Section 3, chapter 15: ecg

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ECGs and Arrhythmias

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Section 3, chapter 15: ecg

  1. 1. Electrocardiogram Section 3, Chapter 15 An electrocardiogram, or ECG (or EKG) is a recording of the electrical changes in the myocardium during the cardiac cycle.
  2. 2. Electrocardiogram P Wave • Represents atrial depolarization • Leads to atrial contraction • Conduction of electrical impulse from right to left and downward QRS Complex • Represents depolarization of ventricles • Leads to ventricular contraction • This massive wave hides the atria repolarization
  3. 3. Electrocardiogram T Wave • Represents repolarization of ventricles
  4. 4. Figure 15.22d. An ECG pattern with the corresponding systole and diastole shown above.
  5. 5. Heart Arrhythmias: normal ECG of a regular heart rhythm at 75 beats per minute Atrial Flutter. Atria fire 250-350 times per minute. For every QRS complex there may be 4 or more P waves.
  6. 6. Examples of Heart Arrhythmias. Arrows indicate p Wave. Bradycardia – cardiac rhythm less than 60 beats per minute. Tachycardia– cardiac rhythm greater than 100 beats per minute.
  7. 7. Examples of Heart Arrhythmias, fibrillation Atrial fibrillation. Instead of contracting, the atria become quivering chambers. The ventricles respond only to impulses that make it to the AV node. Ventricular fibrillation = Life threatening arrhythmia. Ventricles quiver, and are unable to pump blood properly. Requires immediate defibrillation.
  8. 8. Regulation of cardiac cycle The heart rate is controlled intrinsically by the SA node, but sympathetic and parasympathetic fibers alter the rate at which the pacemaker fires.
  9. 9. Cardiac Control Center •Within Medulla Oblongata •Receives sensory impulses from throughout the cardiovascular system and relays motor impulses to heart in response. •Cardioinhibitor & cardioaccelerator reflex centers
  10. 10. • Cardioinhibitor reflex center – Parasympathetic fibers from vagus nerves innervate SA & AV nodes. – Acetylcholine (ACh) released from fibers decreases the firing rates of SA & AV nodes. – Heart rate decreases
  11. 11. • Cardioaccelerator reflex center – Sympathetic fibers from accelerator nerves innervate SA & AV nodes. – Norepinephrine released from fibers increases the firing rates of SA & AV nodes. – Heart rate and force of contraction increases
  12. 12. Cardioinhibitor & cardioaccelerator reflex centers alter the heart rate in response to sensory impulses from receptors Baroreceptors – monitor blood pressure • Within aortic arch and carotid sinuses • Rising blood pressure stimulates cardioinhibitor center
  13. 13. ` Figure 15.24b Illustration of the baroreflex arc End of Section 3, Chapter 15

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