The electrocardiogram is the most basic tool in a cardiologists diagnostic arsenal. Also referred to as an EKG, the electrocardiogram traces electrical impulses that cause contractions of the heart muscle, known as the myocardium. In a joint effort, the American College of Cardiology and the American Heart Association have outlined the importance of using EKGs as the initial indicator of anatomical and physiological cardiac abnormalities.
Normal EKGs
As noted in the text "ECG Interpretation for the Clinical Exercise Physiologist," the normal EKG consists of three separate waveforms: the P wave, QRS complex and the T wave. These waves are indicative of electrical activity at various points in the cardiac cycle and should occur in sequence such that the P wave precedes the QRS that precedes the T wave. The P wave correlates with contraction of the top chambers of the heart, the atria. The second waveform, the QRS complex, occurs after a short return to baseline following the P wave and is associated with contraction of ventricles, the two lower chambers of the heart. Because the ventricles are much larger and have a greater amount of muscle mass, this wave generally appears as the largest waveform on the EKG. The T wave is indicative of ventricular relaxation and is generally closer to the P wave in size and shape.
Abnormalities in this sequence are termed arrhythmias and can occur due to changes in rate, rhythm or transmission of the electrical impulse throughout the myocardium.
Premature Contractions
Common arrhythmias stem from premature contractions in the atria or the ventricles. These contractions appear on the EKG as extra P or QRS waveforms, respectively, that occur out of sequence. In the event that several P waves or QRS complexes occur consecutively, the resulting arrhythmia will be termed fibrillation or flutter. Fibrillation is a disorganized quivering of the heart chamber where flutter occurs in a rapid yet organized manner. The presence of flutter in the ventricles is generally called ventricular tachycardia, indicating rapid contraction of the ventricles in the absence of atrial contraction. Premature contractions alter the efficiency of the normal cardiac cycle and therefore affect the ability of the heart to adequately deliver nutrients to the body. Because it is the role of the ventricles to deliver nutrients systemically, the ventricular arrhythmias are of greater concern and can rapidly deteriorate to complete cardiac arrest.
Blocks
Occasionally the pause in the transmission of the electrical impulse from the atria to the ventricles is longer than necessary and causes a delay in the initiation of ventricular contraction. These delays are termed atrioventricular blocks and appear on the EKG as an extended time spent at baseline between the P wave and the QRS complex. Advanced atrioventricular blocks can cause the atria and the ventricles to beat out of sync. This leads to P waves occurring without a subsequent QRS complex so the atria and ventricles initiate their own rhythms.
Additional types of blocks prevent the two ventricles from contracting at the same time. These blocks occur once the impulse has traveled from the atria into the ventricles and they disrupt the hearts distinct and efficient pattern of contracting.
Other Conditions
A number of conditions can lead to the appearance of EKG abnormalities. Factors including cardiac medications, electrolyte imbalances related to calcium and potassium, heart attacks and congenital disorders all alter the shape of the various EKG waveforms. While these alterations do not directly impact the sequence of the waveforms in the cardiac cycle, they do indicate changes to the structure and function of the myocardium, and therefore are of equal importance to the cardiologist.
Treatment
The implantable cardioverter defibrillator is instrumental in the treatment of life-threatening cardiac arrhythmias, According to the American Heart Association, The defibrillator is able to detect and correct, or override, abnormal rhythms such as ventricular fibrillation. Keeping the chambers of the heart beating synchronously is the goal in treating arrhythmias.
References
- "Journal of the American College of Cardiology"; ACC/AHA Clinical Competence Statement on Electrocardiography and Ambulatory Electrocardiography; Alan H Kadish, MD; 2001
- "ECG Interpretation for the Clinical Exercise Physiologist"; Christopher C Dunbar, PhD; 2009
- American Heart Association: What is an implantable defibrillator?
