Effects of Carprofen and Morphine on the Minimum Alveolar Concentration of Isoflurane in Dogs

Animals

Eight 1.5- to 3-year-old mixed-breed dogs (four females and four males) with a mean ± standard deviation (SD) body weight of 22.0±1.7 kg were used in the study. All dogs had unremarkable physical examinations, serum biochemical analyses, and complete blood counts prior to the study. All dogs received routine vaccinations and were dewormed on a regular basis; results of an antibody heartworm test were negative. The study protocol was approved by the Oklahoma State University Animal Care and Use Committee.

Study Design and Procedure

A randomized, complete-block, crossover study design was used; MAC of isoflurane was determined in each dog following administration of carprofen alone, morphine alone, carprofen and morphine, and neither drug (control). A minimum of 7 days elapsed between treatments.

Dogs were randomly assigned to one of two groups. Food was withheld from all dogs overnight. Three hours prior to induction of anesthesia, dogs in the first group were given a small amount of canned food containing carprofen^a orally at a dose of 2.2 mg/kg body weight. Dogs in the other group were given an equivalent amount of canned food that did not contain any drug.

Anesthesia was induced by administering isoflurane in oxygen, using a facemask via a nonrebreathing circuit connected to an anesthesia machine. A cuffed endotracheal tube was placed; dogs were positioned in right lateral recumbency; and anesthesia was maintained with isoflurane in oxygen (2 L per minute) delivered via a circle anesthetic rebreathing system and an out-of-circuit vaporizer. Mechanical ventilation was instituted to maintain end-tidal partial pressure of carbon dioxide (PETCO₂) between 35 and 45 mm Hg. Esophageal temperature was maintained between 37°C and 38°C (98.6°F and 100.4°F), using heating pads and blankets. Heart rate, cardiac rhythm, respiratory rate, PETCO₂, indirect blood pressure, and saturation of hemoglobin with oxygen (SpO₂) were monitored. Heart rate and cardiac rhythm were monitored by electrocardiography.^b Blood pressure was measured using an oscillometric device.^c Pulse oximetry^d was used to measure SpO₂. A commercially available adaptor^e modified with a catheter was placed at the Y piece of the breathing circuit. The catheter passed through the endotracheal tube so that its tip rested in the thoracic portion of the trachea. Samples of airway gases were obtained from the catheter and analyzed by use of a side-stream capnograph^f and anesthetic agent monitor,^f which determined respiratory rate, PETCO₂, and end-tidal isoflurane concentration. Using a commercial calibration kit,^g the anesthetic agent monitor was calibrated prior to each determination of MAC of isoflurane in each dog.

Minimum alveolar concentration of isoflurane was determined by use of the tail clamp method,¹⁰ with minor modifications. Briefly, dogs were allowed to equilibrate for 20 to 30 minutes at an end-tidal isoflurane concentration of 1.3%. Hair was clipped from a section of the tail with a diameter approximately equivalent to that of a standard Backhaus towel clamp. A towel clamp was then placed around the tail and closed to the third ratchet. The clamp was left in place for 60 seconds or until gross purposeful movement was evident. The clamp circumscribed the tail and did not puncture the skin of the dog, producing a blunt force on the tail. This method has been used in several previous studies.^8,11,12

If the dog exhibited any purposeful movement in response to tail clamping, the end-tidal isoflurane concentration was increased by 0.1% to 0.2%, and the dog was retested after 15 to 20 minutes of re-equilibration. If the dog did not exhibit any purposeful movement in response to tail clamping, end-tidal isoflurane concentration was reduced by 0.1% to 0.2%, and the dog was retested after 15 to 20 minutes of re-equilibration. Purposeful movement was defined as substantial movement of the head or extremities and did not include coughing, chewing, swallowing, or an increased respiratory effort. Testing was continued until the lowest end-tidal isoflurane concentration at which the dog did not demonstrate any purposeful movement in response to tail clamping. The MAC was then calculated as the mean of the end-tidal isoflurane concentration at which the dog did not demonstrate any purposeful movement and the next lower concentration tested (i.e., the highest concentration at which the dog still demonstrated purposeful movement in response to tail clamping).

For each dog, MAC was determined in triplicate (meaning the clamping procedures were repeated three consecutive times, and the end-tidal concentrations were averaged to yield a mean final MAC). The individual determining the MAC did not know which treatment the dogs had received. Cardiorespiratory data were collected at the time of MAC determination.

After the initial MAC determination, morphine^h was administered at a dose of 1.0 mg/kg intravenously (IV), and MAC was determined again. To avoid potential histamine release, the total volume of morphine was given in small quantities (0.1 to 0.2 mL as a bolus injection over a course of 3 minutes, with normal saline used to flush in between each morphine injection). Determination of MAC after morphine administration was completed within 60 to 80 minutes. The dogs were allowed to recover from anesthesia, and the entire procedure was completed within 6 hours of carprofen administration.

A minimum of 7 days later, the dogs were anesthetized again; however, treatments were reversed so that 3 hours prior to anesthesia, dogs in the first group were given a small amount of canned food that did not contain any drug, and dogs in the other group were given an equivalent amount of canned food containing carprofen^a at a dose of 2.2 mg/kg body weight. The MAC of isoflurane was determined as above.

Statistical Analyses

For each of the three treatments (carprofen alone, morphine alone, and carprofen and morphine together), the percent reduction in the MAC of isoflurane compared with the control value was determined using the following equation:

All results were expressed as mean ± SD. Cardiorespiratory data and MAC were analyzed using analysis of variance for repeated measures.¹³ Least significant difference test was used for multiple comparison between treatment means. The experimental design was a 2 × 2 factorial arrangement of treatments in a randomized, complete-block design. The two factors of interest were carprofen and morphine; dogs served as the blocking factor. The interaction of carprofen and morphine was used to evaluate whether the change in MAC departed from an additive model. If the interaction term was significant, then the effect was synergistic (the combination of carprofen and morphine resulted in a MAC lower than expected if it was assumed that effects of the two drugs were additive) or antagonistic (the combination of carprofen and morphine resulted in a MAC higher than expected if it was assumed that effects of the two drugs were additive). If the interaction term was not significant, then the main effects of carprofen and morphine were examined. For all analyses, values of P<0.05 were considered significant.