Anesthesiology
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In anesthetized cats ventilated with oxygen, 0.5 ml of the inert gas sulfur hexafluoride (SF6) was substituted for vitreous. When the ventilating gas was changed to nitrous oxide (N2O) 66%, balance oxygen, intraocular pressure increased from 14.4 to 30.3 mmHg in 19.5 min. ⋯ This intraocular pressure change secondary to gas volume alteration may adversely affect therapeutic outcome of ophthalmic surgery. Accordingly, N2O should be avoided in patients during and following intravitreal injection of SF6 for up to 10 days.
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We tested the hypothesis that different anesthetic techniques for elective cesarean section would be reflected in the pattern of breathing and its control after birth. The pattern of breathing, including tidal volume, total breath duration (TTOT), minute ventilation, inspiratory (TI) and expiratory times, TI/TTOT ratio, and mouth occlusion pressure, was measured in 27 infants delivered by elective cesarean section during maternal epidural (lidocaine-carbon dioxide-epinephrine, n = 19) or general anesthesia (66% oxygen in N2O and 0.5% halothane, n = 8) at 10, 60, and 90 min and 3-5 days of age. ⋯ In general, at any given age the values of the respiratory parameters measured and their variability were similar between the two groups of infants. These findings indicate that the pattern of breathing after birth is not different following epidural or general anesthesia, and on the basis of our measurements, both epidural or general anesthesia appeared equally suitable for elective cesarean section.
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In a previous study, the authors found that infants, in the first 6 months of life, required the highest minimum alveolar concentration (MAC) of any age group (1.09% halothane). Because only two neonates (0-31 days of age) were included in the original study and because profound depression of blood pressure and heart rate have been reported in neonates, the authors determined 1) whether the MAC of halothane in neonates (n = 12) differs from that in infants (1-6 months of age) (n = 12) and 2) whether the blood pressure and heart rate responses in neonates differ from those in infants at approximately 1 MAC. The authors found that the MAC of halothane in neonates, 0.87% +/- 0.03 SEM, was significantly lower (P less than 0.01) than that in infants, 1.20% +/- 0.06 SEM. ⋯ The authors conclude that the MAC of halothane in neonates is 25% less than that in infants and significantly less than was thought previously. The MAC in infants is the highest of any age group. The decrease in blood pressure and the incidence of hypotension in neonates are similar to those in infants at approximately 1 MAC of halothane.
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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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Compression of gases (Boyle's law) and circuit compliance are major determinants of anesthesia circuit function. The materials of which circuits are constructed and the use of heated humidifiers may result in clinically important variations in delivered minute ventilation (VE) secondary to variations in compression volume. We examined eight anesthetic circuits both with and without a heated humidifier in an in vitro setting. ⋯ Pediatric circle systems were intermediate and adult circle systems had the largest compression volume and were the least efficient. Humidifiers uniformly increased compression volume. The following conclusions were drawn: 1) the anesthetic circuit, its material, and the pressure at which it operates are important determinants of circuit function; 2) humidifiers increase compression volume; 3) Mapleson D circuits had the lowest compression volume and therefore were the most efficient; 4) highly compliant adult circuits may result in compression volume losses that exceed the tidal volume of a pediatric ventilator; 5) humidifiers with low volume and rigid tubing should have the least effect on minute ventilation; and 6) highly compliant adult circuits when used in the care of infants and small children must be used with caution.