Electrocardiogram (ECG) - Recordings and Leads

An ECG reflects and records the electrical activity of the heart muscle; details regarding the physiology of electrical conduction are addressed elsewhere. The movement of action potentials through cardiac muscle cells produces extracellular signals that the ECG records. Waves, which are visible as movement above or below baseline voltage. A typical ECG comprises 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. Depolarization occurs when the membrane potential becomes more positive. Repolarization occurs when the membrane potential returns to negative. Recall that the normal resting potential of ventricular cardiac cells is approximately -90 millivolts. 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. Notice that this arrangement ensures that ventricular depolarization and subsequent contraction begins at the apex and moves towards the atria; this pathway moves blood out of the ventricles, whereas depolarization and contraction that began at the AV node, for example, would only squeeze blood to the bottom of the ventricles! We draw an idealized ECG recording of the electrical events of a single cardiac cycle: The horizontal axis tracks time from 0 to 1.4 seconds; the vertical axis tracks voltage from -0.6 to 1 millivolt. At 0.2 seconds, we see a small positive "wave"; this reflects the period of atrial depolarization; atrial contraction occurs during the latter part of the P wave. 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. Wider and taller than the P wave; reflects the period of ventricular repolarization. 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. 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 (notice that this occurs specifically during the PR segment.). The duration of the PR interval is clinically important; PR intervals lasting longer than 0.12 - 0.20 seconds may indicate AV conduction block, which we learn about elsewhere. The ST segment begins after the QRS complex and ends at the onset to the T wave. 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 (see a portion of a Wigger's Diagram). Occasionally, an additional wave, the U wave, will appear after the T wave. A prominent U wave may reflect bradycardia (slow heart rate), hypokalemia, ischemia, or effects of antiarrhythmic drugs. The ECG can be used to determine heart rate: Calculate the PP interval, which is the distance between P waves of successive cardiac cycles, or the RR interval, which is the distance between successive R waves. Be aware that, unlike in action potential graphs, the shape of the wave does not indicate depolarization or repolarization – notice that both the P wave and T wave are positive (rise above the baseline), but the P wave reflects depolarization and the T wave reflects repolarization. This is because the ECG only shows the direction of current flow relative to the lead's axis.

• Bipolar and Augmented Leads record electrical activity along the Frontal Plane;

Precordial Leads record perpendicular to this. Bipolar Leads I, II, III — Arms (wrists), Left Leg

• Lead I: Negative terminal is connected to the Right Arm; Positive terminal is connected to the Left Arm.
• Lead II: Negative terminal is connected to Right Arm; Positive terminal is connected to Left Leg.
• Lead III: Negative terminal is connected to Left Arm; Positive terminal is connected to Left leg.
• Einthoven's Triangle: Triangle formed around the heart by the terminals.

— Einthoven's Law: Lead I potential + Lead II potential = Lead III potential So, whenever we know the values for two of the bipolar leads, we can calculate the third. Augmented Unipolar Leads aVR, aVL, aVF — Arms (wrists), Left Leg

• a = Augmented; V = Vector
• Named for location of positive terminal (R = Right arm; L = Left arm; F = left leg (think "f' = foot))

Precordial Leads V1-V6 — Chest leads overlie different regions of the heart.

• V1 & V2: Anterior/Septal
• V3 & V4: Anterior/Apical
• V5 & V6: Anterior/Lateral