Survival from cardiac arrest presenting initially with asystole or pulseless electrical activity is very unlikely unless all relevant reversible causes are recognized and treated effectively.1 In some patients, extensive differential diagnosis is needed to ascertain the cause of cardiac arrest, especially if differentiation between cardiovascular and respiratory aetiology is necessary. Routine initial evaluation, including chest X-ray may not be sufficient to diagnose alterations in distribution of ventilation that may be present in some respiratory disorders.