myocardial contusion or air emboli. Pulmonary injury was demonstrated by respiratory acidosis, increases in lung resistance, and decreases in lung compliance and lung volume. Arterial PO2 appeared to be the most sensitive parameter of injury and was decreased for all measurement intervals for all exposure groups.

Methemoglobinemia induced by a benzocaine-based topically administered anesthetic in eight sheep.

Benzocaine-based anesthetic sprays are commonly used in sheep to anesthetize the nasal passages and glottis before intubation. Sprays containing benzocaine have been identified as causing methemoglobinemia in dogs, cats, and human beings. Diagnosis of benzocaine-induced methemoglobinemia was made in 8 Dorset-Finn ewes exposed to a 2-second burst of (approx 56 to 112 mg of benzocaine) anesthetic spray. Venous blood samples taken 10 to 20 minutes after intranasal application of the spray revealed methemoglobinemia of 22.6 +/- 1.8% (mean +/- SD) in 9 samples from 8 ewes. Four other ewes intentionally exposed did not have methemoglobinemia. Topical use of benzocaine-containing anesthetics in sheep is not recommended. The high methemoglobin concentration induced by this product may substantially alter the cardiovascular and pulmonary function, blood gas analyses, and exercise capacity, thereby compromising animal health and/or research results. Although it appears that minimal clinical signs are induced in healthy animals, the risks of compromising a subclinically ill animal do not offset the benefits of this product.

Review of the carotid artery loop procedure in sheep.

Carotid loop (CL) surgery involves the permanent externalization of a common carotid artery in a skin tube. The CL facilitates repeated access to the systemic arterial system for blood sampling and blood pressure measurement in laboratory sheep. It eliminates the need for arterial cut-downs and chronic indwelling catheters, reduces the risk of sepsis and infection, and adds flexibility to research protocols. The surgical procedure is aseptically performed under general anesthesia and involves isolation of the common carotid artery, creation of a bipedicled skin tube, and permanent envelopment of the artery in the skin tube. The primary complication is ischemic necrosis with sloughing of the middle of the loop and is usually due to failure to adhere to the critical length-to-width ratio (2.5:1). We have performed 150 CL procedures with an overall success rate of 94%. Nine CL ablations were required, due to necrosis with exposure of the artery (7/9) or stricture formation with loss of patency (2/9). Twenty-two CLs developed complications secondary to partial necrosis, but did not require ablation. Results indicate that the CL is a reliable method to ensure repeated access to the systemic arterial system in sheep. A modification of the standard CL procedure in which the artery is surrounded by a skin tunnel rather than enclosed in a skin loop was performed in 10 sheep. Preliminary results indicate significant reduction in the incidence of complications associated with the standard CL.

Topical anesthetic-induced methemoglobinemia in sheep: a comparison of benzocaine and lidocaine.

Benzocaine induces methemoglobin (MHb) in several species, whereas lidocaine may increase MHb in cats and human. Elevated MHb (greater than 20%) in sheep after benzocaine exposure was recently recognized. MHb decreases blood oxygen-carrying capacity which can complicate interpretation of experimental data. Sheep are used in research which requires tracheal intubation and blood gas analysis. Since benzocaine and lidocaine are used to provide local anesthesia prior to intubation, we compared MHb production by sheep after exposure to these drugs. A dose-response relationship between benzocaine and MHb was investigated. Eight crossbred Dorset ewes were dosed intranasally with benzocaine for 2 sec or with 40 mg of lidocaine in a randomized crossover design. Sheep with elevated MHb after the 2-sec benzocaine dose were later dosed with benzocaine intranasally for 10 sec. MHb levels were measured periodically on a CO-Oximeter. A quantitative MHb response to an indirect MHb former, p-aminopropiophenone (PAPP), by each sheep was determined 15 min after PAPP (0.6 mg/kg iv). MHb levels remained at baseline (1-2%) after lidocaine exposure in all sheep, as well as in four sheep (nonresponders) after the 2-sec benzocaine dose. Four sheep (responders) demonstrated 16.5-26.4% MHb after the 2-sec benzocaine dose. The responders formed 38.2-50.5% MHb after the 10-sec benzocaine dose. All responders developed high MHb after PAPP, while nonresponders developed slightly elevated MHb after PAPP. An N-hydroxy metabolite of benzocaine is the likely active MHb-forming substance. Benzocaine should be replaced by lidocaine when local anesthesia of the nasal or oropharyngeal region in sheep is required.

Cardiopulmonary effects of a tiletamine-zolazepam combination in sheep.

To assess the effects on heart and lung function, a tiletamine-zolazepam (TZ) anesthetic combination was evaluated in 10 Dorset-type ewes. Ewes were randomly allotted to 2 equal groups. Ewes of groups 1 and 2 were given a single bolus of TZ (12 and 24 mg/kg of body weight, IV, respectively) at time zero. Hemodynamic, pulmonary, and ventilation variables were measured at 15-minute intervals to 120 minutes. Blood gas variables were evaluated at 5-minute intervals for the first 30 minutes, then at 15-minute intervals to 120 minutes. In all sheep, TZ administration induced rapid, smooth induction, with gradual and unremarkable recovery. Anesthesia duration was not significantly different between groups (mean +/- SD, 39 +/- 5 and 40 +/- 14 minutes for groups 1 and 2, respectively). Immediate drug effects included apnea, decreased mean arterial blood pressure, and arterial hypoxemia. Cardiac output was significantly decreased in both groups at all times after drug administration. Significant changes in group-1 ewes included increased pulmonary and systemic vascular resistances and decreased inspired minute ventilation, tidal volume, and respiratory airflow. Significant changes in group-2 ewes included increased systemic vascular resistance and decreased pulmonary arterial pressure, inspired minute ventilation, and respiratory airflow. Both drug dosages induced apneustic breathing patterns and caused significant changes in arterial and venous blood hemoglobin concentrations and PCV. Tiletamine-zolazepam is useful for intermediate-duration anesthesia in sheep.(ABSTRACT TRUNCATED AT 250 WORDS)

The cardiovascular response of sheep to tiletamine-zolazepam and butorphanol tartrate anesthesia.

Butorphanol tartrate (0.5 mg/kg intravenously [IV]) was administered to six ewes (group 1), 10 minutes before administration of tiletamine-zolazepam (12 mg/kg IV). In six ewes (group 2), butorphanol tartrate and tiletamine-zolazepam were administered simultaneously. Time of administration of butorphanol did not alter hemodynamics or duration of anesthesia significantly. Anesthesia was adequate for 25 to 45 minutes (mean, 31 min) in group 1. The sheep in group 2 were anesthetized effectively for 25 to 50 minutes (mean, 39 min). Neither dosing regimen caused significant changes in right atrial pressure, heart rate, pulmonary vascular resistance, or pulmonary capillary wedge pressure. Mean arterial blood pressure (MABP) decreased an average of 18% from baseline values of 113 mm Hg to a minimum of 84 mm Hg at minute 60 in group 1, and from 111 mm Hg to 92 mm Hg at minute 75 in group 2. The decrease was significant only for group 1. Cardiac output (CO) was significantly decreased 24% from 6.6 L/min at minute 45 in group 1, and 32% from 6.3 L/min at minute 15 in group 2. Systemic vascular resistance (SVR) was increased significantly at minute 15, 11% in group 1 and 37% in group 2. Mild respiratory acidosis was measured by significant decreases in arterial pO2 and pH and a significant increase in pCO2 without significant changes in HCO3-. Results of this study show that (1) tiletamine-zolazepam and butorphanol tartrate produce adequate anesthesia for 25 to 50 minutes; (2) the cardiovascular and anesthetic effects of the dosing schedules were similar; and (3) tiletamine-zolazepam and butorphanol result in decreased CO and MABP with a concomitant increase in SVR, and mild respiratory acidosis.