Acute respiratory failure

Hypoxia

Hypoxic respiratory failure is diagnosed in patients without chronic lung disease when, on room air, there is:

• falling pulse oximetry from 92% saturation, or

• initial pulse oximetry <80% saturation.

Severe respiratory failure is diagnosed when arterial blood gas shows arterial partial pressure of oxygen (PaO₂) of <8 kPa (<60 mmHg) on room air.[45]Moss M, Ingram RH. Respiratory failure. In: Dale DC, ed. ACP medicine. New York, NY: WebMD Professional Publishing; 2007:2719-29.

Patients with chronic lung disease may have low pulse oximetry readings and baseline PaO₂ values of 6.7 kPa (50 mmHg), making worsening hypoxia difficult to recognise. Decreases of 10% from baseline oxygenation can indicate impending respiratory failure in patients with chronic lung disease.

Hypercapnia

Hypercapnic respiratory failure is diagnosed in patients without chronic lung disease when there is hypoxia and:

• acute elevation of arterial PaCO₂ >6.0 to 6.7 kPa (>45 to 50 mmHg), and

• associated acidosis (pH <7.35).[45]Moss M, Ingram RH. Respiratory failure. In: Dale DC, ed. ACP medicine. New York, NY: WebMD Professional Publishing; 2007:2719-29.

Patients with chronic lung disease can usually tolerate PaCO₂ levels of up to 10.7 kPa (80 mmHg) with secondary renal compensation. However, increasing acidosis (decreasing pH) in these patients indicates respiratory failure.