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Lecture 17 EKG

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Heart rate Factors Affecting CO Parasympathetic activity decreases HR Sympathetic activity increases HR Stroke volume Depends on force generated by cardiac muscle during contraction Force is affected by: Length-tension tension relation As sarcomere lengthens, tension during contraction increases Starling s law Contractility Controlled by nervous and endocrine systems Contractility increases as available calcium increases

Fig 14.29 Silverthorn 2 nd Ed

Fig 14.27 The Wiggers diagram Silverthorn 2 nd Ed

Introduction Scalar EKG Vector EKG Outline Depolarization of Heart Electric Potential on Body Surface Lead Vectors and Einthoven s Triangle Leads in Horizontal Plane Interpretation of Vector EKG EKG Interpretation Rate Rhythm Axis Blocks

Reading Assignment http://endeavor.med.nyu.edu/student- org/erclub/ekghome.html

Question Orientation of heart in chest cavity?

Introduction The action potential of the heart: Generates an electrical potential field Can be measured on the body surface Scalar EKG Change of potential between 2 points on body vs. time Vector EKG EGK provides information about: Anatomic orientation of heart Relative size of chambers Disturbances of rhythm and conductance Extent and location of ischemic damage to myocardium Effects of altered electrolyte concentrations Influence of certain drugs (e.g. digitalis)

Main Features: Scalar EKG P wave: atrial depolarization QRS complex: ventricular depolarization T wave: ventricular repolarization Magnitude of EKG: 5-10 mv Magnitude of ventricular depolarization: 100 mv

Fig 2.39 Sinoatrial rythms Cardiovascular Physiology

Vector EKG Heart vector: P (t) At rest: Cardiac myocyte has membrane potential of -90 mv AP leads to depol Wavefront of depol spreads across heart, v~0.5-1m/s Can represent this sep of charge as an electric dipole moment When the entire cell is resting, volume average of these dipole moments is zero When the wave of depol spreads, this is not longer true

Heart vector: P (t) Vector EKG At rest: Cardiac myocyte membrane potential of -90 mv Can represent this separation of charge as a dipole moment At rest, volume average of dipole moment is zerp

Fig 2.71a Double layer associated with a single cell Benedek

Heart vector: P (t) Vector EKG Wave of depolarization: AP leads to depol Wavefront of depol spreads across heart v~0.5-1m/s Can represent wavefront of depol as electric dipole Magnitude and direction changes with time

Fig 2.74 Benedek

Fig 2.76 Benedek

Fig 2.77 Benedek

Fig 2.79 Benedek

Fig 2.80 Benedek

Heart vector: P (t) Vector EKG Orientation of heart vector in depolarization: What effect does this produce at body surface?

Electric Potential on Body Surface Assume: Electrical activity of heart acts at point at center of chest Chest is spherical, radius R Want: Electric potential (r, (r, )

Fig 2.81 Benedek

Electric Potential on Body Surface Electric Potential on Body Surface LaPlace s LaPlace s equation: equation: 3 2 2 4 3 4 cos 3, 0 0 ), ( R R R R P R P R r r o o R

Lead Vectors Einthoven s Triangle Place three electrodes: RA, LA, LL V 1 = LA RA V 2 = LL - RA V 3 = LL LA When we measure potential in lead 1 versus time, we measure: P L 1

Fig 2.38 Cardiovascular Physiology

6 Frontal Leads Standard EKG: Use three additional leads in frontal plane avr avl avf Connect three Einthoven leads to a central point with 5000 resistors, create single lead Take difference between this terminal and individual leads in Einthoven triangle

Fig 2.85 Benedek

Fig 14.5 Keener

EKG Interpretation Look for leads with largest and smallest deflections Lead vector with largest mean amplitude is most parallel to heart dipole Lead vector with smallest mean amplitude is most perpendicular to heart dipole

Questions: In normal heart, which leads see largest QRS complex amplitude? In normal heart, which leads see smallest QRS complex amplitude?

Fig 14.6 Keener

6 Leads in Horizontal Plane All connected to central terminal of Einthoven leads

Fig 14.7 Keener

Interpretation of Vector EKG Ventricular Hypertrophy? MI? LV Hypertrophy? RV Hypertrophy?

Fig 14.8 Keener

LVH http://medlib.med.utah.edu/kw/ecg/ecg_outline/lesson8/index.html#lvh

Fig 14.9 Keener

RVH http://medlib.med.utah.edu/kw/ecg/ecg_outline/lesson8/index.html#lvh

Interpretation of Vector EKG Scalar EKG: Rate? Rhythm? Vector EKG: Axis If I and avf are positive, then axis is normal

Interpretation of Vector EKG Blocks: SA node block Missed beat AV node block Primary: PR int > 0.2 s Secondary: more than 1 P wave before each QRS Tertiary: complete dissociation between P waves and QRS complexes Ischemia ST segment elevation or depression Ventricular Fibrillation

SA Node Block http://medlib.med.utah.edu/kw/ecg/mml/ecg_junctional.html http://medlib.med.utah.edu/kw/ecg/ecg_outline/lesson8/index.html#lvh

AV Node Block http://www.ecglibrary.com/chb4.html

Ischemia http://medlib.med.utah.edu/kw/ecg/ecg_outline/lesson8/index.html#lvh

Atrial Fibrillation http://www.ecglibrary.com/af_fast.html

Ventricular Fibrillation http://medlib.med.utah.edu/kw/ecg/mml/ecg_v_fib.html

Outline Introduction Scalar EKG Vector EKG Depolarization of Heart Electric Potential on Body Surface Lead Vectors and Einthoven s Triangle Leads in Horizontal Plane Interpretation of Vector EKG EKG Interpretation Rate Rhythm Axis Blocks