What causes the whooping in whooping cough?

Can't catch breath. When a child has whooping cough he starts to cough and can't stop. As he continues to cough he loses his breath and when he finally stops coughing he takes a deep breath very fast. Since his throat is very small, the large amount of air moving very fast through the small airway creates a "whoop".
Tiny pipe/big breath. The cough in whooping cough empties the lungs of air so completely that the breathe a child finally takes is huge, fast and forced over a tiny swollen airway. That's what makes it so loud.
Hypoxia. The 'whoop' in whooping cough is a forceful inhalation of air after a prolonged paroxysm of cough, often lasting longer than 120 sec. This coughing attack is severe, and can often result in not only hypoxia and apnea, but intra-ocular hemorrhage, abdominal or chest wall muscular sprains, and intercostal injuries. Imagine the first breath that you'd take after choking for 2 min. That's a 'whoop'.
Inhaling strongly. After a coughing and coughing, a child will try to catch his breath by breathing in strongly. This forceful inhalation makes a whooping sound. Babies under 6 months may not whoop even if they have pertussis infection.
A sudden breath. Whooping cough is a potentially fatal but vaccine preventable illness, that is unfortuntunately becoming more commeon due to lower vaccination rates. The whoop is a very alarming sound, that occurs when the person is suddenly able to take a deep breath after a severe coughing fit, also known as a paroxysm. Older patients may actually feel as if they are suffocating during an attack.