EKG Basics # 2
The Precordial Chest Leads
The V Leads Or the Precordial Chest Leads
The precordial chest leads record the heartís electrical activity in the transverse or horizontal plane. To create the six precordial chest leads, each chest lead is made positive and the whole body is considered to be negative.
Leads V3 and V4 lie directly over the interventricular septum. Their line of sight is to view the electrical activity of the interventricular septum. Leads V5 and V6 lie over the left ventricle. Therefore, these leads view the electrical activity of the left ventricle.
In Review :
The P Wave
The P wave is the signal that the electrical potential has left the SA node, swept across the atria, and has initiated atrial contraction. A normal P wave has a duration of 2.0 - 2.5 mm (.04 - .1 sec). If it is greater than 2.75 mm (.11 sec) it is abnormal. Normal amplitude for a P wave is 2-3 mm. The P wave should always be gently rounded - never pointed or peaked.
Abnormal amplitude for the P wave is often seen in cor pulmonale, A-V valve disease, hypertension and in patients with congenital heart disease. P waves within the same lead that are multiform (having different shapes) indicate the presence of possibly multiple ectopic pacemakers in the right atrium. In the six limb leads, you will generally see P waves that are positively deflected above the isoelectric line except in aVR where the P wave usually is upside down.
You will frequently see biphasic P waves in Lead III, Lead V1, Lead V2 and occassionally in Lead aVL.
The PR Interval
After the P wave, there is a "silent period" where nothing is happening in the EKG tracing. This quiescent period is called the PR interval. The PR interval represents a refractory period in which the AV node captures the SA node signal and holds it for a short time. The PR interval allows the atria to contract and "top off" the ventricles with blood - an event called atrial kick. If there was no time lag after the P wave, the ventricles would pump prematurely before being adequately filled by atrial systole
The PR interval is measured on the EKG tracing from the beginning of the P wave to the beginning of the Q wave or the beginning of the R wave if the Q wave is absent. The PR interval represents the time period encompassing atrial depolarization up to but not including the start of ventricular depolarization.
The normal duration of the PR interval is between 3-5 mm or .12 - .20 seconds in duration. In adults, if the PR interval is longer than 5 mm, then it is considered a prolonged PR interval and may be diagnostic for the presence of atrioventricular blocks (AV blocks).
The PR interval does shorten during exercise as a means of allowing the heart rate to accelerate. The PR interval continues to shorten with heart rates up 140-150 bpm. With higher heart rates, the PR interval does not get noticeably shorter. In young children, the PR interval is shorter than in adults. In very young children ages 1-2 years, the PR interval may be as short as .11 seconds (2.75 mm). At the age of 6 years, the PR interval has lengthened out to .13 seconds (3.25 mm) and by the time the child is 12 years of age, the PR interval is .14 seconds (3.5 mm).
Very short PR intervals are seen in patients with pheochromocytoma and in patients with Wolfe-Parkinson-White Syndrome. Ocassionally, individuals with normal hearts will have very short PR intervals as a normal variant. Prolonged PR intervals are seen in individuals with 1st degree AV block, 2nd degree AV block (Mobitz I), and in patients with rheumatic heart disease
The QRS complex is the electrical wave that signals the depolarization of the myocardial cells of the ventricles. The duration for a normal QRS is no greater than 3 mm or about .06 - .12 seconds (1.5 - 3.0 mm). If the duration is greater than 3 mm (.12 seconds), then you have to suspect an abnormal intraventricular conduction velocity.
The Q Wave
The Q wave is defined as the first downward deflection after the P wave. It may be present and it can be absent in a normal EKG. The Q wave represents the depolarization of the intraventricular septum. It is never considered to be abnormal if it is missing from any particular lead. However, it is considered very significant if it is greater than 1 mm wide and 25% - 33% of the total amplitude of the R wave. If its amplitude is 25% - 33% of the R wave, it is diagnostic for a myocardial infarction in the lead that it is found. If a significant Q wave appears in aVF or Lead II or III, then there has been an infarction in the inferior portion of the left ventricular wall. If a significant Q wave appears in aVL or Lead I, then an infarction has occured in the lateral wall of the left ventricle. Q waves are therefore only significant if they are present and have large amplitudes.
The R wave
The R wave is the first upward deflection after the P wave. It represents part of the ventricular depolarization cycle. The R wave takes on a life of its own when seen in the precordial chest leads. Normally the R wave is very small to non-existant in V1. In successive chest leads, the R wave amplitude continues to grow until it reaches its greatest amplitude in V5. This increasing R wave amplitude that is seen in the precordial chest leads is called R wave progression. As the R wave gets larger, the S wave gets smaller. If the R wave progression is reversed or greatly disturbed, it frequently suggests the presence of a bundle branch block. Usually when the the R wave is disturbed, there are also bizzare reciprocal changes in the S wave.
The S Wave
The S wave is defined as the first downward deflection after the R wave. It represents the remaining time period for ventricular depolarization. S waves are disturbed and have bizarre shapes in the presence of bundle branch blocks.
QRS complexes are dominently upwardly deflected in Leads I, II, III, aVL, and V4-V6. QRS complexes are downwardly deflected in aVR and V1. Chest leads V2-V3 are transition leads in that they are usually isoelectric in appearance.
The ST segment is the portion of the EKG tracing that begins from the J point to the beginning of the T wave. It is a pause after the QRS complex. It is essentially a period of diastole for the heart and represent the period from the end of systole to the beginning of repolarization of the ventricles. It may appear as a flat line between the QRS and the T wave or it may be upsloping from the J point from 1-2 mm in its amplitude and may be 2-3 mm in its duration.
The ST segment is a crucial part of the EKG tracing. The appearance of the ST segment changes dramatically in the presence of ischemia or during a myocardial infarction. During ischemia, the ST segment will become depressed and have a long duration and a large amplitude before it joins the T wave. The ST segment is elevated during an acute myocardial infarction. The ST segment is, therefore, a diagnostic segment of the EKG strip that is very important in the diagnoses of heart problems.
The T wave represents the repolarization of the ventricles after systole is completed. Repolarization begins in the apex of the heart and moves up through the heart muscle to the base of the heart. Because of this distal to proximal movement of the repolarization wave front, the T wave is dominently upwardly deflected in Leads I, II, III, aVF, aVL, and in V3-V6. It is downwardly deflected in aVR and in V1. the normal duration for the T wave is .04 - .08 seconds (1 - 2 mm). Amplitudes vary. Unusually tall T waves sometimes appear in patients with potassium intoxication and in psychotic patients (probably due to the medications they are on).
T waves can be deranged (flipped or inverted) during an acute myocardial infarction. The classic signs of a myocardial infarction in progress are ST segment elevation, T wave inversion and the presence of significant Q waves.