The Science Journal of the American Association for Respiratory Care

2004 OPEN FORUM Abstracts


Edward L Schneider RRT-NPS, Greg Schears MD, Steve Sittig RRT-NPS, Grant Wilson RRT-NPS Mayo Clinic Rochester Minnesota

HFOV/NO is commonly used in infants with severe reversible hypoxemia of various causes. It is expected that set O2/ NO levels are delivered to the patient. Described is a critically ill patient in whom this was not true.

Case Summary: A newborn infant with Ebstein’s Anomaly and severe pulmonary hypoplasia was placed on a Sensormedics 3100A and NO to optimize oxygenation. The patient failed conventional ventilation and required prostaglandin to maintain ductal patency to augment pulmonary blood flow. It was noted with adjustment of the mean airway pressure (MAP) using the pressure limit knob that a paradoxical drop in the patient’s delivered O2 and NO measured at the distal end of the circuit occurred.

A bench test was set up and equipment calibrated prior to testing. Bench test settings for both the ventilator and NO delivery system were adjusted to match those during the clinical experience. An end tidal CO2 detector was placed between the test lung and the ventilator circuit. MAP adjustments that duplicated those made during the clinical experience were performed. The oscillator settings were FiO2 1.0, nitric oxide 20 parts per million (PPM) and the MAP of 8cm H2O. Recorded measurements at the distal end of the circuit revealed a delivered FIO2 of 0.78 and the NO of 14 PPM. We theorized that entrainment of ambient air was occurring in the circuit. To test this theory, carbon dioxide gas was placed near the mushroom valves. The presence of carbon dioxide in the circuit confirmed entrainment of ambient air. We then tried to determine where this entrainment was occurring. Carbon dioxide gas was placed near each mushroom valve individually. We did not encounter entrained carbon dioxide until the gas was placed near the limit valve.

Discussion: The fact that the Sensormedics 3100A oscillator ventilator has an active inspiratory and expiratory phase, we theorized that ambient air was being entrained into the circuit under certain conditions. We found that adjusting the MAP with the limit knob may in certain conditions cause this entrainment to occur. In checking the ventilator operator’s manual (1991 edition) the practice of using the limit knob to adjust MAP was described. A subsequent manual (1996 edition) does not mention the use of the limit knob to adjust MAP. In light of these findings we suggest that the limit knob not be used to adjust MAP. If the limit knob is used in adjusting MAP it is our recommendation to consider measuring the delivered oxygen proximately at the patient connection. The manufacturer has been notified of our findings.