Why do some asthmatics develop a cardiac arrest?

Mucous plugging from severe bronchospasm causing asphyxia is the commonest modality of death in asthmatics.

Prolonged hypoxia can precipitate cardiac arrhythmia causing death.

Because of bronchoconstriction, airway pressures in the lungs in acute asthma are high. Air can enter the lungs, but cannot escape. This leads to “breath stacking” which causes an increase in intrathoracic pressure. Raised intrathroracic pressure consequently can cause pneumothoraces to develop or decrease venous return to the heart precipitating circulatory collapse.

Reference: Soar, Perkins, Abbas et al. European Resucitation Council Guidelines for Resucitation 2010. Cardiac Arrest in special circumstances: Electrolyte abnormalities, poisoning, drowning, accidential hypothermia, hyperthermia, asthma, anaphylaxis, cardiac surgery, trauma, pregnancy, electrocution.