2003 OPEN FORUM Abstracts
EFFECT OF OXYGEN FLOW ON PERFORMANCE OF A BiPAP VENTILATOR.
Mark J. Sollars, RRT; Dean R. Hess, PhD, RRT, FAARC. Massachusetts General Hospital
and Harvard Medical School, Boston MA.
Background: Noninvasive ventilation is commonly used for selected patients with acute
respiratory failure. The BiPAP ventilator (Respironics, Pittsburgh, PA) is commonly used for
this purpose. Supplemental oxygen is provided by titrating it into the circuit. Oxygen flows
=15 L/min are usually used, which provide modest inspired oxygen concentrations. When a
very high inspired oxygen concentration is needed, some clinicians have titrated high oxygen
flows into the circuit.
Hypothesis: Oxygen flows >15 L/min titrated into the BiPAP circuit
will affect the performance of the ventilator.
Methods: A bellows-in-a-box lung model
(compliance 100 mL/cm H2O, resistance 20 cm H2O/L/s) was used to simulate spontaneous
breathing at a rate of 30/min. A single limb circuit was placed between the BiPAP and
oronasal mask. The mask was glued to a Plexiglas plate, from which a 15 mm hole led to the
test lung. Two mask types were used; the Respironics Spectrum in which the leak port is
incorporated into the circuit and the ResMed Mirage in which the leak port is incorporated
into the mask. A Respironics BiPAP S/TD 30 was attached to the model and 3 settings were
evaluated: IPAP/EPAP of 10/5, 20/10, and 25/10 cm H2O. Oxygen was titrated into the
circuit at the ventilator outlet at flows of 15, 30, 45, 60, and 75 L/min. A Novametrix Ventrak
(Novametrix, Wallingford, CT) was used to measure flow, tidal volume, airway pressure, and
pressure in the bellows of the lung model. FIO2 was measured with a Puritan-Bennett 7820
oxygen analyzer.
Results: Little additional increase in FIO2 was achieved by using flows
of 60 and 75 L/min (see figure). The oxygen flow had no clinically important
effect
on triggering or auto-PEEP. However, IPAP and EPAP pressures increased significantly
(P < 0.001) and by a clinically important amount as the oxygen flow increased
(see
figures).
Conclusions: Use of very high oxygen flows into the BiPAP circuit achieves
little increase in FIO2. Moreover, this practice increases the ventilating pressures,
which may go undetected by the clinician. We suspect that the increased arterial
oxygenation noted by some clinicians when very high oxygen flows are titrated
into the circuit might be the result of an increase in EPAP and IPAP rather than
a higher FIO2. Consistent with the recommendations of the manufacturer, high
oxygen
flows should not be titrated into the circuit of the BiPAP ventilator.