Introduction

Oxygenation is one of the primary gas exchange functions of the lung. Acute hypoxaemic respiratory failure is defined as an arterial partial pressure of oxygen (PaO2) of less than 8 kPa. This specific value is to a degree arbitrary, but reflects the beginning of the relatively steep portion of the oxy-haemoglobin dissociation curve (Figure 6.1).

This chapter will briefly review how to assess the adequacy of oxygen uptake and, in the context of each of the mechanisms of arterial hypoxaemia, examine how this can be improved in the mechanically ventilated patient.

Is the patient adequately oxygenated?

The assessment of oxygenation has two facets, one pulmonary, and one extra-pulmonary. The pulmonary facet is asking the question ‘how well are this patient's lungs able to take up the oxygen I am supplying?’ This is an important question to answer because it provides information on how sick the patient is, and provides an impetus for further action to improve pulmonary function. The extrapulmonary facet is asking the question ‘is enough oxygen being supplied to the patient's vital organs?’ This too is an important question, because inadequate oxygen delivery will lead to organ failure, but in the absence of arterial hypoxaemia the management of this problem cannot be addressed by ventilatory strategies.