ECG

Those are the exam questions:

1. Cause of variable PR interval is Mobitz I.
2. Cause of sinus tachycardia. Physiological (e.g., exercise) and pathological (e.g., pulmonary embolism).
3. P wave morphology is atrial depolarization.
4. QRS low voltage is caused by pericardial effusion, obesity, and pleural effusion (pneumothorax, hydrothorax...).