A doctor might give bicarbonate (HCO₃¯) to a patient who is breathing very rapidly. What is the doctor assuming about the patient’s blood chemistry?
if a person is both (1) hyperventilating and (2) has a low blood pH then this is a case of metabolic acidosis... in metabolic acidosis the patient compensates by breathing heavy... why? because hydrogen ions are captured by bicarbonate (the conjugate base of carbonic acid) which is then exhaled as carbon dioxide... 

metabolic acidosis is not caused by excess carbon dioxide... this is the case of respiratory acidosis... but under respiratory acidosis the breathing is supressed, however tachypnea with frequent shallow breaths could lead to poor removal of carbon dioxide...

here are some causes of metabolic acidosis:
http://en.wikipedia.org/wiki/Metabolic_acidosis#Causes