11Detecting the BodyDetecting the Body’’s Electricals ElectricalSignalsSignalsPhysics 208September 15, 2006Professor Richard MossDepartment of PhysiologyUW School of Medicine and Public Health23Information from ECGInformation from ECG••Heart rate, rhythmHeart rate, rhythm••Pattern of conductionPattern of conduction••DiseasesDiseases––arrhythmiasarrhythmias––conduction abnormalitiesconduction abnormalities––hypertrophy (not always bad)hypertrophy (not always bad)––ischemiaischemia––necrosisnecrosis4ECG LeadsECG Leads••Lead = any 2 electrodes on skinLead = any 2 electrodes on skin••potential differences between electrodespotential differences between electrodes••Recording electrode indicated as (+)Recording electrode indicated as (+)••Reference electrode indicated as (-)Reference electrode indicated as (-)••pd = (recording pd = (recording elecelec) ) –– (reference (reference elecelec))••Between 1 and 15 leads!Between 1 and 15 leads!••12-lead ECG is standard12-lead ECG is standard5Normal 12-lead ECGNormal 12-lead ECG6Lead 1 ECG27891011123131415QuestionWhat would happen to the amplitude of awave if the mass of tissue undergoingdepolarization doubled?16QuestionWhat would happen to the amplitude of awave if the mass of tissue undergoingdepolarization doubled?amplitude would approximately doubledue to doubling of charge density17QuestionWhat would happen to the amplitude of thewave if depolarization were perpendicularto the lead rather than parallel to the lead?18QuestionWhat would happen to the amplitude of thewave if depolarization were perpendicularto the lead rather than parallel to the lead?amplitude would be zero419Key rules of electrocardiography• Depol toward lead → positive wave20Key rules of electrocardiography• Depol toward lead → positive wave••Repol Repol toward lead toward lead → → negative wavenegative wave21Key rules of electrocardiography• Depol toward lead → positive wave• Repol toward lead → negative wave22 Key rules of electrocardiography• Depol toward lead → positive wave• Repol toward lead → negative wave••Depol Depol perpendicular to lead perpendicular to lead → → 00amplitudeamplitude23Key rules of electrocardiography• Depol toward lead → positive wave• Repol toward lead → negative wave• Depol perpendicular to lead → 0amplitude••↑ ↑ tissue mass tissue mass → → ↑ ↑ amplitudeamplitude2452526Lead I ECGLead I ECGP-wave:P-wave:atrial atrial depolarizationdepolarizationQRS complex:QRS complex:ventricular depolarizationventricular depolarizationT-wave:T-wave:ventricular ventricular repolarizationrepolarization2728Precordial leads29Precordial Precordial LeadsLeads••6 leads in transverse plane6 leads in transverse plane––VV11 to right of sternum over right ventricle to right of sternum over right ventricle––VV66 on left side over lateral wall of left ventricle on left side over lateral wall of left ventricle••Record activity front to back, back to frontRecord activity front to back, back to front••Reference is sum of std limb leadsReference is sum of std limb leads30Axis of ventricular depolarizationAxis of ventricular depolarization• Net direction of depolarization• Usually toward lower left quadrant• Pathologies shift the axis:– left ventricular hypertrophy– right ventricular hypertrophy– death of tissue, e.g., heart attack• Pregnancy shifts the axis631Left Ventricular Hypertrophy (LVH)Left Ventricular Hypertrophy (LVH)32There is a complicated hypothesis which usually entails anelement of mystery and several unnecessary assumptions.This is opposed by a more simple explanation which containsno unnecessary assumptions. The complicated one is alwaysthe more popular one at first, but the simpler one, as a rule,eventually is found to be correct. This process frequentlyrequires 10 to 20 years to complete.J.H. Northrop33There is a complicated hypothesis which usually entails anelement of mystery and several unnecessary assumptions.This is opposed by a more simple explanation which containsno unnecessary assumptions. The complicated one is alwaysthe more popular one at first, but the simpler one, as a rule,eventually is found to be correct. This process frequentlyrequires 10 to 20 years to complete.The reason for this long time lag was explained by MaxPlanck. He remarked that “Scientists never change theirminds, but eventually die.”J.H.
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