The Science Journal of the American Association for Respiratory Care
INTRODUCTION: The immediate survival of infants with hypoplastic left heart syndrome (HLHS) is dependent upon the success in achieving several therapeutic goals. These are: (1) maintaining patency of the ductus arteriosis, (2) assuring adequate mixing of blood at the atrial level, and (3) establishing and maintaining a balance between systemic and pulmonary blood flow at or near unity. In accomplishing the final goal, different ventilatory strategies have been used to alter the physiologic modifiers of pulmonary vascular resistance and to thus maintain balanced circulation. One method is breathing small amounts of carbon dioxide (Respir Care 1994;39:1039). Another strategy employs oxygen delivered at concentrations below atmospheric pressure to achieve this goal. However, commercial oxygen analyzers are specified for use within the range of 0.21 to 1.0 fractional concentration of inspired oxygen (FiO2), leaving the accuracy and precision of subambient oxygen delivery systems somewhat uncertain. The purpose of this study was to evaluate the performance of oxygen analyzers below this specified range in an effort to improve the regulation of subambient oxygen delivery systems.
Methods: Two commercial brands of analyzers were used in the study: the TED-190 (Teledyne Brown Engineering) and the Mini-Ox III (Mine Safety Appliances Company). Five new analyzers of each model were tested. After calibration with 100% oxygen, all 5 analyzers of the same model were simultaneously exposed to precision blended gases at 6 different concentrations of oxygen. Steady state was maintained for at least 2 minutes at each concentration before readings were recorded. Calibration was verified at 21% oxygen between each level.
Results: The maximum difference between measured and known oxygen concentrations was 1%. Data below are expressed as the mean difference (as % of full scale) with error bars representing the 95% confidence interval. The dotted lines represent the manufacturers' total error specifications.
CONCLUSION: These two commercial oxygen analyzers maintained linearity and accuracy from 0 to 21% oxygen at ambient pressure and temperature. All measurements were within the manufacturers' error specifications. The Teledyne units were modified by the manufacturer so that the alarm thresholds could be adjusted down to 0%. Because even minor fluctuations in FiO2 can have a significant effect on patient condition, the Mini-Ox III, with readings to the nearest 0.1%, might allow more precise control.
(See Original for Figure)