Oxygenation is the immediate priority in most medical emergencies and is more important than ventilation. The common concern that health care professionals have in relation to oxygen dependency ('hypoxic drive') can sometimes lead to neglect of this priority. 

Patients can be oxygenated without being intubated.

There can be confusion between critical illness driven by impaired airway, oxygenation or ventilation. When assessing a patient on the ICU who is receiving respiratory support, it is useful to decide which of these three issues predominates. Good examples of scenarios to understand the difference include brain-stem testing, where no ventilation occurs, but oxygenation is intact. In asthma, there may be no difficulty oxygenating, but ventilation cn be extremely challenging. In ARDS, an initial difficulty in oxygenating can be complicated by subsequent difficulty ventilating. 

Oxygen delivery involves the whole pathway of  from inspired gas (air/supplemented air) through gas exchange in the lungs; to delivery to tissues and utilisation within mitochondrial aerobic respiration (oxidative phosphorylation). 

How much Oxygen? Many ICU patients get more oxygen than they will get during health and circulating haemoglobin can be over saturated compared to the normal situation.  Conversely chronic 'permissive' hypoxia (targeted in severe ARDS in an effort to avoid excessive stress/strain on lung tissue) related to critical illness itself may have long-term consequences eg in relation to long-term cognitive recovery which is now only being understood.

How to give Oxygen? There is a spectrum of delivery systems that are used to deliver oxygen to critically ill patients. These range from simple systems like nasal specs/ variable performance masks-venturi 'fixed performance masks-high flow therapy-CPAP-invasive ventilation-oxygen rescue techniques (proning/nebulised prostacyclin/ECMO). 

The interaction of the heart and lungs in relation to oxygen delivery is poorly understood, particularly in relation to pulmonary hypertension/RV strain which is probably underestimated in patients with injured lungs. 

Intubation is frequently associated with an initial worsening of oxygenation due to loss of inspired oxygen during the procedure; collapse of lungs (derecruitment); hypotension during anaesthetic induction.

Depending on the clinical context, a rapid sequence induction may need to be modified to prevent loss of control of oxygenation during intubation. Many ICU physicians will gently ventilate patients after induction to hold the lung of the patient open; continue oxygen delivery and crucially buy time for the induction agents to fully work to ensure ideal intubating conditions.

Critically ill patients often improve oxygenation post intubation, simply by applying PEEP and waiting for lungs to open. 

The COVID19 pandemic was caused by a disease that resulted in acute oxygenation failure due to viral pneumonitis in patients who were unable to prevent viral spread (ineffective innate/adaptive immunity) and demonstrated the fundamental importance of oxygenation in modern medicine.