1996 OPEN FORUM Abstracts
DIFFERENCES BETWEEN TIME-CYCLED, AND VOLUME CYCLED BREATHS WHICH MAY AFFECT OXYGENATION IN SEVERE NEONATAL RESPIRATORY FAILURE: A TEST-LUNG STUDY
Craig Black, PhD, RRT, Pulmonary Services Division and Douglass Laboratory, St. Vincent Medical Center, and Department of Biology, University of Toledo, Toledo, Ohio.
Introduction: Published studies suggest that volume control (VC) ventilation is superior to traditional time-cycled, pressure limited (TC) ventilation in treatment of several disorders which cause severe neonatal respiratory failure; specifically, VC yields superior oxygenation. Analysis of flow-volume (F/V) and pressure-volume (P/V) curves generated from these two modes with a neonatal test lung should suggest a possible explanation for clinical observations. Methods: A BiCore CP-100 measured respiratory mechanics of VC and TC breaths generated by a VIP Bird hooked to a two- compartment neonatal test lung (INGMAR MEDICAL). Three levels of airway resistance (Raw) and system dynamic compliance (Cdyn) typical of neonatal respiratory failure were utilized, including one where one compartment was set to mimic "slow" lung units (high Raw/Low Cdyn) and the other "fast" lung units (lower Raw/higher Cdyn). Constant respiratory rate (20/min), PEEP (3cmH20), and flow rate (8lpm) were maintained. Achieved tidal volume (20cc) was maintained constant by adjusting PIP for TC breaths and delivered VT for VC breaths. Results: No significant differences (P < 0.05) between TC and VC breaths were observed for Raw, Cdyn, Time Constant, or PIP, at any of the three test lung settings. MAP was significantly (P < 0.05) lower for VC breaths in all instances, and F/V curves of VC breaths showed a marked flattening of the inspiratory limb in all instances. Two representative F/V curves are shown below.
(See original for figure)
Conclusions: Analysis of F/V curves shows that maximal flow is more sustained during the VC breaths than during the TC breaths. This could result in increased recruitment and stabilization of lower compliance/higher resistance lung units and thereby improve oxygenation.