cough cough WHEEZE

A patient in the middle of an asthma attack, has respiratory alkalosis.

In respiratory alkalosis, ABG  values are:

 

pH 7.40

pCO2<40 (due to the rapid breathing rate that expels CO2)

elevated pO2 

(remember, with asthma it's problem getting the air OUT, hence the expiratory wheeze)

The work of breathing during an asthma attack is increased to overcome the constricted airways leading to an increase in lactic acid in the respiratory muscles.

Intubation is usually avoided in asthmatics, but cannot be avoided if the breathing muscles are exhausted. This "tiring out" is seen on ABG when the pH begins to normalize because this "normal" pH is a sign that the patient is unable to remove CO2 and lactic acid is building up. 

So let's say, you have an asthmatic patient, having an asthma attack and his pH is 7.39 (usually considered normal), it is a ominous sign that the patient is going into respiratory failure.

INTUBATE! 

*of course, i must mention: if the pH normalizes because you have administered treatment, it means the treatment is adequate and there's no need to stick a tube down your patient's throat. the 'normal-abnormal pH rule' (i just coined that term...hee) only applies if the patient presents to the ER, with acute asthma, but ABG shows a normal pH.