Cardiovascular Lab: Electrocardiogram: Basics
imply endorsement by the provider or ANCC of any commercial .. C. No correlation between the P waves and the QRS complexes (correct). The P-wave is generated by activation of the muscle of both atria. if the first deflection in the QRS complex happens to be an R-wave (i.e. no Q-wave present) . Signs of heart block lie in the PR interval and P to QRS relationship. of the heart is interrupted intermittently, P wave of atrial contraction is no longer followed.
Also note that this chapter is accompanied by a video lecture: The Normal ECGwhich covers all topics discussed below.
Therefore, ECG interpretation requires a structured assessment of the waves and intervals. Before discussing each component in detail, a brief overview of the waves and intervals is given. The classical ECG curve with its most common waveforms. Important intervals and points of measurement are depicted.
ECG interpretation requires knowledge of these waves and intervals. The P-wave reflects atrial depolarization activation.
CV Physiology: Electrocardiogram (EKG, ECG)
The PR interval is assessed in order to determine whether impulse conduction from the atria to the ventricles is normal. The flat line between the end of the P-wave and the onset of the QRS complex is called the PR segment and it reflects the slow impulse conduction through the atrioventricular node.
The PR segment serves as the baseline also referred to as reference line or isoelectric line of the ECG curve.
Refer to Figure 1. Since the electrical vector generated by the left ventricle is many times larger than the vector generated by the right ventricle, the QRS complex is actually a reflection of left ventricular depolarization.
A short QRS complex is desirable as it proves that the ventricles are depolarized rapidly, which in turn implies that the conduction system functions properly. Wide also referred to as broad QRS complexes indicate that ventricular depolarization is slow, which may be due to dysfunction in the conduction system.
The ST segment must always be studied carefully since it is altered in a wide range of conditions. Many of these conditions cause rather characteristic ST segment changes.
The ST segment is of particular interest in the setting of acute myocardial ischemia, because ischemia causes deviation of the ST segment ST segment deviation. There are two types of ST segment deviations. ST segment depression implies that the ST segment is displaced, such that it is below the level of the PR segment.
QRS complex - Wikipedia
ST segment elevation implies that the ST segment is displaced, such that it is above the level of the PR segment. The J point is the point where the ST segment starts.
If the baseline PR segment is difficult to discern, the TP interval may be used as the reference level. The T-wave The T-wave reflects the rapid repolarization of contractile cells phase 3 and T-wave changes occur in a wide range of conditions.
T-wave changes are frequently misunderstood in clinical practice, which the discussion below will attempt to cure. The transition from the ST segment to the T-wave should be smooth and not abrupt. The normal T-wave is slightly asymmetric, with a steeper downward slope.
The U-wave The U-wave is seen occasionally. It is a positive wave occurring after the T-wave. The U-wave is most frequently seen in leads V2—V4. Individuals with prominent T-waves, as well as those with slow heart rates, display U-waves more often.
The genesis of the U-wave remain elusive. QT duration and QTc duration QT duration reflects the total duration of ventricular depolarization and repolarization.
It is measured from the onset of the QRS complex to the end of the T-wave. The QT duration is inversely related to heart rate; i. Therefore to determine whether the QT interval is within normal limits, it is necessary to adjust for the heart rate. A long QTc interval increases the risk of ventricular arrhythmias.
Now follows the detailed discussion of each ECG of these components. The P-wave is a small, positive and smooth wave. It is small because the atria make a relatively small muscle mass. If the rhythm is sinus rhythm i. The P-wave is always positive in lead II during sinus rhythm. The P-wave vector is slightly curved in the horizontal plane. It is initially directed forward but then turns left to activate the left atrium Figure 2, left hand side.
Lead V1 might therefore display a biphasic diphasic P-wave, meaning that the greater portion of the P-wave is positive but the terminal portion is slightly negative the vector generated by left atrial activation heads away from V1. Occasionally, the negative deflection is also seen in lead V2. P-wave morphology in chest and limb leads. Figure 2 above does not show that the P-wave in lead II might actually be slightly asymmetric by having two humps.
ECG interpretation: Characteristics of the normal ECG (P-wave, QRS complex, ST segment, T-wave)
This is often but not always seen on ordinary ECG tracings and it is explained by the fact that the atria are depolarized sequentially, with the right atrium being depolarized before the left atrium. The first half of the P-wave is therefore a reflection of right atrial depolarization and the second half is a reflection of left atrial depolarization. This is shown in Figure 3 upper panel. Recall that the P-wave in V1 is often biphasic, which is also shown in Figure 3.
The contour of the normal and abnormal P-wave P pulmonale and P mitrale. If an atria becomes enlarged typically as a compensatory mechanism its contribution to the P-wave will be enhanced. Enlargement of the left and right atria causes typical P-wave changes in lead II and lead V1 Figure 3. Human Resuscitation The first electrical resuscitation of a human took place almost certainly in The resuscitation of a drowned girl with electricity is described by Guillaume Benjamin Amand Duchenne de Boulogne, a pioneering neurophysiologist, in the third edition of his textbook on the medical uses of electricity.
Although it is sometimes described as the first artificial pacing, the stimulation was of the phrenic nerve and not the myocardium. Occasional septal Q waves can be seen in other leads. ST elevation following an S wave "high take off" is common in leads V2—V4 and is quite normal. Differentiating this from pathological ST elevation can be difficult and relies on the patient's history and the availability of a previous ECG. These "repolarization abnormalities" are more common in the young and in athletes.
T-wave inversion is common in Afro-Caribbean blacks. U waves — small extra waves following T waves — are seen in hypokalemic patients, but can also represent a normal variant.
Ventricular extrasystoles — no P waves, broad and abnormal QRS complexes, and T waves interspersed between normal sinus rhythm — sometimes occur and do not require further investigation unless they are associated with symptoms such as dizziness, palpitations, exercise intolerance, chest pain, shortness of breath or occur several times every minute.
Pathological variations Long PR interval A distance of more than five small squares from the start of the P wave to the start of the R wave or Q wave if there is one constitutes first-degree heart block see Figure It rarely requires action, but in the presence of other abnormalities might be a sign of hyperkalemia, digoxin toxicity, or cardiomyopathy.Cardiac Conduction System and Understanding ECG, Animation.
ECG demonstrating first-degree heart block. There is some debate over exactly who invented the electrocardiogram. The Dutch "K" elektrokardiogram is often used as a tribute to the Indonesian-born physician Wilhelm Einthoven who, while working in The Netherlands inreceived the Nobel prize for "the discovery of the mechanism of the electrocardiogram". A downward deflection immediately following a P wave that is wider than two small squares or greater in height than a third of the subsequent R wave is significant: This is indicative of a previous infarction.
The Sokolow—Lyon index is the most commonly calculated index of estimation. If so, the patient has LVH by voltage criterion. Right ventricular hypertrophy is indicated by a dominant R wave in V1 ie, R wave bigger than following S wave; Sokolow—Lyon index: ECG demonstrating left ventricular hypertrophy.
- Electrocardiogram (EKG, ECG)
- QRS complex
- Differential Diagnosis of Wide QRS Complex Tachycardias
Note also the T-wave inversion in leads V4—V6. This is often labeled "strain". The method of determining QRS axis will be explained in a later section. ST segment reflects the current flow associated with phase 2 of ventricular repolarization. Since there is no current flow during this plateau phase of repolarization, the ST segment is normally isoelectric with the baseline. The T Wave The T wave represents the current of rapid phase 3 ventricular repolarization see diagram above.
The polarity of this wave normally follows that of the main QRS deflection in any lead. The ventricles are electrically unstable during that period of repolarization extending from the peak of the T wave to its initial downslope.
This interval measures the time from the initial depolarization of the atria to the initial depolarization of the ventricles and reflects a physiological delay in AV conduction imposed by the AV node.
Normal range is — ms 3 to 5 1-mm-divisions and no longer. It represents the time in which the ventricles depolarize and repolarize and is a measure of ventricular action potential AP duration.
This interval should be determined in the ECG lead where it is longest.