Unipolar leads record the difference between an active limb electrode and an indifferent (zero potential) electrode at the centre of Einthoven's triangle (Figure HE.11). These signals are of lower amplitude than other leads and require increased amplification (hence referred to as augmented leads):
• aVR—the augmented unipolar right arm lead faces the heart from the right side and is usually orientated to the cavity of the heart. Therefore, all the deflections P, QRS and T are normally negative in this lead
• aVL—the augmented unipolar left arm lead faces the heart from the left side and is orientated to the anterolateral surface of the left ventricle
• aVF—the augmented unipolar left leg lead, orientated to the inferior surface of the heart Precordial Chest Leads
These are horizontal plane, unipolar leads, placed as follows:
• V1—fourth intercostal space immediately right of the sternum
• V2—fourth intercostal space immediately left of the sternum
• V3—exactly halfway between the positions of V2 and V4
• V4—fifth intercostal space in the mid-clavicular line
• V5—same horizontal level as V4 but on the anterior axillary line
• V6—same horizontal level as V4 and V5 on the mid-axillary line Anatomical Orientation of ECG Electrodes
Although there is considerable variation in the position of the normal heart the atria are usually positioned posteriorly in the chest while the ventricles form the base and anterior surface. The right ventricle is anterolateral to the left ventricle. The ventricles consist of three muscle masses. These are the free walls of the left and right ventricles and the interventricular septum. Electrical activity in the left ventricle and the interventricular septum are predominant. ECG electrodes will pick up signals from the closest structures and those producing the greatest electrical signals. Therefore, ECG signals recorded from the anterior aspect of the heart are mainly due to activity in the interventricular septum with only a small contribution from the right ventricular wall. Since ECG lead signals reflect activity in different parts of the heart because of their position, they are said to 'look' at different aspects of the heart:
• sll, sill and aVF look at the inferior surface of the heart
• si and aVL are orientated towards the superior left lateral wall
• aVR and V1 face the cavity of the heart and the deflections are mainly negative in these leads
• Leads V1-6 are orientated towards the anterior wall. Vi and V2 are anterior leads, V3 and V4 are septal leads, V5 and V6 are lateral leads
• V1 and V2 examines the right ventricle, while V4-6 are orientated towards the septum and left ventricle There is no lead orientated directly to the posterior wall of the heart.
The heart rate (bpm) can be determined from the ECG by measuring the time interval (s) between two successive beats (R-R interval), and dividing this into 60 s. Thus, if R-R interval = 0.6s heart rate = 60/0.6 = 100 bpm.
The time scale of the ECG depends on the recording paper speed. Normal paper speed is 2.5 cm/s, meaning that each large square (5 mm) of the ECG trace represents 0.2s (Figure HE.10). In the above example, the R-R interval is three large squares (15 mm), which is equal to 0.6 s. If the R-R interval were to become five large squares (1 s) this would give a heart rate of 60 bpm.
The direction of the electrical axis of the heart is usually calculated in the frontal plane only and can be done using two of the frontal ECG leads (these are the standard limb and unipolar limb leads). It is determined as an angle referred to the axes shown in Figure HE.12, the normal range lying between 0 and +90°.
The cardiac vector, like any vector, can be resolved to give an effect or "component' in any given direction. The amplitudes of the QRS complexes in the frontal leads represent components of the cardiac vector in the direction of the leads. The cardiac axis can, therefore, be found by the vector summation of any two of these components to find their resultant (here sI and aVF are taken for convenience since they lie at 0 and 90° respectively). The direction of the cardiac axis is then given by the angle (theta), of the resultant. A simple algorithm is presented to determine the cardiac axis from si and aVF.
Calculation Algorithm for Cardiac Axis
An example illustrating the calculation of the cardiac axis from ECG leads si and aVF is shown in Figure HE.13. To obtain the axis:
• Determine the amplitudes of the QRS complexes in si and aVF by subtracting the height of the S wave from the height of the R wave in each lead
• Construct a rectangle with the sides in proportion to the amplitudes of si and aVF. The diagonal then represents the resultant of si and aVF (i.e. the cardiac vector)
• Determine the direction of the cardiac axis is from the angle 9 by taking the tangent of 9 as the ratio of [amplitude of aVF]/[amplitude of si]
Figure HE.13 Calculation of cardiac axis
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