The Science Journal of the American Association for Respiratory Care

Original Contributions

October 2002 / Volume 47 / Number 10 / Page 1168

Accuracy of Oxygen Analyzers at Subatmospheric Concentrations Used in Treatment of Hypoplastic Left Heart Syndrome

Timothy R Myers RRT and Robert L Chatburn RRT FAARC

INTRODUCTION: The immediate survival of infants with hypoplastic left heart syndrome depends on success in achieving several therapeutic goals: (1) maintain patency of the ductus arteriosus, (2) assure adequate mixing of blood at the atrial level, and (3) establish and maintain a balance between systemic and pulmonary blood flow at or near unity. In accomplishing that final goal, various ventilatory strategies have been used to alter the physiologic modifiers of pulmonary vascular resistance and thus maintain balanced circulation, including ventilation with gas of subatmospheric oxygen concentration. However, no data on this subject have been published in the scientific literature, and commercial oxygen analyzers are specified for use within the range of 0.21 to 1.0 fraction of inspired oxygen (FIO2), leaving the accuracy of hypoxic gas delivery somewhat uncertain. We evaluated the performance of oxygen analyzers below FIO2 0.21. METHODS: Two commercially available analyzers were studied: the TED-190 (Teledyne) and the Mini-OX III. Five new analyzers of each model were tested. After a 2-point calibration (FIO2 1.0 and 0.21), all 5 analyzers of the same model were simultaneously exposed to precision-blended gases at 6 different concentrations of oxygen in nitrogen. Steady state was maintained for at least 2 min at each concentration before readings were recorded. Calibration was verified at FIO2 0.21 between each level. RESULTS: The mean ± SD error was 0.0013 ± 0.0021 for the Mini-OX III analyzers and -0.0004 ± 0.0009 for the Teledyne analyzers. The upper and lower limits of the 95% confidence interval were 0.39% and -0.13% for the Mini-OX III analyzers and 0.07% and -0.15% for the Teledyne analyzers. The maximum difference between measured and known oxygen concentrations was 1% of full scale. CONCLUSIONS: The Mini-OX III and the Teledyne TED-190 provide accurate and reliable FIO2 readings between 0 and 0.21 that are within the manufacturers' specifications for maximum error. These 2 analyzers are therefore acceptable for use in delivering subambient oxygen concentrations. The Mini-OX III displays oxygen concentration to the nearest 0.1% and may be more appropriate for precise control.
Key words: oxygen analyzer, subatmospheric concentration, hypoplastic left heart syndrome.
[Respir Care 2002;47(10):1168–1172]

Introduction

Hypoplastic left heart syndrome (HLHS) is a potentially fatal congenital heart defect that occurs with failure of the systemic or left heart structures to develop adequately. HLHS describes a spectrum of cardiac abnormalities characterized by marked hypoplasia of the left ventricle and ascending aorta. HLHS affects 0.016-0.036% of live births and 1.4-3.8% of patients with congenital heart defect. Congenital heart defect is more prevalent in males than females, with a male predominance of 60-70%. Without surgical intervention, the majority of infants born with HLHS will die within a month of birth.

Infants born with HLHS have various degrees of hypoplasia or atresia of the aorta, aortic and mitral valves, and left ventricle. Typically, a patent foramen ovale or ductus arteriosus or a true atrial septal defect must be present to allow for left-to-right intracardiac shunting. Four distinct anatomical subtypes, based on the morphology of the left heart valves, have been described: (1) aortic and mitral stenosis, (2) aortic and mitral atresia, (3) aortic atresia and mitral stenosis, and (4) aortic stenosis and mitral atresia.

Because of these complex congenital malformations, oxygen-saturated pulmonary venous blood returning to the left atrium cannot flow into the left ventricle. This intracardiac shunting results in pulmonary venous blood flowing across the atrial septum and mixing with desaturated, systemic venous blood in the right atrium. This desaturated, mixed blood gets pumped to both the pulmonary (via the branch pulmonary arteries) and systemic (via the ductus arteriosus and descending aorta) circulations in parallel by the right ventricle. Thus, blood flow into each circulation depends on the pulmonary or systemic vascular resistance.

Frequently, optimization of systemic oxygenation and perfusion is achieved with very little medical intervention other than intravenous administration of prostaglandins to maintain ductal patency. However, with clinical signs of poor perfusion, metabolic acidosis, or oliguria it becomes medically necessary to increase the pulmonary vascular resistance by active respiratory management.

In most neonatal intensive care units oxygen analyzers are routinely used with oxygen hoods and mechanical ventilators to measure concentrations of oxygen administered to acutely ill patients. Four types of oxygen analyzer are commonly available: polargraphic, galvanic cell, paramagnetic, and Wheatstone bridge. We chose to bench test 2 polargraphic analyzers because of their response time, design for continuous use, and the availability of both high and low oxygen concentration alarms. Commercially available oxygen analyzers are specified for use within the range of 0.21 to 1.0 fraction of inspired oxygen (FIO2). Within that FIO2 range both manufacturers of the analyzers we studied list an accuracy of ± 2%. With the accuracy of the analyzer uncertain we were unsure of our ability to adequately control a subatmospheric oxygen delivery system. The purpose of this bench study was to evaluate the accuracy of 2 oxygen analyzers below the measurement range specified by the manufacturer.

The entire text of this article is available in the printed version of the October 2002 RESPIRATORY CARE.

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