OXYGEN

Scenario 3 Solution

You are called to the emergency room (ER). A 62 year old male with a history of chronic bronchitis has been admitted with a lower respiratory tract infection. The ER resident is requesting admission to intensive care for mechanical ventilation. On examination, the patient is tachypneic and cyanosed. His blood gas is pH 7.34 PCO₂ 54 PO₂ 45 HCO₃ 30 BE-2.

You request a piece of information, perform a therapeutic maneuver and 30 minutes later the patient’s blood gas is: pH 7.32 pCO₂ 60 PO₂ 55 HCO₃ 31 BE 0. The nurse wishes to reduce the FiO₂.

What information did you look for? What therapeutic maneuver did you perform, and what do you plan to do about the last blood gas?

What you were interested in was the patient’s baseline blood gas (from a previous admission): it was pH 7.38 PCO₂ 55 PO₂ 55 HCO₃ 32. The patient is hypoxemic at baseline, and retains CO₂.

You increased the FiO₂ to 40%, and have returned the PaO₂ to baseline for this patient.

The nurses concern about the blood gas is inappropriate: this relates to the widely held misconception that inspired oxygen tension should be minimized in COPD patients to prevent loss of respiratory drive. This is incorrect, even if there is some truth in the basic science (questionable), chemoreceptors respond to arterial oxygen tension, not what is given at the mouth. In this case, the patients PaO₂ is normal for him, and he is receiving the appropriate amount of oxygen. The raised CO₂ relates to ventilation-perfusion mismatch, resulting from the underlying acute injury, and release of hypoxic pulmonary vasoconstriction. The administration of oxygen may have increased the amount of dead space ventilation. In addition, raised PACO₂ displaces O₂ at alveolar level, requiring a higher FiO₂.

Copyright 2002 All rights reserved