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Cardiac TissueBioengineering 6000 CV PhysiologyElectrophysiology of Cardiac TissueCardiac TissueBioengineering 6000 CV PhysiologyStructureBioengineering 6000 CV PhysiologyCardiac TissueCardiac Tissue• Syncytium (to a point)• Fibers• Anisotropy• Gap junctionsBioengineering 6000 CV PhysiologyCardiac TissueGap Junctionsrg = 4 Ohm-cm2rm = 5000 Ohm-cm2Pore size: 1.5 nmBioengineering 6000 CV PhysiologyCardiac TissueMyocardial Anisotropy• In ventricles ranges from 90–180 degrees of rotation• Rotation more rapid near ventricle walls• Imbrication angle of 3--5 degreesCardiac TissueBioengineering 6000 CV PhysiologyTheory/MechanismsBioengineering 6000 CV PhysiologyCardiac TissueLocal Circuit CurrentsOutsideInsideCharging Currents+++---Depolarizing CurrentsCell MembraneGap Junctions+-+-+-+-Bioengineering 6000 CV PhysiologyCardiac TissuePropagation Example 1t=0t=1t=2t=3t=4Excited regionRefractory regionTimeSpaceBioengineering 6000 CV PhysiologyCardiac TissuePropagation Example 2 (collision)Excited regionRefractory regionTimeSpacet=0t=1t=2t=4t=5t=3v =KD dV/dtRiCmBioengineering 6000 CV PhysiologyCardiac TissuePropagation Velocity• K = constant• D = diameter of fiber (or nerve axon)• dV/dt = rate of rise of action potential•Ri = intracellular resistivity•Cm = membrane capacitance• Approximate but qualitatively correctBioengineering 6000 CV PhysiologyCardiac TissueCardiac Propagation: Essential Summary• Requires a network of connected, excitable cells• Driven by an all-or-nothing phenomena– no superposition (unlike other waves)– collisions result in annihilation– amplitude does not diminish with distance• Anisotropic due to preferential connections between cells– wavefronts are elliptical and not spherical– propagation depends on muscle structure (fiber orientation)• Exhibits refractoriness• Can be blockedBioengineering 6000 CV PhysiologyCardiac TissueElectrophysiology of the Whole Heart• Specialized Conduction system– sinoatrial (SA) Node– atrioventricular (AV) node– Purkinje system• Pacemaker functions– SA Node– AV Node– Purkinje Fibers• The Electrocardiogram (ECG)Bioengineering 6000 CV PhysiologyCardiac TissueCardiac Activation SequenceBioengineering 6000 CV PhysiologyCardiac TissueCardiac Activation SequenceBioengineering 6000 CV PhysiologyCardiac TissueCardiac Activation SequenceBioengineering 6000 CV PhysiologyCardiac TissueCardiac Activation SequenceBioengineering 6000 CV PhysiologyCardiac TissueCardiac Activation SequenceBioengineering 6000 CV PhysiologyCardiac TissueCardiac Activation SequenceBioengineering 6000 CV PhysiologyCardiac TissueAnother View of Conduction SystemBioengineering 6000 CV PhysiologyCardiac TissueCardiac Activation SequenceCardiac TissueBioengineering 6000 CV PhysiologyMeasurementBioengineering 6000 CV PhysiologyCardiac TissueExtracellular PotentialsOutsideInsideCharging Currents+++---Depolarizing CurrentsCell MembraneGap Junctions+-+-+-+-VBioengineering 6000 CV PhysiologyCardiac TissueMeasuring in Cardiac Tissue• Electrodes– extracellular – unipolar versus bipolar– surface, volume, catheter• Cardiac Mapping– multiple recording sites– isopotential maps– isochrone maps• Applications– experimental– clinicalEntry pointElectrode spacing1.0--1.6mmHeart surfaceBioengineering 6000 CV PhysiologyCardiac TissueUnipolar vs. Bipolar• Potential differences between a single electrode site and a remote reference. • Features– Recording field is infinite and uniform in all directions, hence no directional sensitivity– Signals contain far field components and are sensitive to distant electrical activity.– Signal morphology indicates wavefront direction– Morphology depends on choice of referenceBioengineering 6000 CV PhysiologyCardiac TissueUnipolar vs. Bipolar• Potential differences between two closely spaced electrodes.• Features– Recording field falls off quickly, much less sensitive to distance activation fronts, hence recording of local events. – Method is sensitive to directional differences of the wavefront with respect to the axis of the bipole.– Morphology does not indicate wavefront directionBioengineering 6000 CV PhysiologyCardiac TissueElectrogram ExampleUnipolarBipolar ECGBioengineering 6000 CV PhysiologyCardiac TissueCardiac Mapping• Coverage • Sampling Density• Surface or volumeBioengineering 6000 CV PhysiologyCardiac TissueCardiac Needle ElectrodesEntry pointElectrode spacing1.0--1.6mmHeart surfaceRVLVBioengineering 6000 CV PhysiologyCardiac TissueCardiac Sock ElectrodesBioengineering 6000 CV PhysiologyCardiac TissueActivation Sequence: Human Heart• Durrer et al., 1970• Three sites of earliest activation• Anisotropic conduction21 3Bioengineering 6000 CV PhysiologyCardiac TissueOptical Mapping• Fluorescent Dyes– voltage sensitive– calcium sensitive– pH sensitive• Confocal microscopy• Multiple sensors– photodiodes– CCD cameras• Advantages– no impalement or contact– can measure membrane potential• Disadvantages– measures relative change so calibration a challenge– motion artifacts Bioengineering 6000 CV PhysiologyCardiac TissueOptical Mapping Setuphttp://cogprints.org/4242/1/wu.htmBioengineering 6000 CV PhysiologyCardiac TissueOptical Imaging Example (Rat)http://ajpheart.physiology.org/content/290/3/H1298Bioengineering 6000 CV PhysiologyCardiac TissueExample: Spread of ActivationBioengineering 6000 CV PhysiologyCardiac TissueClinical MappingBioengineering 6000 CV PhysiologyCardiac TissueClinical MappingBioengineering 6000 CV PhysiologyCardiac TissueSignal Processing of Cardiac Data• Signal conditioning– Filtering– Baseline correction• Feature extraction– Beat identification– Morphological measurements• Visualization– Space and time preserving– Overlays, interaction, feature presentationBioengineering 6000 CV PhysiologyCardiac TissueSignal Processing of Cardiac DataActivationMin(dV/dt)RecoveryMax(dV/dt)BaselineARIQRSExampleBioengineering 6000 CV PhysiologyCardiac TissueInterpreting ElectrogramsActivation Time• Wavefront approaching = positive potentials• Wavefront passing = rapid downward deflection• Wavefront receding = negative potentialsBioengineering 6000 CV PhysiologyCardiac TissueRoles of Tissue Anisotropy++--0-30 mVVoltage Voltage + CurrentSite of stimulationCardiac TissueBioengineering 6000 CV PhysiologyArrhythmia MechanismsBioengineering 6000 CV PhysiologyCardiac


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U of U BIOEN 6000 - Electrophysiology of Cardiac Tissue

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