23.9: Electrocardiogram
An electrocardiogram (ECG or EKG) is a critical diagnostic tool that records the electrical signals produced by the heart during each heartbeat. This recording is achieved through electrodes placed strategically on the arms, legs, and chest. The electrocardiograph amplifies these signals and produces 12 distinct tracings, offering a comprehensive understanding of the heart's electrical activity.
Three major waveforms are present in a typical ECG recording: the P wave, the QRS complex, and the T wave. The P wave signifies atrial depolarization, indicating the spread of action potentials through the atria. The QRS complex represents rapid ventricular depolarization, reflecting the propagation of action potentials through the ventricular contractile fibers. The T wave, smaller and wider than the QRS complex, denotes ventricular repolarization, occurring as the ventricles start to relax.
The size of these waves can provide valuable insights into cardiac health. Larger P waves may suggest an enlarged atrium, an enlarged Q wave could indicate myocardial infarction and an enlarged R wave typically points to enlarged ventricles. A flatter-than-normal T wave may suggest oxygen deprivation in the heart muscle, while an elevated T wave could be a sign of high blood potassium levels.
ECG analysis also involves measuring intervals or segments between waves. The P-Q interval represents the conduction time from the start of atrial excitation to the start of ventricular excitation. The S-T segment signifies the time when the ventricular contractile fibers are depolarized. An elevated S-T segment may indicate acute myocardial infarction, whereas a depressed S-T segment might suggest an insufficient oxygen supply to the heart muscle. The Q-T interval, extending from the start of the QRS complex to the end of the T wave, represents the time from the start of ventricular depolarization to the end of ventricular repolarization.
Stress testing can be beneficial for evaluating the heart's response to physical exercise. This test helps identify whether narrowed coronary arteries can meet the heart's increased oxygen need during strenuous activity. Continuous ambulatory electrocardiography, where an individual wears a Holter monitor for 24 hours to record continuous ECG data, is used for detecting brief or unpredictable heart issues.
