Beachey: Respiratory Care Anatomy and Physiology, 2nd Edition

Answers to Concept Questions

Chapter 18: The Electrocardiogram and Cardiac Arrhythmias

18-1	Amplitude or voltage on the electrocardiogram is a function of muscle mass. Thus an abnormally high P-wave amplitude means the atrial muscle mass is greater than normal (atrial hypertrophy). High-amplitude P waves may be seen in cor pulmonale, in which high pulmonary vascular resistance strains the right side of the heart and causes atrial hypertrophy.

18-2	The increased number of muscle fibers in an enlarged right ventricle generates abnormally large amounts of current during depolarization. This causes the mean direction of current flow (the mean cardiac vector) to shift to the right, creating a right axis deviation.

18-3	The aVR lead would record a negative QRS complex. During depolarization in the direction of +60 degrees, current flows away from this unipolar lead’s positive electrode.

18-4	People with these conditions develop enlarged atria because of chronically high atrial distending pressures. Stretched muscle fibers lengthen the depolarization pathway, prolonging conduction time. As a result, depolarizing impulses may travel around the atria so slowly that they restimulate areas that have already repolarized. Thus a cycle of unorganized fibrillation develops.

18-5	Parasympathetic stimulation decreases the heart rate (the opposite of sympathetic stimulation).

18-6	Multifocal premature ventricular contractions mean ectopic impulses arise from a number of different sites in the ventricles. This means ventricular muscle irritability is widespread and not limited to one point.

18-7	In third-degree atrioventricular block, no atrial impulses reach the ventricles. Eventually, the most excitable ventricular tissue spontaneously depolarizes and paces the ventricles. This spontaneous depolarization rate is slower than that of sinoatrial node or atrial tissue. Thus the heart rate slows.