Normal Cardiac Electrical Function

Question 1

how does the conduction system of the heart work

Answer 1

• generates its own electrical activity which is conducted via a specialized set of cells from the atria to the ventricles
• eletrical activity of this conduction system is passed to the muscle cells (cardiomyocytes) which then contract via the process of excitation-contraction coupling
  1. SA node generates spontaneously depolarizing cells (pacemaker cells)
  2. when they fire the conduction passes from the right atrium to the left atrium and the AV node
  3. the AV node has pacemaker cells but they are slower –> down to the Bundle of His
  4. conduction reaches the purkinje fibres
  5. passes to outer wall of the heart (epicardium)
  6. occurs every half beat

Question 2

what is the dipole concept

Answer 2

1. an electrical dipole is a negative and positive charge separated by a distance
2. dipole creates a voltage
3. voltage can be measured using a voltmeter
4. voltmeter has two electrodes one denoted as (+) and the other (-)
5. if positive end of the dipole is nearer the (+) electrode then an upward deflection in the voltage trace occurs (vice versa)

Question 3

what is the resting membrane potential of a cardiomyocte

Answer 3

-80mV

inside is negative and outside is positive

Question 4

what occurs during depolarization in a cardiomyocyte

Answer 4

positive ions enter the cardiomyocte –> inside becomes positive and the outside becomes negative

Question 5

what occurs during repolarization

Answer 5

interior of cardiomyocyte becomes negative again and outside becomes positive

Question 6

explain the dipole concept in the whole heart (5)

Answer 6

1. volmeter is the ECG machine and the electrodes are placed on the body
2. ECG machine measures the changes in voltage created by depolarization and repolarization of the different regions of the heart during the cardiac cycle
3. one electrode is placed on the left forelimb and one on right
4. ECG is record of voltage in the left forelimb with respect to the right therefore the left forelimb is denoted as the (+) electrode
5. if a relatively more positive region of the heart is closer to the left electrode an upward deflection of the ECG trace will result (vice versa)

Question 7

explain what occurs in atrial depolarization and what will appear on the ECG trace (6)

Answer 7

1. the SA node cells in the RA spontaneously depolarize and result in depolarization of the adjacent RA cells
2. the wave of depolarization moves towards the left as demonstrated by the arrow
3. the cells in the LA are at rest hence a dipole is created
4. positive end of the dipole is closer to the (+) electrode hence an upward deflection occurs in ECG trace
5. when all the RA and LA is depolarized the trace returns to base-line
6. next delay is the wave of depolarization passing through the AV node to ventricles

Question 8

explain what occurs during early ventricular depolarization

Answer 8

1. wave of depolarization passes down interventricular septum
2. wave of depolarization spreads from left to right creating a dipole
3. the negative end of the dipole is closer to the (+) electrode hence a downward deflection in the ECG trace occurs

*left hand side of septum depolarizes first so dipole –> negative end closer to the voltmeter –> trace goes down whole septum depolarizes and trace goes to 0 –> early ventricular depolarization (Q wave)

Question 9

what occurs during ventricular depolarization (4)

Answer 9

1. endocardium depolarizes before the epicardium
2. a dipole is created which is very large since the number of cardiomyocytes within the left ventricle are numerous
3. the positive end of the dipole is closer to the (+) electrode hence a upward deflection in the ECG trace occurs
4. a dipole is directed towards the left since –> the ventricular apex is directed towards the left and the left ventric is much larger than the right and therefore dominates electrically

**

once its gone thru septum –> goes down bundle branches to the purkinje bundles –> the outside starts to depolarize –> dipole positive end is closer to the positive end –> trace end goes up –> very high because greater number of dipoles created compared to the atrium (R wave)

the left apex is closer the left side of the chest (apex beat)

this has direction and magnitude –> vector

Question 10

explain what occurs in late ventricular depolarization and what it appears like on the ECG

Answer 10

1. wave of depolarization finishes spreading from the endocardium to the epicardium of both ventricles
2. the ECG returns to the baseline point and sometimes goes negative
3. the negative end of the dipole is closer to the (+) electrode hence a downward deflection in ECG trace

S wave: small negative deflection –> negative end is closer to the positive end

Question 11

explain what occurs in ventricular repolarization and what it appears on the ECG

Answer 11

1. the epicardium is the last to depolarize but the first to repolarize
2. epicardial cells are now positive on the extracellular surface and create a dipole with endocardial cells which are still depolarized
3. positive end of the dipole is closer to the (+) electrode hence a upward deflection in the ECG trace occurs

**the first cells to repolarize are on the outer surface of the myocardium –> cells go back to negative and the outside is positive –> closer to the positive electrode –> trace goes up even though its repolarization –> eventually whole thing repolarizes and the dipole is lost

Question 12

what is the P wave

Answer 12

atrial depolarization

Question 13

what is the QRS complex

Answer 13

ventricular depolarization

Question 14

what is the T wave

Answer 14

ventricular repolarization

Question 15

what is the PR interval

Answer 15

time between atrial depolarization and ventricular depolarization

Question 16

what is the QT interval

Answer 16

length of time that the ventricles remain depolarized

Question 17

what is the QRS complex

Answer 17

time taken for ventricular depolarization to occur once the wave of depolarization has passed through the AVN from the atria

Question 18

what is the PP interval

Answer 18

time between atrial depolarizations

Question 19

what is the RR interval

Answer 19

time between ventricular depolarization

Question 20

what are the 3 lead systems

Answer 20

lead I: right forelimb (-), left forelimb (+)

lead II: right forelimb (-), left hindlimb (+)

lead III: left forelimb (-), left hindlimb (+)

Question 21

what are the clinical significances of ECGs

Answer 21

1. heart rate (HR)
2. normal sinus rhythm –> dysrhythmia (arrhythmia) means an abnormal rhythm
3. chamber enlargement
4. myocardial ischemia
5. electrolyte imbalance

Question 22

how is the heart rate calculated using ECGs

Answer 22

RR interval –> becomes shorter when heart rate increases (tachycardia)

Question 23

how can arrhythmias be detected using ECGs

Answer 23

normal heartbeat initiates in the SAN since the cells in that region normally have the fastest intrinsic pacemaker activity

a dysrhythmia means abnormal rhythm

Question 24

how can ECG be used to determine chamber enlargement

Answer 24

using the lead system a 3D appreciation of the eletrical activity of the heart is acquired

can be used to determine chamber enlargement (not horse, but dog)

**

vector quantity –> the R wave is very tall because it has more muscle compared to atrium

if the atria are enlarged there may be a change in the P wave

can tell this in the dog but not horse

Question 25

how can ECG be used to determine if there is myocardial ischemia

Answer 25

determines whether part of the ventricle has undergone myocardial ischemia not used in vet med \*\* reduced blood flow leads to heart attach --\> first things they do is stick ECG the ST segment is elevated because of dead heart tissue

Question 26

how can ECG be used to determine electrolyte imbalance

Answer 26

electrical activity of the heart is dependent upon changes in ion concentration ex. action potentials generated by changes in Na, K and Ca ex. hyperkalemia --\> what phase is K involved in? T phase (will be larger)

Question 27

what information does the ECG not provide

Answer 27

whether or not the heart is contracting or not ## Footnote **always say depolarization/repolarization in exams (not contraction of atrium/ventricle)**

Question 28

describe the features of this ECG

Answer 28

T wave is inverted in dogs (not humans) --\> starts from endocardium and goes to epicardium (purkinje fibre network is different in other species) not pathalogical

Question 29

describe the features of this trace

Answer 29

the RR is irregular (slow, fast) --\> not equal distance --\> arythmia (but not random, it goes slow fast slow fast) each one has a P wave --\> originates from SA node --\> SA arhythmia heart rate increases when you breath in --\> when breaths out the heart rate lowers --\> physiological arythmia \*regularly irregular --\> sinus arythmia