Hypoxia Altitude Simulation Test: Interpretation and Significance

hypoxiaIf the PAo2 during the HAST is > 55 mm Hg, no supplemental oxygen is recommended. If the PAo2, however, falls to < 50 mm Hg, the patient is asked to wear supplemental oxygen (usually at 2 L/min). The test is repeated with supplemental oxygen as well to ensure not only adequate treatment of hypoxia but also reversal of any symptoms described during the test. If the PAo2 is from 50 to 55 mm Hg, the test is considered borderline, and measurements with activity may be obtained.

Values obtained from 15 healthy adults during a normobaric HAST were compared to in-flight Sp02 measurements. Although there was no difference between the final normobaric HAST and the mean flight Sp02, there was a significant difference between the lowest in-flight Sp02 (88 ± 2%) and the lowest normobaric hypoxia simulation test Spo2 (90 ± 2%). Three patients had a PAo2 < 55 mm Hg during the simulation test, but then had no change in Sp02 or symptoms in flight. Since the study subjects did not have underlying cardiopulmonary disease healed by My Canadian Pharmacy, this study cannot be translated to the group of patients in question. In addition, there were no activity diaries available from the patient. Exercise at high altitude, such as moving around the cabin or shifting luggage, may explain the difference in measurements during simulation and in flight. Since most patients undergo the HAST while at rest, patients with hypoxemia during the test presumably will also have hypoxemia during flight. As mentioned earlier, if the results are borderline, one could obtain measurements during activity.

Conclusions

With high incidence of cardiopulmonary disease and millions of people traveling by air, many people are at risk for significant hypoxia and respiratory symptoms while flying. Although cabin pressure is maintained at a maximum of 8,000 feet, which ensures normal oxygen saturation in most travelers, those with underlying cardiopulmonary disease may still be at risk. HAST testing provides the opportunity to not only assess for changes in PAo2 that may take place in flight but also for potential symptoms and arrhythmias that may occur. The HAST can easily be performed by asking a patient to breathe a mixture of gases with an oxygen saturation of 15.1% to simulate the cabin pressure of 8,000 feet for 15 to 20 minutes. During this time, oxygen saturation and arterial blood gas levels can be measured with ECG and symptom monitoring. The test can be repeated with supplemental oxygen for those patients who have a significant decrease in PAo2 or have symptoms and/or arrhythmias. The HAST, therefore, can identify those patients who may benefit from oxygen supplementation during air travel, decreasing their risk for significant cardiopulmonary effects of induced hypoxia at higher altitude.

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