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.
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
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
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)
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)
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