2006 OPEN FORUM Abstracts
EFFECTS OF CHANGING FREQUENCY, DELTA-P AND INSPIRATORY TIME ON DISTAL LUNG PRESSURE DURING HFOV - A BENCH STUDY
Gary Martin BS RRT; Allan
Andrews MS RRT; Carl Haas MLS RRT FAARC
Critical Care Support Services, University of Michigan Hospitals & Health Center, Ann Arbor MI
Background: High frequency oscillatory ventilation (HFOV) is increasingly being used as a lung protective ventilation mode although the protective nature may be lost as the ventilator settings approach maximum limits (low frequency (Hz), high amplitude (delta-P), long inspiratory time (insp-%)). STUDY
Objective: To determine the relative proximal airway, tracheal and distal lung pressure changes as frequency, delta-P and insp-% are manipulated using an ARDS lung model.
Methods: A lung simulator (ASL 5000, IngMar Medical) with adjustable resistance and compliance of a two-lung model was used. The resistance of an 8 ETT was measured (15 cm H2O/L/sec) and used for upper airway resistance. Resistance to each lung was set to 10 cm H2O/L/sec and compliance to 10 mL/cm H2O. The ventilator (3100B, VIASYS) was set to MAP =25 cm H2O, bias flow =35 L/min, and 50% oxygen. The following combinations of settings were tested: 1) frequency of 3, 5, and 7 Hz; 2) delta-P of 50, 75 and 100 cm H2O, and 3) insp-% of 33% and 50%. Approximately 200-250 breaths of data for each combination were captured via the ASL software and converted to Excel for analysis.
OBSERVATIONS: The data suggests: For a given frequency, as delta-P increased: 1) lung MAP decreased, 2), VT increased, 3) end-expiratory lung volume (FRC) decreased, and 4) the increased lung delta-P (and VT) was due to a decrease in minimum lung pressure, not an increase in maximum lung pressure. For a given delta-P, as frequency was reduced: 1) VT increased, 2) lung MAP was below target MAP and remained constant across all frequencies, and 3) the increase in lung delta-P resulted from an equal increase in minimum and maximum pressure. For a given frequency and delta-P, as insp-% increased: 1) VT increased, and 2) lung MAP was closer to the set target. For all conditions, distal lung pressure fluctuated ± half the distal delta-P around the lung MAP. CLINICAL IMPLICATIONS: This study helps appreciate how lung volume and pressure is affected by changes in ventilator settings. Although the observed volume and pressure changes may not be exactly what occurs clinically, the direction and general magnitude are most likely correct.