ECG Curve: Tracing the Heart’s Conduction Pathway
One of the amazing characteristics of the human heart is its ability to independently switch on its own electrical impulses – signals which are readily detected by the electrocardiogram (ECG) machine and are responsible for the heart’s mechanical activity. To learn ECG analysis, it is important to be familiar with these electrical phenomena.
Cardiac impulse is represented graphically on the ECG curve as a series of waves and complexes including the P wave, QRS complex and T wave. Each curve has different characteristics depending on the location of the electrode recording it. A curve that falls below the base line is called a negative deflection, whereas one that rises above the base line is a positive deflection. A negative deflection indicates that the recorded wave has traveled away from the electrode and a positive deflection means it has traveled toward it.
In the heart, electrical currents are continuously generated and conducted to the myocardium resulting in rhythmic and regular contractions. To have an adequate and efficient pumping mechanism, a specialized conduction system for this electrical activity is necessary. Thus, the myocardium is equipped with a conduction system composed of specialized structures and fibers including:
- Pacemaker structures
- Specialized conductive fibers
- Conductive cardiac muscle tissue
A pacemaker structure is defined by its ability to generate action potentials autonomously, which is then conducted into the myocardium. The normal pacemaker of the heart is the sinoatrial (SA) node, a small oval structure located in the right atrium near the opening of the superior caval vein. The SA node generates action potentials at an average frequency of 60-70 per minute.
An action potential generated by the SA node excites adjacent cells of the right atrium. Subsequently these cells excite adjacent cells until all cells of the atrial syncytium are depolarized.
From the atria the cardiac impulse travels down the atrioventricular (AV) node, the His bundle, and through the left and right bundle branches. Eventually, the wave of depolarization reaches the ventricular myocardium, depolarizing the cells of the ventricular syncytium and resulting in ventricular contraction.
Knowing the physiology of the heart’s electrical conduction pathway is a prerequisite to fully understand the ECG curve. For a review of cardiac anatomy, physiology and ECG interpretation, you may visit ExpertCollege.com for its comprehensive lessons and interactive exercises.