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BIOC33H3 (100)
Lecture

Lecture 2


Department
Biological Sciences
Course Code
BIOC33H3
Professor
Stephen Reid

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BIOC33 H3 Lecture 2
ECG- measure of the surface electrical activity from the heart allowing to quantify heart
rate and seeing how heart activity can change; looking at heart function and cardiovascular
activity
3 components of ECG (+/- deflection) are important because they tell us about the heart
function
Electrical Axis- heart depolarizes, electrical activity going off in all directions ultimately
there is a mean axis at which the electrical activity flows and that axis which goes at about
60 degrees from the middle of heart can shift left or right and shifting is important for
determining heart disease
ECG can also be used to determine heart rate :
Bradycardia: decrease in heart rate
Tachycardia: increase in heart rate
Arrhythmias: rhythm can be disrupted by processes that are atrial or ventricular in origin;
Ventricular in origin, will be more problematic as compared to atrial
Sequence activation disorders:
So we know that activity starts are sinoatrial node, goes to the intermodal pathways, then
down to the atrioventricualr node, down the branch bundles and up to the purkinje fibres;
Normal sequence of events can be disrupted with electrical activity starting somewhere
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else; or if pathways are blocked; Branch bundle block (RBBB or LBBB)
ECG can also tell us if the heart has grown or gone under hypertrophy; heart growing in
mass will lose its ability to contract properly; any kind of strain on the heart will impinge on
its ability to pump blood effectively
ECG can also be used for coronary circulation disrupted; heart can become ischemic; heart
tissue dying- cardiac infarct (heart attack)
Drugs such as Digitalis and Quinidine can have effects on heart rhythm and rate;
Electrolyte imbalance and fluid imbalance can also disrupt heart rate
Carditis (myocarditis and pericarditis)- infection of the myocardiac muscle or pericardial
cavity also altering the ECG trace
ECG is a powerful at measuring surface electricity on the body surface that originates from
the heart
As the heart depolarizes in its normal sequence, the electrical activity spreads throughout
the body
In the simplistic form of conducting the ECG we can use bipolar limb leads (LA, RA and LL)
LL= + for 2 and 3 ; RA= - for lead 1 and 2; LA= + for Lead 1 and - for lead 3
This forms an equilateral triangle- Einthoven's Triangle
Combining all the lead and making a axial reference system depicts that lead 1 is 0
degrees, 2 is 60 degrees and 3 is 120 degrees at an angle away from the heart
Deflections of electrical axes above or beyond these different range can be used as
diagnostic tools for different diseases
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Lead 2 is the most important one
Imporant: Looking to see what exactly is causing positive or negative deflections
So we have an normal ECG with deflections 1, 2 and 3 (2 and 3 look very similar)
Looking at lead 1 (having a negative deflections in the right and positive in the left
arm)
If a wave of depolarization spreads towards the positive electrode (towards left arm)
then we are going to get a + deflection on the ECG
The maximum deflection that you can see either + or would be if the depolarization
passed right along the axis of depolarization (i.e. max would be seen if the wave of
depolarization passed at 120 degrees from RA to LL)
In our standard bipolar limb lead ECG, 3 major deflections:
1)P wave small deflection away from base line; indications of electrical activity
associated with depolarization of the atria
3 general stages of the P wave:
Pacemaker potentials
Spread of electrical activity down the internodal pathways
Polarizations of the muscle tissue within the atria
2)QRS Complex- predominately deflects the depolarization of the ventricles also the
repolarisation of the atria (ventricular muscle is bigger though)
3)T wave- represents the repolarisation of the ventricle
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