Blood concentrations of remifentanil during and after infusion in horses anesthetized with isoflurane and dexmedetomidine.

OBJECTIVE: To determine blood remifentanil concentration in isoflurane-anesthetized horses during and after a 1h remifentanil and dexmedetomidine infusion. STUDY DESIGN: Prospective study. ANIMALS: Six adult mixed breed horses with (mean±SD) bodyweight of 507±61kg and 14±4years of age. METHODS: Following sedation with xylazine IV, anesthesia was induced with ketamine IV mixed with diazepam IV. Anesthesia was maintained with isoflurane in oxygen. After 52±7min for instrumentation, dexmedetomidine (0.25µgkg(-1) followed by 1.0µg(-1)kg(-1)h(-1)) and remifentanil infusions (6µgkg(-1)h(-1)) were administered for a minimum of 60min and horses recovered from anesthesia. Drug infusions were administered into the left jugular vein. Blood was sampled (4mL) from the right jugular vein at predefined intervals before and during administration of remifentanil infusion. Following catheter flush, blood was sampled from the left jugular vein after the infusion was terminated while the horse was recovering from anesthesia. Blood was placed into tubes containing sodium heparin with citric acid, flash frozen in liquid nitrogen, and stored at -80°C until analysis. Blood remifentanil concentration was measured using high performance liquid chromatography and tandem mass spectrometry. RESULTS: Mean peak remifentanil concentration was 7.14ηgmL(-1) at 50min after start of infusion. Mean volume of distribution was 268±40mLkg(-1) and mean half-life was 12.8min. Blood concentration decreased to 1ηgmL(-1) 27min after termination of infusion. Limit of quantification was 0.2ηgmL(-1).

Cardiopulmonary effects of an infusion of remifentanil or morphine in horses anesthetized with isoflurane and dexmedetomidine.

OBJECTIVE: To examine the cardiopulmonary effects of infusions of remifentanil or morphine, and their influence on recovery of horses anesthetized with isoflurane and dexmedetomidine. STUDY DESIGN: Randomized crossover study with 7-day rest periods. ANIMALS: Six adult horses (507 ± 61 kg). METHODS: After the horses were sedated with xylazine, anaesthesia was induced with ketamine and diazepam, and maintained with isoflurane. After approximately 60 minutes, a dexmedetomidine infusion was started (0.25 µg kg(-1) then 1.0 µg(-1) kg(-1) hour(-1) ) in combination with either saline (group S), morphine (0.15 mg kg(-1) then 0.1 mg kg(-1) hour(-1) ; group M), or remifentanil (6.0 µg kg(-1) hour(-1) ; group R) for 60 minutes. Mean arterial pressure, heart rate, end-tidal carbon dioxide tension, and end-tidal isoflurane concentration were recorded every 5 minutes. Core body temperature, cardiac output, right ventricular and arterial blood-gas values were measured every 15 minutes. Cardiac index, systemic vascular resistance (SVR), intrapulmonary shunt fraction, alveolar dead space, oxygen delivery and extraction ratio were calculated. Recoveries were videotaped and scored by two observers blinded to the treatment. Data were analyzed using repeated measures anova followed by Dunnett's or Bonferroni's significant difference test. Recovery scores were analyzed using a Kruskal-Wallis test. RESULTS: No significant differences were found among groups. Compared to baseline, heart rate decreased and SVR increased significantly in all groups, and cardiac index significantly decreased in groups S and M. Hemoglobin concentration, oxygen content and oxygen delivery significantly decreased in all groups. The oxygen extraction ratio significantly increased in groups M and R. Lactate concentration significantly increased in group S. Recovery scores were similar among groups. CONCLUSIONS AND CLINICAL RELEVANCE: Dexmedetomidine alone or in combination with remifentanil or morphine infusions was infused for 60 minutes without adverse effects in the 6 healthy isoflurane-anesthetized horses in this study.

Stimulating thoracic epidural placement via a lumbar approach causes significant spinal cord damage in a porcine model.

BACKGROUND: Continuous thoracic epidural analgesia is a valuable and common technique for analgesia but involves risk to the spinal cord. There is significant pediatric experience safely placing thoracic epidurals via a caudal approach. The use of a stimulating catheter offers the advantage of real-time confirmation of appropriate catheter placement. We hypothesize that the tip of a stimulating epidural catheter can be reliably advanced to the thoracic epidural space with lumbar insertion in a porcine model. METHODS: This prospective experimental porcine study evaluated the feasibility of placing the tip of a stimulating epidural catheter to a predefined thoracic epidural location after percutaneous lumbar epidural access in six live pigs. After the lumbar epidural space was accessed, a stimulating epidural catheter was advanced until the targeted thoracic myotome was stimulated. The final position of the catheter in relation to the targeted location was determined by fluoroscopy. All animals were euthanized at the end of the experiment, necropsy and spinal cord histology were then performed to assess the extent of spinal cord damage. RESULTS: In all animals the epidural catheter tip could be accurately advanced to the targeted thoracic myotome. Gross subdural bleeding occurred in three of the six animals and deep spinal damage was observed in two of the six animals. In one animal, the catheter was placed in the subarachnoid space. CONCLUSIONS: Accurate access to the thoracic epidural space is possible via a lumbar approach using a stimulating epidural catheter. Based on gross and histopathological examination, this technique resulted in frequent complications, including subdural hemorrhage, deep spinal cord damage, and subarachnoid catheter placement.

Prolonged anesthesia using sevoflurane, remifentanil and dexmedetomidine in a horse.

HISTORY: A 10-year old Arabian mare had a slow-growing mass on the lower right mandible and required a large partial mandibulectomy. PHYSICAL EXAMINATION: No abnormalities were detected apart from the mass. MANAGEMENT: A temporary tracheostomy was performed pre-operatively. Anesthesia was induced with xylazine followed by ketamine and diazepam. For 13 hours, anesthesia was maintained using sevoflurane, dexmedetomidine and remifentanil infusions, with the exception of surgical preparation time. Intra-operatively, ventilation was delivered through the cuffed tracheotomy tube. Heart and respiratory rates, ECG, arterial pressures, inspired and expired gases, pulse oximetry values and body temperature were monitored. Dobutamine and whole blood were necessary, and romifidine was used to control recovery. Post-operatively, phenylbutazone and buprenorphine given systemically and bupivacaine administered through a wound soaker catheter were used to provide analgesia. Head-shaking from buprenorphine was controlled with acepromazine and detomidine once standing after 87 minutes in recovery. For 3 days after surgery, analgesia was provided with butorphanol, phenylbutazone and bupivacaine. The mare recovered well, appeared comfortable and started eating the following day with no signs of ileus. FOLLOW-UP: Seven months later, the mare was doing well. CONCLUSIONS: Sevoflurane, dexmedetomidine and remifentanil infusions were suitable for a long and invasive procedure.