To initiate the neural conduction process throughout the myocardium, the specialized neural cells possess the ability to spontaneously originate electrical impulses. This action is independent of any nerves or hormones; however, actual firing rates can be influenced by the autonomic nervous system. The autonomic nerves include sympathetic and parasympathetic fibers. Sympathetic nerves can increase activity or rate of cardiac impulses, whereas parasympathetic nerves slow their rate. Each cardiac cycle begins with a spontaneous neural impulse generated by the SA node. Subsequently this impulse spreads throughout the remainder of the cardiac neural conductive tissues, stimulating the muscle (myocardial) cells to contract. Abnormalities within the neural conduction system will adversely affect cardiac output. These abnormalities are called arrhythmias or dysrhythmias; the terms are used synonymously.
Electrical Membrane Potentials
To comprehend the electrical impulses and the information provided by an ECG, one must understand the basic concepts regarding electrical membrane potentials. All cell membranes are positively charged on their outer surface owing to the relative distribution of cations. This resting membrane potential is maintained by an active transport mechanism called the sodium-potassium ion pump. When the cell is stimulated, movement of ions in and out of the membrane alters its charge, reversing the resting potential. This period is referred to as depolarization. When depolarization occurs, myocardial cells are stimulated to contract. The depolarization period is short. Reversal of ion flow reestablishes a positive charge to the outside of the membrane, a process called repolarization, which returns the membrane to its resting membrane potential. During repolarization, myocardial muscle cells relax. The processes of depolarization and repolarization are collectively referred to as an action potential. This event perpetuates itself as an “impulse” along the entire surface of a cell and from one cell to another, provided cell membranes are connected to one another. The following steps summarize the action potential sequence:
Unique to the tissue of the cardiac neural conduction system is a process called automaticity, which allows cell membranes to spontaneously depolarize at recurrent periods. Another unique feature of this specialized tissue is that cell membranes are in direct contact with cardiac muscle, and their action potential directly initiates depolarization of the cardiac muscle cells. Cardiac muscle cells are fused to one another, which allows the cells to function as a continuous sheet of cells (syncytium). The sheet of cells of the atria is separated from that of the ventricles by a layer of connective tissue that acts as an insulator. The SA node initiates depolarization of the atrial muscle, but the insulation prevents transmission into the ventricles. The AV node delays and finally relays the impulse along the common bundle of His, which penetrates the connective tissue to enter the ventricles. The impulse continues along the common bundle of His and its branches until it finally reaches the Purkinje fibers, stimulating the ventricular muscular sheet.
Action potentials that spread throughout the muscle sheets of the heart may be detected by surface electrodes to record and produce a tracing known as an electrocardiogram. What is observed in an ECG tracing is the action potentials of the atrial and ventricular muscle cells. However, other events can be deduced from the tracing.
The electrical sequence of a cardiac cycle is initiated by the sinoatrial node, known as the pacemaker of the heart. This is because the SA node has a faster rate of spontaneous firing than the remaining specialized tissues (see Fig. 16-1).
The baseline of an ECG is called the isoelectric line and signifies resting membrane potentials. Deflections are the positive or negative changes in the tracing relative to the isocenter over the time of the cycle. These deflections are lettered in alphabetic order, and after each the tracing normally returns to the isoelectric point.
P wave. The first deflection is the P wave and represents depolarization of atrial muscle cells. It does not represent contraction of this muscle, nor does it represent firing of the SA node. These events are deduced, based on the shape and consistency of the P waves. The assumption is that the SA node fires at the start of the P wave and atrial contraction begins at the peak of the P wave. Although atrial repolarization follows depolarization, the ECG provides no evidence of this event because atrial repolarization is too minor in amplitude to be recorded by surface electrodes.
QRS complex, T and U waves. The QRS complex represents depolarization of ventricular muscle cells. The Q portion is the initial downward deflection, the R portion is the initial upward deflection, and the S portion is the return to the baseline (isoelectric point). Often the Q portion is not evident, and the depolarization manifests as only an RS complex. In any case the complex does not represent ventricular contraction. The assumption is that contraction will commence at the peak of the R portion of the complex. After depolarization, ventricular muscle repolarizes, and this event is significant enough in amplitude to generate the T wave on the ECG tracing. Although not always seen, the U wave is typically small and, when evident, follows the T wave. The U wave is theorized to represent repolarization of the papillary muscles and Purkinje fibers.
PR interval. The PR interval is measured from the beginning of the P wave to the beginning of the R portion of the QRS complex. (This is conventional practice because the Q portion of the complex is so frequently indiscernible.) Because the PR interval commences with atrial muscle depolarization and ends with the start of ventricular depolarization, it can be assumed that the electrical impulse passes through the AV node into the ventricle during this interval. If the PR interval is prolonged, it can be deduced that AV block is present.
The electrical events of an ECG are illustrated and summarized in Fig. 16-2.
Summary of Events of a Cardiac Cycle
Of the following eight physiologic events listed for a cardiac cycle, only three are actually observed on an ECG tracing.