|syllabus | home page | BME Home Page | search the web | e-mail Doug|
Under just the right conditions, it is possible for the heart to undergo contractions that fail to properly propel blood along; in essence, they twitch and quiver, instead of pump. This type of heart arrhythmia is known as fibrillation and can occur in both the atria and the ventricles. It is thought that this is a problem with reentry where the loops breaks up into a number of irregular circuits.
In atrial fibrillation, the atrial contractions are not synchronized and, therefore, are ineffective at contributing to ventricular filling. In the electrocardiogram, this shows up as a lack of P waves, which are replaced with irregular fluctuations known as f waves. (Refer to figure 2-42A on page 48 of text) Symptoms of atrial fibrillation include: fatigue, dizziness, shortness of breath, and palpitations--a sensation that the heart is "flip-flopping." In a person with an otherwise normal heart, atrial fibrillation may go away by itself. Further episodes may be prevented by controlling stress and stopping the use of alcohol, caffeine, nicotine, and cocaine. More intensive medical therapy is often required if atrial fibrillation is caused by other heart conditions.
Contrary to atrial fibrillation, which is possible to live with, ventricular fibrillation is considerably more serious, leading to loss of consciousness wihtin seconds, and death if not treated immediately. The severity is obvious in that when the ventricles twitch, no blood is pumped through the system. Ventricular fibrillation occurs when a premature impulse arrives at the ventricle during a vulnerable period during which the excitability of the cardiac cells is highly variable - some cells are refractive, some are completely recovered, while still others are only able to conduct slow, partial pulses. As a result, potentials are still carried along the ventricle, but in chaotic wavelets that roam around the ventricles, self-sustaining the process. Without the synchronized effort of all the cells, the ventricles don't contract properly and don't pump any blood. Death immediately follows this form of fibrillation unless resuscitation is performed. For a sample EKG waveform, refer to figure 2-42B on page 48 of the text.
Animation of Ventricular Fibrillation
1. True or False: Fibrillation is most likely a result of a re-entry mechanism. 2. If a subject experiences atrial fibrillation: (a) death ensues (b) no blood flows through the system (c) the atria don't contribute to maximizing ventricular filling (d) immediate treatment is necessary 3. To prevent death from ventricular fibrillation, a patient would require (circle any and all that apply): (a) drugs to prolong the refractory period (b) strong electrical shock to the heart (c) a pet to keep them happy (d) cardiopulmonary resuscitation
1. True - fibrillation is cause by the re-entry machanism, where a cardiac impulse re-excites a region in the heart that it already passed through. Re-entry in general takes 2 forms, ordered and random, and it's the random form that is characteristic of fibrillation. 2. (C) - Atrial fibrillation does not constitute a medical emergency, and death is not an issue. It simply prevents the atria from adding the last burst of blood into the ventricles before they contract. The other choices (a),(b), and (d) are all characteristic of ventricular fibrillation. 3. (B) and (D) - CPR is necessary to help at least some blood flow until the heart rhythm returns to normal. Spontaneous resynchronization of the heart during ventricular fibrillation is rare; therefore, a strong, well-placed electrical shock to the heart will polarize all cells simultaneously, putting a halt to the fibrillation, and the SA node resumes it's role as the pacemaker. Choice (a) is more suitable to atrial fibrillation, where time is not an issue, and treatment can be administered over time. Choice (c) is obvious. :-)
For more information on this topic, please refer to Berne & Levy , pp. 47-49
Also, check out the following links that may be helpful:
|syllabus | home | BME | search the web | e-mail Doug|
|1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | Exam #1|
|17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | Exam #2|
|28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | Exam #3|
This page was written by Jason Jopling, a student in this course.
BME 403 Pages maintained by the T.A., Douglas Miles.