Case Study 4 Mr. Ortega returned to the hospital after a recent myocardial infar
ID: 51306 • Letter: C
Question
Case Study 4
Mr. Ortega returned to the hospital after a recent myocardial infarction. He has experienced a number of fainting spells. The cardiac rehabilitation nurse reports that his PR intervals were longer than normal. Although his QRS complexes had a normal configuration, there were occasional nonconductive P waves, P waves that are not followed by QRS complexes. His physicians believed that a recent myocardial infarction caused a block in his atrioventricular conducting system
1. What does the PR interval on the ECG represent? What is the normal value?
2. What does the QRS complex on the ECG represent? What is implicit in the information that the QRS complexes on Mr. Ortega’s ECG had a normal configuration?
3. How is it possible to have P waves that are not followed by QRS complexes?
4. Why did Mr. Ortega faint?
Explanation / Answer
1. The PR interval on the ECG represents the time from initial depolarization of the atria to initial depolarization of the ventricles. Therefore, the PR interval includes the P wave (atrial depolarization) and the PR segment, an isoelectric portion of the ECG that corresponds to conduction through the AV node, Because PR interval is a time, its units are given in seconds (sec) or milliseconds (msec). You may have needed to look up the normal value for PR interval, which is 120-200 msec (average, 160 msec)
2. The QRS complex on the ECG corresponds to electrical activation of the ventricles. The normal configuration of Mr. Ortega’s QRS complexes tells that his ventricles were activated in the normal sequence.
3. The slower the conduction velocity through the AV node, the longer the PR interval. Conversely, the faster the conduction velocity through the AV node, the shorter the PR interval. Mr. Ortega’s PR intervals were longer than normal because the conduction velocity through the AV node was decreased, presumably because of myocardial infarction.
4. Mr. Ortega fainted because his arterial pressure was decreased, which caused a decrease in cerebral blood flow. If the ventricles were not activated electrically, they did not contract; if they did not contract, they did not eject blood, and mean arterial pressure decreased.