1. Mechanical & Electrical Activityof the HeartKathleen Brownrigg, BSc, RN, MScNApril 2011

2. Objectives Define the cardiac cycle Identify factors affecting cardiac output Describe the mechanical and electrical activityof the heart 3. Cardiac cycle = Cardiac outputFilling + Contraction = Cardiac CycleSystoleDiastole Lub Dub 4. Cardiac Cycle 5. Mechanical Activity of the HeartDiastole Systole Semilunar valves open Atrioventricular Valves closed 6. Arteries and Veins 7. Normal Intracardiac Pressures 8. Cardiac outputCO = HR X SVThe amount of blood ejected Affected by: The amount of bloodfrom the heart per minute. -Physical factors ejected from theNormal (adults)= 4-6L/min -Nervous systemheart with eachNormal (children) = -Environment contraction.-ElectrolytesAffected by:ectPRELOAD AFTERLOAD CONTRACTILITY 9. Preload Frank Starlings Law of Degree of fiber stretch as a resultof a quantity of blood placed onthe Heart the muscle prior to contraction. The more diastolic volume orfibre stretch at end diastole, thegreater the force of the nextcontraction during systole Measured by LVEDP - leftventricular end diastolic pressure- prior to systole (max. full) Normal value 6-12 mmHgAbility of the muscle fibers to stretch The > stretch = > contractilityaccording to incoming volume. If preload increases so does C.O. 10. AfterloadThe resistance to which the ventricles must overcome to eject blood.Resistance to blood flow as it leaves the ventriclesSVR on the Lt sidePVR on the RT side As resistance , stroke volume 11. ContractilityThe ability of the myofibrils to shorten in length and produce a contraction.Not measured directlyFactors that affect contractility: PRELOAD AFTERLOAD DRUGS CARDIAC OXYGENATION FUNCTIONAL MYOCARDIUM 12. Factors Affecting Cardiac Output 13. Electrical Activity of the Heart 14. Conduction System of the Heart 15. ECG Waves and IntervalsP wave: the sequential activation (depolarization) of the right and left atriaQRS complex: right and left ventricular depolarization (normally theventricles are activated simultaneously)ST-T wave: ventricular repolarization U wave: origin for this wave is not clear - but probably represents"afterdepolarizations" in the ventriclesPR interval: time interval from onset of atrial depolarization (P wave) toonset of ventricular depolarization (QRS complex)QRS duration: duration of ventricular muscle depolarizationQT interval: duration of ventricular depolarization and repolarizationRR interval: duration of ventricular cardiac cycle (an indicator ofventricular rate)PP interval: duration of atrial cycle (an indicator of atrial rate) 16. ECG Waves and Intervals 17. Myocardial Contractile Cell:Action Potential Phase 0: Depolarization phase, Na+ channels open andrapid influx of Na+ into cells Phase 1: Repolarization phase (brief) cause byinactivating Na+ influx and activation of outward K +current. Phase 2: Plateau phase cause by slow inward Ca++current and decreased K+ efflux. Phase 3: Rapid repolarization to resting potential dueto outward K+ current Phase 4: Resting membrane potential, Na+ moves outof cell and K+ moves into cell via an active pump(Na+/K+ pump) 18. ECG Cycle 19. Summary of Electrical Pathway Distinct ion channels drive the depolarization andrepolarization of cardiac cells during actionpotential. Na+ channels (dominant in cardiac cells)depolarize quickly, but Ca++ channels (dominantin the SA and AV nodes) depolarize slowly. The cell cannot respond to stimuli during therefractory period, except late in phase 3 when itcan respond to strong stimuli. The cell can respond again when it is completelyrepolarized. 20. NextECG Level IClass one: Saturday April 16, 2011 21. Questions and Comments Questions and Comments