An electrocardiogram (ECG) records the electrical activity of the heart over a period of time using electrodes placed on the skin. The ECG shown is from a patient whose heart was beating irregularly until it was treated using a defibrillator (arrows) which restored normal electrical activity.

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a. «paddles/electrodes/defibrillator» deliver an electric shock to the heart $\checkmark$ b. depolarizes cardiac muscle $\checkmark$ c. enables the pacemaker/SA node to regain control $\checkmark$

a. impulses initiated from the AV node spread across heart $\checkmark$ b. impulses travel along Purkinje fibres/across ventricles $\checkmark$ c. causing depolarization of the ventricles OR that triggers ventricular contraction $\checkmark$