Physiologic VDD versus nonphysiologic VVI pacing in canine 3rd-degree atrioventricular block.

Historically, ventricular demand, nonphysiologic (VVI) pacing has been the most commonly used modality to treat 3rd-degree atrioventricular (AV) block. The goal of this study was to determine the feasibility of using a commercial, single-lead, physiologic (VDD) pacemaker in dogs with 3rd-degree AV block. Furthermore, we hoped to characterize and identify differences in the radiographic, echocardiographic, neurohormonal, and quality of life consequences of physiologic versus nonphysiologic pacing. We evaluated 10 dogs during a 12-week crossover study. Acutely, rate-matched physiologic pacing reduced pulmonary capillary wedge pressure by 19% compared with nonphysiologic pacing. VDD pacing significantly reduced left atrial size normalized to body weight, left atrial-to-aortic root ratio, and left ventricular end-systolic dimension and increased fractional shortening, aortic Doppler velocity, cardiac output, and stroke volume compared with VVI pacing. Variable rate VDD pacing resulted in a significantly slower heart rate (HR) during echocardiography than fixed-rate (100 bpm) VVI pacing. AV synchronous pacing reduced circulating N-terminal proatrial natriuretic peptide (ANP), norepinephrine (NOR), and epinephrine (EPI) concentrations compared with asynchronous pacing. There were no significant differences in systemic blood pressure, thoracic radiographs, or owner-perceived quality of life. The median percentage of AV synchronous pacing during the VDD modality was 99.8% (range, 1.2 to 99.9%). This study confirms the potential to achieve physiologic pacing with a commercial, single-lead system in dogs. VDD pacing improved hemodynamics and neurohormonal profiles over asynchronous pacing although the long-term clinical benefits of these changes remain to be determined.

Determination of glomerular filtration rate in dogs using contrast-enhanced computed tomography.

The purpose of this project was to establish a procedure and reference values for glomerular filtration rate (GFR) using contrast-enhanced computed tomography (CT) in eight healthy dogs. A single section of the kidney was scanned sequentially after bolus injection (3 ml/s) of iohexol (300 mg/kg). Time-attenuation curves were constructed and the GFR per volume of kidney was calculated using Patlak graphical analysis software. The GFR was then converted from contrast clearance per unit volume (ml/min/ml) to contrast clearance per body weight (ml/min/kg). Individual kidney and global GFR were calculated using both CT and nuclear scintigraphy. Global GFR for each dog was also determined by plasma iohexol clearance. Contrast-enhanced CT underestimated the global GFR compared with the other two methods. The average global GFR was 2.57 +/- 0.33 ml/ min/kg using functional CT and 4.06 +/- 0.37 ml/min/kg using plasma iohexol clearance. There was significant (P < 0.05) interobserver variability of CT GFR of the right kidney and total GFR. There was decreased interobserver variability for the left kidney. There was no difference in the intraobserver variability for CT-determined individual kidney and global GFR. There was no difference between the motion corrected and nonmotion corrected values for individual and global CT GFR. Nuclear scintigraphy produced a slightly higher coefficient of variation than contrast-enhanced CT, 2.9% and 1.0%, respectively. It is hypothesized that altered renal blood flow, hematocrit of the small vessels, and nephrotoxicity play a role in the underestimation of GFR by contrast-enhanced CT.

Relationship of feline bispectral index to multiples of isoflurane minimum alveolar concentration.

The study reported here was done to determine the relationship between bispectral index (BIS) values and minimum alveolar concentration (MAC) multiples of isoflurane in cats. Isoflurane MAC was determined using the tail-clamp method in eight domestic cats. Ten days later, the cats were anesthetized a second time with isoflurane at each of five MAC multiples administered in random order. Ventilation was controlled and, after a 20-min equilibration period at each MAC multiple of isoflurane, BIS data were collected for 5 min and the median BIS value calculated. Data from each isoflurane MAC multiple were compared using analysis of variance for repeated measures, and statistical significance was set at P < 0.05. The MAC of isoflurane (mean +/- 1 standard deviation) was 1.8% +/- 0.2%. BIS values at 0.5 MAC could not be recorded due to spontaneous movement in all eight cats. BIS values at 2.0 MAC were confounded by burst suppression in seven of the eight cats. Over the range of 0.8 to 1.5 MAC, BIS values decreased significantly with increasing end-tidal isoflurane concentrations. Mean (+/- 1 standard deviation) BIS measurements were 32 +/- 3 at 0.8 MAC, 20 +/- 4 at 1.0 MAC, and 5 +/- 3 at 1.5 MAC. Therefore, BIS values are inversely and linearly related to end-tidal isoflurane concentrations in anesthetized cats. However, the consistently low BIS values recorded in this study suggest that clinical BIS endpoints used to titrate anesthetic agents in humans may not be applicable to cats.

Cardiopulmonary effects of fentanyl in conscious dogs and dogs sedated with a continuous rate infusion of medetomidine.

OBJECTIVE: To determine the hemodynamic consequences of the coadministration of a continuous rate infusion (CRI) of medetomidine with a fentanyl bolus in dogs. ANIMALS: 12 healthy sexually intact male dogs weighing 30.3 -/+ 4.2 kg (mean +/- SD). PROCEDURE: Dogs received either fentanyl alone (15.0 microg/kg, i.v. bolus) or the same dose of fentanyl during an 11-hour CRI of medetomidine (1.5 microg/kg/h, i.v.). Prior to drug administration, dogs were instrumented for measurement of cardiac output, left atrial pressure, and systemic arterial blood pressures. Additionally, blood samples were collected from the pulmonary artery and left atrium for blood gas analysis. RESULTS: Medetomidine infusion reduced the cardiac index, heart rate, and O2, delivery while increasing left atrial pressure. Subsequent fentanyl administration further decreased the cardiac index. The Pao2 was not significantly different between the 2 treatment groups; however, fentanyl transiently decreased Pao2 from baseline values in dogs receiving a CRI of medetomidine. CONCLUSIONS AND CLINICAL RELEVANCE: Because of the prolonged hemodynamic changes associated with the CRI of medetomidine, its safety should be further evaluated before being clinically implemented in dogs.

Effect of preoperative administration of tepoxalin on hemostasis and hepatic and renal function in dogs.

Preemptive analgesia is an important part of surgical management, but some NSAIDs can adversely affect platelet function or renal or hepatic status. Tepoxalin is approved in the United States for control of pain and inflammation associated with arthritis and in Europe for relief of pain caused by musculoskeletal disorders. In this study, no significant effects on indices of hemostasis or renal or hepatic function were detected when a single preoperative oral dose of tepoxalin was administered to young healthy dogs undergoing anesthesia and surgery.

Effect of isoflurane, atracurium, fentanyl, and noxious stimulation on bispectral index in pigs.

The study reported here was done to determine the relationship between anesthesia depth and bispectral index (BIS) in stimulated pigs. Isoflurane minimal alveolar concentration (MAC) was determined using the tail-clamp method in 16 Yorkshire/Landrace-cross pigs with mean+/-SEM weight of 27.7+/-1.76 kg. One week later, BIS, ECG, heart rate, arterial blood pressure, esophageal temperature, end-tidal CO2 tension and isoflurane concentration, arterial pH, PaO2, PaCO2, plasma bicarbonate concentration, and base excess were determined at each of five isoflurane MAC-multiples: 0.8, 1.0, 1.3, 1.6, and 2.0. Six treatments were studied: isoflurane; isoflurane and atracurium; isoflurane, atracurium, and fentanyl; isoflurane with noxious stimulation; isoflurane and atracurium with noxious stimulation; and isoflurane, atracurium, and fentanyl with noxious stimulation. The noxious stimulus during BIS measurement was the same as that for MAC determination. Each pig was studied three times (n = 8), and order of MAC-multiples and treatments was randomized. Data were evaluated by use of general linear model analysis of variance and linear regression analysis, with statistical significance set at P < 0.05. Significant differences in BIS values were identified between MAC-multiples within each treatment and between treatment 3 compared with treatments 2 and 4. Significant differences also were observed within and between treatments for heart rate, arterial blood pressure, and PaO2. Use of BIS appears reliable for identification of light versus deep anesthesia, but is of limited use for discrimination between isoflurane MAC-multiples of 1 and 1.6. We conclude that, compared with other treatments, atracurium and noxious stimulation had no significant effect on BIS.

Relationship of bispectral index to minimum alveolar concentration multiples of sevoflurane in cats.

OBJECTIVE: To determine the relationship between bispectral index (BIS) and minimum alveolar concentration (MAC) multiples of sevoflurane in cats. ANIMALS: 8 domestic cats. PROCEDURE: Each cat was anesthetized twice with sevoflurane. First, the MAC of sevoflurane for each cat was determined by use of the tail clamp method. Second, cats were anesthetized with sevoflurane at each of 5 MAC multiples administered in random order. Ventilation was controlled, and after a 15-minute equilibration period at each MAC multiple of sevoflurane, BIS data were collected for 5 minutes and the median value of BIS calculated. RESULTS: The mean (+/- SD) MAC of sevoflurane was 3.3 +/- 0.2%. The BIS values at 0.5 MAC could not be recorded as a result of spontaneous movement in all 8 cats. The BIS values at 2.0 MAC were confounded by burst suppression in all 8 cats. Over the range of 0.8 to 1.5 MAC, BIS values decreased significantly with increasing end-tidal sevoflurane concentrations. Mean (+/- SD) BIS measurements were 30 +/- 3, 21 +/- 3, and 5 +/- 2 at 0.8, 1.0, and 1.5 MAC, respectively. CONCLUSIONS AND CLINICAL RELEVANCE: Values of BIS are inversely and linearly related to end-tidal sevoflurane concentrations in anesthetized cats, and BIS may be a useful predictor of CNS depression in this species. The consistently low BIS values recorded in this study suggest that clinical BIS end points used to titrate anesthetic agents in humans may not be applicable to cats.

Effect of medetomidine administration on bispectral index measurements in dogs during anesthesia with isoflurane.

OBJECTIVE: To determine the relationship between bispectral index (BIS) and minimum alveolar concentration (MAC) multiples of isoflurane after IM injection of medetomidine or saline (0.9% NaCl) solution in anesthetized dogs. ANIMALS: 6 dogs. PROCEDURE: Each dog was anesthetized 3 times with isoflurane. First, the MAC of isoflurane for each dog was determined by use of the tail clamp method. Second, anesthetized dogs were randomly assigned to receive an IM injection of medetomidine (8 microg x kg(-1)) or an equal volume of isotonic saline (0.9% NaCl) solution 30 minutes prior to beginning BIS measurements. Last, anesthetized dogs received the remaining treatment (medetomidine or isotonic saline solution). Dogs were anesthetized at each of 4 MAC multiples of isoflurane. Ventilation was controlled and atracurium (0.2 mg/kg followed by 6 microg/kg/min as a continuous infusion, IV) administered. After a 20-minute equilibration period at each MAC multiple of isoflurane, BIS data were collected for 5 minutes and median values of BIS calculated. RESULTS: BIS significantly decreased with increasing MAC multiples of isoflurane over the range of 0.8 to 2.0 MAC. Mean (+/- SD) MAC of isoflurane was 1.3 +/- 0.2%. During isoflurane-saline anesthesia, mean BIS measurements at 0.8, 1.0, 1.5, and 2.0 MAC were 65 +/- 8, 60 +/- 7 52 +/- 3, and 31 +/- 28, respectively. During isoflurane-medetomidine anesthesia, mean BIS measurements at 0.8, 1.0, 1.5, and 2.0 MAC were 77 +/- 4, 53 +/- 7, 31 +/- 24, and 9 +/- 20, respectively. CONCLUSIONS AND CLINICAL RELEVANCE: BIS monitoring in dogs anesthetized with isoflurane has a predictive value in regard to degree of CNS depression. During isoflurane anesthesia, our results support a MAC-reducing effect of medetomidine.

Doppler echocardiographic effects of medetomidine on dynamic left ventricular outflow tract obstruction in cats.

OBJECTIVE: To evaluate the effects of medetomidine on dynamic left ventricular outflow tract (LVOT) obstruction in cats with left ventricular hypertrophy. DESIGN: Clinical trial. ANIMALS: 6 domestic shorthair cats with echocardiographic evidence of dynamic LVOT obstruction. PROCEDURE: Cats were restrained in lateral recumbency, and baseline M-mode and Doppler echocardiographic examinations were performed. An ECG was recorded continuously, and blood pressure was measured indirectly with Doppler instrumentation. Medetomidine (20 microg/kg 19.1 microg/lb]) was then administered i.m., and examinations were repeated 15 minutes later. RESULTS: Significant decreases in heart rate, LVOT velocity, and the LVOT pressure gradient were documented following medetomidine administration. After adjusting for the effects of heart rate by ANCOVA, there were no significant differences in any other systolic or diastolic indices of left ventricular function. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that administration of medetomidine to cats with dynamic LVOT obstruction may result in elimination of outflow tract obstruction; medetomidine may be a suitable sedative and analgesic agent in this subpopulation of cats.

Relationship of canine bispectral index to multiples of sevoflurane minimal alveolar concentration, using patch or subdermal electrodes.

PURPOSE: The purpose of the study reported here was to determine the relationship of bispectral index (BIS) to multiples of sevoflurane minimal alveolar concentration (MAC), using proprietary patch or subdermal needle electrodes in dogs. METHODS: Eight English Pointers (4 males, 4 females; mean +/- SD age and body weight of 3.9+/-2.2 years and 20.7+/-4.1 kg, respectively) were studied. Sevoflurane MAC was determined in each dog, using the standard tail clamp technique. One week later, BIS was determined in each dog at 0.8, 1.0, 1.5, and 2.0 MAC multiples of sevoflurane, using proprietary patch electrodes and 29-gauge platinum needle electrodes applied in randomized order. Ventilation was controlled, and atracurium (0.2 mg/kg of body weight, followed by 6 microg/kg/min, i.v.) was administered to eliminate the electromyographic artifact from the electroencephalogram. The BIS was determined, using an A-2000 BIS monitor connected to a computer for data logging at five-second intervals. After a 15-min equilibration period at each sevoflurane MAC-multiple, BIS data were collected for five minutes, and median BIS values were calculated. Heart rate, direct mean arterial blood pressure, esophageal temperature, and arterial pH and blood gas tensions were measured immediately after each BIS collection period. End-tidal CO2 and sevoflurane concentrations were continuously monitored, using an infrared gas analyzer. Data were analyzed, using one-way repeated measures analysis of variance (P < 0.05). Agreement of BIS values from each electrode type was determined. RESULTS: Mean +/- SD sevoflurane MAC was 2.1 +/- 0.3%. Mean +/- SD BIS values at 0.8, 1.0, 1.5, and 2.0 MAC were 77 +/- 3, 73 +/- 5, 57 +/- 7, and 53 +/- 7, respectively, for patch electrodes and 80 +/- 6, 72 +/- 7, 56 +/- 4, and 50 +/- 5, respectively, for subdermal needle electrodes. At 2 MAC, BIS could not be determined in six dogs due to presence of burst suppression in the EEG. The regression equation comparing electrodes was: BIS (subdermal) = -5.5 + (1.1 x BIS [patch]); R2= 0.846; bias = -0.192, with a 95% confidence interval of -9.96 to 9.56. Of the other measured variables, none were significantly different between electrode types. Within each group of electrode type, MAP was significantly different among MAC multiples. Within the patch electrode group, PaO2, bicarbonate concentration, and base excess were significantly different among MAC multiples. CONCLUSIONS: Bispectral index significantly decreased with increasing sevoflurane MAC multiples over the range of 0.8 to 2.0 MAC, using patch or subdermal electrodes in dogs. Use of subdermal needle electrodes is a reliable and practical alternative to use of patch electrodes for measurement of BIS in dogs.