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SUMMARY

Objective

To evaluate effect of incremental doses of alfentanil on isoflurane minimum alveolar concentration (MAC) in cats to determine whether alfentanil reduces isoflurane MAC and, if so, maximal isoflurane MAC reduction.

Animals

6 healthy spayed female cats.

Procedure

Cats were anesthetized with isoflurane and instrumented to allow collection of arterial blood for measurement of gas tensions, pH, and plasma alfentanil concentration and to measure arterial blood pressure. Isoflurane MAC was determined in triplicate, and alfentanil was administered IV, using a computer-driven syringe pump to achieve estimated plasma alfentanil concentrations of 50, 100, 250, 500, 750, and 1,000 ng/ml; isoflurane MAC was determined at each alfentanil concentration. Cats were allowed to recover, and the process was graded as poor, good, or excellent.

Results

Alfentanil had a significant dose effect on isoflurane MAC reduction. Significant regression was found for normalized isoflurane MAC versus estimated plasma alfentanil concentration. A quadratic term was necessary to fit the model and, using this curve, MAC reduction (35.0 ± 6.6%) was estimated to be maximal at a plasma alfentanil concentration of 500 ng/ml. Significant differences were evident in rectal temperature, bicarbonate concentration, base deficit, arterial carbon dioxide and oxygen tensions, and arterial pH between isoflurane alone and some plasma alfentanil concentration and the corresponding reduction in isoflurane concentration.

Conclusions

Infusion of alfentanil resulted in maximal MAC reduction midway between that reported for horses and dogs. At such plasma alfentanil concentration, adverse effects were minimal, but included increase in rectal temperature, metabolic acidosis, and decrease in Pao2 . Provided cats were not handled during the recovery period, recovery was smooth and quiet.

Clinical Relevance

Infusion of alfentanil decreases the need for potent inhalant anesthetics in cats and could potentially be a clinically useful anesthetic regimen in sick cats. (Am J Vet Res 1997;58:1274–1279)

Free access
in American Journal of Veterinary Research

SUMMARY

Objective

To determine whether administration of opioids to anesthetized cats induced less cardiovascular depression than that induced by an equivalent amount of anesthetic alone, and to measure endocrine responses to a noxious stimulus.

Animals

6 healthy female cats.

Procedure

Anesthesia was induced with isoflurane and was maintained for 60 minutes at 1.3 isoflurane MAC. Blood gas tensions, pH, and plasma alfentanil and hormone concentrations, blood pressures, and cardiac output were measured. A noxious stimulus was applied for 5 minutes, while blood acquisition and measurements were repeated. Alfentanil was administered IV to achieve estimated plasma concentration of 500 ng/ml, and end-tidal isoflurane concentration was reduced by 35%. After another 60 minutes, blood was obtained and measurements were taken, then a second 5-minute noxious stimulus was applied while blood acquisition and measurements were retaken.

Results

Alfentanil administration and reduction of isoflurane concentration significantly increased body temperature, heart rate, mean arterial pressure, mean pulmonary arterial pressure, stroke index, cardiac index, hemoglobin, oxygen delivery index, Pvo2 and Pvco2 , dopamine, epinephrine (EPI), norepinephrine (NOREPI), and cortisol values, and significantly decreased arterial and venous pH. Application of a noxious stimulus significantly increased heart rate, stroke index, cardiac index, Pao2 , oxygen delivery index, arterial and venous pH, and NOREPI values, and decreased bicarbonate, Paco2 , Pvco2 , and EPI values. Alfentanil administration blunted cardiac index, Paco2 , oxygen delivery index, arterial pH, Pao2 , and EPI, and NOREPI responses to a noxious stimulus.

Conclusions

Compared with isoflurane alone, alfentanil administration and reduction of isoflurane MAC improved cardiovascular variables, and blunted respiratory, hormonal, and most hemodynamic responses to a noxious stimulus in cats.

Clinical Relevance

Use of the balanced opioid anesthesia regimen induced some beneficial effects in healthy cats; effects were similar to, although greater in nature, than effects induced by a noxious stimulus. (Am J Vet Res 1997;58:1267–1273)

Free access
in American Journal of Veterinary Research

SUMMARY

The compound nerve action potential (cnap) of the superficial peroneal nerve of dogs was investigated to determine: (1) the influence of the stimulation technique on the configuration of the cnap, with particular attention to late components; (2) the fiber diameter (fd) distribution; and (3) the relationship between fd distribution and cnap configuration, by reconstruction of cnap made on the basis of fd distributions.

The cnap were evoked in 9 dogs under halothane anesthesia by 2 stimulation methods: percutaneous needle electrode stimulation and direct stimulation of the exposed superficial peroneal nerve. Recordings were made with percutaneous needle electrodes. Full nerve cross sections of 7 superficial peroneal nerves were prepared for fd morphometric analysis. Reconstruction of cnap were made on the basis of the fd distributions.

Late components of the cnap could be evoked with either stimulation method, but only with a stimulus intensity of 3 to 5 times maximal for the main (early) component of the cnap. The fd histograms of 7 analyzed nerves had bimodal distribution. In 5 nerves, peaks were at 4.2 to 4.5 μm and 9.0 to 10.0 μm, with 60% of the fibers in the small-diameter group. In 2 nerves with lower maximal conduction velocities, peaks were shifted toward smaller values.

The cnap reconstructions made by use of fd data closely resembled actual recordings when a fifth-order polynomial function was applied to the relationship between nerve conduction velocity and fd. Reconstructions made by use of 1 or 2 linear functions did not accurately resemble actual recordings.

The results indicate clinical sensory electroneurographic recordings can provide accurate information regarding both large- and small-diameter fibers, if adequate stimulus intensities are used. To understand the recorded potential more completely, further studies are needed to determine the effects of volume conduction on configuration of the cnap. It should then be possible to estimate fd distributions even more accurately by analyzing cnap of normal nerves, or of diseased nerves in which the normal relation between fd and conduction velocity is preserved.

Free access
in American Journal of Veterinary Research

Abstract

Objective—To evaluate sensitivity of 4 commercially available microchip scanners used to detect or read encrypted and unencrypted 125-, 128-, and 134.2-kHz microchips under controlled conditions.

Design—Evaluation study.

Sample Population—Microchip scanners from 4 manufacturers and 6 brands of microchips (10 microchips/brand).

Procedures—Each microchip was scanned 72 times with each scanner passed parallel to the long axis of the microchip and 72 times with each scanner passed perpendicular to the long axis of the microchip. For each scan, up to 3 passes were allowed for the scanner to read or detect the microchip. Microchip and scanner order were randomized. Sensitivity was calculated as the mean percentage of the 72 scans for each microchip that were successful (ie, the microchip was detected or read).

Results—None of the scanners had 100% sensitivity for all microchips and both scanning orientations, and there were clear differences between scanners on the basis of operating frequency of the microchip, orientation of the microchip, and number of passes used to detect or read the microchip. For the 3 scanners designed to detect or read microchips of all 3 frequencies currently used in the United States, sensitivity was highest for 134.2-kHz microchips and lower for 125- and 128-kHz microchips. None of the scanners performed as well when only a single pass of the scanner was used to detect or read the microchips.

Conclusions and Clinical Relevance—Results indicated that use of multiple passes in different directions was important for maximizing sensitivity of microchip scanners.

Full access
in Journal of the American Veterinary Medical Association