Pathophysiology

Pathophysiology Study Guide

Chapter 3: The Cardiovascular System

Objectives 1. Identify disturbances in the cardiac excitation. 2. Understand the anatomy and physiology of the heart. 3. Discuss infections and diseases of the heart and how they impact heart functioning. 3.1 Cardiac Excitation/Rhythmic Disturbances The activity of the heart is normal due to the generation of electrical impulses and the spread of myocytes with unique electrical characteristics such that it facilitates the organized contractions of myocardia in a particular sequence. The generation or conduction of these electrical impulses gets impaired and leads to disorders in the normal cardiac excitation. It should be noted that the rhythm of the heart gets disturbed by any heart disorder, including genetic disorders related to the structure or function of the heart. Some causes of this disturbance include abnormalities in the electrolyte, hypoxia, imbalance in hormones, drugs, and toxins. Action Potentials The action potential of the cardiac system has a prolonged plateau phase that lasts about 300 ms. There are five phases of the cardiac action potential. In the first phase, known as phase 0, the permeability of the membrane to potassium falls, and the fast sodium channels open up. This activity produces rapid depolarization from minus 90 mV to +10 mV. In the second phase, phase 1, the permeability to sodium falls and leads to partial repolarization. Next is the plateau phase in which the permeability of calcium increases and depolarization is sustained, which prolonged the action potential. By the end of this phase, the permeability of calcium falls but the plateau is maintained due to the inward sodium current. In phase 4, sodium and calcium channels close down, repolarization also stops, and the potential of the membrane comes down to baseline. The last phase, phase 4, shows the resting membrane potential at -90 mV. It is produced by selective permeability of the cell membrane to potassium, which closes to the Nernst equilibrium potential. This entire cycle of cardiac potential takes about 300ms. It should be noted that until the repolarization is complete, the next action potential does not begin. It may be possible that a supramaximal stimulus causes the action potential to be slow, and the rate of depolarization slows down, showing smaller amplitude as the contraction gets weak. Disorders of Conduction Atrial One common atrial disorder is paroxysmal atrial tachycardia in which sudden, abrupt, and spontaneous pounding of the heart occurs. Young and healthy people are more vulnerable to such attacks and in extreme cases, it may lead to periodic disability. Ways to terminate this attack are by applying pressure to the carotid artery residing in the neck, holding breath, inducing forced

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