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Intrinsic Cardiac Conduction System Approximately 1% of cardiac muscle cells are autorhythmic rather than contractile 75/min 40-60/min 30/min Intrinsic Conduction System Function: initiate & distribute impulses so heart depolarizes & contracts in orderly manner from atria to ventricles. SA node AV node Bundle of His Bundle Branches Purkinje fibers ECG Deflection Waves (Pacemaker) Atrial repolarization ECG Deflection Waves 60 seconds ÷ 0.8 seconds = resting heart rate of 75 beats/minute 1st Degree Heart Block = P-Q interval longer than 0.2 seconds. ECG Deflection Wave Irregularities Enlarged QRS = Hypertrophy of ventricles ECG Deflection Wave Irregularities Prolonged QT Interval = Repolarization abnormalities increase chances of ventricular arrhythmias. ECG Deflection Wave Irregularities Elevated T wave : Hyperkalemia ECG Deflection Wave Irregularities Flat T wave : Hypokalemia or ischemia Heart Blocks P T Normal ECG QRS 2nd Degree Block 3rd Degree Block Not a QRS for each P wave No P waves. Rate determined by autorhythmic cells in ventricles Cardiac Cycle • All events associated with a single heart beat including atrial systole & diastole followed by ventricular systole & diastole. (V. Systole) Systolic BP Diastolic BP (V. Diastole) ECG Deflection Waves 60 seconds ÷ 0.8 seconds = resting heart rate of 75 beats/minute Frank Starling Law of the Heart • The more cardiac muscle is stretched within physiological limits, the more forcibly it will contract. • Rubber band analogy • Increasing volumes of blood in ventricles increase the stretch & thus the force generated by ventricular wall contraction. • Greater stretch means more blood volume is pumped out, up to physical limits. Frank Starling Law of the Heart = Increased blood volume = increased stretch of myocardium Increased force to pump blood out. Terms, Definitions & Units • Blood Pressure - force generated against arterial walls per unit of area in mm Hg. • Systolic Pressure - peak arterial pressure. Averages about 120 mm Hg in healthy adults. • Diastolic Pressure - lowest arterial pressure. Averages between 70 - 80 mm Hg in healthy adults. • Blood Volume - quantity of blood in cardiovascular system. Varies from 4-5 L. in females to 5-6 L. in males. Terms, Definitions & Units • Cardiac Output - the amount of blood pumped by a ventricle per minute. Units may be in milliliters or Liters per minute. • Heart Rate - number of cardiac cycles per minute. Average for males = 64-72/min. Average for females = 72-80/min. • Stroke Volume - amount of blood pumped out of a ventricle each beat. Average resting stroke volume = 70 ml. Factors influencing blood pressure Blood Pressure = Blood Volume × Peripheral Resistance • Blood volume loss due to injuries, hemorrhages, use of diuretics, etc. = BP • Blood volume increases due to increased water retention from increased ADH production, IVs or transfusions = BP Factors influencing blood pressure Blood Pressure = Blood Volume × Peripheral Resistance Cardiac Output = circulating blood volume Cardiac Output = Heart Rate × Stroke Volume Increased heart rate caused by the release of epinephrine into blood by the adrenal glands = increased cardiac output, which increases circulating blood volume, to increase blood pressure. Factors influencing blood pressure Blood Pressure = Blood Volume × Peripheral Resistance Peripheral Resistance affected by: •blood viscosity (thickness) (Polycythemia) •diameter of vessels (vasoconstriction/vasodilation) Vasoconstriction = diameter = resistance = BP Vasodilation = diameter = resistance = BP •elasticity of arterial walls Elastic Arterial Walls = BP Homeostatic Blood Pressure Regulation Mechanisms • Medullary Reflex Centers: – Cardioacceleratory - increases heart rate – Cardioinhibitory - decreases heart rate – Vasomotor - changes diameter of vessels • Baroreceptors in aortic arch & carotid sinuses: – sensitive to changes in blood pressure. BP - Stimulates Cardioinhibitory center to heart rate & Vasomotor center to diameter. BP - Stimulates Cardioacceleratory center to heart rate & Vasomotor center to diameter. Homeostatic Blood Pressure Regulation Mechanisms • Medullary Reflex Centers: – Cardioacceleratory - increases heart rate – Cardioinhibitory - decreases heart rate – Vasomotor - changes diameter of vessels • Chemoreceptors in aortic bodies & carotid bodies: – sensitive to changes in CO2 & O2 in blood. in CO2 or in O2 stimulates Vasomotor center to diameter (vasoconstrict) of vessels to BP. in CO2 stimulates Vasomotor center to diameter (vasodilate) of vessels to BP. Aneurysm • Weakness of the wall of an artery causing an abnormal enlargment or bulge. • The aorta or the arteries that supply the heart, brain, legs or kindeys are most commonly affected. Angina Pectoris • Medical term for chest pain due to coronary heart disease. • It occurs when the myocardium doesn’t get as much blood (Oxygen) as it needs. • Insufficient blood supply is called ischemia. • May initially occur during physical exercise, stress, or extreme temperatures. • It is a sign of increased risk of heart attack. Hypertension • High blood pressure. • Sustained arterial blood pressure of 140/90 mm Hg or above. • Rising diastolic pressure generally indicative of progressive hardening of arteries. • Since the heart must work harder to pump blood against higher pressures, there is increased risk of a cardiovascular accident. Hypotension • Abnormally low blood pressure. • Sustained systolic blood pressure of below 100 mm Hg. • Generally associated with lower risk of cardiovascular accidents & long life providing that the tissues are adequately perfused.. Circulatory Shock • Blood vessels inadequately filled to enable normal circulation & supply of O2 & nutrients. • May result in death of cells & damage to organs. • • • • • Common Types: Hypovolemic - severe blood loss Cardiogenic - heart (pump) failure Vascular - excessive vasodilation Septicemic - vasodilation due to bacterial toxins produced during an infection. Atherosclerosis (Arteriosclerosis) • Narrowing and hardening of arteries and impairment of blood flow due to the deposition of fatty materials and calcium in their walls. • Risk factors include: – – – – – smoking inactivity diabetes high blood cholesterol personal or family history of heart disease Arteriosclerosis (Atherosclerosis): All images copyright © Camera M.D. Studios. Special thanks to Gregory Curfman, M.D.. Acknowledgements Most of the figures used in this presentation came from the Benjamin Cummings Digital Library Version 2.0 for Human Anatomy & Physiology, Fifth Edition. Other figures came from public domain internet sources and software in the possession of the author.