Review of Medical Physiology - William F. Ganong 2002

Blood Circulation
The Nature of Cardiac Contraction and Electrical Activity of the Heart
Electrocardiogram

Body Fluids are good electrical conductors (the body acts as a volume conductor), which makes it possible to record biopotential fluctuations extracellularly. These fluctuations represent the algebraic sum of the myocardial fiber action potentials. The continuous recording of these biopotential changes over the course of the cardiac cycle is known as an electrocardiogram (ECG). Most electrocardiographs record these fluctuations on a moving paper strip.

An ECG can be recorded using an active electrode connected to an indifferent electrode with zero potential (unipolar recording) or by using two active electrodes (bipolar recording). In a volume conductor, the sum of potentials at the vertices of an equilateral triangle with a current source at its center is always zero. The triangle with The Heart at its center (Einthoven's triangle) is formed by placing electrodes on both arms and the left leg. These constitute the three standard limb leads used in electrocardiography. Connecting these electrodes to a common terminal yields an indifferent electrode with a potential close to zero. Depolarization spreading toward the active electrode in a volume conductor produces an upward deflection, whereas depolarization spreading in the opposite direction produces a downward deflection. The various waves and segments of the human ECG are illustrated in Fig. 28-5. By convention, if the active electrode becomes positive relative to the indifferent electrode, an upward deflection is recorded; if the active electrode becomes negative, a downward deflection is recorded. The P wave is caused by atrial depolarization, the QRS complex by ventricular depolarization, and the ST segment and T wave by ventricular repolarization. Normally, atrial repolarization is not visible because it is masked by the QRS complex. The U wave is not always present and is thought to be caused by the repolarization of the papillary Muscles. The intervals between the various ECG waves and the underlying physiological processes are summarized in Table 28-2.

Class="center">Table 28-1. Conduction Velocity in the Heart

Fig. 28-4. Normal propagation of electrical activity in the heart (reproduced with permission from Goldman MJ: Principles of Clinical Electrocardiography, 12th ed. McGraw-Hill, 1986).

Bipolar Leads

Bipolar leads were used prior to the Introduction of unipolar leads. In this configuration, standard limb leads record potential differences between two limbs. Because body fluids are good electrical conductors, the exact placement of electrodes on the limbs is not critical. In lead I, the electrodes are connected such that when the left arm is positive relative to the right arm, an upward deflection is recorded (left arm positive). In lead II, the electrodes are placed on the right arm and left leg (leg positive); in lead III, they are placed on the left arm and left leg (leg positive).

Unipolar (V) Leads

Clinical electrocardiography routinely utilizes nine additional unipolar leads that record the potential difference between an active and an indifferent electrode. There are six chest ( precordial) unipolar leads designated as V₁-V₆ (Fig. 28-6), and three unipolar limb leads: VR (right arm), VL (left arm), and VF (left leg). Most commonly, augmented limb leads, denoted by the prefix a (aVR, aVL, aVF), are employed. These augmented leads record the potential difference between one limb and the other two combined, thereby increasing the voltage magnitude by 50% without altering the waveform configuration of the unstandardized leads.

Unipolar electrodes can also be attached to the tips of catheter electrodes and introduced into the Esophagus or the heart chambers.

The Normal Electrocardiogram

A normal human ECG is shown in Fig. 28-7. When interpreting the waveform configuration in each lead, one must consider both The sequence of depolarization across different Regions of the heart and the spatial orientation of the heart relative to the electrodes. The atria are located posteriorly in the chest. The ventricles form the base and anterior surface of the heart, with the right ventricle occupying an anterolateral position and facing left. Consequently, aVR "faces" the ventricular cavities. Because atrial depolarization, ventricular depolarization, and ventricular repolarization all spread away from this active electrode, the P wave, QRS complex, and T wave are predominantly negative (downward) deflections. Conversely, aVL and aVF "face" the ventricles, resulting in deflections that are primarily positive or biphasic. Leads V₁ and V₂ lack a Q wave, and the initial portion of the QRS complex exhibits a minor upward deflection. This occurs because ventricular depolarization initially spreads through the middle portion of the septum from left to right, heading toward the active electrode. The excitation wave then travels downward along the septum and into the left ventricular wall away from the electrode, producing a prominent S wave. Finally, the impulses propagate backward along the ventricular walls toward the electrode, causing the trace to return to the isoelectric line. In contrast, left ventricular leads (V₄-V₆) may display a small Q wave (representing septal depolarization from left to right) followed by a prominent R wave (representing septal and left ventricular depolarization), after which leads V₄ and V₅ reveal a moderate S wave (reflecting late ventricular wall depolarization spreading retrogradely toward the AV junction).

Fig. 28-5. ECG waves.

Table 28-2. ECG Intervals

¹ Measured from the onset of the P wave to the beginning of the QRS complex.

² Decreases as heart rate increases, dropping from a mean value of 0.18 at a rate of 70 beats/min to 0.14 at 130 beats/min.

Normal anatomical variations in THE POSITION OF the heart can be substantial, which directly affects the configuration of the electrocardiographic complexes across different leads.

Bipolar Limb Leads and the Electrical Axis of the Heart

Because standard leads record potential differences between two points, the deflection in any given lead at any moment represents the magnitude and direction—projected onto the lead axis—of the electromotive force generated by the heart (the cardiac axis, or vector). The instantaneous vector in a two-dimensional frontal plane can be determined for any given moment using any two standard limb leads, assuming that the three electrodes are placed at the vertices of an equilateral triangle (Einthoven's triangle) with the heart at its center. Although these assumptions are not strictly exact, the calculated vectors are useful approximations. An approximate mean QRS vector (the "electrical axis of the heart") can be determined from the mean QRS deflection in each lead, as illustrated in Fig. 28-8. This mean vector differs from instantaneous vectors. Mean QRS deflections can be calculated precisely by integrating the QRS complexes; however, adequate approximations can be obtained simply by measuring the differences between the positive and negative QRS peaks. Normally, the orientation of the mean QRS vector on the coordinate system shown in Fig. 28-8 ranges from -30° to +110°. A shift of the QRS vector to the left of -30° is termed left axis deviation, whereas a shift to the right of +110° is termed right axis deviation. Right axis deviation typically indicates right ventricular hypertrophy. Left axis deviation may result from Left Ventricular Hypertrophy, although more sensitive and reliable electrocardiographic criteria exist for this condition.

Vectorcardiography

Connecting the tips of all instantaneous cardiac vectors in the frontal plane throughout the cardiac cycle, from start to finish, traces out a continuous curve known as the vectorcardiogram, which forms three distinct loops: the first corresponds to the P wave, the second to the QRS complex, and the third to the T wave. This can be accomplished electronically using a cathode-ray oscilloscope.

His Bundle Electrogram

In patients with heart blocks, electrical changes within the AV node, His bundle, and Purkinje fibers are frequently studied using an electrode-tipped catheter introduced through the Veins into the right side of the heart near the tricuspid valve. Three or more standard electrocardiographic leads are recorded simultaneously. Recording electrical activity with a catheter near the tricuspid valve is termed a His bundle electrogram (HBE). Normally, it reveals deflection A during AV nodal excitation, wave H during impulse transmission through the His bundle, and deflection V during ventricular depolarization (Fig. 28-9). Using the His bundle electrogram and standard electrocardiographic leads, the duration of three intervals can be precisely determined: the PA interval, from the onset of atrial depolarization to wave A on the His bundle electrogram, corresponds to the conduction time of impulses from the SA node to the AV node; the AH interval, from wave A to the onset of the H deflection, corresponds to the transit time through the AV node; and the HV interval, from the onset of wave H to the onset of the QRS complex on the ECG, indicates the conduction time of impulses through the His bundle and its branches. Approximate normal values for these intervals in milliseconds are: PA, 27; AH, 92; and HV, 43. These values indicate relatively slow impulse propagation through the AV node (see Table 28-1).

Fig. 28-7 Normal ECG (reproduced with permission from Goldman MJ: Principles of Clinical Electrocardiography, 12th ed. McGraw-Hill, 1986)

Fig. 28-8. Electrical axis of the heart. Left: Einthoven's triangle. Perpendicular lines dropped from the midpoints of the sides of the equilateral triangle intersect at the center of electrical activity. RA - right arm; LA - left arm; LL - left leg. Center: Calculation of the mean QRS vector. For each lead, a segment is plotted from the midpoint of the triangle's side, with a length equal to the difference between the height of the R wave and the height of the largest negative deflection in the QRS complex. A vector originating at the center of electrical activity and terminating at the intersection of the perpendiculars dropped from the points obtained on the sides of the triangle determines the length and direction of the mean QRS vector. Right: Axes for calculating vector direction.

Monitoring

Continuous ECG monitoring is frequently performed in cardiac care units. In such cases, the onset of life-threatening arrhythmias triggers an audible alarm. Using a portable recording device (Holter monitor), ambulatory patients can have their ECG recorded during normal daily activities. The recording is subsequently played back at high speed and analyzed, thereby providing a continuous ECG record over an extended period. There are also portable ECG devices that are activated by the patient upon the onset of symptoms, after which the recording is transmitted via telephone to a physician's office or laboratory. The clinical utility of monitor-based ECG recording is well established for diagnosing arrhythmias and planning management strategies in post-myocardial infarction patients.

Fig. 28-9 Normal His bundle electrogram (HBE) with a simultaneously recorded ECG