Cardiorespiratory effects of variable pressure thoracic insufflation in cats undergoing video-assisted thoracic surgery.

OBJECTIVE: To evaluate the effects of intrathoracic insufflation on cardiorespiratory variables and working space in cats undergoing video-assisted thoracic surgery. STUDY DESIGN: Prospective randomized study. ANIMALS: Six healthy cats. METHODS: Cats were anesthetized using a standardized protocol. A Swan-Ganz catheter was positioned in the pulmonary artery under fluoroscopic guidance for measurement of cardiac output. Intrathoracic pressures (ITP) of 0 (baseline), 3, and 5 mm Hg were induced with CO2 and maintained for 30 minutes. Statistical comparison of cardiorespiratory variables was performed. After the procedures, all cats were recovered from anesthesia. Videos of thoracic working space at each ITP level were scored in a blinded fashion by 3 board-certified surgeons using a numerical scale from 0-10. RESULTS: All cats tolerated insufflation with 3 and 5 mm Hg for 30 minutes without oxygen desaturation, although ventilatory levels had to be increased substantially to maintain eucapnia and oxygenation. Cardiac index was not significantly different from baseline after 30 minutes at 3 mm Hg but was significantly lower after 30 minutes at 5 mm Hg compared with 3 mm Hg. Oxygen delivery was unaffected by 3 or 5 mm Hg compared with baseline. Scores for working space increased between baseline and 3 and 5 mm Hg but were not different between 3 and 5 mm Hg. CONCLUSION: CO2 insufflation to 5 mm Hg seems well tolerated in healthy cats, provided ventilatory settings are substantially increased as ITP increases. CLINICAL SIGNIFICANCE: Thoracic CO2 insufflation of 3 mm Hg in cats during video-assisted thoracic surgery is associated with less hemodynamic perturbation than 5 mm Hg insufflation and may provide the benefit of improved working space compared with baseline.

Video-assisted thoracoscopic thoracic duct sealing is inconsistent when performed with a bipolar vessel-sealing device in healthy cats.

OBJECTIVE: To describe a technique for video-assisted thoracoscopic (VATS) thoracic duct ligation (TDL) in normal cats with a bipolar vessel-sealing device and to assess durability of the seal. STUDY DESIGN: Prospective case series. ANIMALS: Six healthy research cats. METHODS: Cats were placed under anesthesia for computed tomography lymphangiography (CTLA) to identify thoracic duct anatomy. On the basis of CTLA findings, cats were positioned in either right or left lateral recumbency for a 3-port VATS TDL. Thoracic duct branches were dissected from the aorta after subcutaneous indirect near-infrared fluorescence (NIRF) lymphography with indocyanine green was performed to optimize detection. A vessel-sealing device was used to seal each thoracic duct in 1 or more locations. Postattenuation, indirect NIRF lymphography was repeated to confirm complete occlusion of thoracic duct flow. CTLA was repeated in all cats 3 months postoperatively. RESULTS: The thoracic duct was surgically approached from the right in 3 cats and from the left in 3 cats. A median of 2.5 (range 1-6) TDL seal sites were applied. In 2 cats, leakage of chyle was detected during dissection. At 3 months postoperatively, CTLA confirmed reestablished chylous flow in 5 of 6 cats, appearing to occur through recanalization of previously sealed sites rather than through development of de novo lymphatic vessels. CONCLUSION: VATS TDL is feasible in cats, although the variable and delicate nature of feline thoracic duct anatomy should be considered preoperatively. CLINICAL SIGNIFICANCE: Bipolar vessel-sealing devices are not a durable modality for thoracic duct sealing in healthy cats in a seal only fashion.

A comparison of retrobulbar and two peribulbar regional anesthetic techniques in dog cadavers.

OBJECTIVE: To compare injectate distribution and likelihood of regional anesthesia to the orbit following retrobulbar (RB) or peribulbar (PB) injections in dog cadavers. STUDY DESIGN: Randomized, masked study. ANIMALS: Twenty-four dog cadavers (aged 5.5-17 years, 2.0-36.3 kg). METHODS: Orbits underwent one of three injection techniques with bupivacaine 0.5% and iohexol (1:1): ventrolateral RB injection (1-2 mL; 15 orbits), medial canthal PB injection (2-8 mL; PB-1; 16 orbits), or dorsomedial and ventrolateral PB injections (each 1-4 mL; PB-2; 16 orbits). The likelihood of successful regional anesthesia was estimated based on computed tomographic images scored for injectate volume of distribution at the base and within the extraocular muscle cone (EOMC), and injectate distribution around the optic nerve. Intraocular pressure (IOP) was measured before and after injections. Mixed-effects linear regression with post hoc Bonferroni contrast adjustments was performed. Significance was set at 0.05. RESULTS: A difference in injectate volume of distribution within or at the base of the EOMC was not detected among groups. The median optic nerve circumference of injectate distribution was significantly higher in the RB injected group than in the PB-2 group. Injectate distribution following RB, PB-1 and PB-2 injections was graded as likely to provide regional anesthesia within the EOMC in 40%, 19% and 31% of eyes, and at the EOMC base in 60%, 63% and 50% of eyes, respectively. The probability of likelihood to provide regional anesthesia was lower in dogs of higher body weights. The IOP was significantly higher than baseline following PB-1 (18 ± 14 mmHg) and in comparison with RB (2 ± 3 mmHg), but not different from PB-2 injection (10 ± 11 mmHg). CONCLUSIONS AND CLINICAL RELEVANCE: None of the techniques reliably produced 'successful' injectate distribution based on this study's definitions; however, clinical assessment of anesthetic success is required.

Pain and analgesia following onychectomy in cats: a systematic review.

OBJECTIVES: To systematically review published studies evaluating pain associated with onychectomy in cats, and to assess the efficacy of the analgesic therapies applied. DATABASES USED: Four sources were used to identify manuscripts for review. Databases searched were those of the National Library of Medicine, EMBASE and CAB International. In addition, pertinent references in the bibliographies of included articles were retrieved. RESULTS: Twenty manuscripts published in refereed journals were reviewed. These included papers reporting 18 clinical trials and two studies conducted in conditioned research cats. Twelve analgesics were evaluated, including seven opioids, four non-steroidal anti-inflammatory drugs and one local anesthetic. Nine studies involved a direct comparison of analgesic agents. Limb use was abnormal when measured at 2 and 12 days following onychectomy, and neither fentanyl patch nor butorphanol administration resulted in normal use of the surgical limb. In another study, cats evaluated at 6 months after this surgery were not lame. Differing surgical techniques were compared in six studies; the results indicated that pain scores were lower after laser surgery than after scalpel surgery. The difficulties associated with assessing pain in cats and the lack of sensitivity of the evaluation systems utilized were highlighted in many of the studies. Huge variations in dose and dosing strategies had significant impacts on drug efficacy. Statistically significant differences among treatments were found in most studies; however, no clearly superior analgesic treatment was identified. A combination of meloxicam or robenacoxib with an opioid may provide more effective analgesia and should be evaluated.

Endoscopic treatment of an intrathoracic tracheal osteochondroma in a dog.

CASE DESCRIPTION: A 1.5-year-old spayed female Bernese Mountain Dog was examined for a 6-month history of intermittent vomiting, regurgitation, wheezing, and coughing. Initially, a diagnosis of gastroesophageal reflux disease with secondary aspiration pneumonitis was made but clinical signs did not resolve with treatment. CLINICAL FINDINGS: Thoracic and cervical radiography and CT revealed a sessile, irregularly marginated soft tissue opacity at the level of the fourth rib. Results of a CBC, serum biochemical analysis, and urinalysis were within reference limits. Results of abdominal ultrasonography were normal. TREATMENT AND OUTCOME: Tracheoscopy revealed a firm, irregularly marginated mass apparently originating from the ventral aspect of the trachea, occluding approximately one-half of the tracheal lumen, and located 2 cm cranial to the carina. Cytologic and histopathologic examination of fine-needle aspirate and biopsy samples suggested a benign etiology; therefore, endoscopic minimally invasive laser and electrocautery resection of the mass was scheduled. A total IV anesthetic protocol was administered with an oxygen-air mixture used to decrease the risk of fire during tracheal surgery. The mass was successfully resected, and histopathologic examination confirmed a diagnosis of osteochondroma. Clinical signs resolved, and at follow-up 32 months later, no regrowth of the mass was evident. CLINICAL RELEVANCE: Tracheoscopy-guided electrocautery and surgical diode laser resection was successful in removing an obstructive tracheal mass that was not resectable by means of a conventional open surgical approach. Minimally invasive procedures may decrease morbidity and mortality and improve outcome in appropriately selected small animal patients.

Effect of One-Lung Ventilation With or Without Low-Pressure Carbon Dioxide Insufflation on Cardiorespiratory Variables in Cats Undergoing Thoracoscopy.

OBJECTIVES: To document a technique for one-lung ventilation (OLV) in cats and evaluate the effect of low-pressure carbon dioxide insufflation and OLV (OLV-CDI) on cardiorespiratory variables in cats. STUDY DESIGN: Prospective randomized study. ANIMALS: Cats (n = 6). METHODS: General anesthesia was induced using a standardized anesthetic protocol. A thermodilution catheter was placed into the pulmonary artery using fluoroscopic guidance. Two 6 mm thoracoscopic cannulae were placed at a subxiphoid and intercostal location, respectively, to allow direct observation of all lung lobes. OLV was induced using an endobronchial blocker, which was placed into the right and left main stem bronchi in randomized order using bronchoscopic guidance. Cardiorespiratory variables were measured at 5 and 30 minutes after induction of OLV and subsequently at 5 and 30 minutes after initiation of OLV-CDI at intra-thoracic pressures of 3 mmHg. RESULTS: Left-sided OLV was successful in 5 cats, right-sided OLV was successful in 2 cats, and neither was successful in 1 cat. No significant effects on hemodynamic variables or oxygen delivery were observed after right or left-sided OLV alone. Hemodynamic variables were also well-preserved after OLV-CDI; however, oxygen delivery was significantly lower after left OLV-CDI compared with right OLV-CDI, and in 2 cats, severe desaturation occurred after L-OLV-CDI before the 30 minute time point. CONCLUSIONS: OLV can be used in cats for thoracoscopic interventions. Although right OLV-CDI was tolerated better than left OLV-CDI, the technique requires further investigation before it can be recommended for widespread clinical use.

Prospective evaluation of access incision position for minimally invasive surgical organ exposure in cats.

OBJECTIVE: To evaluate the exposure obtained for minimally invasive abdominal organ biopsy (MIOB) from 3 access incisions in cats. DESIGN: Prospective experimental study and clinical case series. ANIMALS: 6 purpose-bred research cats and 6 feline clinical patients with indications for abdominal organ biopsy. PROCEDURES: Three 3-cm incisions into the peritoneal cavity were created at different locations along the linea alba in research cats in randomized order. A wound retraction device was inserted in each incision. Ability to exteriorize various abdominal organs to the extent required to reasonably perform a surgical biopsy was recorded, and results were compared among incision sites. On the basis of results obtained, the access incision that provided exposure of the most frequently biopsied abdominal organs was used to perform MIOB in 6 feline clinical patients with various underlying pathological conditions. RESULTS: On the basis of experiments with research cats, a 3-cm access incision centered midway between the caudal margin of the xiphoid cartilage and the umbilicus was found to provide access for MIOB for most organs. In 5 of 6 clinical patients, all of the organs of interest were biopsied successfully via this incision location, although access to all hepatic lobes and all parts of the pancreas was inconsistent. In 1 cat, conversion to an open approach was performed because a palpable mass was detected in the area of the duodenocolic ligament. CONCLUSIONS AND CLINICAL RELEVANCE: Optimization of access incision location for MIOB allowed biopsy specimen collection from organs of interest to be performed in a minimally invasive manner in cats.

The challenge of evaluating pain and a pre-incisional local anesthetic block.

Background. Our objective was to test the effectiveness of a local anesthetic line block administered before surgery in reducing postoperative pain scores in dogs undergoing ovariohysterectomy (OVHX). Methods. This study is a prospective, randomized, blinded, clinical trial involving 59 healthy female dogs. An algometric pressure-measuring device was used to determine nociceptive threshold, and compared to three subjective pain scales. Group L/B received a line block of lidocaine (4 mg/kg) and bupivacaine (1 mg/kg) subcutaneously in the area of the incision site and saline subcutaneously as premedication; group L/BM (positive control) received a similar block and morphine (0.5 mg/kg) subcutaneously for premedication; and group SS (negative control) received a saline line block and saline premedication. Criteria for rescue analgesia were defined before the study. Dogs were assessed prior to surgery, at extubation (time 0) and at 2, 4, 6, 8 and 24 h post-recovery. The data were analyzed with one-way ANOVA, and a Split Plot Repeated Measures ANOVA with one grouping factor and one repeat factor (time). P < 0.05 was considered statistically significant. Results. Approximately 33% of dogs required rescue analgesia at some point during the study, with no significant difference between groups. There was no significant difference between treatment groups with any assessment method. Conclusions. As there were no statistically significant differences between positive and negative controls, the outcome of this technique cannot be proven.

Effects of pneumoperitoneum induced at various pressures on cardiorespiratory function and working space during laparoscopy in cats.

OBJECTIVE: To evaluate the effect of pneumoperitoneum on cardiorespiratory variables and working space during experimental induction of 3 intra-abdominal pressures (IAPs) in cats. ANIMALS: 6 healthy young adult neutered male domestic shorthair cats. PROCEDURES: All cats were anesthetized through use of a standardized protocol. A catheter was placed in the right femoral artery for blood pressure and blood gas monitoring. A thermodilution catheter was placed in the right jugular vein via fluoroscopic guidance. Cardiopulmonary variables were measured before (baseline) and 2 and 30 minutes after initiation of pneumoperitoneum at IAPs of 4, 8, and 15 mm Hg; these were created through the use of a mechanical insufflator. At each IAP, abdominal dimensions (height, width, and circumference) were measured at a standardized location. RESULTS: At 4 mm Hg and 8 mm Hg IAP, no clinically important changes were identified in cardiorespiratory values. Heart rate, cardiac index, and stroke volume index remained unchanged throughout the study at all IAPs. Mean arterial blood pressure began to increase at 8 mm Hg and was significantly higher, compared with baseline, at both time points at 15 mm Hg. At 15 mm Hg, Paco2 was significantly higher and cats were more acidotic than at baseline. Working space was subjectively greater at 8 mm Hg than at 4 mm Hg IAP; however, at 15 mm Hg, no clinically important enlargement of the working space was identified, compared with at 8 mm Hg. CONCLUSIONS AND CLINICAL RELEVANCE: Values of cardiopulmonary variables were largely unchanged by induction of pneumoperitoneum in healthy cats up to an IAP of 8 mm Hg, and no clinically important increases in working space were evident at an IAP of 15 versus 8 mm Hg. These findings provide little justification for use of IAPs > 8 mm Hg in healthy cats undergoing laparoscopic procedures; however, whether the situation is similar in diseased or elderly cats remains to be determined.

Evaluation of short-term outcome after lung lobectomy for resection of primary lung tumors via video-assisted thoracoscopic surgery or open thoracotomy in medium- to large-breed dogs.

OBJECTIVE: To describe clinicopathologic features of dogs that underwent lung lobectomy for resection of primary lung tumors via video-assisted thoracoscopic surgery (VATS) or open thoracotomy (OT) and to compare short-term outcomes for dogs following these procedures. DESIGN: Retrospective cohort study. ANIMALS: 46 medium- to large-breed dogs with primary lung tumors. PROCEDURES: Medical records of dogs that underwent a lung lobectomy via VATS (n = 22) or OT (24) for resection of primary lung tumors between 2004 and 2012 were reviewed. Dogs were included if they weighed > 10 kg (22 lb) and resection of a primary lung tumor was confirmed histologically. Tumor volumes were calculated from preoperative CT scans where available. Surgical time, completeness of excision, time in the ICU, indwelling thoracic drain time, postoperative and total hospitalization time, incidence of major complications, and short-term survival rate were evaluated. RESULTS: VATS was performed with a 3-port (n = 12) or 4-port (10) technique and 1-lung ventilation (22). In 2 of 22 (9%) dogs, VATS was converted to OT. All dogs survived to discharge from the hospital. There were no significant differences between the VATS and OT groups with regard to most variables. Surgery time was significantly longer for VATS than for OT (median, 120 vs 95 minutes, respectively). CONCLUSIONS AND CLINICAL RELEVANCE: In medium- to large-breed dogs, short-term outcomes for dogs that underwent VATS for lung lobectomy were comparable to those of dogs that underwent OT. Further studies are required to evaluate the effects of surgical approach on indices of postoperative pain and long-term outcomes.

Evaluation of blind thoracoscopic-assisted placement of three double-lumen endobronchial tube designs for one-lung ventilation in dogs.

OBJECTIVE: To describe a technique for blind thoracoscopic-assisted double-lumen endobronchial tube (DLT) placement for achieving one-lung ventilation (OLV) using 3 different DLT designs and to evaluate whether thoracic auscultation could reliably confirm OLV in dogs. STUDY DESIGN: Prospective randomized study. ANIMALS: Mature female hound dogs (n = 6). METHODS: Physical examination, baseline blood work and thoracic radiography confirmed the absence of preexisting disease in all dogs. Thoracoscopic observation was established through a subxiphoid portal and used for evaluation of differential lung lobe ventilation. Each dog was sequentially intubated using 1 of 3 DLT designs in random order; Robertshaw left-sided tube (RS-L), Carlens left-sided tube (C-L), Dr. White right-sided tube (DW-R). Incidence of initial and overall (after a maximum of 3 DLT manipulations) correct and complete OLV (CC-OLV) was recorded. After each blind thoracoscopic-assisted DLT placement, bronchoscopic evaluation was performed to document correct DLT position. RESULT: Blind DLT placement achieved overall CC-OLV in all dogs using RS-L, 66% using C-L, and all using DW-R. Successful initial left-sided OLV (L-OLV) was statistically more likely when RS-L DLTs were used compared to C-L or DW-R. Of cases where overall CC-OLV was achieved, correct DLT position was present in only 44% of intubations. CONCLUSIONS: Blind DLT placement produced successful CC-OLV in all dogs using at least one DLT design evaluated. Use of thoracoscopic assistance may obviate the need for bronchoscopic observation during DLT placement in dogs. However, bronchoscopic observation may still refine DLT positioning in dogs after blind thoracoscopic-assisted DLT placement.

Use of the Ligasure vessel-sealing device for thoracoscopic peripheral lung biopsy in healthy dogs.

OBJECTIVE: To evaluate the use of a bipolar vessel-sealing device for peripheral lung lobe biopsy in healthy dogs. STUDY DESIGN: Experimental study. ANIMALS: Mature female hound dogs (n = 6). METHODS: Thoracoscopic access was obtained by placement of a subxiphoid telescope portal with instrument portals at the right 4-6th intercostal spaces and the left 10th intercostal space. The 10-mm Ligasure Atlas device was used to collect a biopsy specimen from the periphery of the left cranial lung lobe. Dogs were recovered from anesthesia. Histologic evaluation was used to characterize the effect of biopsy technique on pulmonary tissue. Thoracic radiography was performed at 1, 2, 4, and ∼180 days to evaluate for air leakage or other postoperative complications. RESULTS: Lung specimens were collected without complications. Postoperatively, 1 dog had some residual pneumothorax thought to be secondary to incomplete thoracic evacuation of air postoperatively. No evidence of air leakage was detected at any time point. Histologically, specimens had 3 distinct zones: a crush zone representing the tissue within the jaws of the device, a transition zone of thermally damaged parenchyma, and beyond that a viable zone of normal parenchyma. CONCLUSION: From these limited data, it appears that the Ligasure Atlas vessel-sealing device can be used for peripheral lung biopsy in healthy dogs. Further studies are required to evaluate the efficacy of the device in dogs with parenchymal disease.

Development of an avian brachial plexus nerve block technique for perioperative analgesia in mallard ducks (Anas platyrhynchos).

Surgical procedures of the wing are commonly performed in companion, captive, and wild avian species. To develop a clinically applicable brachial plexus nerve block technique for perioperative analgesia in birds, 8 adult female mallard ducks (Anas platyrhynchos) were anesthetized and used in several local anesthetic trials with bupivacaine (2 or 8 mg/kg) or a combination of lidocaine (15 mg/kg) and epinephrine (3.8 microg/kg) perineurally; equal volumes of saline were administered as control treatments. Both axillary and dorsal approaches to the brachial plexus were evaluated. With the axillary approach, radial and ulnar compound nerve action potentials (CNAP), sensory nerve conduction velocities (SNCVs), and cord dorsum potentials (CDPs) were recorded after distal sensory nerve stimulation. Values were recorded before and at 5, 30, and 60 minutes after injection of local anesthetic or saline. Birds were monitored for the presence of a wing droop and a change in motor function on recovery from anesthesia. Results were highly variable for all techniques. No technique significantly decreased CDPs or resulted in consistent wing droop. Radial and ulnar CNAPs, SNCVs, and CDPs were consistently recorded in all birds. Variable results might indicate that the treatment, concentration, or volume of local anesthetic used was ineffective in producing local anesthesia. Electrodiagnostic methods used in these ducks to assess loss of sensory nerve conduction might not be sensitive enough to assess the effects of local anesthesia. Further research is needed to identify methods for assessing the efficacy of brachial plexus nerve blockade in birds.

Changes in the minimum alveolar concentration of isoflurane and some cardiopulmonary measurements during three continuous infusion rates of dexmedetomidine in dogs.

OBJECTIVE: To measure the change in the minimum alveolar concentration of isoflurane associated with three constant rate infusions of dexmedetomidine. STUDY DESIGN: Prospective, randomized, and blinded experimental trial. Animals Six healthy 6-year-old Beagles weighing between 13.0 and 17.7 kg. METHODS: The dogs received each of four treatments; saline or dexmedetomidine at 0.1, 0.5 or 3 microg kg(-1) loading dose given intravenously (IV) over 6 minutes followed by infusions at 0.1, 0.5 or 3 microg kg(-1) hour(-1), respectively. There were 2 weeks between treatments. The dogs were mask-induced with and maintained on isoflurane in oxygen. Acetated Ringer's (5 mL kg(-1) hour(-1)) and saline or dexmedetomidine (each at 0.5 mL kg(-1) hour(-1)) were given IV. Pulse rate, blood pressure, samples for the measurement of blood gases, pH, lactate, packed cell volume (PCV), total protein (TP) and dexmedetomidine concentrations were obtained from an arterial catheter. Sixty minutes after induction minimum alveolar concentration (MAC) was determined by intermittently applying supramaximal electrical stimuli to the thoracic and pelvic limbs. Cardiopulmonary measurements and arterial blood samples were collected before each set of stimuli. Statistical analyses were conducted with analysis of variance or mixed models according to the experimental design. RESULTS: There was a significant decrease in the MAC of isoflurane associated with 0.5 and 3 microg kg(-1) hour(-1) but not with 0.1 mg kg(-1)hour(-1). Serum concentrations of dexmedetomidine were not measurable at the 0.1 mg kg(-1) hour(-1) and averaged 0.198 +/- 0.081 and 1.903 +/-0.621 ng mL(-1) for the 0.5 and 3 microg kg(-1) hour(-1) infusion rates, respectively. Heart rate decreased with increasing doses of dexmedetomidine while blood pressure increased. Packed cell volume increased at 3 microg kg(-1) hour(-1) but not with other doses. CONCLUSIONS AND CLINICAL RELEVANCE: Dexmedetomidine infusions decrease the intra-operative requirement for isoflurane and may be useful in managing dogs undergoing surgery, where the provision of analgesia and limitation of the stress response is desirable.

Effect of transdermally administered fentanyl on the minimum alveolar concentration of isoflurane in cats.

OBJECTIVE: To determine the effect of two doses of fentanyl, administered transdermally, on the minimum alveolar concentration (MAC) of isoflurane in cats. STUDY DESIGN: Prospective, randomized study. ANIMALS: Five healthy, spayed, female cats. METHODS: Each cat was studied thrice with at least 2 weeks between each study. In study 1, the baseline isoflurane MAC was determined in triplicate for each cat. In studies 2 and 3, isoflurane MAC was determined 24 hours after placement of either a 25 or 50 microg hour(-1) fentanyl patch. In each MAC study, cats were instrumented to allow collection of arterial blood and measurement of arterial blood pressure. Twenty-four hours prior to studies 2 and 3, a catheter was placed and secured in the jugular vein and either a 25 or 50 microg hour(-1) fentanyl patch was placed in random order on the left thorax. Blood samples for plasma fentanyl determination were collected prior to patch placement and at regular intervals up to 144 hours. After determination of MAC in studies 2 and 3, naloxone was administered as a bolus dose (0.1 mg kg(-1)) followed by an infusion (1 mg kg(-1) hour(-1)) and MAC redetermined. RESULTS: The baseline isoflurane MAC was 1.51 +/- 0.21% (mean +/- SD). Fentanyl (25 and 50 micro g hour(-1)) administered transdermally significantly reduced MAC to 1.25 +/- 0.26 and 1.22 +/- 0.16%, respectively. These MAC reductions were not significantly different from each other. Isoflurane MAC determined during administration of fentanyl 25 micro g hour(-1) and naloxone (1.44 +/- 0.16%) and fentanyl 50 micro g hour(-1) and naloxone (1.51 +/- 0.19%) was not significantly different from baseline MAC (1.51 +/- 0.21%). CONCLUSIONS AND CLINICAL RELEVANCE: Fentanyl patches are placed to provide long-lasting analgesia. In order to be effective postoperatively, fentanyl patches must be placed prior to surgery. Plasma fentanyl concentrations achieved intraoperatively decrease the need for potent inhalant anesthetics in cats.