Detecting the BodyDetecting the Body’’s Electricals ElectricalSignalsSignalsPhysics 208October 12, 2007Professor Richard MossDepartment of PhysiologyUW School of Medicine and Public HealthEquivalent circuit of excitablecell membraneInformation 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––necrosisnecrosisECG 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 elec) pd = (recording elec) –– (reference elec) (reference elec)••Between 1 and 15 leads!Between 1 and 15 leads!••12-lead ECG is standard12-lead ECG is standardNormal 12-lead ECGNormal 12-lead ECGLead 1 ECGQuestionWhat would happen to the amplitude of awave if the mass of tissue undergoingdepolarization doubled?QuestionWhat would happen to the amplitude of thewave if depolarization were perpendicularto the lead rather than parallel to the lead?Key rules of electrocardiography• Depol toward lead → positive wave• Repol toward lead → negative wave• Depol perpendicular to lead → 0amplitude••↑ ↑ tissue mass tissue mass → → ↑ ↑ amplitudeamplitudeLead I ECGLead I ECGP-wave:P-wave:atrial depolarizationatrial depolarizationQRS complex:QRS complex:ventricular depolarizationventricular depolarizationT-wave:T-wave:ventricular repolarizationventricular repolarizationPrecordial leadsPrecordial LeadsPrecordial Leads••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 leadsNormal 12-lead ECGNormal 12-lead
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