EKG Deflections, Segments, and Intervals and their Clinical Implications

Objectives

• Activation of atria: Pwave
• Activation of AV node
• PR interval
• Activation of ventricles: QRS complex
• J point
• ST segment
• T wave
• QT interval
• U wave

Activation of the atria: P wave

• Due to the activation of atria
• Configuration: The configuration of the normal sinus P wave is smooth and well-rounded. Because the sinus node is located at the upper border of the right atrium, the sinus impulse has to travel from the right atrium to the left atrium on its way to the ventricles. The first half of the P wave, therefore, is due to the activation of the right atrium (Fig.A). The second half is due to activation of the left atrium (Fig. B).
• Duration/ Amplitude: The P wave duration should not exceed 2.5 small blocks (=<0.10 seconds or 100 milliseconds). The height of the P wave also should not exceed 2.5 small blocks (<2.5 mm) and is measured vertically from the top of the baseline to the top of the P wave. 
• Lead II usually records the most upright P wave deflection and is the most important lead in recognizing that the rhythm is normal sinus. If the P wave is inverted in lead II, the impulse is unlikely to be of sinus node origin.

Activation of AV node

• Propagation of the impulse at the atrioventricular node and His-Purkinje system will not cause any deflection in the electrocardiogram and is represented as an isoelectric or flat line after the P wave.

PR interval

• measured from the beginning of the P wave to the beginning of the QRS complex. 
• If the QRS complex starts with a Q wave, the PR interval is measured from the beginning of the P wave to the beginning of the Q wave (P-Q interval), but is nevertheless called PR interval. 
• It includes the time it takes for the sinus impulse to travel through the atria, AV node, bundle of His, bundle branches, fascicles of the left bundle branch, and the Purkinje network of fibers until the ventricles are activated.
• The normal PR interval measures 0.12 to 0.20 seconds in the adult. It includes the time it takes for the sinus impulse to travel from atria to ventricles. 
• Prolonged PR interval:  when there is delay in conduction of the sinus impulse to the ventricles.
• Short PR interval: This usually occurs when an accessory pathway or bypass tract is present connecting the atrium directly to the ventricle or when conduction of the impulse across the AV node is enhanced because of a small AV node or from pharmacologic agents that speed AV nodal conduction. This will also occur when there is an ectopic impulse, meaning that the P wave originates from the atria or AV junction and not from the sinus node.

Activation of the Ventricles- QRS complex

• Activation of the ventricles is represented as a QRS complex in the electrocardiogram. Because the Purkinje fibers are located in the endocardium, the endocardium is the first to be activated. The impulse spreads from the endocardium to the epicardium in an outward direction.
• The QRS complex generates the largest deflection in the ECG because the ventricles contain the largest mass of muscle cells in the heart.
• Because the ventricles consist of a thick layer of myocardial cells, not all cells are depolarized at the same time.

• The QRS complex has various waves that go up and down from baseline. These waves are identified as follows: Q, R, S, R’, and S’. If additional waves are present, R’’or S’’ designations may be added. Regardless of the size of the deflections, capital and small letters are used empirically mainly for convenience, although, generally, capital letters designate large waves and small letter, small waves.
• The Q wave is defined as the first wave of the QRS complex below the baseline.
• The R wave is defined as the first positive (upward) deflection of the QRS complex
• The S wave is the negative deflection after the R wave.
• The QRS complex is identified as a QRS complex regardless of the number of waves present. Thus, a tall R wave without a Q wave or S wave is still identified as a QRS complex.
• The QRS complex is measured from the beginning of the first deflection, whether it starts with a Q wave or an R wave, and extends to the end of the last deflection. 
• The normal QRS duration varies from 0.06 to 0.10 seconds. 
• The QRS duration is increased when there is ventricular hypertrophy, bundle branch block, or when there is premature excitation of the ventricles because of the presence of an accessory pathway.

J Point

• marks the end of the QRS complex and the beginning of the ST segment
• marks the end of depolarization and the beginning of repolarization of the transmembrane action potential.
• The T-P or TQ segment is used as the isoelectric baseline for measuring deviations of the J point or ST segment (elevation or depression) because the transmembrane action potential is at baseline and there is no ongoing electrical activity at this time. Thus, the TQ interval is not affected by other waves.

ST segment

• starts from the J point to the beginning of the T wave
• The ST segment is flat or isoelectric and corresponds to phase 2 (plateau phase) of the action potential of the ventricular myocardial cells. It represents the time when all cells have just been depolarized, and the muscle cells are in a state of sustained contraction
• An ST segment is considered abnormal when it deviates above or below this baseline by 1 mm. 
• ST elevation in normal individuals
• often seen in normal healthy individuals, especially in men (younger, African American)
• ST-segment elevation in normal healthy individuals is most often seen in precordial leads V1 to V4 and was most marked in V2. The morphology of the normal ST elevation is concave.
• The ST segment elevation in men is much more pronounced than that noted in women, with most of the men having ST elevation of =>1 mm. Most women have ST elevation measuring <1 mm. Thus, ST elevation of <1 mm has been designated as a female pattern and ST elevation of at least 1 mm associated with a sharp take-off of the ST segment of at least 20 degrees from baseline, has been designated as a male pattern.
• Another ST elevation considered normal variant is the presence of ST elevation accompanied by inversion of the T wave in precordial leads V3 to V5.

T Wave

• represents rapid ventricular repolarization.
• This segment of ventricular repolarization corresponds to phase 3 of the transmembrane action potential.
• The different layers of the myocardium exhibit different repolarization characteristics.
• Repolarization of the myocardium normally starts from epicardium to endocardium because the action potential duration of epicardial cells is shorter than the other cells in the myocardium. Thus, the onset of the T wave represents the beginning of repolarization of the epicardium and the top of the T wave corresponds to the complete repolarization of the epicardium.
• Repolarization of the endocardium takes longer than repolarization of the epicardium. Therefore, the repolarization of the endocardium is completed slightly later at the downslope of the T wave.
• Generally, the direction (axis) of the T wave in the 12-lead ECG follows the direction of the QRS complex.
• Thus, when the R wave is tall, the T wave is upright, and when the R wave is smaller than the size of the S wave, the T wave is inverted.
• The duration and amplitude of the T wave is variable. The shape of the normal T wave is rounded and smooth and slightly asymmetric with the upstroke inscribed slowly and the downslope more steeply. 
• The T wave is considered abnormal if the shape becomes peaked, notched, or distorted or if the amplitude is increased to more than 5 mm in the limb leads and >10 mm in the precordial leads. It is also abnormal when the T wave becomes symmetrical or inverted.

QT Interval

• measured from the beginning of the QRS complex to the end of the T wave (U waves not included)
• In assessing the duration of the QT interval, multiple leads should be selected and the QT interval is the longest QT that can be measured in the whole 12-lead ECG recording.
• QTc: QT interval is affected by heart rate
• becomes longer when the heart rate is slower and shorter when the heart rate is faster. The QT interval therefore should always be corrected for heart rate
• Overall, the average QTc in healthy persons after puberty is 420±20 ms
• The normal range for QTc is similar in males and females from birth until the start of adolescence, while after puberty and in adults, females have slightly longer QT intervals than males. After puberty in males, a QTc between 460 and 469 ms is borderline and ≥470 ms is considered prolonged. In postpubertal females, 460 to 479 ms is borderline and ≥480 ms is considered prolonged. 
• When the heart rate is >70 bpm, one can “eyeball” that the QTc is normal if the QT interval is equal to or less than half the R-R interval. When this occurs, no calculation is necessary. If the QT interval is more than half the R-R interval, the QTc may not be normal and should be calculated.

Example: Calculating QTc (Bazett Formula)

• In the figure below: The QT interval (10 small blocks) is more than half the preceding R-R interval (14 small blocks). Thus, the QTc may not be normal and needs to be calculated.
• First: Measure the QT interval: The QT interval measures 10 small blocks. This is equivalent to 0.40 seconds.
• Second:Measure the R-R interval: The R-R interval measures 14 small blocks, which is equivalent to 0.56 seconds. The square root of 0.56 seconds is 0.75 seconds.
• Finally: Calculate the QTc: Using the Bazett formula as shown below: QTc = 0.40 divided by 0.75 = 0.53 seconds. The QTc is prolonged.

U  Wave

• T waves may often be followed by a small positive deflection called the U wave. The U wave is not always present, but it may be the last complex in the ECG to be recorded.
• Cause of the U wave is most likely the repolarization of the His-Purkinje system.
• Size of the normal U wave is small, measuring approximately one-tenth of the size of the T wave. 
• best recorded in the anterior precordial leads V2 and V3 because these chest leads are closest to the ventricular myocardium.
• Usually visible when the heart rate is slow (<65 bpm) and rarely visible with faster heart rates (>95 bpm).
• U wave follows the direction of the T wave and QRS complex.
• U waves are abnormal when they are inverted or become unduly prominent, as may be seen in the setting of hypokalemia.