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. 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 PR interval , which normally ranges from 0. This interval represents the time between the onset of atrial depolarization and the onset of ventricular depolarization. The QRS complex represents ventricular depolarization. Ventricular rate can be calculated by determining the time interval between QRS complexes.
Click here to see how ventricular rate is calculated. The duration of the QRS complex is normally 0. This relatively short duration indicates that ventricular depolarization normally occurs very rapidly. A QRS duration of greater than 0. The QRS duration will lengthen when electrical activity takes a long time to travel throughout the ventricular myocardium. The normal conduction system in the ventricles is called the His-Purkinje system and consists of cells that can conduct electricity quite rapidly.
Dr Araz Rawshani May 22, Table of Contents. The following rules apply when naming the waves: A deflection is only referred to as a wave if it passes the baseline. If the first wave is negative then it is referred to as Q-wave.
All positive waves are referred to as R-waves. Any negative wave occurring after a positive wave is an S-wave. Large waves are referred to by their capital letters Q, R, S , and small waves are referred to by their lower-case letters q, r, s. Figure 5 shows examples of naming of the QRS-complex. Figure 5. Naming of the QRS complex. Figure 6. Approximations of the net direction of the QRS-complex. The positive areas are yellow and the negative areas are green. Electrical vectors that engender the QRS complex Depolarization of the ventricles generate three large vectors, which explains why the QRS complex is composed of three waves.
Figure 7. The second vector: the ventricular interventricular septum The ventricular septum receives Purkinje fibers from the left bundle branch and therefore depolarization proceeds from its left side towards its right side. The third vector: the ventricular free wall The vectors resulting from activation of the ventricular free walls is directed to the left and downwards Figure 7. The explanation for this is as follows: The vector resulting from activation of the right ventricle does not come to expression , because it is drowned by the many times larger vector generated by the left ventricle.
Thus, the vector during activation of the ventricular free walls is actually the vector generated by the left ventricle. Activation of the ventricular free wall proceeds from the endocadrium to the epicardium. This is because the Purkinje fibers run through the endocardium, where they deliver the action potential to contractile cells. The subsequent spread of the action potential occur from one contractile cell to another, starting in the endocardium and heading towards the epicardium.
The fourth vector: basal parts of the ventricles The final vector stems from activation of the basal parts of the ventricles. Implications and causes of wide broad QRS complex Prolongation of QRS duration implies that ventricular depolarization is slower than normal. The following causes of wide QRS complexes must be familiar to all clinicians: Bundle branch block: The left and the right bundle branch consists of Purkinje fibers which spread out into the ventricular myocardium.
The Purkinje network enables fast impulse conduction so that the action potential can be delivered to the whole myocardium at the same time approximately. Bundle branch block occurs if a bundle branch is dysfunctional and unable to transmit the impulse. The ventricle whose bundle is blocked will have to wait for electrical impulses to spread from the other ventricle.
Because spread of the impulse from the other ventricle will take place partly or entirely outside of the conduction system, it will be slow and therefore the QRS duration is prolonged.
Left and right bundle branch block are discussed in separate articles. Hyperkalemia: Hyperkalemia causes slow impulse transmission in all myocardial and conduction cells and prolongation of the QRS duration.
Drugs: class I antiarrhythmic drugs, tricyclic antidepressants and other medications can cause widening of the QRS complex. Ventricular rhythm, ventricular ectopy and pacemaker with ventricular stimulation: Spontaneous action potentials discharged within the ventricles may depolarize the ventricles. Such a focus may fire a single or multiple impulses either consecutively or intermittently.
A single impulse gives rise to a premature ventricular beat, whereas multiple impulses may establish a ventricular rhythm , or even ventricular tachycardia.
In all these instances the QRS complex will be broad because the depolarizing impulse arises and spreads outside of the normal conduction system. External artificial pacemakers have an electrode inserted in the right ventricular apex. This end of the QRS Complex is the pone there the last wave of the complex begin to flatten out, sharply or gradually, at, above, or below the baseline.
These will be discussed later in the course. Repolarization of the ventricles begins at the epicardial surface of the ventricles and progresses inwardly through the ventricular walls to the endocardial surface. The T Wave occurs during the last part of the ventricular systole. The onset of the T Wave is the first or abrupt or gradual deviation from the S-T segment; or from the point where the slope of the S-T segment appears to become abruptly of gradually steeper.
The point where the T Wave returns to the baseline marks the end of the T Wave. Often the onset and end of the T Wave are difficult to determine with certainty.
The U Wave probably represents repolarization of a small segment of the ventricles, such as the papillary muscles or ventricular septum, after most of the right and left ventricles have been repolarized. Although uncommon, and not easily identified, the U Wave can best be seen when the heart rate is slow. A U Wave indicates that the repolarization of the ventricles has occurred. An abnormally tall U wave may be present in hypokalemia, cardiomyopathy, left ventricle hypertrophy, diabetes, and may follow administration of digitalis and quinidine.
The lead EKG will be discussed in greater detail later in this course. However, at this time we will present an introduction to the EKG leads simply to help explain the basics of EKG interpretation that will follow. Later, the specifics of leads and lead placement will be discussed.
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