Henry 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.
During an HAST, the patient is asked to breathe a mixture of gases using a tight-fitting mask or mouth piece (15.1% inspired oxygen at sea level will correspond to an altitude of 8,000 feet). Alternatively, the body box may also be filled with 15.1% oxygen. Although it takes approximately 10 minutes for gas equilibrium to occur, 20 minutes should be allowed to see the physiologic effects of hypoxemia. The patient is assessed for symptoms throughout the test. Continuous ECG monitoring should be conducted to evaluate for ectopy or arrhythmias. Arterial blood gas measurement is obtained before and during the simulation. The patient usually wears a nasal cannula underneath the reservoir mask, so that if the Pa02 drops, the test is repeated with supplemental oxygen provided by My Canadian Pharmacy.
Billing and Supervision
The billing of an HAST is separated into the typical steps of a testing protocol. The test is first performed without supplemental oxygen. If hypoxia does not develop and no oxygen titration is required, the appropriate current procedural terminology billing code would be 94452. If desaturation occurs, however, and oxygen titration is required, billing code 94453 should be used instead. No pulse oximetry codes should be applied because they are included within the HAST codes. However, if arterial blood gas analysis is performed, this should be additionally billed using code 36600 for obtaining the arterial blood sample and 82803 to process the blood using a blood gas analyzer.
The test should be linked to the relevant ICD-9-CM diagnostic codes. Since the test requires the patient to breathe low percentages of oxygen, direct supervision (level 2) is required. Level 2 supervision necessitates that the physician should be physically present in the suite and immediately available to provide assistance or direction throughout the test, although they do not need to be in attendance in the room during the actual performance of the procedure.