The Electrical Axis

The concepts of the heart vector and the lead vector allow calculation of the mean electrical axis of the heart. The process for computing the axis of the mean force during activation (the mean electrical axis) and is the reverse of that used to compute potential magnitudes in the leads from the orientation and moment of the heart vector. The mean force during activation is represented by the area under the QRS waveform measured as millivolt-milliseconds. Areas above the baseline are assigned a positive polarity and those below the baseline have a negative polarity; the overall area equals the sum of the positive and the negative areas.

Calculation of the mean electrical axis during the QRS complex from the areas under the QRS complex in leads I and III. Magnitudes of the areas of the two leads are plotted as vectors on the appropriate lead axes, and the mean QRS axis is the sum of these two vectors.

The area in each lead (typically two are chosen) is represented as a vector oriented along the appropriate lead axis in the hexaxial reference system, and the mean electrical axis equals the resultant or the sum of the two vectors. An axis directed toward the positive end of the lead axis of lead I, that is, oriented horizontally away from the right arm and toward the left arm, is designated as an axis of zero degrees. Axes oriented in a clockwise direction from this zero level are assigned positive values and those oriented in a counterclockwise direction are assigned negative values.

The mean electrical axis in the horizontal plane can be computed in an analogous manner by using the areas and the lead axes of the six precordial leads. A horizontal plane axis located along the lead axis of lead V₆ is assigned a value of zero degrees, and those directed more anteriorly have positive values.

This process can be applied to compute the mean electrical axis for other phases of cardiac activity. Thus, the mean force during atrial activation will be represented by the areas under the P wave, and the mean force during ventricular recovery will represented by the areas under the ST-T wave. In addition, the instantaneous electrical axis can be computed at each instant during ventricular activation by using voltages or amplitudes at a specific point in time rather than using areas to calculate the axis.

The orientation of the mean electrical axis represents the direction of the activation front (or recovery direction) in an “average” cardiac fiber. The direction of the front, in turn, is determined by the interaction of three factors—the anatomical position of the heart in the chest, the properties of the cardiac conduction system, and the activation and recovery properties of the myocardium. Differences in the anatomical position of the heart within the chest would be expected to change the relationship between cardiac regions and the lead axes and would thus change recorded voltages. Similarly, changes in conduction patterns, even of minor degree, can significantly alter relationships between activation (or recovery) of various cardiac areas and, hence, the direction of instantaneous as well as mean electrical force. In practice, differences in anatomy contribute relatively little to shifts in axis; the major influences on the mean electrical axis are the properties of the conduction system and cardiac muscle.

ELECTROCARDIOGRAPHIC DISPLAY SYSTEMS

Another group of factors that determines ECG waveforms includes the characteristics of the electronic systems used to amplify, filter, and digitize the sensed signals. ECG amplifiers are differential amplifiers, that is, they amplify the difference between two inputs. For bipolar leads, the differential output is the difference between the two active leads; for unipolar leads, the difference is between the exploring electrode and the reference electrode. This differential configuration significantly reduces the electrical noise that is sensed by both inputs and hence is canceled. The standard amplifier gain for routine electrocardiography is 1000 but may vary from 500 (half-standard) to 2000 (double standard).

Amplifiers respond differently to the range of signal frequencies included in an electrophysiological signal. The bandwidth of an amplifier defines the frequency range over which the amplifier accurately amplifies the input signals. Waveform components with frequencies above or below the bandwidth may be artifactually reduced or increased in amplitude. In addition, recording devices include high- and low-pass filters that intentionally reduce the amplitude of specific frequency ranges of the signal. Such reduction in amplitude may be done, for example, to reduce the effect of body motion or line voltage frequencies, that is, 60-Hz interference. For routine electrocardiography, the standards of the American Heart Association require a bandwidth of 0.05 to 100 Hz.

Amplifiers for routine electrocardiography include a capacitor stage between the input and the output terminals; that is, they are capacitor coupled. This configuration blocks direct-current (DC) voltage while permitting flow of alternating-current (AC) signals. Because the ECG waveform may be viewed as an AC signal (which accounts for the waveform shape) that is superimposed on a DC baseline (which determines the actual voltage levels of the recording), this coupling has significant effects on the recording process. First, unwanted DC potentials, such as those produced by the electrode interfaces, are eliminated. Second, elimination of the DC potential from the final product means that ECG potentials are not calibrated against an external reference level. ECG potentials must be measured in relation to an internal standard. Thus, amplitudes of waves are measured in millivolts or microvolts relative to another portion of the waveform. The TP segment, which begins at the end of the T wave of one cardiac cycle and ends with onset of the P wave of the next cycle, is usually the most appropriate internal ECG baseline.

An additional issue is the digitizing or sampling rate for computerized systems. Too low a sampling rate will miss brief signals such as notches in QRS complexes or brief bipolar spikes and reduce the precision and accuracy of waveform morphologies. Too fast a sampling rate may introduce artifacts, including high-frequency noise, and requires excessive digital storage capacity. In general, the sampling rate should be at least twice the frequency of the highest frequencies of interest in the signal being recorded. Standard electrocardiography is most commonly performed with a sampling rate of 500 Hz, with each sample representing a 2-millisecond period.

Cardiac potentials may be processed for display in numerous formats. The most common of these formats is the classic scalar ECG. Scalar recordings depict the potentials recorded from one lead as a function of time. For standard electrocardiography, amplitude is displayed on a scale of 1 mV to 10-mm vertical displacement and time as 200 msec/cm on the horizontal scale. Other display formats are used for ambulatory electrocardiography, and for bedside ECG monitoring.