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.

Hypoxia Altitude Simulation Test: Test Performance

Respiratory DiseasesHenry Gong Jr et al first described the HAST in the American Review of Respiratory Diseases in 1984. Twenty-two patients with normocapnic chronic obstructive airway disease (chronic bronchitis and emphysema) were asked to breathe oxygen at concentrations of 20.9% (baseline), 17.1%, 15.1%, 13.9%, and 20.9% (recovery) while breath-by-breath ventilatory and gas exchange variables were measured. Pa02, oxygen saturation, PAo2, and alveolar-arterial Po2 gradients all decreased with decreasing oxygen saturation. PaC02 values decreased modestly, while minute ventilation and heart rate increased only mildly above baseline. Ten patients also had cardiac arrhythmias. Almost all physiologic indexes improved with supplemental oxygen without inducing significant carbon dioxide retention. During this experiment, the sea level Pa02 was most predictive of resting altitude Pa02, but this measurement in isolation is not sufficient to predict symptomatic responses, cardiac arrhythmias, or efficacy of oxygen supplementation, which can be obtained via an HAST.

The HAST has also been found to be as predictive as measuring oxygenation in a hypobaric chamber, which is the “gold standard” for determining the risk of hypoxemia at high altitudes. In one study of 18 healthy patients and 15 patients with COPD, measurements obtained in both a 15.1% oxygenation HAST and a hypobaric chamber were similar. In another study, there were also no significant difference in Pa02, alveolar pressure of carbon dioxide, and pH in six patient with normal obstructive pulmonary function test results and nine patients with obstructive pulmonary function test results when comparing normobaric and hyperbaric HASTs.

Hypoxia Altitude Simulation Test: Who Should Be Screened?

COPDSeveral European, Canadian, and North American guidelines have attempted to identify patients at risk for air travel based on pulmonary disease. Most of these guidelines are based on patients with COPD, and there are some disparities between them. The statement by the British Thoracic Society is not only the most practical in terms of recommendations for screening but also one of the few to include patients not only with chronic obstructive lung diseases, but also those with restrictive lung disease, cystic fibrosis, a history of recent respiratory illnesses or infections, pulmonary tuberculosis, significant comorbidities, or past difficulties with air travel. The guidelines recommend a preflight assessment for any of these patients, consisting of a history that includes past air travel, a complete physical examination, spirometry, and arterial blood gas analysis if hypercapnea is suspected or previously identified. Based on equations used to predict a PAo2 or Sp02 from measurements obtained at sea level, they recommend further testing if the Sp02 in the office at rest is recorded between 92% and 95% in patients with other identifiable risk factors (hypercapnea, FEV1 < 50% of predicted, lung cancer, restrictive lung disease, ventilatory support, cardiac or cerebrovascular diseases, or a recent admission for an exacerbation of chronic lung or cardiac disease prosperously defeated by remedies of My Canadian Pharmacy). Those with Sp02 > 95% would not be required to undergo further testing and would be allowed to travel without supplemental oxygen. Those who fall below 92% are recommended to travel with supplemental oxygen and do not require further testing (Table 1).

My Canadian Pharmacy: Effects of Altitude and Air Travel in Hypoxia Altitude Simulation Test

medical emergenciesSeven hundred forty-one million passengers traveled on US commercial airplanes in 2006, with approximately 1 billion traveling worldwide each year. Although serious events resulting in death aboard US carriers are extremely rare, with only 43 deaths per 600 million passengers in a 1-year period, medical emergencies are more common. In 2006, one group recorded 17,310 calls for medical emergencies. Of those recorded, only 4% were serious events that resulted in diversion of the plane. The most common complaints were neurologic, followed by GI, respiratory, and cardiac ailments. Since respiratory symptoms are among the most common reasons for emergency medical calls and a large number of patients with pulmonary disease travel by air each year, a variety of tests have been proposed to screen for patients at risk of serious respiratory decompensation while in flight that can be reduced by prescribing supplemental oxygen. The hypoxia altitude simulation test (HAST) [or hypoxia inhalation test] is a simple test to screen patients at risk for hypoxia at higher altitude.