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.
Results:
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.