1995 OPEN FORUM Abstracts
VOLUME ACCURACY OF THE SIEMENS SERVO 900C AND NOVAMETRIX VENTRAK WHEN DELIVERING HELIUM-OXYGEN MIXTURES.
Mark Rogers BS RRT RCP Randy Scott BS RRT RCP Tom Malinowski BS RRT RCP Leo Langga BS RRT RCP Shamel Abd-Allah MD Ronald Perkin MD Daved vanStralen MD Loma Linda University Children's Hospital, Loma Linda, California.
BACKGROUND: Mechanically ventilated patients with lower airway obstruction (AO) are prone to air-trapping, increased peak inspiratory pressures, prolonged expiratory phase, and auto-PEEP. Low density helium-oxygen (HO) gas mixtures have been advocated to aid ventilation in patients with AO. We sought to determine the accuracy of the Novametrix VenTrak and Siemens 900C pneumotachs using various HO mixtures. DESCRIPTION OF TECHNIQUE: The Servo 900C is set up using a helium and oxygen mixture supplied via the ventilator's low pressure inlet. The helium and the oxygen flows are titrated to maintain desired FIO_2. Total HO flow is maintained greater than the patient's minute ventilation and adjusted to assure working pressure does not drop during a breath cycle. The Servo's working pressure is set approximately 10 cmH_2O greater than the patient's PIP to ensure adequate ventilating pressure. EVALUATION
Methods: We performed a bench evaluation using a Servo 900C and VenTrak monitor at five concentrations of helium (FIHe%): 0, 0.2, 0.5, 0.8, and 1.0. The ventilator circuit wye was connected to a VentAid Training Test Lung (TTL) with the compliance set to 0.45 L/cmH_2O. Expired gases from the ventilator were collected in a tissot spirometer. A VenTrak flow sensor was placed between the patient wye and the TTL and was calibrated to room air prior to testing. The ventilator was set at an IMV rate of 10 and a displayed tidal volume of 500 cc. 100 samples were taken at each HO mixture tested. The minute ventilation control of the 900C was adjusted to maintain a "delivered inspiratory" tidal volume of 500 ml. We considered the tissot spirometer an actual representation of delivered volumes.
Comparison of mean tidal volumes at varying FIHe%
FIHe% 0 0.2 0.5 0.8 1.0
Tissot 500 ml 503 ml 555 ml641 ml797 ml
Servo500 ml 500 ml[symbol??]500 ml [symbol??]500 ml [symbol??]500 ml
VenTrak [symbol??]446 ml [symbol??]405 ml [symbol??]356 ml [symbol??]307 ml [symbol??]239 ml
[symbol??] = differs significantly from tissot volume (p > 0.05 t-test)
Conclusions: 1) The Servo 900C ventilator can be used to deliver HO mixtures. 2) Volumes displayed by the Servo and VenTrak will read significantly less than actual delivered volumes. 3) The error introduced by HO increases with increasing helium concentrations.