Ch 31- Understanding EKG Waves, Segments and Intervals

• Electrical activity of the heart is displayed on the EKG

EKG Waves

• The P wave represents atrial depolarization
• The Q wave represents depolarization of the intraventricular septum
• The R wave represents depolarization of the main mass of the ventricles
• The S Wave represents the last phase of ventricular depolarization at the base of the heart
• The T-wave represents ventricular repolarization

EKG Segments and Intervals

• The PQ segment represents the time it takes for the signal to pass from the SA node to the AV node
• The PR interval measures the time from the onset of atrial activation to the onset of ventricular activation, with a normal duration of 0.12 – 0.20 seconds; represents the time necessary for the signal to travel from the sinus node through the atrium, AV node, and His-Purkinje system to activate the ventricular myocardial cells
• The QRS complex represents the sum of all ventricular muscle cell depolarizations, with a normal duration of 0.06 – 0.10 seconds; configuration and amplitude may vary among individuals; atrial repolarization occurs during the QRS complex but is masked by the large QRS
• During the ST-interval, the entire ventricular myocardium is depolarized which causes the ventricle to contract and pump blood
• The QT interval, sometimes called the "electrical systole" of the ventricles, represents the time for both ventricular depolarization and repolarization to occur, with a normal duration of 0.04 seconds but varies inversely with heart rate