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Unformatted text preview: Page 1 of 3 BIOE 322/BIOS 332 Fundamentals of Systems Physiology Spring 2008 Problem Set #5 Solutions 1. 1.1 The PR interval on the ECG represents the time from initial depolarization of the atria to initial depolarization of the ventricles (i.e., beginning of the P wave to beginning of the R wave). 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 or milliseconds. The PR interval is 120-200ms, with an average of 160 ms. 1.2 Conduction velocity, as applied to myocardial tissue, has the same meaning that it has in nerve or skeletal muscle. It is the speed at which action potentials are propagated within the tissue from one site to the next. Thus, the units for conduction velocity are distance/time (e.g., meters/sec). Conduction velocity in the AV node is the slowest of all of the myocardial tissues (0.01-0.05 m/s). Compare this value in the AV node with the much faster conduction velocities in atria and ventricles (1m/s) and in His-Purkinje tissue (2-4 m/s). 1.3 The slower the conduction velocity through the AV node, the longer the PR interval (since the length of the PR segment is increased). Conversely, the faster the conduction velocity through the AV node, the shorter the PR interval. Charles PR intervals were longer than normal because the conduction velocity through the AV node was decreased, presumably due to tissue damage caused by the myocardial infarction. 1.4 The QRS complex on the ECG corresponds to electrical activation of the ventricles. The normal configuration of Charles QRS complexes implies that his ventricles were activated in the normal sequence (i.e., spread of activation was from the AV node through the bundle of His to the ventricular muscle). 1.5 Charles ECG showed some P waves that were not followed by QRS complexes. AV nodal conduction was slowed so much that some impulses were not conducted at all from atria to ventricles. This observation is consistent with increased AV delay and increased PR interval. 1.6 Charles fainted because his arterial pressure was decreased, which caused a decrease in cerebral blood flow. The decrease in arterial pressure is likely related to the absent QRS complexes on the ECG. Each cardiac cycle without a QRS complex is a cardiac cycle in which electrical activation of the ventricles did not occur. 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....
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- Spring '08