Class Notes (808,144)
Canada (493,076)
Biology (2,220)
BIO120H1 (1,171)

BIOC33-34 Lecture 2.docx

5 Pages
Unlock Document

University of Toronto St. George
Zachariah Campbell

Lecture 2 – The Electrocardiogram (ECG) 1. The Uses of the Electrocardiogram: - Electrocardiogram (ECG) – a measurement of electrical activity from the heart (that spreads to the surface of the body), allowing you to quantify heart rate o Produces the trace seen on the screen of a heart monitor o Important components to the trace produced by an ECG recording, tell us about the heart function  There are 3 deflections above baseline and between these deflections are segments - One thing we can look at with ECG is the electrical axis of the heart o Electrical activity occurs throughout the heart in all directions but there is a mean axis in which electrical activity flows  It is titled at about 60 degrees through the middle of the heart, and how it shifts left or right can tell us about various disease state in the heart - The ECG can be used to measure heart rate, including analyzing either: o Bradychardia – a slow heart beat ( less than 60 beats/minute) o Tachycardia – an elevated heart beat ( more than 100 beats/minute) o Arrhythmias – the disruption of the heart’s normal rhythm, disruption could be either:  Superventricular – above the ventricles  Ventricular – the ventricular arrhythmias that are dangerous to the health of the heart - Sequence activation disorders: o ECG trace can reveal abnormalities in the conduction of the waves of depolarization through the heart or disruptions in the normal transit of electricity activity in the heart  Abnormalities in movement through the AV node or branch bundles o ECG can tell us whether the heart has undergone hypertrophy  Hypertrophy – when the heart has grown too much muscle which causes difficulty pumping blood properly (when the heart gets too big) - There are changes in ECG: o When the coronary circulation is disrupted and the heart becomes ischemic (decrease in blood supply) o If heart tissue dies, or if there is a heart infarction (heart attack) o Drugs such as digitalis can have effects on heart rhythm and rate and this can be seen on an ECG o Electrolyte imbalances in extracellular fluid and the blood can cause changes in ECG o Infections of the heart such as myocarditis (infection of cardiac muscle) and infections in the peritoneal cavity, peritonitis 2. ECG Measurements, Limb Leads and Einthoven’s Triangle and Law: - The ECG measures electrical activity on the body surface that originates in the heart o As the heart depolarizes, in its normal sequence, the electrical activity spreads through the body and we detect this using electrodes placed upon the body surface. - Simple bipolar ECG limb leads – electrodes are placed on the left and right arms, and the left leg  Lead I – goes from left arm to right arm  Lead II – goes from the heart to the left leg  Lead III – goes from left leg to left arm o These 3 leads form a triangle around the heart called Einthoven’s Triangle  There are positive and negative sides to each lead: • Left leg – positive for both leads • Right arm – negative for both leads • Left arm – has one negative lead (III) and one positive lead (I) o These positive and negative sides brings us to Einthoven’s Law  States that in the ECG, in any given instant, the potential in any wave in lead II is equal to the sum of the potentials in leads I and III (Lead I + III = Lead II) - These 3 leads in an equilateral triangle around the heart forms a star pattern over the heart o Lead I crosses through the center horizontally o Leads II and III, go through the center at opposing 60 degree angles o Therefore:  Lead I is at 0 degrees  Lead II is at 60 degrees  Lead III is at 120 degrees - What causes negative and positive deflections in these traces? (from all 3 leads)  The typical ECG pattern is 2 slightly rounded positive deflections and a sharper one in the middle o If a wave of depolarization heads toward the positive electrode (left arm), then we get a positive deflection in lead I (going from the negative right arm to the positive left arm) o If a wave of depolarization travels away from the left arm, then a negative deflection will appear in lead I (going from positive to negative)  Reverse is true for waves of repolarization o A wave of depolarization travelling toward left leg will appear as positive deflections in lead II and III (both going from negative to positive)  Negative right arm to positive left leg  Negative left arm to positive left leg - The maximum possible deflection will occur when the waves (depolarization or repolarization) occur exactly parallel to the lead o Summary:  Negative to positive = positive deflection  Positive to negative = negative deflection o We can tell which direction current is flowing at any moment by seeing whether a positive or negative deflection occurs on any given lead in the ECG trace 3. Components of the ECG - In the standard bipolar limb lead configuration, there are 3 standard deflections o P-wave – the first small deflection that is associated with the depolarization of the atria  Contains 3 general stages: • First is due to the pacemaker potential • Second is due to the spread of electrical activity through the intermodal pathways • Third phase is due to the depolarization of the muscle tissue within the atria o Next component consists of 3 points: QRS complex, it reflects the depolarization of the ventricles  Q-wave – a small downward deflection  R-wave – a large positive deflection  S-wave – a downward deflection (slightly larger than Q-wave) • Hidden within this blip, the activity is associated with repolarization of the atria but since the ventricular muscle is more massive than the atrial muscle, it is hard to distinguish the two separate events o T-wave – the final deflection that comes a little after the QRS complex  It represents the repolarization of the ventricle  Looks like a larger version of the P-wave - Summary: o P-wave = atrial depolarization o QRS complex = ventricular depolarization, and hidden within is atrial repolarization o T-wave = ventricular repolarization 4. ECG Components and the Stages of Electrical Transmission in the Heart - The P-wave is smaller than the QRS complex because the electrical activity generated (and detected by ECG electrodes) is proportional to the amount of muscle tissue there is to depolarize o The atria has less muscle than the ventricles, so its depolarization produces much smaller deflections than the depolarization of the ventricles o Any disease state that leads to growth of muscle mass in the ventricles will result in a larger QRS complex  It is also affected by hypertrophy of the cardiac muscle - The pacemaker potential in the heart is reflected in the ECG trace as the first phase of the P- wave. o The early phase of the P-wave reflects the electrical activity that is travelling, from the pacemaker cells in the SA node, across the body, to be picked up by the ECG electrodes o The second half of the upward swing in the P-wave is from the depolarization of the pacemaker cells through transmission in the intermodal pathways in the atria o The last phase of the P-wave is after the atria fully depolarizes, the deflection turns downward, but it still represents a depolarization since it is still above baseline  When the atria has depolarized, the waves of depolarization converge onto the AV node, which is the only place electrical activity can move from the atria to the ventricles • This causes a slight delay between the P-wave and the QRS complex - The flat segment on an ECG trace between the P-wave and the QRS complex reflects the t
More Less

Related notes for BIO120H1

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.