Ramez M. Salem

Role of Adenosine Triphosphate-sensitive Potassium Channels in Coronary Vasodilation by Halothane, Isoflurane, and Enflurane

Background Halothane, isoflurane, and enflurane cause coronary vasodilation and cardiac depression. This study was performed to assess the role of adenosine triphosphate (ATP)‐sensitive potassium channels (K (ATP) channels) in these effects. Methods Twenty‐five thoracotomized dogs were anesthetized with fentanyl and midazolam. The left anterior descending coronary artery was perfused via either of two pressurized (80 mmHg) reservoirs. One reservoir was supplied with arterial blood free of a volatile anesthetic, and the second reservoir was supplied with arterial blood equilibrated in an oxygenator with a 1 minimum alveolar concentration of either halothane (0.9%, n = 10), isoflurane (1.4%, n = 28), or enflurane (2.2%, n = 7). Coronary blood flow (CBF) was measured using a Doppler flow transducer, and segmental shortening (SS) was measured with ultrasonic crystals. Responses to the volatile anesthetics were assessed under control conditions, during intracoronary infusion of the KATP channel inhibitor glibenclamide (100 micro gram/min), and after cessation of glibenclamide (recovery). The effectiveness of glibenclamide was verified from inhibition of coronary vasodilator responses to the KATP channel opener cromakalim without effect on those to the KATP channel‐independent vasodilators, sodium nitroprusside and acetylcholine. Results Under control conditions, the volatile anesthetics caused pronounced increases in CBF (isoflurane > halothane = enflurane), and decreases in SS (enflurane > halothane = isoflurane). Glibenclamide blunted significantly (and reversibly) the increases in CBF, but it had no effect on the decreases in SS. Conclusions The KATP channels play an important role in coronary vasodilation but apparently are not involved in cardiac depression caused by halothane, isoflurane, and enflurane in canine hearts in situ.

Efficacy of the Self-inflating Bulb in Confirming Tracheal Intubation in the Morbidly Obese

Background This study was designed to determine the incidence of false‐negative and false‐positive results when the self‐inflating bulb (SIB) is used to differentiate tracheal from esophageal intubation in morbidly obese patients using two techniques. In technique 1, the SIB is compressed before it is connected to the tube; in technique 2, the SIB is compressed after connection to the tube. Methods With institutional review board approval, 54 consenting adult morbidly obese patients (body mass index > 35) undergoing elective surgical procedures were included in the study. After anesthetic induction and muscle relaxation, both the trachea and esophagus were intubated under direct vision with identical cuffed tubes. The efficacy of the SIB in verifying the position of both tubes was tested by a second anesthesiologist. The speed of reinflation was graded as rapid (< 4 s) or none (> 4 s), using both techniques. In the case of tracheal intubation, the absence of reinflation was recorded as a false‐negative, whereas in cases of esophageal intubation, rapid reinflation was recorded as a false‐positive. Identification of tube location by the second anesthesiologist was based on SIB reinflation results from techniques 1 and 2, as well as the presence of a flatuslike sound elicited by technique 2 in esophageally placed tubes. All patients were retested by the SIB after receiving three breaths of 400–500 ml each. In all patients exhibiting false‐negative results, six obese patients exhibiting true‐positive results, and four nonobese patients exhibiting true‐positive results, tracheal responses to the SIB maneuvers were observed directly by a flexible fiberoptic bronchoscope incorporating an airtight system, 15–20 min after mechanical ventilation was instituted. Results The incidence of false‐negative results was initially 30% with technique 1 and 11% with technique 2, but decreased to 4% when technique 2 was used after the delivery of three breaths. The second anesthesiologist initially identified tube location in 92.5% of patients correctly. After the delivery of three breaths, tube location was correctly identified in 96.3% of patients. Fiberoptic bronchoscopic examination of the patients exhibiting false‐negative results revealed exaggerated inward bulging of the posterior tracheal membrane during reinflation of the SIB when technique 1 was used. Conclusions Contrary to previous investigations in healthy patients, the current study demonstrates a high incidence of false‐negative results when the SIB is used to confirm tracheal intubation in morbidly obese patients. If the SIB is used, the technique should include compression of the SIB after connection to the tube and should be used in conjunction with other clinical signs and technical aids. The mechanism of false‐negative results in these patients seems to be related to reduction of caliber of airways secondary to a marked decrease in functional residual capacity, and collapse of large airways due to invagination of the posterior tracheal wall when sub‐atmospheric pressure is generated by the SIB.

Isoflurane-induced Dilation of Porcine Coronary Arterioles Is Mediated by ATP-sensitive Potassium Channels

Background Isoflurane causes increases in coronary blood flow in vivo, which are mediated by the adenosine triphosphate (ATP)‐sensitive potassium channels, but the role of the arterioles (resistance vessels) in these responses is controversial. Methods Medium porcine coronary arterioles (internal diameter, 172 +/‐ 51 [SD] [micro sign]m) were placed in a chamber supplied with Krebs buffer, pressurized (40 mmHg), and preconstricted with acetylcholine (10‐8 ‐10 (‐6) M). Vascular diameter (VD) was assessed using an optical density video‐detection system. Isoflurane (in 95% oxygen and 5% carbon dioxide) was added to buffer using a membrane oxygenator supplied by a calibrated vaporizer. In series 1 (n = 14), 2% isoflurane was administered according to an abrupt (ISO‐A) and gradual (ISO‐G) protocol. In series 2 (n = 13) and 3 (n = 6), ISO‐A (1.5%) was assessed before and after glibenclamide (an ATP‐sensitive potassium channel antagonist) or 8‐phenyltheophylline (a nonselective adenosine receptor antagonist), respectively. In series 4 (n = 5), validation studies were performed using sodium nitroprusside and adenosine diphosphate to verify that the vascular smooth muscle and endothelium of the vessels were functionally intact. In series 5 (n = 6), ISO‐A (0.75 and 1.5%) was compared during preconstriction with acetylcholine and the thromboxane analog U46619 (10‐6 M). Results ISO‐G caused essentially concentration‐dependent increases in VD. At 2% isoflurane, the increases in VD were greater during ISO‐A than ISO‐G. Gilbenclamide, but not 8‐phenyltheophylline, attenuated isoflurane‐induced increases in VD. Both sodium nitroprusside and adenosine diphosphate caused dose‐dependent increases in VD. Isoflurane caused equivalent concentration‐dependent increases in VD during acetylcholine and U46619. Conclusions Isoflurane is a concentration‐dependent dilator of porcine coronary arterioles preconstricted with acetylcholine or U46619. This effect is blunted by gradual administration, suggesting that the vessels may adapt to the relaxing effects of isoflurane. Isoflurane‐induced dilation of coronary arterioles is mediated by the ATP‐sensitive potassium channels but not by the adenosine receptors.

The Combitube™ oesophageal-tracheal double lumen airway for difficult intubation

5 Mason DG, Bingham RM. The laryngeal mask airway in children. Anaesthesia 1990; 45: 760-3. 6 Collier C. A hazard with the laryngeal mask airway (Letter). Anaesth Intensive Care 1991; 19: 301. 7 Silk JM, Hill HM, Calder I. Difficult intubation and the laryngeal mask. Eur J Anaesth 1991;

Is calcium a coronary vasoconstrictor in vivo

4: 47-51. 8 Nandi PR, Charlesworth CH, Taylor S J, Nunn JE Dore CJ, Effect of general anaesthesia in the pharynx. Br J Anaesth 1991; 66: 157-62. 9 Sivarajan M, Fink R. The position and the state of the larynx during general anesthesia and muscle paralysis. Anesthesiology 1990; 72: 439-42.

The Self-Inflating Bulb Versus End-Tidal Capnography for Confirming Tracheal Intubation in the Parturient Undergoing Cesarean Section

Background Calcium produces constriction in isolated coronary vessels and in the coronary circulation of isolated hearts, but the importance of this mechanism in vivo remains controversial. Methods The left anterior descending coronary arteries of 20 anesthetized dogs whose chests had been opened were perfused at 80 mmHg. Myocardial segmental shortening was measured with ultrasonic crystals and coronary blood flow with a Doppler flow transducer. The coronary arteriovenous oxygen difference was determined and used to calculate myocardial oxygen consumption and the myocardial oxygen extraction ratio. The myocardial oxygen extraction ratio served as an index of effectiveness of metabolic vasodilation. Data were obtained during intracoronary infusions of CaCl2 (5, 10, and 15 mg/min) and compared with those during intracoronary infusions of dobutamine (2.5, 5.0, and 10.0 micro gram/min). Results CaCl2 caused dose‐dependent increases in segmental shortening, accompanied by proportional increases in myocardial oxygen consumption. Although CaCl2 also increased coronary blood flow, these increases were less than proportional to those in myocardial oxygen consumption, and therefore the myocardial oxygen extraction ratio increased. Dobutamine caused dose‐dependent increases in segmental shortening and myocardial oxygen consumption that were similar in magnitude to those caused by CaCl2 In contrast to CaCl2, however, the accompanying increases in coronary blood flow were proportional to the increases in myocardial oxygen consumption, with the result that the myocardial oxygen extraction ratio remained constant. Conclusions Calcium has a coronary vasoconstricting effect and a positive inotropic effect in vivo. This vasoconstricting effect impairs coupling of coronary blood flow to the augmented myocardial oxygen demand by metabolic vascular control mechanisms. Dobutamine is an inotropic agent with no apparent direct action on coronary resistance vessels in vivo.

Critical Hemoglobin Desaturation Can Be Delayed by Apneic Diffusion Oxygenation

In a previous report, we demonstrated that the use of a self-inflating bulb (SIB) to confirm tracheal intubation in the pregnant woman at term is unreliable (1). The present report compares SIB with capnography for confirming tracheal intubation in 22 parturients presenting for cesarean section. After preoxygenation, anesthesia was induced by thiopental 3-4 mg/kg and succinylcholine 1.5 mg/kg. The trachea was then intubated with a 7.0-mm inner diameter tube under direct vision laryngoscopy, and the cuff was inflated. Before ventilation was initiated, the position of the tracheal tube was checked by using a SIB. The SIB was squeezed before it was connected to the tube. The speed of reinflation of the SIB was graded as rapid (~4 s), delayed (4-10 s), and none (the SIB did not reinflate, or reinflated in >lO s). Rapid reinflation of the SIB was considered as positive, while delayed or no reinflation as false negative. The SIB was then removed, the tube was connected to the anesthesia circuit, and controlled ventilation was started; tracheal tube placement was then confirmed by capnography (Hewlett Packard, Boeblingen, Germany). The SIB was associated with a high incidence of false-negative results (Table 1). The false negatives can be attributed to decreased functional residual capacity by the gravid uterus, which is further decreased by the SIB-induced subatmospheric pressure (1). This can result in airway closure, with a consequent delayed reinflation of the SIB. In contrast, capnography was positive in all parturients, whether the SIB reinflated rapidly, slowly, or did not reinflate at all