As noted in Chapter 2, the normal ST segment, representing the early phase of ventricular repolarization, is usually isoelectric (flat on the baseline). Slight deviations of the ST segment (usually less than 1 mm) may be seen normally. As described in Chapter 10, certain normal subjects will show more marked ST segment elevations as a normal variant (early repolarization pattern). Finally, examine the ST segments in the right chest leads (V1 to V3). Notice that in these examples the ST segment is short and the T wave appears to take off almost from the J point (junction of QRS complex and ST segment). This pattern of an early takeoff of the T wave in the right chest leads is mot an uncommon finding in normal subjects.
THE NORMAL T WAVE
Up to this point, we have deferred discussion of ventricular repolarization-the return of stimulated muscle to the resting state, which produces the ST segment, T wave, and U wave. Deciding whether the T wave in any lead is normal or not is generally straightforward. As a rule, the T wave follows the direction of the main QRS deflection. Thus, when the main QRS deflection id positive (up-right), the T wave is normally positive. We can also make some more specific rules about the direction of the normal T wave.
The normal T wave in lead aVR is always negative, while in lead II it is always positive. Left-sided chest leads, such as V4 to V6, normally always show a positive T wave.
The T wave in the other leads may be variable. In the chest leads the T wave may be normally negative, isoelectric, or positive in leads V1 and V2. in most normal adults, the T wave becomes positive by lead V3. Furthermore, if the T wave is positive in any chest lead, it must remain positive in all chest leads to the left of that lead. Otherwise, it is abnormal. For example, if the T wave is negative in chest leads V1 and V2 and becomes positive in lead V4, it should normally remaim positive in leads V4 to V6.
The polarity of the T wave in the extremity leads depends on the electrical position of the heart. With a horizontal heart, the main QRS deflection is positive in leads I and aVL, and the T wave is also positive in these leads. However, in some normal ECG with a vertical axis, the T wave may be negative in lead III.
v.Electrical Axis and Axis Deviation
MEAN QRS AXIS
The depolarization stimulus spreads through the ventricles in different directions from instant to instant, for example, the depolarization wave may be directed toward lead I at one moment and toward lead III at the next . we can also talk about the mean direction of the QRS complex or mean QRS electrical axis. If you could draw an arrow to represent the general, or, mean, direction in which the QRS is pointed in the frontal plane of the body, you would be drawing the electrical axis of the QRS complex.
The term “mean QRS axis,” therefore, describes the general direction in the frontal plane toward which the QRS complex is predominantly pointed.
Since we are defining the QRS axis in the frontal plane, we are describing the QRS only in reference to the six extremity leads (the six frontal plane leads). Therefore the scale of reference used to measure the mean QRS axis is the diagram of the frontal plane leads . We also know the Einthoven triangle and how the triangle can easily by simply having the three axes (leads I, II, and III) radiate from a central point. Similarly we showed how the axes of the three unipolar extremity leads (aVR, aVL, and aVF) also form a triaxial lead diagrams were combined to produce a hexaxial lead diagram. This is the lead diagram we shall use in determining the mean QRS axis and in describing axis deviation.
Each of the leads has a positive and a negative pole. As a wave of depolarization spreads toward the positive pole, an upward (positive) deflection occurs. As a wave of depolarization spreads toward the negative pole, a downward (negative) deflection is inscribed.
Finally, in order to determine or calculate the mean QRS axis, we need a scale. By convention, the positive pole of lead I is said to be at 00; all point below the lead I axis are negative. Thus, as we move toward lead aVL (-300), the scale becomes more positive-lead II at +600, lead aVF at +900, lead III at +1200.
The completed hexaxial diagram used to measure the QRS axis. By convention again, we can say that an electrical axis that points toward lead aVL is leftward or horizontal. An axis that points toward leads II, III, and aVF is rightward or vertical.
In calculating the mean QRS axis you are answering the question: in what general direction or toward which lead axis is the QRS complex predominantly oriented? For example, notice that there are tall R waves in leads II, III, and aVF, indication that the heart is electrically vertical (vertical electrical axis). Furthermore, the T wave is equally tall in leads II and III. Therefore, by simple inspection, the mean electrical QRS axis can be seen to be directed between leads II and III and toward lead aVF. Lead aVF on the hexaxial diagram is at +900.
As a general rule the mean QRS axis will point midway between any two leads that show tall R waves of equal height.
In the preceding example the mean electrical axis could have been calculated a second way. Recall from Chapter 3 that if a wave of depolarization is oriented at right angles to any lead axis a biphasic complex (RS or QR) will be recorded in that lead. Reasoning in a reverse manner, if you find a biphasic complex in any of the extremity leads, then the mean QRS axis must be directed at 900 to that lead. Are there any biphasic and shows an RS pattern. Therefore, the mean electrical axis must be directed at right angles to lead I. Since lead I on the hexaxial lead scale is at oo, the mean electrical axis must be at right angles to 00 or at either –900 or + 900. if the axis were –900, then the depolarization forces would be oriented away from the positive pole of lead aVF and lead aVF would show a negative complex. In this case lead aVF shows a positive complex (tall R wave), so the axis must be + 900.
Another example. In this case, by inspection, the mean QRS axis is obviously horizontal since leads I and aVLare positive and leads II, III, and aVF are predominantly negative. The precise electrical axis can be calculated by looking at lead II, which shows a biphasic RS complex. Therefore, using the same logic as before, we can say that the axis must be at right angles to lead II. Since lead II is at + 600 on the hexaxial scale, the axis must be either –300 or + 1500. if the axis were + 1500, then leads II, III, and aVF would be positive. Clearly in this case the axis is –300.
Another example is presented the QRS complex is positive in leads II, III, and aVF. Therefore we can say that the axis is relatively vertical. Since the R waves are of equal magnitude in leads I and III, the mean QRS axis must be oriented between these two leads, or at + 600.
Alternatively, we could have calculated the axis by looking at lead aVL (in Fig. 5-5), which shows a biphasic RS-type complex. The axis must be at right angles to lead aVL (-300), that is either –1200 or +600. Obviously, in this case, the answer is + 600. The electrical axis must be oriented toward lead II, which shows a tall R wave.
We now describe a second general rule: the mean QRS axis will be oriented at right angles to any lead showing a biphasic complex. In such cases the mean QRS axis will point in the direction of leads showing tall R waves.
Still another case is, by inspection, the electrical axis can be seen to be oriented away from leads II, III, and aVF and toward leads aVR and aVL, which show positive complexes. Since the R waves ate of equal magnitude in leads aVR and aVL, the axis must be oriented precisely between these leads, or at –900. Alternatively, look at lead I, which shows a biphasic RS complex. In this case the axis must be directed at right angles to lead I (00); that is, it must be either –900. lf +900, since the axis is oriented away from the positive pole of lead aVF and toward the negative pole of lead aVF, it must be –900.
Still another case can be noted. There are two ways of approaching the calculation of the mean QRS axis in this case. Since lead aVR shows a biphasic RS-type complex, the electrical axis must be right angles to the axis of lead aVR. Since the axis of lead aVR is at –1500, the electrical axis in this case must be either –600 or +1200. clearly it must be –600 in this caser since lead aVL is positive and lead III shows a negative complex.
These basic examples should establish the ground rules for estimating the mean QRS axis. It is worth emphasizing that such calculations are generally only an estimate or a near approximation. And error of 100 or 150 is not significant. Therefore, it is perfectly acceptable to calculate the axis from leads in which the QRS is nearly biphasic or from two leads where the R (or S) waves are approximately equal in amplitude.
The mean QRS axis is a basic measurement that should be made in every ECG you read. In most normal individuals the mean QRS axis will lie between –300 and +1000. and axis of –300 or more negative is described as left axis deviation (LAD). The term right axis deviation (RAD) refers to an axis of +1000 or more positive. In other words, left axis deviation is an abnormal extension of the mean QRS axis found in persons with an electrically horizontal heart; right axis deviation is an abnormal extension of the QRS axis found in persons with an electrically vertical heart.
The mean QRS axis is determined by two major factors: (1) the anatomic position of the heart and (2) the direction of ventricular depolarization (the direction in which the stimulus spreads through the ventricles).
The influence of cardiac anatomic position on the electrical axis can be illustrated by the effects of respiration. With inspiration, the diaphragm descends and the heart becomes more vertical in the chest cavity. This change in cardiac position generally shifts the electrical axis vertically (to the right). (Patients with emphysema and chronically hyperinflated lungs also have anatomically vertical hearts and electrically vertical QRS axes.) Conversely, with complete expiration, the diaphragm ascends and the heart assumes a more transverse or horizontal position in the chest. With expiration, the electrical axis generally shifts horizontally (to the left).
The second major determinant, the direction of depolarization through the ventricles, can be illustrated by left anterior hemiblock (Chapter 7), where there is a delay in the spread of stimuli through the left ventricle and the mean QRS axis is shifted to the left. On the other hand, right ventricular hypertrophy shifts the QRS axis to the right.
Recognition of right and left axis deviations is easy.
RAD, as stated before, is defined as a QRS axis more positive than +1000. Recall that if leads II and III show tall R waves of equal height then the axis must be +900. as an approximate rule, if leads II and III show tall R waves and the R wave in lead III exceeds that in lead II, then right axis deviation is present. In addition, lead I will show an RS pattern, with an S wave that is deeper than the R wave is tall.
The cutoff for LAD is –300. Notice that lead II shows a biphasic complex (RS complex). Remember that the location of lead II is at +600 and a biphasic complex indicates that the electrical axis must be at right angles to lead II or at –300 (or at +1500). Thus, with an axis of –300, lead II will show an RS complex where the R wave equals the S wave in amplitude. If the electrical axis is more negative than –300 (left axis deviation) then lead II will show an RS complex where the S wave is deeper than the R wave is tall.