The next 3 tutorials will be on the impact of Carbon Dioxide in Acid Base. This is different from the tutorials in the Acid Base series – and is complementary – but you don’t have to review that series to follow the tutorials. The tutorials follow directly on from those in the Vapors and Gases series.

I will be speaking at the ASA in San Antonio – on Stewart Acid Base on Oct 11th at 4pm. My updated chapter on that topic in the new edition of Miller should be available shortly. Drop by and say hello if you are in Texas.

I will be instructing on the JFICMI Mechanical Ventilation course in Galway on October 24th.

I will also be co-hosting this year’s Hi-Impact Critical Care Conference 6.0 Sepsis in Galway, at the Clayton Hotel, on November 7th.

I am organizing next year’s Western Anaesthesia Symposium at the Lake House Glasson, near Athlone, Ireland, on April 17th and 18th. It will be an awsome – fully interactive highly clinical meeting in a great location.

I am the co-convenor of the College of Anaesthesiologists Annual Congress, and organizer of the ICSI (Intensive Care Society of Ireland) ASM (at the same conference) on May 21st and 22nd 2026. This will be a really useful meeting for Anaesthesiologists and Intensive Care Specialists, and we (the CAI Council) hope you will be able to attend.

I am delighted to receive feedback on the tutorials and videos that I have posted, so if you come across me at any of those conferences – please tap me on the shoulder and we will have a chat over coffee, beer or wine.

Finally, the Eagle eyed of you will note that I am posting videos on @or2icu and @ccmtutorials. The or2icu channel will be used for rough cuts of videos (to get them out earlier – but there may be some mistakes), and random thoughts and (sometimes unedited) short videos that I will not be posting on the main channel. I will use it for audio only podcasts. On occasion I will post clips from other long videos that I think are worth watching. So, it is worth subscribing to both channels.

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.