My doc says that respiraory alkalosis is a sign of a tension pneumothorax but why?

Can be: Hyperventilation can lead to a respiratory alkalosis by increasing the amount of carbon dioxide that you exhale. When this happens the overall ph of your increases because co2 elimination is linked to the activity of enzymes in your blood that convert hydrogen ions to co2. Even a small pneumothorax may result in a degree of respiratory distress, leading to high breathing rate.