Cardiopulmonary Effects of the a2-Adrenoceptor Agonists Medetomidine and ST-91 in Anesthetized Sheep

To test the hypothesis that pulmonary alterations are more important than hemodynamic changes in a2-agonist-induced hypoxemia in ruminants, the cardiopulmonary effects of incremental doses of (4-[1-(2,3-dimethylphenyl)ethyl]-1H-imadazole) hydrochloride (medetomidine; 0.5, 1.0, 2.0, and 4 mg/kg) and 2-(2,6-diethylphenylamino)-2-imidazol (ST-91; 1.5, 3.0, 6.0, and 12 mg/kg) were compared in five halothane-anesthetized, ventilated sheep using a placebo-controlled randomized crossover design. Pulmonary resistance (RL), dynamic compliance, and tidal volume changes in transpulmonary pressure (DPpl) were determined by pneumotachography, whereas cardiac index (CI), mean pulmonary artery pressure (Ppa), and pulmonary artery wedge pressure (Ppaw) were determined using thermodilution and a Swan-Ganz catheter. The most important finding was the fall in partial pressure of oxygen in arterial blood (PaO2) after administration of medetomidine at a dose (0.5 mg/kg) 20 times less than the sedative dose. The PaO2 levels decreased to 214 mm Hg as compared with 510 mm Hg in the placebo-treated group. This decrease in PaO2 was associated with a decrease in dynamic compliance and an increase in RL, DPpl, and the intrapulmonary shunt fraction without changes in heart rate, CI, mean arterial pressure, pulmonary vascular resistance, Ppa, or Ppaw. On the other hand, ST-91 only produced significant changes in PaO2 at the highest dose. After this dose of ST-91, the decrease in PaO2 was accompanied by a 50% decrease in CI and an increase in mean arterial pressure, Ppa, Ppaw, and the intrapulmonary shunt fraction without significant alterations of RL and DPpl. The study suggests that the mechanism(s) by which medetomidine and ST-91 produce lower PaO2 are different and that drug-induced alterations in the pulmonary system are mainly responsible for the oxygenlowering effect of medetomidine. The a2-adrenoceptor agonists (a2-agonists) are becoming increasingly popular in veterinary and human medicine for use as anxiolytics, analgesics, and preanesthetic sedatives (Maze and Tranquilli, 1991). In veterinary practice, the relatively new a2-agonist, (4-[1-(2,3-dimethylphenyl)ethyl]-1Himadazole) hydrochloride (medetomidine), is widely used in Europe and Australia for sedation and to reduce the general anesthetic requirement (Cullen, 1996). This drug is 7 times more selective for a2-adrenergic receptors than the prototype a2-agonist, clonidine (Virtanen, 1989). The dextroisomer of medetomidine, dexmedetomidine, is being developed for use in human anesthetic practice. Dexmedetomidine has been used in the perioperative period to provide sedation and anxiolysis (Aantaa et al., 1990a), to reduce opioid, thiopental, and inhalation anesthetic requirements (Aantaa et al., 1990b), and to reduce hemodynamic instability in humans (Aantaa et al., 1990b). It has been known for some time that the a2-agonist xylazine decreases partial pressure of oxygen in arterial blood (PaO2) in cattle (DeMoor and Desmet, 1971), goats (Kumar and Thurmon, 1979), and sheep (Doherty et al., 1986; Nolan et al., 1986). The degree of hypoxemia in sheep after sedation with clonidine (Eisenach, 1988) or xylazine (Nolan et al., 1986; Doherty et al., 1986) is quite severe. We recently reported that three newer a2-agonists (detomidine, medetomidine, and romifidine) also produce severe hypoxemia when administered i.v. at equipotent sedative doses in conscious sheep (Celly et al., 1997a,b). The level of hypoxemia was similar with the five a2-agonists studied, irrespective of their differences in selectivity for a2versus a1-adrenoceptors Received for publication May 18, 1998. 1 This work was supported by the Canadian Commonwealth Scholarship and Fellowship program (to C.S.C.), and the Ontario Ministry of Agriculture, Food and Rural Affairs. ABBREVIATIONS: medetomidine, (4-[1-(2,3-dimethylphenyl)ethyl]-IH-imidazole) hydrochloride; ST-91, (2-(2,6-diethylphenylamino)-2-imidazol); a2-agonist, a2-adrenoceptor agonist; Cdyn, dynamic compliance; CI, cardiac index; DPpl, maximum change in transpulmonary pressure; IPPV, intermittent positive pressure ventilation; MAP, mean arterial pressure; PaO2, partial pressure of oxygen in arterial blood; PaCO2, partial pressure of carbon dioxide in arterial blood; Pv̄O2, partial pressure of oxygen in mixed venous blood; PAO2, alveolar oxygen tension; P(A-a)O2, alveolar to arterial oxygen tension gradient; Ppa, pulmonary artery pressure; Ppaw, pulmonary artery wedge pressure; PVR, pulmonary vascular resistance; RL, pulmonary resistance; SV, stroke volume; SVR, systemic vascular resistance; TXB2, thromboxane2; VT, tidal volume; Qs/Qt, shunt fraction; HR, heart rate; TPP, total plasma protein. 0022-3565/99/2892-0712$03.00/0 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 289, No. 2 Copyright © 1999 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A. JPET 289:712–720, 1999 712 at A PE T Jornals on O cber 2, 2017 jpet.asjournals.org D ow nladed from (Celly et al., 1997a). A selective a2-agonist that does not cross the blood brain barrier, 2-(2,6-diethylphenylamino)-2-imidazol (ST-91), also produced a comparable level of hypoxemia, suggesting involvement of a peripheral component in the development of hypoxemia (Eisenach, 1988; Celly et al., 1997b). The hypoxemia was not caused by hypoventilation and was not due to postural changes after drug administration (Celly et al., 1997a,b). It was accompanied by significant changes in respiratory frequency and in the maximum change in transpulmonary pressure (DPpl) required for tidal volume (VT) breathing; heart rate (HR) and mean arterial pressure (MAP) were less affected. Others have reported that the development of hypoxemia occurs even when the animal is ventilated artificially (Nolan et al., 1986; Eisenach, 1988). Although these studies suggest a drug-induced increase in the proportion of blood flow going through the lungs without becoming oxygenated, i.e., intrapulmonary shunt fraction (Qs/Qt), as one possible mechanism underlying a2-agonistinduced hypoxemia, the origin of this increase in shunt fraction remains unclear. From the studies reported to date, it is not possible to separate the relative contribution of pulmonary and cardiovascular alterations with respect to the development of hypoxemia in sheep. The doses used were clinically useful sedative doses, and these doses produce significant circulatory alterations (Campbell et al., 1979; Cullen, 1996). It is quite possible that a fall in cardiac output (Q̇t) and mixed venous oxygen saturation magnified the apparent degree of venous admixture associated with any ventilation/perfusion mismatch (McDonell, 1996). The present study was conducted to investigate the relative contribution of the cardiovascular and respiratory systems to a2-agonist-induced hypoxemia in ruminants. To test the hypothesis that the pulmonary alterations are paramount and that these effects can occur without central a2receptor involvement, the effect of incremental doses of the central and peripheral acting a2-agonist, medetomidine, the peripherally acting a2-agonist, ST-91, and a saline placebo were studied in halothane anesthetized, ventilated sheep. Materials and Methods Experimental Animals. Five adult female Arcot sheep weighing 80 to 90 kg [mean body weight 84 6 1.7 S.E.] were used in the study. Each animal was used on three different occasions to study the cardiopulmonary effects of placebo (physiological saline), medetomidine, and ST-91. A minimum of 7 days separated each experiment and the order of treatment was randomized. The study was approved by the institutional Animal Care Committee and followed the guidelines of the Canadian Council on Animal Care. At least 1 month before experimentation, the carotid artery was relocated under halothane anesthesia to a s.c. position in all animals. Health status was established on the basis of physical examination, a complete blood count, arterial blood gas analysis, and chest radiography. Water was available ad libitum but feed was withheld 20 to 24 h before each experiment. Instrumentation. The sheep were positioned in a custom designed restraint device that served to minimize the positional effects of anesthesia on pulmonary function. The base of the wooden stock had four holes cut to permit the animal’s legs to protrude such that the sheep rested comfortably on its sternum and abdomen on a pad of foam 15 cm thick. Sternal recumbency is associated with less interference of gas exchange in anesthetized large mammals than lateral or dorsal recumbency (McDonell, 1996). Anesthesia was induced with pentobarbital sodium (20 mg/kg i.v.), a cuffed endotracheal tube (diameter 9.5–10.5 mm) was inserted, and halothane/O2 was used for maintenance of anesthesia with intermittent positive pressure ventilation (IPPV) to maintain eucapnia. Muscle paralysis was induced with 0.2 mg/kg atracurium (Tracrium, Burroughs Wellcome, Research Triangle Park, NC) administered i.v., and the level of muscle relaxation was monitored with a peripheral nerve stimulator (Innervator, Fisher & Paykel Electronics Ltd., Auckland, NZ) using electrodes applied to the ulnar nerve. Total absence of muscle response to a train of four 20-mA stimuli was maintained throughout the study by the supplemental administration of atracurium (usually 0.1 mg/kg i.v.) as needed. Airway gas concentrations (inspired O2 and end-tidal halothane and CO2) were monitored on a breath-to-breath basis using a rapid response monitor (Criticare 1100 Patient Monitor, Criticare System Inc., Waukesha, WI). The gas analyzer was calibrated at the beginning and end of each experiment using known concentrations of halothane, O2, and CO2 (Anesthesia calibration gas, Criticare System Inc., Waukesha, WI). A constant volume, electronically cycled, sinusoidal airflow ventilator (Harvard pump, Harvard Apparatus Co., Inc., Dover, MA) was inserted into the inspiratory side of the anesthetic rebreathing circuit to produce constant volume IPPV. The pump was set to deliver a constant VT (700 ml) with respiratory frequency at approximately 8/min. Breathing frequency was adjusted as needed to maintain eucapnia [partial pressure of carbon dioxide in arterial blood (PaCO2) ranging between 40–45 mm Hg], whereas VT was kept constant. A 14-gauge, 10-cm long catheter (Angiocath, Becton Dickinson, Sandy, UT) was inserted in the rumen to provide a means of venting for any gas production in the rumen. A 20-gauge, 8-cm long catheter (Becton Dickinson) was introduced percutaneously into the relocated carotid artery to measure MAP and to collect arterial blood samples. The scapulohumeral joint was used as the zero reference point for MAP measurements. A base apex lead system of electrocardiography (ECG) was used for recording HR and rhythm. Copper alligator clip leads were attached to stainless steel wire loops placed s.c.. An 8.5-F introducer catheter (Arrow International Inc., Reading, PA) was placed in the jugular vein to permit insertion of a 130-cm thermistor catheter (Swan Ganz, American Edwards Laboratories, Irvine, CA) into the pulmonary artery. This catheter was used to record mean pulmonary artery pressure (Ppa), pulmonary artery wedge pressure (Ppaw), and Q̇t using thermodilution and a computer system (Com-2, Edwards Critical Care, Irvine, CA), which provided a visual display of the thermal curve. Iced 5% dextrose solution (10 ml) was used as the thermal indicator and the mean of three Q̇t determinations made in rapid succession was taken as the representative Q̇t for that sampling interval. To minimize the effect of ventilation on Q̇t measurements, estimation of Q̇t was done by timing each injection to start at the same point in the ventilatory cycle. Likewise, Ppaw was recorded at the end of expiration. A five-channel monitor (Criticare 1100 patient monitor, Criticare System Inc., Waukesha, WI) was used to record MAP, Ppa, Ppaw, and ECG continuously. The blood pressure measurement system was calibrated before and after completion of each experiment with a mercury manometer. From recorded variables, the cardiac index (CI), stroke volume (SV), systemic vascular resistance (SVR), pulmonary vascular resistance (PVR), and Qs/Qt were derived using standard equations (Martin, 1987). Three-milliliter arterial and mixed venous blood samples were drawn simultaneously into heparinized air tight glass syringes containing washers to facilitate thorough mixing of the specimen before analysis. Blood samples were stored on ice for not more than 30 min before blood gas and acid base analysis using an automated blood gas analyzer (Stat Profile Plus 9, Nova Biomedical, Waltham, MA). The packed cell volume and total plasma protein (TPP) levels were estimated by microhematocrit and refractometry methods, respectively. Blood gas and acid base values were corrected to body temperature. The alveolar oxygen tension (PAO2) was calculated using the alveolar gas equation (Martin, 1987). The PaO2 value was subtracted from 1999 a2-Agonist-Induced Hypoxemia in Sheep 713 at A PE T Jornals on O cber 2, 2017 jpet.asjournals.org D ow nladed from the calculated value of PAO2 to estimate the alveolar-to-arterial oxygen tension gradient [P(A-a)O2]. The blood gas analyzer was calibrated daily with known serum equivalents (Nova Stat Profile Control, Nova Biomedical, Waltham, MA) and between samples with precision gas mixtures. Paired arterial and mixed venous blood samples were collected in tubes containing EDTA and indomethacin to measure thromboxane B2 (TXB2) levels. The samples were spun to collect plasma, which was stored at 280°C until analyzed using a radioimmunoassay (Viinikka and Ylikorkala, 1980) with a radioimmunoassay kit (Thromboxane B2 [ I]assay system, Amersham International Plc, Buckinghamshire, UK). Air flow was measured with a screen pneumotachograph (Gould Electronics, Biltholven, the Netherlands) positioned between the endotracheal tube and anesthetic Y piece and connected to a differential pressure transducer (Validyne, #6–14, range 6 2.25 cm of H2O, Validyne Engineering Corp., Northridge, CA). Tidal volume was derived through integration of the measured flow signals (Pulmonary Mechanics Analyzer, model 6, Buxco Electronics, Inc., Sharon, CT). Transpulmonary pressure was the difference between the pressure at the airway opening (endotracheal tube) and the pleural pressure, as estimated with a thin latex esophageal balloon (7 cm long) affixed to the end of a 130-cm polyethylene catheter (2 mm i.d. and 3 mm O.D.) passed to the mid-thorax (Wanner and Reinhart, 1978). A volume of 1.5 ml of air was maintained in the balloon; this volume was within the high compliance range of the pressure recording system, and was close to the minimal relaxed volume of the balloon. The two pressure ports were connected to a differential pressure transducer (Validyne, #6–32, range 6 140 cm of H2O, Validyne Engineering Corp., Northridge, CA) to measure change in pressure. The flow, volume, and pressure signals were processed through an analyzer (Pulmonary Mechanics Analyzer, model 6) and stored on a computer for subsequent breath-by-breath analysis using software from Buxco Electronics. Dynamic compliance (Cdyn), total pulmonary resistance (RL), and DPpl were obtained from pressurevolume loop analysis (Tesarowski et al., 1996). A minimum of nine breaths were analyzed and used to provide the mean value for the respiratory variable at each sampling period. The frequency response of the transducer/catheter system was tested as described elsewhere (Young and Tesarowski, 1994), and was linear up to 6 Hz. Before and after each experiment, the system was calibrated for pressure using a water manometer, and for volume using a 1-l calibration syringe (Collins, model #5540, Hans Rudolph Inc., Kansas City, MO). The volume calibration of the pneumotachograph was performed using a gas mixture (1% halothane in O2) similar to the gas mixture inspired during the experiment (Horbes, 1967). Drugs. The drugs tested were the relatively selective a2-adrenoceptor agonists medetomidine (molecular weight 5 236.7) and ST-91 (molecular weight 5 253.8). Medetomidine was available as a 1 mg/ml solution (Domitor, Orion Corporation, Farmos, Turku, Finland), whereas ST-91 (Boehringer Ingelheim, Ridgefield, CT) was dissolved in 0.9% saline to make a final concentration of 1 mg/ml. All doses have been expressed as salts. Experimental Protocol. After the onset of anesthesia and instrumentation, more than 1 h was provided as a stabilization period. Any gas accumulation in the rumen was expressed and then the lungs were “sighed” as follows to ensure a previous volume history for pulmonary mechanics measurements (Wheeler et al., 1990). The animal was disconnected from the pneumotachograph and anesthetic machine/ventilator at the level of the endotracheal tube and any secretions in the upper airways were suctioned out. The endotracheal tube was then attached to a second anesthetic circuit containing 1% halothane in oxygen and a 3-l rebreathing bag. Using this circuit, the sheep’s lungs were expanded to an inflation pressure of 30 cm of H2O over 3 to 5 s, after which passive exhalation to functional residual capacity (FRC) was permitted. This maneuver was repeated three times before the animal was reconnected to the pneumotachograph and primary anesthetic circuit. A 2-min period of constant volume IPPV was permitted to provide for stabilization of cardiovascular, respiratory, and end-tidal gas measurement before baseline (pretreatment) measurements of respiratory data, vascular pressures, and Q̇t were obtained. At this time, arterial and mixed venous blood samples were collected simultaneously for subsequent blood gas analysis and TXB2 determination. After baseline sampling, the first dose of the test drug (0.5 mg/kg for medetomidine, 1.5 mg/kg for ST-91, or 2.0 ml saline) was then given i.v. diluted to a 2.0 ml volume. Thereafter, cardiovascular measurements were made at 3, 10, and 20 min, and respiratory measurements were made at 2, 5, 10, and 20 min. After the 20-min post-treatment sampling period for the first dose of drugs, the catheters were flushed with saline, and the anesthetic concentration and IPPV rate was adjusted if needed to ensure conditions of eucapnia and a stable end-tidal halothane concentration (1.2%). There was some variation among sheep in the time required to achieve this degree of stability; usually 20 to 25 min was required after the last measurements were made. Before administration of the second dose of drugs (1.0 mg/kg medetomidine, 3.0 mg/kg ST-91, 2.0 ml saline), suction of upper airway secretions and sighing of the lungs was carried out as described above. A 2-min stabilization postsigh period was again utilized, followed by administration of the second drug dose and post-treatment sampling at the same time intervals as for the first dose of the drugs. The same sequence of events was repeated for the third dose (2.0 mg/kg medetomidine, 6.0 mg/kg ST-91, or 2.0 ml saline) and fourth dose (4 mg/kg medetomidine, 12 mg/kg ST-91, or 2.0 ml saline) of each drug. Mean elapsed times between the end of recordings after one dose and the administration of the next dose was similar: 24.4 6 17 min between the first and second dose; 30.8 6 13.3 min between the second and third doses; and 26.5 6 12.4 min between doses III and IV. Statistical Analysis. A general survey of the data showed that, after administration of each dose, the peak drug effect for the variables in individual animals was either 3 or 5 min postdrug administration, irrespective of the variable. The effects tended to decrease in intensity by 20 min. To represent the overall response of a variable after each dose administration, a weighted mean was calculated from the values at 3, 10, and 20 min postdrug administration for cardiovascular variables and from values calculated at 2, 5, 10, and 20 min postdrug administration for respiratory variables. This resulted in five sampling interval values; i.e., pretreatment baseline values and weighted mean values after dose I, II, III, and IV. The data were then subjected to two-way ANOVA for repeated measures to test for significance (p # .05) of treatment over time, as well as for differences between treatments and the placebo (Dawson-Saunders and Trapp, 1990). When a significant effect of treatment was observed, comparisons were performed between treatments using oneway ANOVA and a post hoc least significant difference (LSD) test. To account for repeated measures in the experimental design, the LSD was calculated using a values corrected by Bonferroni’s method to control the overall level of significance (p # .05; Dawson-Saunders and Trapp, 1990). The results have been presented as the average of weighted mean 6 S.E.