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Mam Phys lecture notes

# Mam Phys lecture notes - Lecture 24 The EKG records the...

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Lecture 24 The EKG records the passage of current from one part of the heart to the other and each lead records the component of the total current vector parallel with the lead. Lead one- recording current vectors that are traveling between the shoulders. Each lead gives us a different view of exactly the same event Leads I, II, and III are recording simultaneously don’t look alike b/c they are all different views of the same electrical event (looking at same person but from different angles) No one tells the whole story, so we would need to integrate all the info we got Vector summation of all of the leads to know the total current vector Full blown EKG includes the limb leads and chest leads These are extracellular recordings How come the EKG looks the same if you could hook up the wires randomly? Because everyone who does it hooks up the wires in the very same way to get the same recordings Rule: (for every single one of the leads) -hook up the EKG so that depolarization moving from the right shoulder to the left leg makes the pen go up -if depolarization moving from the left leg to the right shoulder- pen goes down -repolarization moving from the left leg to right shoulder-pen goes up QRS complex- depolarization in the ventricles -QR depolarization is moving upward (right shoulder to left leg) -RS segment moves down (left leg to right shoulder depolarization) -P wave: pen going up and down (depolarization atrial) upward (rt shoulder to left leg) - pathway: 1 st depol moves from the right shoulder to left leg through the atrium then left leg to right shoulder but this is not the case in the atrium! -if we are looking at lead II, the size of the recording reflects the size of the vectorial component parallel to the line connected through the electrode -vector starts small then gets bigger, less and less current moving because it is getting depolarized -direction is constant but amplitude changes in time -current vector stays constant in amplitude but direction changes -overtime we would see a variety of components -the only way to tell is by looking at all of the leads doing the vector sum -in the P Wave we have a vector that changes its amplitude and some direction -baseline (no transmembrane potential of 0mv) telling us electrical fields produced by passage of depol and repol through the heart from cell to cell -pen at baseline, there is no net current passing through the heart all of the cells might be at rest or depol and there’s no way to tell the difference, all cells are in the same electrical state all are depol or repolarized - time b/w the T wave (ventricular repol) and the next P wave pen sits at baseline all the cells are repolarized but if the ST segment is long enough in time we often see that the pen returns to baseline and all of the cells are depolarized in the ventricle -the only fields that are big enough to be recorded from leads on the skin are due to currents passing through contractile fibers to contractile fibers, the (nodal fiber and conducting system)

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