Using Hypoxic and Hypercarbic Challenges in the Diagnosis of Ventilatory Receptor Dysfunction

Abstract

Respiratory center disorders are frequent causes of periodic apnea, and assessing ventilatory responses to hypoxemia and hypercarbia has become clinically important. Hypoxic and hypercarbic challenges can be executed in a hospital pulmonary function laboratory. In hypoxic challenge the patient breathes from a spirometer with an initial FO2 of 0.30. Helium replaces the oxygen used by the patient and the test is terminated if system FO₂ drops to 0.06, if the patient's blood oxygen saturation drops to 70%, or if minute ventilation triples. In hypercarbic challenge the patient breathes from a spirometer with no carbon dioxide absorber. System FO2 of 0.50 is maintained and F CO2 is monitored. The test is terminated if the patient experiences hypercarbia, if the FECO2 increases 3%, or if minute ventilation triples. Combined hypoxic-hypercarbic challenge is used to determine central depression secondary to hypoxemia in patients with intact responses to hypercarbia. In this test the patient breathes from a spirome-ter with no carbon dioxide absorber and an initial system Fo₂ of 0.30 is used. Helium is bled into the spirometer to replace oxygen used. Criteria for termination of the test are the sum of those used for hypoxic and hypercarbic challenges. In a patient with diabetic neuropathies and nephropathy who was experiencing periodic apnea, hypoxic and hypercarbic challenges revealed absent response to hypoxemia but rela-tively normal response to hypercarbia. Oxygen cannula therapy at 5 1/min was initiated to prevent central depression and eliminated the apneic spells for five months following treatment.