Arterial blood gas analysis should be considered in patients who are criticallyil land have oxygen saturation sofless than92% or an FEV1 of less than 30% who do not respond to intensive conventional treatment. Proper interpretation of the pH, arterial oxygen pressure, and partial pressure of arterial carbon dioxide (PaCO2) might help further assess the severity of an acute asthma exacerbation (Fig 1). For example, a breathless asthmatic patient presenting with a PaCO2 of 45 mm Hg or greater indicates a life-threatening attack and the need for transfer to a medical intensive care unit for further care. Less than 10% of asthmatic patients presenting to the emergency department have arterial oxygen values of less than 50 mm Hg and carbon dioxide levels of greater than 45 mm Hg. Lactic acidosis is common in patients with severe acute asthma. However, increased lactic acid levels are also associated with high doses of inhaled b2-agonist treatment