AN INTRODUCTION TO CARDIAC ELECTROPHYSIOLOGY
To understand the electrocardiogram (ECG), you need a little background in electrophysiology. After all, it’s electricity that makes the heart work. The ECG is nothing more than a recording of the heart’s electrical activity. By analyzing the pattern of these recordings, you can diagnose numerous abnormalities of the heart’s structure and function. At first you may think this basic science has little to do with “real world” electrocardiography, but have patience. In time you will realize that these concepts come in handy when we start interpreting actual ECG tracings and discussing cardiac medications.
Electrical Properties of Cardiac Cells
There are four inherent electrical properties of cardiac cells: (1) automaticity, (2) excitability, (3) conductivity, and (4) refractoriness. All have important implications with regard to the generation of normal and abnormal cardiac rhythms and each property can vary in health and disease.
Automaticity refers to the ability of cardiac pacemaker cells to initiate electrical activity without an outside stimulus. We read in Chapter 1 that Galvani needed to apply an external electrical stimulus to make his frog leg contract. Nature has provided us with special, self-stimulating, pacemaker cells that can start the electrical process on their own.
The SA node is the dominant pacemaker of the heart because its natural rate of discharge is greater than any other potential pacemaker. We’re fortunate, however, that the SA node is not the only pacemaker available to maintain cardiac function. If illness or disease causes the SA node to fail, another latent pacemaker can take over, acting like a “backup generator” if the main power goes out. The automatic rates of these subsidiary pacemakers are slower than the SA node, so the heart rate will be lower. For example, the natural automatic rate of the SA node is 60 to 100 bpm, the AV junction 40 to 60 bpm, and the Purkinje system 20 to 40 bpm. Slower pacemakers are inhibited from firing after stimulation by a faster pacemaker, an effect termed overdrive suppression. This is nature’s way of keeping these latent pacemakers inactive as the faster pacemaker maintains control.
Overdrive suppression is also the term used by electrophysiologists to describe the technique of artificially stimulating the heart at a rapid rate, temporarily suppressing normal cardiac pacemaker activity. Measuring the rate of spontaneous return of the natural pacemaker is used to assess normal or abnormal function. This technique is used to evaluate sick sinus syndrome by determining the sinus node recovery time.
Excitability is the property of all cardiac cells to respond to an outside stimulus.
Conductivity refers to the ability of cardiac cells to transmit an electrical impulse from one cell to another. The speed of conduction is not the same in all areas of the heart. Cells with the fastest conduction are found in ...