The doctor is assuming that the patient's blood has a lower than normal pH.
Work Step by Step
Remember section 42.6 and the questions therein. An increase in CO$_2$ and decrease in oxygen causes decreased blood pH and stimulates processes to try to gain more oxygen, such as increased heart and breathing rates. A patient who has a low blood pH would likely also be showing signs of their body trying to rebalance. This is not the sole cause for such a symptom, however-- a doctor would use more information than just breathing rate to determine a diagnosis and treatment plan.