The Basics

Day 1: The Basics

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Cardiac muscle has two unique properties that predispose it to the two common types of arrhythmias
1. Automaticity
  1. As opposed to skeletal muscle, all myocardial cells exhibit spontaneous depolarization.
  2. This is normally a beneficial property because:
    1. It obviates the need for central nervous system initiation of myocardial depolarization.
    2. It allows for “backup” pacemakers to take over if there is sinus node dysfunction or failure of propagation of depolarization.
  3. Disorders of this property can result in automatic or ectopic arrhythmias.
2. Gap junction transmission
  1. Again, as opposed to skeletal muscle, myocardial cells can transmit electrical signals from one to another via gap junctions.
  2. This is normally beneficial as nervous tissue is not required for the propagation of depolarization.
  3. However, as will be seen, the requirements for reentrant arrhythmias include having at least two pathways for electrical current, a condition facilitated by gap junctions.
Components of the conduction system
1. The conduction system consists of modified cardiac muscle cells that have unique electrical properties.
2. Sinoatrial (SA) node
  1. The SA node is a collection of cells in the upper right corner of the right atrium.
  2. The SA node controls the rhythm of the heart by virtue of having the fastest intrinsic rate of depolarization (60–100 beats/min).
  3. The SA node starts the cardiac cycle by initiating atrial systole.
3. The atrioventricular (AV) node
  1. The AV node is located near the inferior portion of the interatrial septum.
  2. The AV node serves two functions:
    1. It provides a physiological conduction delay to allow the atria to fill the ventricles prior to ventricular systole.
    2. It also protects the ventricles from excessive stimulation from the atria, such as in atrial fibrillation.
4. The His-Purkinje system
  1. The His bundle divides into the right and left bundles.
  2. The left bundle further divides into the left anterior and posterior fascicles.
  3. The His-Purkinje system provides for the orderly depolarization of the ventricles.
Genesis of the surface electrocardiogram (ECG)
1. A wave of negative electric potential spreads across contracting myocardium.
2. This potential can be detected by electrodes placed at various locations on the skin, the signal amplified, and displayed as an ECG.
3. The components of the ECG represent various cardiac events.
  1. The P wave corresponds to atrial systole.
  2. The PR interval represents the physiological delay in the AV node and His bundle.
  3. The QRS complex results from ventricular systole.
  4. The T wave represents ventricular repolarization.
  5. The cause of the inconsistently present U wave is controversial.
4. The ECG paper
  1. The ECG is recorded on moving paper ruled at 1 mm intervals with darker lines every 5 mm.
  2. At the standard paper speed of 25 mm/sec, each I mm horizontally represents 40 msec and each 5 mm interval 200 msec.
  3. In the vertical dimension, each 10 mm represents 0.1 mv of electrical potential.
The standard ECG leads
1. There are six standard leads (the “limb leads”) that depict cardiac electrical events from six angles in the frontal or vertical plane.
2. There are six precordial leads (the “chest leads”) that depict electrical events in the horizontal plane.
Genesis of the ECG wave forms in the various leads and the concept of axis
1. The P wave, QRS complex, and T wave for any lead can be derived from the vector representation of electrical activity in the appropriate plane.
  1. For instance, ventricular depolarization in the frontal plane can be displayed by a series of vectors representing the mathematical sum of all the electrical activity occurring at that instant.
  2. The tips of these vectors can be connected to form a loop.
  3. The loop can be superimposed on the frontal plane axis.
    1. To form the QRS complex of Lead I, a line is drawn perpendicular to that lead.
    2. By convention, all electrical forces on the side of the perpendicular towards Lead I are designated as positive, and those away as negative.
2. The electrical axis is the sum of all the vectors in that plane.
  1. The normal QRS axis is between -30° and +90°.
  2. An easy way to determine the QRS axis is normal is to examine Leads I and aVF, and, if necessary, Lead II.
  3. Summary and examples of QRS axes.
    
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3. The chest leads are constructed similarly from the vector loop viewed in the horizontal plane.
4. The P wave axis
  1. The normal P wave axis is quite restricted in its range (15°–75°) because of the location of the SA node in the upper right corner of the right atrium.
  2. The P wave should obviously be upright in Leads I and aVF.
Determining heart rate
1. If the rhythm is regular, the rate can be determined by the distance between complexes.
2. If the rhythm is irregular, the rate can be determined by counting the number of beats in 6 sec and multiplying by 10.
Sinus rhythm
1. Sinus rhythm is defined as:
  1. Regularly recurring P waves of the same morphology
  2. A normal P wave axis
  3. A rate between 60 and 100 beats/min
2. In addition, if each P wave is followed by a QRS complex, then there is normal sinus rhythm.
DAY 1-06
Systematic approach to ECG interpretation
1. Rate (may be different, e.g., atrial flutter with 4:1 AV block)
  1. Supraventricular
  2. Ventricular
2. Rhythm (may be different, e.g., sinus rhythm and ventricular tachycardia)
  1. Supraventricular
  2. Ventricular
3. P wave morphology (e.g., right or left atrial abnormalities)
4. PR interval
5. QRS complex
  1. Axis
  2. Voltage [e.g., in left ventricular hypertrophy (LVH), right ventricular hypertrophy (RVH), or low voltage]
  3. Duration [e.g., with right bundle branch block [RBBB], left bundle branch block (LBBB), or fascicular blocks]
  4. Morphology (e.g., the presence of Q waves or a tall R wave in V₁)
6. ST segment
7. T wave
8. QT interval
9. U wave
Normal values