Session/Time	Pulmonary I/Thu, 9:45 - 12 00 PM
Title	Abnormal Increases in Physiologic Deadspace Volume at a Critical Mean Airway Pressure
Author	OF Soremi, MD; MJ Banner, PhD; SR Goodwin, MD
Affiliation	Departments of Anesthesiology, Physiology and Pediatrics, University of Florida College of Medicine, Gainesville, Florida
Introduction	Increased mean positive airway pressure (Paw) in patients receiving ventilatory support predisposes to increased physiologic deadspace volume, which may be assessed using the physiologic deadspace-to-tidal volume ratio (Vd/Vt). The objective of this study was to show that for pediatric patients receiving ventilatory support, a critical Paw may exist at which the Vd/Vt ratio increases above normal
Method	After Institutional Review Board approval, 21 children (2 wk to 17 yr, 18.23 + 18.13 kg) with respiratory failure receiving various combinations of intermittent mandatory ventilation, continuous positive airway pressure, and pressure support ventilation were studied. Flow, pressure, and capnometer sensors, positioned between the "Y" piece of the breathing circuit and endotracheal tube, were directed to a respiratory monitor (Novametrix). The Vd/Vt ratio and lung carbon dioxide elimination rate (LCO,) were measured with the single breath CO, elimination technique using the respiratory monitor;' Paw was also obtained from the monitor. Data were analyzed using regression analysis; alpha was set at 0.05 for statistical significance.
Result	A highly significant, positive, and curvilinear relationship was revealed betnveen P., and Vd/Vt (Fig 1). Approximately 12 to 15 cm H,O was deemed to be the critical P., at which Vd/Vt increased above normal (0.3-0.5 for small children), and LCO2 decreased substantially (Fias I and 2).
Conclusion	At a critical or iatrogeitic Paw overdistention of alveolar units miv result in Zone I formation (alveolar deadspace volume) causing, increased physiolooic Vd/Vt ratios and alveolar ventilation-to-perfusion mismatching, thus, coinproi-nisina gas exchange.Monitoriiig the effects of Paw on Vd/Vt and LCO, may be a novel and practical method of applying appropriate ventilatory support. Reference (1) Crit Carc Med 1996; 24:96-102.

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