The American review of respiratory disease
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Am. Rev. Respir. Dis. · Jul 1989
Comparative StudyIn vitro versus in vivo comparison of endotracheal tube airflow resistance.
The mechanics of gas flow in endotracheal (ET) tubes have been evaluated extensively in vitro under static and dynamic conditions. Previous bedside determinations of respiratory system mechanics in patients with acute respiratory failure have been based on assumptions derived from in vitro measurements without direct measurement of in vivo ET tube resistance (RET). We hypothesized that the RET measured in vivo would be greater than those values obtained in vitro when peak flow rates and ET tube size were held constant. ⋯ Although there was considerable individual variation, values of RET measured in vivo were generally higher than those derived from in vitro measurements at both peak flow rates tested, perhaps because of secretions, head or neck position, tube deformation, or increased turbulence. We conclude that ET tubes contribute significantly to total airflow resistance and that RET is often significantly greater than indicated by in vitro studies. Estimates of work of breathing in critically ill patients must take into consideration the contribution of in vivo RET on total pulmonary system resistance.
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Am. Rev. Respir. Dis. · Jul 1989
Detrimental effects of positive end-expiratory pressure during controlled mechanical ventilation of patients with severe airflow obstruction.
Positive end-expiratory pressure (PEEP) in treatment of asthma may be beneficial by dilating airways or detrimental by increasing hyperinflation. Several studies have reported beneficial results but with conflicting effects on lung volume. We studied the effects of PEEP on pulmonary hyperinflation, gas exchange, and circulation in six patients (59 +/- 19 yr, four men, two women) with severe airflow obstruction requiring mechanical ventilation (four with asthma, two with an exacerbation of chronic airflow obstruction). ⋯ Two patients were not studied at 15 cm H2O PEEP because of hypotension. Without PEEP, all patients showed gas trapping above FRC that increased progressively as R was increased (i.e., expiratory time decreased). At each R, increases in PEEP progressively increased FRC up to 1.42 +/- 0.43 L (mean +/- SD) at 15 cm H2O PEEP (n = 4) and progressively reduced the degree of gas trapping above the PEEP FRC.(ABSTRACT TRUNCATED AT 250 WORDS)