Fig. 16.28A), the deflection recorded in lead I is upward because the positive arrowhead lies closer to the left arm than to the right arm. The deflection in lead II is also upright because the arrowhead lies closer to the left leg than •Fig. 16.28 MagnitudeanddirectionoftheQRScomplexesinlimbleadsI,II,andIIIwhenthemeanelectricalaxis(θ)is60degrees(A), 120degrees(B), and0degrees(C).
to the right arm. The magnitude of the lead II deflection is greater than that in lead I because in this example, the direction of the vector parallels that of lead II; therefore, the magnitude of the projection on lead II exceeds that on lead I. Similarly, in lead III, the deflection is upright and its magnitude equals that in lead I.
If the vector in
Fig. 16.27A is the result of electrical events that occur during the peak of the QRS complex, the orientation of this vector is said to represent the mean electrical axis of the heart in the frontal plane. The positive rotatory direction of this axis is assumed to be in the clockwise direction from the horizontal plane (contrary to the usual mathematical convention). In normal individuals, the average mean electrical axis is approximately +60 degrees (as in
Fig. 16.28A ). Therefore, QRS complexes are usually upright in all three leads and largest in lead II.
If the mean electrical axis shifts substantially to the right (as in
Fig. 16.28B , in which θ= 120 degrees), projections of the QRS complexes on the standard leads change considerably. In this case, the largest upright deflection is in lead III, and the deflection in lead I is inverted because the arrowhead is closer to the right arm than to the left arm. Such a shift is termed right axis deviation and occurs with hypertrophy (i.e., increased thickness) of the right ventricle. When the axis shifts to the left, as occurs with hypertrophy of the left ventricle (see
Fig. 16.28C , where θ= 0 degrees), the largest upright deflection is in lead I, and the QRS complex in lead III is inverted.
In addition to limb leads I, II, and III, other limb leads that are also oriented in the frontal plane are routinely recorded in patients. These leads are (1) aVR, for which the right arm is defined as the positive lead and the middle of the heart is defined as the negative lead (i.e., the left arm and ankle leads are connected together); (2) aVL, for which the left arm is the positive lead and the middle of the heart is defined as the negative lead (i.e., the right arm and ankle leads are connected together); and (3) aVF, for which the ankle (foot) lead is defined as positive and the middle of the heart is defined as the negative lead (i.e., the two arm leads are connected together). The axes of these leads form angles of +90 degrees for aVF, −30 degrees for aVL, and −150 degrees for aVR (all with respect to the horizontal axis).
Leads can also be applied to the surface of the chest, so-called precordial leads, to determine the projections of the cardiac vector on the sagittal and transverse planes of the body. These precordial leads are recorded from six selected points on the anterior and lateral surfaces of the chest in the vicinity of the heart. The leads extend from the right border of the sternum in the fourth intercostal space (lead V1) to under the left arm (midaxillary line) in the fifth intercostal space (lead V6). Each precordial lead (V1 to V6) is defined as a positive lead, whereas the middle of the heart is defined as the negative lead. Detailed analysis of the ECG, as detected by the various lead systems just described, is beyond the scope of this book. Interested students are referred to textbooks on electrocardiography for more information.