Misunderstanding the arterial blood oxygen saturation

Case report

An 88-year-old woman was admitted to the emergency department of our hospital with an itchy skin rash that developed while she was on antibiotics (quinolones), administered for an episode of acute cystitis, asthenia and fever. She was suspected to have an allergic reaction and treated with i.v. methylprednisolone 40 mg per day. After 24 hours, skin rash and fever had subsided and the woman was normally oriented, easily breathing in the supine position, without peripheral oedema.

A pneumologist consultation was requested by the internal medicine ward because the patient had a history of chronic obstructive pulmonary disease (COPD), and very low oxygen saturation while breathing oxygen 1 L·min⁻¹ through a nasal cannula. On clinical examination, she presented cyanosis and a marked reduction of muscle mass. The chest examination was consistent with the diagnosis of COPD. Respiratory rate was 25 beats·min⁻¹ while breathing air and without paradox breathing at rest. She reported moderate dyspnoea while walking. Arterial blood pressure and cardiac frequency were 140/70 mmHg and 85 beats·min⁻¹, respectively. The oxygen saturation, captured from one finger with a pulse oximeter while breathing air was 81%. It was stable and did not change when the finger or the probe was changed.

We need to both increase the oxygen content of blood and the oxygen delivery to the tissues and to detect which kind of respiratory failure we are facing.

As previously mentioned, the patient had a history of COPD. An excess of oxygen concentration in the breathing mixture can reduce the hypoxic drive to ventilation. …