The breaking of an intrathecally-placed epidural catheter during extraction

IMPLICATIONS

Misplacement of an epidural catheter into the subarachnoid space is a recognized complication. However, breakage of an intrathecal epidural catheter during removal presents a dilemma. Appropriate imaging, a neurosurgical consultation, and aggressive surgical exploration and extraction of the retained piece are warranted, even in the asymptomatic patient.

Isoflurane inhibits neutrophil-endothelium interactions in the coronary circulation: lack of a role for adenosine triphosphate-sensitive potassium channels

Isoflurane protects myocardium during ischemia-reperfusion via a mechanism involving the adenosine triphosphate-sensitive potassium channels. We tested the hypothesis that an inhibition of the neutrophil-endothelium interactions by isoflurane contributes to this effect. Polymorphonuclear neutrophils and coronary artery segments were obtained from 35 healthy dogs. Superoxide production by neutrophils, stimulated with platelet-activating factor (PAF; 1.0 microM), was measured spectrophotometrically. Adherence of PAF-activated neutrophils to the endothelium of coronary segments was assessed by direct counting of neutrophils labeled with fluorescent dye. Coronary artery rings were exposed to PAF-activated neutrophils, and, after washing and preconstriction with U46619, they were evaluated for vasorelaxation responses to acetylcholine (endothelium dependent) and sodium nitroprusside (endothelium independent). Measurements were performed in the absence and presence of isoflurane (1 and 2 minimum alveolar anesthetic concentration) both with and without glibenclamide (10 microM). Isoflurane inhibited superoxide production and adherence by neutrophils and abolished neutrophil-induced reductions in coronary vascular relaxation responses to acetylcholine. Glibenclamide did not alter the effects of isoflurane on neutrophils or coronary artery endothelium. In conclusion, isoflurane had an inhibitory action on neutrophil-endothelium interactions and neutrophil-mediated coronary endothelial dysfunction--effects that may be involved in its cardioprotective action in vivo. These inhibitory actions of isoflurane were not mediated by adenosine triphosphate-sensitive potassium channels.

IMPLICATIONS

Isoflurane inhibited neutrophil-endothelium interactions and the inflammatory response in vitro via a pathway independent of the adenosine triphosphate-sensitive potassium channels. This action could be involved in the cardioprotection by isoflurane observed in vivo.

Tracheal intubation conditions and cardiovascular effects after modified rapid-sequence induction with sevoflurane-rapacuronium versus propofol-rapacuronium

STUDY OBJECTIVES

To compare intubation conditions and hemodynamic effects resulting from rapid-sequence induction of anesthesia with sevoflurane-rapacuronium and propofol-rapacuronium.

DESIGN

Randomized, blinded study.

SETTING

Operating suites of a large university-affiliated medical center.

PATIENTS

40 ASA physical status I and II adult patients without airway abnormalities who were scheduled for elective surgery requiring endotracheal intubation.

INTERVENTIONS

Patients were randomly allocated to receive either sevoflurane inhalational induction (Group 1) or propofol (2 mg/kg) intravenous induction (Group 2). Group 1 patients were coached on how to perform vital capacity breathing and the anesthesia machine was primed with sevoflurane 8%, N2O:O2 3.5:1.5 L/min. In both groups, when loss of consciousness was established, rapacuronium 1.5 mg/kg was administered. After 50 seconds, an anesthesiologist blinded to the study entered the room and attempted laryngoscopy and intubation.

MEASUREMENTS

Intubation conditions were graded as excellent, good, poor, or impossible according to Good Clinical Research Practice (GCRP) criteria. Arterial blood pressure and heart rate changes accompanying both induction techniques were also monitored and recorded.

MAIN RESULTS

All patients were successfully intubated within 60 seconds. Clinically acceptable intubating conditions (excellent or good scores) were obtained in 19 of 20 Group 1 patients and in 19 of 20 Group 2 patients. Moderate tachycardia was encountered in both groups and mild systolic hypotension in the Group 2 patients. There were no complications.

CONCLUSIONS

Modified rapid-sequence inhalational induction using sevoflurane and rapacuronium produced clinically acceptable intubation conditions within 60 seconds of muscle relaxant administration. The intubation conditions were similar to those produced after intravenous propofol and rapacuronium.

Verification of endotracheal tube position

The goals of tracheal intubation are to place the tube in the trachea and to position the tube at an appropriate depth inside the trachea. Various clinical signs and technical aids are described to verify tracheal intubation and to diagnose esophageal intubation. Many of these methods fail under certain circumstances. Not all these methods can be applied in every intubation, but it is essential that the clinician involved in tracheal intubation have the necessary airway management skills, perform these tests accurately, and interpret the results correctly. Prioritization of these tests depends on many factors, including familiarity, availability of monitors, and the location of intubation. Viewing the tube passing between the cords during direct laryngoscopy and visualization of the tracheal rings and carinae with a fiberoptic scope after intubation are the only fullproof methods of confirming tracheal intubation. In the nonarrested patient, carbon dioxide monitoring quickly can differentiate tracheal from esophageal intubation. In the arrested patient, however, carbon dioxide monitoring can be unreliable, although it can be useful as a prognostic indicator of the efficacy of resuscitation. Devices such as [figure: see text] the self-inflating bulb and esophageal detector device may be more useful in patients with cardiac arrest, but they also can yield false results. Placing the distal tip of the tube in the middle of the trachea can be accomplished by positioning the upper end of the cuff 2 cm below the cords during direct laryngoscopy or by placing the distal tip of the tube 4 cm above the carinae with the aid of a fiberoptic scope. The position of the tube always should be verified by clinical assessment (e.g., auscultation). If direct visualization cannot be done, referencing the marks on the tube, transillumination techniques, or cuff maneuvers can be helpful. In the emergency and critical care settings, a chest radiograph easily can detect malpositioned tracheal tubes that may not be detected by routine clinical assessment. Other techniques (e.g., use of fiberoptic scopes, cuff maneuvers, transillumination) can decrease the need for frequent chest radiographs. Based on available information, two algorithms are proposed: one for emergency intubation (Fig. 9) and the other for verification of tracheal tube position in elective intubation (Fig. 10). These algorithms are designed [figure: see text] to assist the clinician and should not be substituted for clinical judgment. Under no circumstances should clinical signs be ignored in the presence of conflicting information from monitors and technical aids.

Preoxygenation with tidal volume and deep breathing techniques: the impact of duration of breathing and fresh gas flow

Various techniques of "preoxygenation" before anesthetic induction have been advocated, including tidal volume breathing (TVB) for 3-5 min, four deep breaths (DB) in 0.5 min, and eight DB in 1 min. However, no study has compared the effectiveness of these techniques, assessed extending deep breathing beyond 1 min, or investigated the influence of fresh gas flow (FGF) in the same subjects using a circle absorber system. In 24 healthy adult volunteers breathing oxygen from a circle absorber system by tight-fitting mask, we compared TVB/5 min and deep breathing at a rate of 4 DB/0.5 min for 2 min at 5, 7, and 10 L/min FGF. Inspired and end-tidal respiratory gases were measured at 0.5-min intervals. During TVB, end-tidal oxygen (ETO2) increased rapidly and plateaued by 2.5 min at 86%, 88%, and 88% with 5, 7 and 10 L/min FGF, respectively. ETO2 values of > or =90% were attained between 3 and 4 min. Four DB/0.5 min increased ETO2 to 75%, 77%, and 80% at 5, 7, and 10 L/min FGF. Eight DB/min resulted in ETO2 values of 82% and 87% at 7 and 10 L/min, respectively. Extending deep breathing to 1.5 and 2 min with 10 L/min FGF increased ETO2 by > or =90%, although a decrease in ETCo(2) was noted. We concluded that TVB/3-5 min was effective in achieving maximal "preoxygenation" whereas 4 DB/0.5 min resulted in submaximal "preoxygenation," and thus should be used only when time is limited. Increasing FGF from 5 to 10 L/min does not enhance "preoxygenation" with either TVB or 4 DB/0.5 min. Deep breathing yields maximal "preoxygenation" when extended to 1.5 or 2 min, and only when high (10 L/min) FGF is used.

IMPLICATIONS

Using a circle absorber system, normal breathing of oxygen for 3-5 min achieves optimal oxygenation of the lungs; whereas 4 deep breaths in 30 s does not. However, extending deep breathing to 1.5-2 min and using a high flow of oxygen improves oxygenation of the lungs to the same degree as normal breathing for 3-5 min. This may have important implications for patient safety.

Efficacy of preoxygenation with tidal volume breathing. Comparison of breathing systems

BACKGROUND

Preoxygenation before tracheal intubation is intended to increase oxygen reserves and delay the onset of hypoxemia during apnea. Various systems are used for preoxygenation. Designed specifically for preoxygenation, the NasOral system uses a small nasal mask for inspiration and a mouthpiece for exhalation. One-way valves in the nasal mask and the mouthpiece ensure unidirectional flow. This investigation compares the efficacy of preoxygenation using the standard circle system with the NasOral system and five different resuscitation bags.

METHODS

Twenty consenting, healthy volunteers were studied in the supine position for 5-min periods of tidal volume breathing using the circle absorber system, the NasOral system, and five resuscitation bags in a randomized order. Data were collected during room air breathing and at 30-s intervals during 5 min of oxygen administration. Inspired oxygen, end-tidal oxygen, and end-tidal nitrogen were measured by mass spectrometry.

RESULTS

At 2. 5 min of oxygenation, end-tidal oxygen plateaued at 88.1 +/- 4.8 and 89.3 +/- 6.4% (mean +/- SD) for the circle absorber and NasOral systems, respectively. This was associated with inverse decreases in end-tidal nitrogen. At no time did these end-tidal oxygen or nitrogen values differ from each other. Three of the resuscitation bags (one disk type and two duck-bill type with one-way exhalation valves) delivered inspired oxygen more than 90%, and the end-tidal oxygen plateaued between 77 and 89% at 2 min of tidal volume breathing. The other two resuscitation bags (both duck-bill bags without exhalation valves) delivered inspired oxygen less than 40%, and the end-tidal oxygen values ranged between 21.8 +/- 5.0 and 31.9 +/- 8.7%.

CONCLUSIONS

The circle absorber and NasOral systems were equally effective in achieving maximal preoxygenation during tidal volume breathing. Resuscitation bags differed markedly in effectiveness during preoxygenation; those with duck-bill valves without one-way exhalation valves were the least effective. Thus, the use of these bags should be avoided for preoxygenation.

Direct coronary vasomotor effects of sevoflurane and desflurane in in situ canine hearts

BACKGROUND

An extracorporeal system was used to investigate the direct coronary vasomotor effects of sevoflurane and desflurane in vivo. The role of the adenosine triphosphate-sensitive potassium channels (KATP channels) in these effects was evaluated.

METHODS

Twenty-one open-chest, anesthetized (fentanyl-midazolam) dogs were studied. The left anterior descending coronary artery was perfused at controlled pressure (80 mmHg) with normal arterial blood or arterial blood equilibrated with either sevoflurane or desflurane. Series 1 (n = 16) was divided into two groups of equal size on the basis of whether sevoflurane (1.2, 2.4, and 4.8%) or desflurane (3.6, 7.2, and 14.4%) was studied. The concentrations for the anesthetics corresponded to 0.5, 1.0, and 2.0 minimum alveolar concentration (MAC), respectively. Coronary blood flow (CBF) was measured with an ultrasonic, transit-time transducer. Local coronary venous samples were obtained and used to evaluate changes in myocardial oxygen extraction (EO2). In series 2 (n = 5), changes in CBF by 1 MAC sevoflurane and desflurane were assessed before and during intracoronary infusion of the KATP channel inhibitor glibenclamide (100 microg/min).

RESULTS

Intracoronary sevoflurane and desflurane caused concentration-dependent increases in CBF (and decreases in EO2) that were comparable. Glibenclamide blunted significantly the anesthetic-induced increases in CBF.

CONCLUSIONS

Sevoflurane and desflurane have comparable coronary vasodilative effects in in situ canine hearts. The KATP channels play a prominent role in these effects. When compared with data obtained previously in the same model, the coronary vasodilative effects of sevoflurane and desflurane are similar to those of enflurane and halothane but considerably smaller than that of isoflurane.

Nitric oxide does not modulate whole body oxygen consumption in anesthetized dogs

The effects of the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) and the NO donor sodium nitroprusside (SNP) on whole body O2 consumption (VO2) were assessed in 16 dogs anesthetized with fentanyl or isoflurane. Cardiac output (CO) and mean arterial pressure (MAP) were measured with standard methods and were used to calculate VO2 and systemic vascular resistance (SVR). Data were obtained in each dog under the following conditions: 1) Control 1, 2) SNP (30 microg. kg-1. min-1 iv) 3) Control 2, 4) L-NAME (10 mg/kg iv), and 5) SNP and adenosine (30 and 600 microg. kg-1. min-1 iv, respectively) after L-NAME. SNP reduced MAP by 29 +/- 3% and SVR by 47 +/- 3%, while it increased CO by 39 +/- 9%. L-NAME had opposite effects; it increased MAP and SVR by 24 +/- 4% and 103 +/- 11%, respectively, and it decreased CO by 37 +/- 3%. Neither agent changed VO2 from the baseline value of 4.3 +/- 0.2 ml. min-1. kg-1, since the changes in CO were offset by changes in the arteriovenous O2 difference. Both SNP and adenosine returned CO to pre-L-NAME values, but VO2 was unaffected. We conclude that 1) basally released endogenous NO had a tonic systemic vasodilator effect, but it had no influence on VO2; 2) SNP did not alter VO2 before or after inhibition of endogenous NO production; 3) the inability of L-NAME to increase VO2 was not because CO, i.e., O2 supply, was reduced below the critical level.

Influence of nitric oxide on vascular, metabolic, and contractile responses to dobutamine in in situ canine hearts

The left anterior descending coronary arteries of 30 anesthetized, open-chest dogs were perfused via an extracorporeal circuit. Coronary blood flow (CBF), myocardial oxygen consumption (MVO2), and segmental shortening (SS) were measured. Studies were performed with coronary perfusion pressure (CPP) or CBF constant. With CPP constant, effects of intracoronary (IC) infusions of dobutamine (2.5, 5.0, or 10.0 microg/min) were evaluated alone (control) and after inhibition of nitric oxide (NO) synthase with NG-nitro-L-arginine methyl ester (L-NAME). With CBF constant, a NO donor (sodium nitroprusside [SNP] 80 microg/min IC) or nitroglycerin [NTG] 40 microg/min IC) or a releaser of endogenous NO (acetylcholine [ACh]; 20 microg/ min IC) was infused along with dobutamine. Increases in CBF during dobutamine and isoproterenol were compared before and after blockade of beta1-adrenergic receptors with atenolol. Dobutamine caused proportional, dose-dependent increases in CBF, MVO2, and SS, which were not altered by L-NAME. Administration of the NO donors or ACh during dobutamine markedly decreased CPP, but only ACh also reduced SS and MVO2. These latter effects persisted after L-NAME. Atenolol blunted increases in CBF by dobutamine more than those by isoproterenol. We conclude that endogenous NO did not modulate the coronary vasodilation or the increases in myocardial contractility and MVO2 during dobutamine. In addition, neither SNP nor NTG altered myocardial contractility or MVO2 in dobutamine-stimulated myocardium, whereas ACh had a negative inotropic effect in dobutamine-stimulated myocardium that was independent of NO.

IMPLICATIONS

Endogenous nitric oxide (NO) did not modulate increases in coronary blood flow, myocardial contractility, or myocardial oxygen consumption during intracoronary infusions of dobutamine. The NO donors sodium nitroprusside and nitroglycerin had no effect on contractility or oxygen consumption in dobutamine-stimulated myocardium. Acetylcholine had negative inotropic effect in dobutamine-stimulated myocardium that was independent of NO.

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] microm) were placed in a chamber supplied with Kreb's 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. Glibenclamide, 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.

Is calcium a coronary vasoconstrictor in vivo?

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 microg/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.