Hyperoxia Test
1. Introduction
The hyperoxia test (also known as the 100% Oxygen Challenge Test) is a traditional bedside diagnostic tool used to differentiate between cyanosis of cardiac origin (Right-to-Left shunt) and non-cardiac origin (pulmonary or CNS causes).
2. Physiological Basis
- Pulmonary Disease: Cyanosis is usually due to V/Q mismatch or diffusion defects. Increasing the inspired oxygen concentration (FiO2) to 100% can overcome these barriers, significantly increasing the PaO2.
- Cardiac Disease: Cyanosis is due to an anatomic Right-to-Left (R-L) shunt. Since a portion of the systemic venous blood bypasses the lungs entirely, increasing FiO2 to the lungs does not significantly raise the arterial oxygen tension.
3. Procedure
- Baseline: Obtain a baseline Arterial Blood Gas (ABG) while the neonate is breathing room air (FiO2 0.21).
- Intervention: Administer 100% humidified oxygen for 10β15 minutes (via headbox or ET tube if ventilated).
- Measurement: Repeat the ABG (preferably from the Right Radial artery - pre-ductal).
- Alternative: In resource-limited settings, pulse oximetry (SpO2) is used, though it is less sensitive than PaO2.
4. Interpretation
| Result | PaO2 (mmHg) | Interpretation |
|---|---|---|
| Pass | > 150 - 250 | Likely Pulmonary/CNS cause |
| Inconclusive | 100 - 150 | Consider PPHN or mild CHD |
| Fail | < 100 | Likely Cyanotic Congenital Heart Disease (CCHD) |
Note:
If PaO2 remains < 100 mmHg despite 100% FiO2, the probability of a duct-dependent cardiac lesion is very high.
5. Specific Scenarios & Limitations
- PPHN (Persistent Pulmonary Hypertension): May initially fail the test. However, if hyperventilation or vasodilators (Nitric Oxide) improve PaO2, it suggests PPHN rather than structural CHD.
- Mixing Lesions: Conditions like TAPVC or HLHS may show a "blunted" response (PaO2 ~100 mmHg) because of high pulmonary blood flow.
- Methemoglobinemia: Shows low SpO2 but a normal PaO2 (the "Saturation Gap").
- Risks: Prolonged 100% oxygen can cause closure of the ductus arteriosus in duct-dependent lesions or cause retinopathy of prematurity (ROP) in preterms.
6. Current Status
With the widespread availability of bedside 2D-Echocardiography, the hyperoxia test is less frequently used in tertiary centers. However, it remains a critical "bridge" test in primary care to initiate life-saving Prostaglandin E1 (PGE1) infusion before transport.