Articles: general-anesthesia.
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Associations between airway closure, alveolar-arterial oxygen tension difference (A-aDO2), and positive end-expiratory pressure (PEEP) were investigated in anesthetized, paralyzed, artifically ventilated patients. The difference between closing capacity (CC) and functional residual capacity (FRC) was measured with a modified standard technique using a bolus of N2 to detect airway closure in denitrogenated patients. At FIO2 = 0.4 during anesthesia before application of PEEP, A-aDO2 was larger than expected in comparable conscious subjects and increased at about 1 mmHg/yr of age. ⋯ Patients in whom CC was initially below FRC failed to improve oxygenation with PEEP. At least half of the decrease in A-aDO2 associated with application of PEEP persisted for 20-30 min after the withdrawal of PEEP, although the withdrawal resulted in an immediate recurrence of airway closure above FRC. The authors conclude that closure of pulmonary units operates in some circumstances to contribute to pulmonary dysfunction in anesthetized patients but is neither the only nor necessarily the most important such mechanism.
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The intraoperative and postoperative effects of fentanyl and ketamine administered continuously by infusion were compared with those produced by conventional intermittent bolus administration in 100 patients. After a standardized induction with thiopental 4 mg/kg intravenously, patients received either fentanyl (50 micrograms boluses vs. 2 micrograms/ml infusion) or ketamine (25 mg boluses vs. 1 mg/ml infusion) as intravenous adjuvants to nitrous oxide, 70% in oxygen. With continuous infusion, the doses of fentanyl and ketamine required were decreased 45% and 43%, respectively. ⋯ However, excessive sedation was noted in 48% and 52% of patients in the fentanyl and ketamine bolus groups, respectively, compared with 4% and 8%, respectively, in the infusion groups. Discharge times were decreased by 29% and 13% in the fentanyl and ketamine infusion groups, respectively. The author concludes that continuous infusion fentanyl (0.1 micrograms . kg-1 . min-1) or ketamine (50 micrograms . kg-1 . min-1) significantly decreases the drug dosage requirement, improves intraoperative conditions, and decreases recovery time compared with the traditional intermittent bolus technique.
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Simultaneous measurement of tracheal and esophageal pressures during occluded inspiratory efforts (occlusion test) was used to assess the validity of the esophageal balloon technique in anesthetized supine subjects. Ten ASA 1 patients undergoing general anesthesia (halothane 1 MAC, nitrous oxide 70%, and oxygen) for minor surgery were studied. Esophageal pressure (Pes) was measured using a 5-cm-long balloon and was plotted against tracheal pressure (Pt). ⋯ In the remaining three, however, the difference between delta Pes and delta Pt ranged between +20% and -40%. By repositioning the balloon to 5 or 15 cm above the cardia, a locus was found in all subjects where the difference is less than 10%. We conclude that the esophageal balloon technique can be used in anesthetized supine subjects to give reliable measurements of changes in pleural pressure, provided that it is validated with the occlusion test.