PANCE - ECG

Here are key facts for USMLE Step 1 & COMLEX-USA Level 1 from the ECG tutorial, as well as points of interest that are not directly addressed in this tutorial but should help you prepare for the boards. See the tutorial notes for further details and relevant links. An ECG reflects and records the electrical activity of the heart muscle. The movement of action potentials through cardiac muscle cells produces extracellular signals that the ECG records. A typical ECG comprises:

• Waves: visible as movement above or below baseline voltage. A typical ECG has 5 waves labeled P, Q, R, S, and T.
• Segments: represent time spent at baseline.
• Intervals: include both segments and waves. For example, the ST interval includes the distance encompassing the ST segment and the T wave.

Normal conduction pathway of electrical signals through the heart: 1) Sinoatrial (SA) node is the pacemaker of the heart; it sets the heart's rhythm. It sends electrical signals throughout the atria, and to the atrioventricular (AV) node. 2) The AV node then transmits the signal to the bundle of His (aka, AV bundle). 3) From here, signals travel through the right and left bundle branches to the apex of the heart. 4) The Purkinje fiber network spreads the electrical signals throughout the cardiac muscle cells of the ventricles. This arrangement ensures that ventricular depolarization and subsequent contraction begins at the apex and moves towards the atria. P wave: At 0.2 seconds, a small positive "wave" that reflects the period of atrial depolarization; atrial contraction occurs during the latter part of the P wave. QRS Complex: Peaks at 0.4 seconds; reflects the period of ventricular depolarization; a wide QRS complex indicates impaired conduction within the ventricles, as in bundle branch block. Obscures atrial repolarization. T wave: Wider and taller than the P wave; reflects the period of ventricular repolarization. The PR interval begins at the start of the P wave and ends at the start of the QRS complex. The PR segment is a sub-set of this interval, and encompasses the time between the end of the P wave and the onset of the QRS complex. Within the PR interval, the AV node fires, sending the electrical signal through the bundle of His, bundle branches, and to the Purkinje fibers. The duration of the PR interval is clinically important; PR intervals lasting longer than 0.12 - 0.20 seconds may indicate AV conduction block. The ST segment begins after the QRS complex and ends at the onset to the T wave. Depolarization occurs when the membrane potential becomes more positive. Repolarization occurs when the membrane potential returns to negative. The normal resting potential of ventricular cardiac cells is approximately -90 millivolts. Ventricular contraction begins during the QRS complex and continues through the ST segment. More specifically, isovolumetric contraction begins during the QRS complex, and the ST segment reflects the period of ventricular ejection of blood into the great vessels. Occasionally, an additional wave, the U wave, will appear after the T wave. The sinoatrial node fires just before the P wave; the ECG does not record this event, but recall that the SA node is the pacemaker, and sends the electrical signals that initiate the P wave. Atrial contraction occurs during the latter part of the P wave. QRS complex obscures atrial repolarization.