Anesthesiology
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Comparative Study
Lung volumes, mechanics, and oxygenation during spontaneous positive-pressure ventilation: the advantage of CPAP over EPAP.
To determine if continuous positive airway pressure (CPAP) or expiratory positive airway pressure (EPAP) is superior for achieving or maintaining effective lung volume in spontaneously breathing critically ill patients in acute respiratory failure, the authors measured functional residual capacity (FRC), airway and esophageal pressures, and arterial oxygen tensions when CPAP and EPAP were 5 and 10 cm H2O. Arterial oxygenation, FRC, and transpulmonary pressure at end-expiration were greatest when CPAP was 10 cm H2O. Lung compliance did not change. The authors conclude that CPAP at 10 cm H2O is the more effective technique, either because it allows relaxation of chest wall musculature on expiration, or because EPAP at 10 cm H2O increases chest wall muscle tone.
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Positive end-expiratory pressure (PEEP) has little beneficial effect in improving gas exchange in canine left lower lobe (LLL) pneumonia. This is true because while PEEP improves lobar gas exchange, it also increases relative perfusion (QL) to the diseased lobe. The authors hypothesized that PEEP administered to only the diseased lung would avoid the increased QLLL. ⋯ QLLL per cent did not increase during PEEP. These results suggest that unilateral PEEP improves regional gas exchange within the pneumonia lobe, probably by ventilating units which were previously perfused but not ventilated. Further, this improvement in regional gas exchange occurred without the diversion of blood flow towards consolidated lung that occurs with whole-lung PEEP, and so resulted in a substantial net improvement in overall gas exchange.
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The relationship between plasma cholinesterase genotype and duration and type of succinylcholine neuromuscular blockade was studied in 43 anesthetized patients heterozygous for abnormal plasma cholinesterase using train-of-four nerve stimulation. Twenty-eight patients were heterozygous for the usual and the atypical gene (E1uE1a), eight were heterozygous for the usual and the silent gene (E1uE1s), three were heterozygous for the usual and the fluoride-resistant gene (E1uE1f), three were heterozygous for the fluoride-resistant and the atypical gene (E1fE1a), and one was heterozygous for the fluoride-resistant and the silent gene (E1fE1s). Mean time to 90 per cent recovery of twitch height in patients with genotypes E1uE1a, E1uE1s, and E1uE1f (14.6, 12.4, and 12.0 min, respectively) was significantly prolonged compared to patients with normal cholinesterase genotype (9.3 min). ⋯ The four patients with abnormal genes on both chromosomes (E1fE1a and E1fE1s) all showed significantly prolonged paralysis following the administration of succinylcholine (mean time to 90 per cent twitch recovery was 30 min). Patients heterozygous for the usual and one of the abnormal genes (E1uE1a, E1uE1s, and E1uE1f) had typically depolarizing blocks following the administration of succinylcholine, 1 mg/kg. Patients with abnormal genes on both chromosomes (E1fE1a and E1fE1s), however, all showed desensitization type of neuromuscular blockade (phase II block).
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Based on previous whole lung findings, the authors tested the hypothesis that lobar hypoxic pulmonary vasoconstriction (HPV) would be potentiated by repeated intermittent lobar hypoxic challenges. In sixteen open-chested pentobarbital-anesthetized dogs they found that repetitive hypoxia of the left lower lobe (LLL) (Group I = LLL nitrogen ventilation, n = 8; Group II = LLL absorption atelectasis, n = 8) caused the percentage decrease in the electromagnetically measured fraction of the cardiac output perfusing the LLL (QLLL/Qt) to become progressively greater (increased LLL HPV) through the first three hypoxic challenges in Group I and through the first four hypoxic challenges in Group II. ⋯ There was no significant difference between the eighth LLL HPV response and the subsequent three. These findings indicate that 1) the mechanism of blood flow decrease to atelectatic lung is probably the same as for nitrogen-ventilated lung, namely, by HPV, and 2) in order to maximize HPV in the nonventilated lung during one lung ventilation, several repeated intermittent cycles of deflation-inflation to the lung should be performed during the initiation of one lung ventilation.