Inferior leads Leads II, III and aVF Look at electrical activity from the vantage point of the inferior surface diaphragmatic surface of heart Lateral leads I, aVL, V5 and V6 Look at the electrical activity from the vantage point of the lateral wall of left ventricle Septal leads V1 and V2 Look at electrical activity from the vantage point of the septal surface of the heart interventricular septum Anterior leads V3 and V4 Look at electrical activity from the vantage point of the anterior wall of the right and left ventricles Sternocostal surface of heart In addition, any two precordial leads next to one another are considered to be contiguous. For example, though V4 is an anterior lead and V5 is a lateral lead, they are contiguous because they are next to one another. Cardiac electrophysiology The formal study of the electrical conduction system of the heart is called cardiac electrophysiology EP.
General Description As the heart undergoes depolarization and repolarizationthe electrical currents that are generated spread not only within the heart, but also throughout the body.
This electrical activity generated by the heart can be measured by an array of electrodes placed on the body surface. A "typical" ECG tracing is shown to the right.
The different waves that comprise the ECG represent the sequence of depolarization and repolarization of the atria and ventricles.
Therefore, each small 1-mm square represents 0. Because the recording speed is standardized, one can calculate the heart rate from the intervals between different waves. P wave atrial depolarization The P wave represents the wave of depolarization that spreads from the SA node throughout the atria, and is usually 0.
The brief isoelectric zero voltage period after the P wave represents the time in which the impulse is traveling within the AV node where the conduction velocity is greatly retarded and the bundle of His.
Atrial rate can be calculated by determining the time interval between P waves. Click here to see how atrial rate is calculated. The period of time from the onset of the P wave to the beginning of the QRS complex is termed the P-R interval, which normally ranges from 0.
This interval represents the time between the onset of atrial depolarization and the onset of ventricular depolarization.
Ventricular rate can be calculated by determining the time interval between QRS complexes. Click Ecg electrocardiogram to see how ventricular rate is calculated. The duration of the QRS complex is normally 0. This relatively short Ecg electrocardiogram indicates that ventricular depolarization normally occurs very rapidly.
This can occur with bundle branch blocks or whenever a ventricular foci abnormal pacemaker site becomes the pacemaker driving the ventricle. Such an ectopic foci nearly always results in impulses being conducted over slower pathways within the heart, thereby increasing the time for depolarization and the duration of the QRS complex.
The shape of the QRS complex in the above figure is idealized. In fact, the shape changes depending on which recording electrodes are being used. The shape also changes when there is abnormal conduction of electrical impulses within the ventricles.
ST segment The isoelectric period ST segment following the QRS and ending at the beginning of the Twave is the time at which both ventricles are completely depolarized. This segment roughly corresponds to the plateau phase of the ventricular action potentials. The ST segment is very important in the diagnosis of ventricular ischemia or hypoxia because under those conditions, the ST segment can become either depressed or elevated.
T and U waves The T wave represents ventricular repolarization. Generally, the T wave exhibits a positive deflection.
The reason for this is that the last cells to depolarize in the ventricles are the first to repolarize. This occurs because the last cells to depolarize are located in the subepicardial region of the ventricles and these cells have shorter action potentials than found in the subendocardial regions of the ventricular wall.
So, although the depolarization of the subepicardial cells occurs after the subendocardial cells, the subepicardial cells undergo phase 3 repolarization before the subendocardial cells.
Therefore, repolarization waves generally are oriented opposite of depolarization waves green versus red arrows in figureand repolarization waves moving away from a postive recording electrode produce a positive voltage.
The T wave is longer in duration than the QRS complex that represents depolarization. The longer duration occurs because conduction of the repolarization wave is slower than the wave of depolarization.
The reason for this is that the repolarization wave does not utilize the high-velocity bundle branch and purkinje system, and therefore primarily relies on cell-to-cell conduction. Sometimes a small positive U wave may be seen following the T wave not shown in figure at top of page.
This wave represents the last remnants of ventricular repolarization. Inverted T waves or prominent U waves indicates underlying pathology or conditions affecting repolarization.
Q-T interval The Q-T interval represents the time for both ventricular depolarization and repolarization to occur, and therefore roughly estimates the duration of an average ventricular action potential. This interval can range from 0. At high heart rates, ventricular action potentials shorten in duration, which decreases the Q-T interval.
Because prolonged Q-T intervals can be diagnostic for susceptibility to certain types of tachyarrhythmias, it is important to determine if a given Q-T interval is excessively long.
In practice, the Q-T interval is expressed as a "corrected Q-T QTc " by taking the Q-T interval and dividing it by the square root of the R-R interval interval between ventricular depolarizations. This allows an assessment of the Q-T interval that is independent of heart rate.
Normal corrected Q-Tc intervals are 0.64 Chapter 4: An ECG Primer Cardiac Monitoring System Since Dr. Willem Einthoven invented the first “electrokardiogram” in , the electrical activity of the heart has been recorded.
Stress tests don't always predict heart attacks.(SEAN JUSTICE/CORBIS)Your doctor can't tell how well your heart is working until it's put to the test.
If you have heart disease or if you're at. ECG apps & weekly challenge for those learning and reading Paediatric ECG's. An electrocardiogram (ECG) is a simple test that can be used to check your heart's rhythm and electrical activity. Sensors attached to the skin are used to detect the electrical signals produced by your heart each time it beats.
ECG The following is a summary of Current Procedural Terminology (procedure ®) codes commonly used for various electrocardiograph procedures performed with a Midmark ECG device. Jul 30, · An electrocardiogram — abbreviated as EKG or ECG — is a test that measures the electrical activity of the heartbeat.
With each beat, an electrical impulse (or “wave”) travels through the heart. This wave causes the muscle to squeeze and pump blood from the heart. A normal heartbeat on ECG will show the timing of the top and lower chambers.