Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program 8 PR Intervals Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program

PR Intervals Another important step of assessing the ECG is analyzing the PR intervals Ask the students “What are the 1st, 2nd, 3rd and 4th steps of analyzing an ECG tracing?” Answer: Assess the rate, regularity, P waves and QRS complexes. Q

PR Interval Distance from the beginning of the P wave to the beginning of the Q wave Morphology is a P wave and a flat line Instructional points: A normal PR interval indicates the impulse originated from SA node (or close to it) and traveled through the atria and AV node in a regular and unobstructed course. Remind the student we measure the R wave if the Q wave is absent. I

PR Interval Denotes depolarization of the heart from the SA node through the atria, AV node and His-Purkinje system

PR Intervals Their presence or absence as well as duration can help us identify if and how well the impulses are traveling through the heart’s conductive pathway Important to know because delays or interruption of conduction through AV node and/or junction can lead to serious or even life-threatening conditions

Measuring PR Intervals If the PR intervals are identifiable, measure from where the P wave begins to where the Q wave begins To make it easier, find a P wave that begins on a vertical line and then measure to where the Q or R wave begins

PR Intervals Are considered abnormal if they are shorter, longer, progressively longer, vary or absent

Shorter P’R Intervals Shorter P’R intervals occur when the impulse originates in the atria close to the AV junction or in the AV junction

Shorter P’R Intervals Can occur when an impulse arises from a supraventricular site but travels through abnormal accessory pathways to the ventricles Leads to premature ventricular depolarization called preexcitation

Longer P’R Intervals Can occur when there is a delay in impulse conduction through the AV node

Varying P’R Intervals Can occur when the pacemaker site moves from beat to beat causing the P’ waves to appear different and the P’R intervals to vary

Varying P’R Intervals In one form of AV heart block, the PR intervals are progressively longer until a QRS complex is dropped and then the cycle repeats

Absent PR Intervals Occurs with: atrial rates that exceed 250 BPM and produce flutter waves or a chaotic baseline and absence of P waves preceding the QRS complexes ventricular dysrhythmias

Absent PR Intervals Also seen in the most severe form of AV heart block

More P Waves and Constant PR Intervals In on form of AV heart block, some sinus beats are blocked in the AV node and do not reach the ventricles PR intervals associated with P waves that are conducted through to the ventricles are constant

Practice Makes Perfect In this tracing, determine the type of PR interval Answer: PRI 0.08 seconds in duration. I

Practice Makes Perfect In this tracing, determine the type of PR interval Answer: PRI has a longer than normal duration at 0.30 seconds. I

Practice Makes Perfect In this tracing, determine the type of PR interval Answer: PRI 0.22 seconds in duration I

Practice Makes Perfect In this tracing, determine the type of PR interval Answer: PRI 0.16 seconds in duration. I

Practice Makes Perfect Determine the type of PR interval Answer: PRI is immeasurable (as the P waves are not associated with the QRS complexes) I

Summary Another important step of analyzing an ECG rhythm is examining the PR intervals PR interval is the distance from the beginning of the P wave to the beginning of the Q wave Duration of the PR interval is normally 0.12 to 0.20 seconds PR intervals are considered abnormal if they are shorter, longer, absent or vary

Summary Shorter P’R intervals occur when the impulse originates in the atria close to the AV junction or in the AV junction itself AV heart block is the most common cause of longer PR intervals When the pacemaker site moves from beat to beat it causes the P’ waves to appear different and the P’R intervals to vary

Summary In one for of AV heart block, the PR intervals are progressively longer until a QRS complex is dropped and then the cycle repeats There is an absence of PR intervals in atrial rates that exceed 250 BPM producing flutter waves or a chaotic looking baseline and with ventricular dysrhythmias In the most severe form of AV heart block the PR intervals are not measurable

Summary In one form of AV heart block, some of the sinus beats are blocked in the AV node and do not reach the ventricles. The PR intervals associated with the P waves that are conducted through to the ventricles are constant in duration