This program is designed to teach you the essentials of 12-lead ECG interpretation. Following this introduction and brief overview, we will review the cardiac conduction system. You will then review the features of the ECG grid paper and complexes that make up the ECG tracing. The main section of the introductory program will review the orderly approach for proper interpretation of the ECG.
This program is not a review on electrophysiology or cell biology; there are many excellent resources to learn these fundamental concepts.
Conduction system
Before we can evaluate the clinical ECG, we now need to review our understanding of the cardiac conduction system. The electrical activation of the heart begins with the SA (or sino-atrial) node, a collection of specialized pacemaker cells located in the right atrium near its junction with the superior vena cava. Subsequently, the stimulus spreads to the right and left atrium causing atrial contraction, and then to the AV node. The electrical stimulus then spreads into the intraventricular conduction system, composed of Purkinje fibers (the His Purkinje system). These fibers are organized into the His Bundle and the left and right bundle branches; the left bundle branch has two major subdivisions supplying the left ventricle: the anterior and posterior fascicles. Note that once the electrical impulse enters the His bundle, it is insulated from ventricular muscle and it is not until it reaches the distal Purkinje fibers (Purkinje terminals) that it begins to activate ventricular muscle and cause ventricular contraction. The following animation illustrates what we have just described.
Components
In order to begin interpreting the ECG, first let us note the 2 main components of the clinical ECG: the ECG grid upon which it is inscribed and the individual ECG complexes.
Grid
Let us take a closer look at the grid; it consists of symmetrical lightly lined 1 mm squares, and every 5 mm (up or across) the lines are darker. Each mm vertically represents 0.1 millivolts in amplitude, and each mm horizontally represents 0.04 seconds in time. One large box with dark lines therefore represents 0.5 mV in amplitude and 0.2 seconds in time.
Complex
We will now define the parts of the ECG complex and correlate these with the underlying conduction system and the resulting electrical correlates.
The P wave represents atrial depolarization.
The PR segment is between the end of the P wave and the beginning of the QRS, primarily reflecting the time to conduct through the AV node and the His-Purkinje system.
The QRS complex represents ventricular depolarization. Any initial negative deflection is called a Q wave and a positive deflection is called an R wave. Any negative wave after an R wave is called an S wave. Some ECG leads may not show all 3 of these waves.
The ST segment represents the beginning of ventricular repolarization; it begins with the J point, which is at the end of the QRS complex, and it ends with the beginning of the T wave.
The T wave follows the ST segment and represents the major period of ventricular repolarization.
The U wave may follow the T wave. It is best seen in the mid-precordial leads and its origin is controversial.
Approach for interpretation
Now let us look at the orderly approach for interpreting the 12-Lead ECG. Note the synchronized rhythm strips below that are part of the typical ECG, to help rhythm analysis. First we should include any available clinical history, including the age and gender. Next we assess the standardization of the ECG; heart rate, rhythm, and intervals; mean QRS frontal plane axis; and morphology of the individual complexes of the ECG (the P, QRS, ST and T waves). Lastly we arrive at the final interpretation, which in this case is a normal ECG.