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Mechanical & electrical activity of the heart

Mechanical & electrical activity of the heart






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    Mechanical & electrical activity of the heart Mechanical & electrical activity of the heart Presentation Transcript

    • Mechanical & Electrical Activity of the Heart Kathleen Brownrigg, BSc, RN, MScN April 2011
    • Objectives• Define the cardiac cycle• Identify factors affecting cardiac output• Describe the mechanical and electrical activity of the heart
    • Cardiac cycle = Cardiac outputFilling + Contraction = Cardiac Cycle •Systole •Diastole Lub Dub
    • Cardiac Cycle
    • Mechanical Activity of the Heart Diastole Systole Semilunar valves open Atrioventricular Valves closed
    • Arteries and Veins
    • Normal Intracardiac Pressures
    • Cardiac output CO = 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 system heart with eachNormal (children) = -Environment contraction. -Electrolytes Affected by: …ect… PRELOAD AFTERLOAD CONTRACTILITY
    • Preload• Frank Starling’s Law of  Degree of fiber stretch as a result of a quantity of blood placed on the Heart the muscle prior to contraction.  The more diastolic volume or fibre stretch at end diastole, the greater the force of the next contraction during systole  Measured by LVEDP - left ventricular 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.
    • AfterloadThe resistance to which the ventricles must overcome to eject blood.Resistance to blood flow as it leaves the ventriclesSVR on the Lt sidePVR on the RT side As resistance , stroke volume 
    • ContractilityThe ability of the myofibrils to shorten in length and produce a contraction.Not measured directlyFactors that affect contractility: PRELOAD AFTERLOAD DRUGS CARDIAC OXYGENATION FUNCTIONAL MYOCARDIUM
    • Factors Affecting Cardiac Output
    • Electrical Activity of the Heart
    • Conduction System of the Heart
    • 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)
    • ECG Waves and Intervals
    • Myocardial Contractile Cell: Action Potential• Phase 0: Depolarization phase, Na+ channels open and rapid influx of Na+ into cells• Phase 1: Repolarization phase (brief) cause by inactivating 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 due to outward K+ current• Phase 4: Resting membrane potential, Na+ moves out of cell and K+ moves into cell via an active pump (Na+/K+ pump)
    • ECG Cycle
    • Summary of Electrical Pathway• Distinct ion channels drive the depolarization and repolarization of cardiac cells during action potential.• Na+ channels (dominant in cardiac cells) depolarize quickly, but Ca++ channels (dominant in the SA and AV nodes) depolarize slowly.• The cell cannot respond to stimuli during the refractory period, except late in phase 3 when it can respond to strong stimuli.• The cell can respond again when it is completely repolarized.
    • Next… ECG Level IClass one: Saturday April 16, 2011
    • Questions and Comments Questions and Comments