In some patients with severe lung disease, the hypoxic drive to ventilation becomes very important. These patients have chronic CO2 retention, and the pH of their brain extracellular fluid has returned to near normal in spite of a raised pCO2  (renal compensation). Thus, they have lost most of their increase in the stimulus to ventilation from CO2. Under these conditions, the arterial hypoxemia becomes the chief stimulus to ventilation. If such a patient is given a high O2 mixture to breathe to relieve the hypoxemia, ventilation may become grossly depressed. Pulmonary edema and convulsion are side effects of pure oxygen therapy.