AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |
Back to Blog
Pics of a cardiograph1/2/2023 ![]() ![]() At high heart rates, ventricular action potentials shorten in duration, which decreases the QT interval. This interval can range from 0.20 to 0.40 seconds depending upon heart rate. The QT 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 wave represents the last remnants of ventricular repolarization. Inverted T waves or prominent U waves indicates underlying pathology or conditions affecting repolarization. Sometimes a small positive U wave may be seen following the T wave (not shown in figure at top of page). 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. The longer duration occurs because conduction of the repolarization wave is slower than the wave of depolarization. The T wave is longer in duration than the QRS complex that represents depolarization. Therefore, repolarization waves generally are oriented opposite of depolarization waves (green versus red arrows in figure), and repolarization waves moving away from a postive recording electrode produce a positive voltage. ![]() So, although the depolarization of the subepicardial cells occurs after the subendocardial cells, the subepicardial cells undergo phase 3 repolarization before the subendocardial cells. 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. The reason for this is that the last cells to depolarize in the ventricles are the first to repolarize. Generally, the T wave exhibits a positive deflection. The T wave represents ventricular repolarization. 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. This segment roughly corresponds to the plateau phase of the ventricular action potentials. The isoelectric period (ST segment) following the QRS and ending at the beginning of the T wave is the time at which both ventricles are completely depolarized. The figure to the right summarizes the nomenclature used to define the different components of the QRS complex as may occur in different ECG recording leads and/or with abnormal conduction within the ventricles. The shape also changes when there is abnormal conduction of electrical impulses within the ventricles. In fact, the shape changes depending on which recording electrodes are being used. The shape of the QRS complex in the above figure is idealized. 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. This can occur with bundle branch blocks or whenever a ventricular foci (abnormal pacemaker site) becomes the pacemaker driving the ventricle. If the QRS complex is prolonged (> 0.10 sec), conduction is impaired within the ventricles. This relatively short duration indicates that ventricular depolarization normally occurs very rapidly. The duration of the QRS complex is normally 0.06 to 0.10 seconds. Click here to see how ventricular rate is calculated. The QRS complex represents ventricular depolarization. Ventricular rate can be calculated by determining the time interval between QRS complexes. If the PR interval is >0.20 sec, there is an AV conduction block, which is called a first-degree heart block if each impulse from the atria is still able to be conducted into the ventricles. ![]() The period of time from the onset of the P wave to the beginning of the QRS complex is termed the PR interval, which normally ranges from 0.12 to 0.20 seconds in duration. This interval represents the time between the onset of atrial depolarization and the onset of ventricular depolarization. Click here to see how atrial rate is calculated. 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. ![]() The P wave represents the wave of depolarization that spreads from the SA node throughout the atria, and is usually 0.08 to 0.10 seconds (80-100 ms) in duration. ![]()
0 Comments
Read More
Leave a Reply. |