Key Points

• Deflections in the ECG correspond to electrical events on the heart. Action potentials on cardiac muscle cells create extracellular voltages. The thorax acts as a volume conductor so that the voltages generated by the cells are conducted to the body surface.

• The P wave represents atrial depolarization; the QRS complex represents ventricular depolarization; the T wave represents ventricular repolarization.

• The PR interval is influenced by the conduction time in the AV node.

• The QT interval is influenced by the duration of ventricular action potentials and conduction time over the ventricle.

• There are six limb leads, which can be used to calculate the electrical vectors of the waves in the frontal plane.

• Unipolar leads are used to explore the chest, which gives front-to-back information about the electrical vectors.

• Bundle branch block or hypertrophy of a chamber can alter the angle of the QRS wave's electrical vector, which is normally in the left lower quadrant.

• Myocardial ischemia produces a current of injury that moves the ST segment off of the baseline.

Essential Medical Physiology, Third Edition

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