Atrioventricular Conduction Block
Charles Doucette, who is 68 years old, retired from a middle management position in the automotiveindustry following an acute myocardial infarction. He was recovering in a local hospital, where the physicians closely monitored his ECG .
Mr. Doucette’s PR intervals were normal and his QRS complexes had a normal configuration.
However, occasional P-waves occurred that were not followed by QRS complexes (nonconducted P-waves). He fainted twice in the hospital. The physicians believed that the myocardial infarction caused a block in his AV conduction system called a Mobitz type II AV block. Since Mr. Doucette’s conduction block could become more severe, his physicians planned to treat him by implanting a pacemaker.
1. Describe and explain the physiology of the waves and intervals of the normal ECG.
2. What does the PR interval on the ECG represent? What units are used to express the PR interval? What is the normal value?
3. What does the term “conduction velocity” mean, as applied to myocardial tissue? What is the normal conduction velocity through the AV node? How does conduction velocity in the AV node compare with conduction velocity in other portions of the heart?
4. How does AV nodal conduction velocity correlate with PR interval? Since Mr. Doucette’s physicians believe he has a block in his AV conduction system, why are his PR intervals normal (rather than increased)?
5. What does the QRS complex on the ECG represent? What is implied in the information that the QRS complexes on Mr. Doucette’s ECG had a normal configuration?
6. How is it possible to have a P-wave that is not followed by a QRS complex, as seen on Mr. Doucette’s ECG? Propose a mechanism to explain the P-wave that is not followed by a QRS complex.
7. Why did Mr. Doucette faint?