Target blood gases

The goal of respiratory support is an adequate gas exchange at the lowest iatrogenic cost. Traditionally, 'adequate' normal blood gases were considered to be a PaO2 of about 100 mmHg (13.3 kPa) and a PaCO2 of about 40 mmHg (5.3 kPa). However, age affects gas exchange even in normal individuals, so that PaO2 is usually given by

Moreover, it is obvious that it is nonsense to set a target of normal blood gases in a patient who, before the acute lung insult, had abnormal blood gas values because of pre-existing pulmonary disease. It follows that the target blood gases should be those that the patient is believed to have had before the acute event.

However, in the last few years the implementation of the philosophy of 'lung rest' has led to the acceptance of definitively abnormal blood gas values (particularly PaCO 2, permissive hypercapnia) as a side-effect of a gentler treatment of diseased lungs. Although it may be reasonable to accept high PaCO2, we believe that PaO2 should be maintained at about 80 mmHg (10.7 kPa) (rather than 60 mmHg (8 kPa) as suggested by others) to avoid the risk of sudden deterioration of O 2 saturation.

Thus, in summary:

1. target the blood gases at normal values (considering the age);

2. adjust the target if the patient had previous pulmonary disease;

3. accept elevated PaCO2 if the iatrogenic cost of ventilation is too high.

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