Preparatory Activity: The Electrical System of the Heart

Transcription

1 Preparatory Activity: The Electrical System of the Heart 1. Before class label this heart diagram using each bold term described below: The SA node (sinoatrial node) is located in the right atrium near the entrance of the superior vena cava. The SA node contains cells that spontaneously generate action potentials at a rate of beats/minute. Action potentials propagate throughout the atrial myocardium from cell to cell via intercalated discs and through specialized internodal pathways. (no label) The AV node (atrioventricular node) is located at the junction between the atria and ventricles. Action potentials are propagated through the AV node very slowly. The AV node contains cells that spontaneously generate action potentials at a rate of beats/minute. The AV bundle in the interventricular septum receives electrical activity from the AV node. This is the only pathway for electrical activity to move from the atria to the ventricles. Action potentials propagate from the AV bundle through the bundle branches to Purkinje fibers, which are large-diameter cells that propagate action potentials very rapidly to myocardial cells throughout the ventricles. Purkinje fibers spontaneously generate action potentials at a rate of beats/minute.

2 Model 1: Sequencing Electrical Excitation in the Heart (darkened areas indicate electrical excitation) 2. Examine Model 1 and describe what is happening in each of the figures (A-E). In your descriptions include the names of specific intrinsic electrical system structures when they are involved. A. rest B. SA node generates an action potential which begins to spread over atria C. atrial myocardium is excited D. AV bundle and bundle branches are excited E. both ventricles are excited via Purkinje fibers 3. Before continuing, decide as a group which part of the intrinsic electrical system serves as the normal pacemaker for the heart. Explain your answer. The SA Node is the normal pacemaker because it has the fastest frequency of action potentials. 4. A. Mechanical events of the heart must be preceded by an electrical event. Explain why. Electrical events, i.e. action potentials, trigger muscle contractions by changing membrane permeability to calcium (Ca +2 ) ions. B. Depolarization and repolarization are the two electrical events of the heart muscle. What do depolarization and repolarization mean? Depolarization means the myocardium is becoming electrically excited; the membrane potential becomes positive during this early phase of the action potential. Repolarization means the membrane potential of the myocardium is returning to its original, resting level. What mechanical event follows each of these electrical events? Depolarization, as described in 4A, triggers the increase in membrane permeability to calcium ions. The increased calcium ion concentration leads to interaction of actin and myosin; muscle contraction occurs. Repolarization is return of the membrane potential to resting levels. Membrane permeability to calcium ions decreases, calcium ion concentration decreases, interaction of actin and myosin returns to resting state; muscle relaxation occurs. C. Predict the mechanical event that follows figure C. atrial contraction D. Predict the mechanical event that follows figure E. ventricular contraction

3 5. The part of the intrinsic electrical excitation system with the fastest rate of spontaneously generated action potentials serves as the pacemaker. If the normal pacemaker stopped functioning, how would heart rate be affected? Explain which cells would take over the role of pacemaker. Pacemaker cells in the AV Node would become the pacemaker resulting in a slower heart rate. 6. Why is it important for impulses from the atria to be delayed at the AV node before passing to the AV bundle? The delay ensures that atrial contraction precedes ventricular contraction; this is important for ventricular filling. 7. Predict potential consequences of a heart attack causing blockage of the AV node. Blocking action potentials traveling through the AV node is heart block. Third degree or complete heart block occurs when no impulses travel through the AV node. The SA node may serve as the pacemaker for the atria, and a group of cells in the AV node or AV bundle may serve as the pacemaker for the ventricles. An artificial pacemaker is typically required because ventricular rhythm is so slow. Model 2: Electrocardiogram (EKG or ECG) The electrocardiogram (ECG) records electrical activity in the heart. Scientists and clinicians make assumptions about mechanical activity based on the ECG, but it does not directly measure mechanical events. The P wave represents atrial depolarization; the QRS complex represents ventricular depolarization; the T wave represents ventricular repolarization. 8. Name the part of the ECG representing: A. atrial depolarization P wave B. ventricular depolarization QRS complex C. ventricular repolarization T wave 9. Atrial repolarization is usually not visible on the ECG. Predict during which wave/complex atrial repolarization occurs. Atrial repolarization occurs during the QRS complex. 10. Place an arrow on the ECG recording in Model 2 to indicate depolarization of the SA node.

4 Model 3: ECG and Blood Pressure Changes during Two Heart Beats 11. Model 3 shows an ECG and blood pressure changes during two heart beats. Name the three blood pressure locations shown in Model 3. left atrium, left ventricle, aorta 12 A. Describe the change in left atrial pressure that occurs following the P wave. left atrial pressure increases B. Explain what causes this change. P wave = atrial depolarization atrial contraction increased left atrial pressure 13 A. Describe the change in left ventricular pressure that occurs following the R wave (QRS complex). left ventricular pressure increases B. Explain what causes this change. QRS complex = ventricular depolarization ventricular contraction increased left ventricular pressure 14 A. Describe the change in left ventricular pressure that occurs following the T wave. left ventricular pressure decreases B. Explain what causes this change. T wave = ventricular repolarization ventricular relaxation decreased left ventricular pressure 15. Based on the ECG and aortic pressure, which small letter (a-f) in Model 3 represents when the ventricle starts ejecting blood into the aorta? Explain. Ejection starts at letter c. Ventricular contraction begins during the QRS complex, but aortic pressure does not rise until blood leaves the left ventricle and enters the aorta at letter c. (The left ventricle starts to contract at letter b, but the aortic valve is still closed. The interval b-c is isovolumetric ventricular contraction.)

5 Voltage in Millivolts Model 4: ECG and Selected Arrhythmias ECGs are helpful in determining changes and irregularities (arrhythmias) in the heart s excitation-conduction system. Below are 4 ECG recordings: a normal heart rate of 75 beats/min and three examples of normal changes in SA Node activity. Time in Milliseconds 16. Lance Armstrong has a resting heart rate of 35 beats/minute. Which recording above could be Lance s at rest? Explain. Sinus bradycardia = heart rate <60 bpm 17. When Lance is pedaling up a mountainside, his heart rate is 150 beats/minute. Which recording above could be Lance s during strenuous exercise? Explain. Sinus tachycardia = heart rate >100 bpm 18. A. Most people have a change in heart rate associated with breathing, e.g. heart rate often increases with inhalation and decreases with exhalation. Which recording above shows this condition? Sinus arrhythmia B. Do you have the condition described in 18A? How do you know? Probably (nearly everybody does); Can tell by checking your pulse and noting rate changes relative to respiration

6 19. People have been known to live without atrial depolarization. What would an ECG look like in a person with this condition? Write a sentence or draw an ECG to explain your answer. There would be no P wave if there were no atrial depolarization. 20. Given that people can live without atrial depolarization, do you think people can live without ventricular depolarization? Explain. No, because ventricular depolarization triggers the ventricular myocardium to contract, and without ventricular contraction there would be no blood pumped out of the heart. Without medical treatment people will die within a few minutes. MODEL 5: Ventricular Fibrillation Basketball player Hank Gathers collapsed and died during a college basketball game. The cause of his collapse was an irregular heartbeat. He suffered from exercise-induced ventricular tachycardia but developed ventricular fibrillation during the game. An automated external defibrillator (AED) was used to try and treat him for this condition. 21. Describe how this ECG of ventricular fibrillation compares to the normal ECG in Model 4. No normal P-QRS-T waves. Very irregular electrical activity. 22. What is the heart rate of the ECG shown in Model 5? Explain. Undetectable or Zero 23. Ventricular fibrillation (V-fib) is a condition of uncoordinated contraction of the cardiac muscle of the ventricles, making them quiver rather than contract properly. People who are not health professionals usually cannot feel a pulse. Such an arrhythmia is only confirmed by an ECG. As a group, explain why this is a medical emergency that requires prompt treatment. No blood leaves the heart (cardiac output). Blood pressure falls and flow to the body (including the brain) decreases.

7 24. Is V-fib the same as or different from the person in question 20 who had no ventricular depolarization? Explain. Although the outcome (no blood leaving the heart and death without medical treatment) is the same, the mechanism is different. In V-fib the ventricular cells are depolarizing and contracting but in such an asynchronous manner that no blood leaves the heart. With no ventricular depolarization there is no contraction of ventricular cells. 25. An automated external defibrillator or AED is a portable electronic device that automatically diagnoses the potentially life threatening cardiac arrhythmias of ventricular fibrillation and ventricular tachycardia. It is also able to treat them through defibrillation, which stops the arrhythmia, allowing the heart to reestablish an effective rhythm. List three places in your community that you might find an AED. School, Mall, Airports, Churches, YMCA, etc.