The majority of patient who are admitted to ICU require targeted oxygen therapy during the course of their stay. This tutorial looks at how we assess oxygenation.

The easiest method for assessing oxygenation is to use the PaO2/FiO2 ratio (PFR) as a method of aligning the inspired oxygen tension (the therapy) to the PaO2 (the goal and the response). The PF ratio has been a key component of the diagnostic criteria for ARDS for decades. The PaO2 is measured by performing a blood gas. It represents dissolved oxygen in plasma rather than the oxygen content of blood that is determined by the oxyghemoglobin concentration and saturation (SaO2). The SaO2 can be helpfully estimated using non invasive pulse oximetry (SpO2).

Oxygen is taken up from the lungs continuously and carbon dioxide is excreted. Breathing is cyclical. Most gas exchange occurs during expiration, as inspiration only occupies 10 seconds or so per minute. The lungs hang down in the chest and hang out at a resting volume known as Functional Residual Capacity (FRC) at end expiration. Anything that reduces FRC, reduces the surface area for gas exchange and results in stale gas in or atelectasis of the alveoli. The consequence is ventilation perfusion mismatch and hypoxemia.

The initial treatment for hypoxemia is oxygen therapy delivered through nasal cannula. If that fails, or the patient is distressed, then high flow nasal oxygen (HFNO) is delivered. This improves oxygenation and reduces the work of breathing. If high flow fails, CPAP is delivered. CPAP applies positive pressure throughout the respiratory cycle, preventing phasic atelectasis and redistributing gas withing the lung. FRC is restored. If the patient fails HFNO, then they are intubated and ventilated.

Oxygen is assessed using FiO2, oxygen flow rate, PEEP or CPAP and mean airway pressure.