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The alkalosis persists as long as the initiating disorder is acting
The alkalosis persists as long as the initiating disorder persists unless some other disorder or complication causing impairment of the hyperventilation intervenes. For example, a hyperventilating head injury patient may develop acute neurogenic pulmonary oedema and this complication would tend to cause the arterial pCO2 to rise.
This is different to the situation with a metabolic alkalosis where maintenance of the disorder requires an abnormality to maintain it as well as the problem which initiated it.
Only one respiratory acid-base disorder can be present at one time.
A patient cannot have both a respiratory alkalosis and a respiratory acidosis. There may of course be multiple factors acting to alter an individual's alveolar ventilation but each of these various factors are not considered separate respiratory acid-base disorders. Essentially this is because a person cannot be both hyperventilating and hypoventilating at the same time.
Using the above hyperventilating head injured patient example: This patient has a neurogenic cause for hyperventilation and if the arterial pCO2 is lowered, then she is said to have a respiratory alkalosis. If neurogenic pulmonary oedema develops subsequently and decreases alveolar ventilation to normal and returns arterial pCO2 to 40mmHg (assuming no metabolic acid-base disorders are present), then she now has no respiratory acid-base disorder.
More than one metabolic acid-base disorder can be present at the one time
The above respiratory situation is different to that occurring with a metabolic disorder. A patient can have a lactic acidosis and then develop a metabolic alkalosis (eg due to vomiting) and end up with a HCO3- level & pH which are normal. This is possible if the acidosis and the alkalosis exactly balance each other. This patient is then said to have both a metabolic acidosis AND a metabolic alkalosis. It is therapeutically useful to know this rather then to say there is no acid-base disorder present.