The Conduction System of the Heart

1. The Conduction System of the Heart Anatomy The Circulatory System

2. The Conduction System • The heart is controlled by the ANS (autonomic nervous system) – it increases/decreases contraction, but it does NOT initiate it. • The heart has its own regulating system = conduction system • The conduction system is composed of specialized muscle tissue that generates action potentials within cardiac tissue.

3. Components of the conducting system • SA Node • AV Node • Conducting cells (distribute the stimulus)

4. Components of the Conduction System 1. SA NODE • Located in the right atrial wall below the opening to the superior vena cava • Initiates/establish heartbeat pacemaker cells • Known as the natural pacemaker • Average = 75 beats/minute 2. AV NODE • Located in the interatrialseptum (floor of the right atrium) • Receives the action potential from the SA node • Action potential slows considerably  allows time for the atria to empty into the ventricles

5. Components of the Conduction System 3. AV BUNDLE • Receives impulse from the AV node • Located at the interventricular septum • Passes impulse onto the bundle branches 4. BUNDLE BRANCHES • Located below the AV bundle • Passes impulse onto the conduction myofibers 5. CONDUCTION MYOFIBERS (Purkinje Fibers) • Located in the ventricular myocardium • Receives action potential/impulse from bundle branches and passes it onto ventricular myocardial cells to complete ventricle contraction

6. ECG or EKG • Impulses can be detected on the body surface and recorded with an electrocardiograph. • The recording that is made, the electrocardiogram (ECG/EKG), traces the flow of current through the heart. • Each time the heart beats, a wave of depolarization radiates through the atria, pauses at the AV node, then travels down the interventricular septum to the apex, turns, and spreads through the ventricular myocardium toward the base • An ECG integrates electrical information obtained by placing electrodes at different locations on the body surface • The typical ECG has three recognizable waves: • 1st Wave: P wave (depolarization of the atria) • 2nd Wave: QRS complex (ventricular depolarization) • 3rd Wave: T wave (ventricular repolarization)

7. Abnormalities in the shape of the wave and changes in the timing could indicate something is wrong. Example: An excessively large QRS complex often indicates that the heart has become enlarged.

8. The Cardiac Cycle • Each heartbeat is followed by a brief resting phase, allowing time for the chambers to relax and prepare for the next heartbeat • The period between the start of one heartbeat and the beginning of the next is a single cardiac cycle • Includes alternating periods of contraction and relaxation

9. The Cardiac Cycle Phases Systole: the phase of contraction of a chamber (the chamber contracts and pushes blood into an adjacent chamber or into an arterial trunk) Diastole: the phase of relaxation (the chamber fills with blood and prepares for the next cardiac cycle)

10. Blood Pressure • Pressure that blood exerts against the inner walls of the blood vessels • Systolic Number– This is the TOP number in your blood pressure…related to the pressure of contraction in the heart that is placed on the inside of the vessels • Diastolic Number– This is the BOTTOM number in your blood pressure…related to the relaxed phase of your heart that is then transferred to your vessels • Normal blood pressure is 120/80

11. Cardiac Output • Cardiac Output: the amount of blood pumped by the left ventricle in one minute • It is determined by: 1. The amount of blood pumped by the left ventricle during each beat (stroke volume) 2. The number of heart beats per minute • The amount of blood ejected by a ventricle during each contraction (single beat) is called = stroke volume • Cardiac output is an indication of the blood flow through peripheral tissues; without adequate blood flow, homeostasis cannot be maintained

12. Cardiac Output • For an adult at rest… • Stroke volume = 80 mL (average) • Heart rate = 75 beats per minute (average) • Therefore, Cardiac Output (CO) = Stroke Volume x Beats per Minute CO = 80 mL/beat x 75bpm CO = 6000mL/min (6L/min) • Factors that increase or decrease stroke volume and/or heart rate will also tend to increase or decrease cardiac output, respectively

13. Arrhythmia • Abnormal heart rhythm (irregular heartbeats and heart rates) • Bradyarrhythmia(Bradycardia) – heart rates that are slow (less than 50bpm) • Tachyarrhythmia (Tachycardia) – rapid heart rates (faster than 100 bpm) • About 5% of healthy individuals experience a few abnormal heartbeats each day

14. Causes of Arrhythmia • Coronary artery disease (partial or complete blockage of coronary circulation) • Electrolyte imbalances in your blood • Injury from a heart attack • Healing process after heart surgery • Example: Fibrillation (atrial or ventricular) – rapid, uncoordinated shuddering of the heart

15. High Blood Pressure/Hypertension(Silent Killer) • Normal bp = 90/60 (birth) and 120/80 (adults) • High blood pressure = 140/90 consistently • High blood pressure can threaten healthy arteries and lead to life-threatening conditions (leading cause of stroke and major cause of heart attack) • Significantly increases the workload on the heart, and the left ventricle gradually enlarges • More muscle mass means a greater oxygen demand • Places physical stress on the walls of blood vessels

16. Hypertension

17. What causes hypertension? • Smoking • Being overweight or obese • Lack of physical activity • Too much salt in the diet • Too much alcohol consumption • Stress • Genetics • Older age

18. Atherosclerosis • Usual cause of heart attacks & strokes • High blood pressure, smoking or high cholesterol damage the endothelium (lines the arteries) & cholesterol plaque forms • The arteries harden and narrow and block blood flow • Plaque behaves in different ways: can stay within the artery wall, cause blockages, or rupture which allows blood to clot in the artery

19. Heart Attack/Myocardial Infarction • Part of the coronary circulation becomes blocked, and cardiac muscle cells die from lack of oxygen • Coronary heart disease causes the arteries to become narrow and blood cannot flow as well • Fatty matter, calcium, proteins, and inflammatory cells build up within the arteries to form plaque

20. Heart attack continued • When the plaque that forms becomes hard, the outer shell cracks, and blood clots form around the plaque • If a blood clot blocks the artery, the heart muscle becomes “starved” for oxygen • Within a short time, death of heart muscle cells occurs, causing permanent damage

21. Risk Factors of a Heart Attack 1. Smoking 2. High blood pressure 3. High blood cholesterol levels 4. Diabetes 5. Obesity 6. Sedentary lifestyle 7. Genetics

22. Pulmonary Edema • An abnormal buildup of fluid in the air sacs of the lungs, which leads to shortness of breath • Caused by heart failure, which causes pressure in the veins going through the lungs to rise • As pressure in the blood vessels increase, fluid is pushed into the air spaces (alveoli) in the lungs • This fluid interrupts normal oxygen movement in the lungs

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