ECG Part 1 - Obtaining & Reading an ECG

Question 1

what is an ecg?

Answer 1

graphic tracing of voltage fluctuations at the body surface that represents how action potentials are generated & distributed through cardiac myocytes

Question 2

what is the standard electrode placement used for ECGs?

Answer 2

left forelimb - black

right forelimb - white

left hindlimb - read

Question 3

what is einthoven’s triangle?

Answer 3

standard convention for interconnecting electrodes to record the ECG in 3 leads - heart sits in a triangle between the forelimbs & hindlimb

Question 4

what does each lead represent in regards to voltage differences?

Answer 4

lead I - voltage difference between left forelimb & right forelimb

lead II - voltage difference between the left hindlimb & right forelimb

lead III - voltage difference between the left hindlimb & left forelimb

Question 5

how does voltage deflection change with each lead?

Answer 5

voltage deflection (positive or negative) on an ECG will change according to each lead

so in lead II, the R wave may be positive but in lead III, the R wave could be negative

Question 6

what is the ideal set up for recording an ECG for a small animal patient?

Answer 6

ideally in right lateral recumbency placing the patient on a non-conductive surface to minimize interference

place leads on the distal limbs to avoid respiratory artifact - restrain patient appropriately to reduce motion artifact

ensure electrodes have good contact - gel or isopropyl alcohol (only if electrical defibrillation isn’t anticipated, can cause burns/fire) & clip hair as needed

adjust filters to limit 60 Hz interference from other electrical devices in the area

don’t let wires/clips touch each other

ideally monitor for 3 minutes & then record paper strip

Question 7

how is base-apex set up used for ecgs on horses?

Answer 7

appropriate to screen for arrhythmias

positive lead - left 5th ICS at the PMI of the heart’s apex
negative lead - right jugular furrow about 2/3 way down towards the heart

ground lead - over the withers on the right side

Question 8

what is the ecg recording when you see a p wave?

Answer 8

cardiac action potential originates in the SA node in the right atrium with the wave of atrial depolarization moving from right to left causing a POSITIVE voltage difference between the left & right forelimbs

p wave = atrial depolarization

Question 9

what happens at the end of atrial depolarization that is too small to detect on ECG?

Answer 9

voltage difference returns to zero at the end of atrial depolarization & then atrioventricular node depolarization occurs but it is too small to detect

Question 10

what is the ecg recording when you see a q wave?

Answer 10

q wave, negative - interventricular depolarization, voltage moves from left to right across the interventricular septum, so you see a small negative voltage difference between the left & right forelimb

Question 11

what is the ecg recording when you see a r wave?

Answer 11

r wave, positive - depolarization spreads through the rest of the ventricle via the left & right bundle branches & purkinje fibers, so you see a large positive deflection on ECG from the left to right forelimb because the cardiac axis is tilted to the left & the left ventricle is significantly bigger than the right, so the action potential causes a positive voltage difference between the left & right forelimbs

Question 12

what is the ecg recording when you see an s wave?

Answer 12

s wave, negative, depolarization finishes & the left ventricle becomes slightly negative compared to the right

Question 13

what is does the QRS complex represent on an ecg?

Answer 13

ventricular depolarization - r wave is the predominant deflection

Question 14

what is the ecg recording when you see an t wave?

Answer 14

t wave, can be positive or negative - represents the repolarization of the ventricles

waves of repolarization doesn’t travel predictably & varies between animals

Question 15

is atrial repolarization seen on ECG readings?

Answer 15

no - too small for the ECG to detect

Question 16

what does the PR interval represent?

Answer 16

time between the start of atrial depolarization & ventricular depolarization

Question 17

what does the QRS wave represent?

Answer 17

time of ventricular depolarization

Question 18

what does the QT interval represent?

Answer 18

time from the beginning of ventricular depolarization to the end of ventricular repolarization - approximately the duration of ventricular action potential

Question 19

what does the PP interval represent?

Answer 19

time between atrial depolarization - atrial rate

Question 20

what does the RR interval represent?

Answer 20

time between ventricular depolarizations - ventricular rate

Question 21

T/F: in a normal heart, the PP interval is equal to the RR interval

Answer 21

true

Question 22

what does the vertical axis of the ECG measure? what is the standard calibration?

Answer 22

measures the strength of each electrical impulse

standard - 10 mm (10 small boxes) = 1 millivolt

Question 23

what does the horizontal axis of the ECG measure? what are the standard calibrations?

Answer 23

measures timing of cardiac events

25 mm/s = 25 small boxes =1 sec

50 mm/s = 50 small boxes = 1 sec

Question 24

when would you want a 50 mm/s speed over a 25 mm/s speed on an ECG?

Answer 24

you would want it for a patient with an abnormality associated with a higher heart rate such as a cat with tachyarrhythmias

Question 25

what is MEA?

Answer 25

mean electrical activity - sum of all the waves of depolarization that occur simultaneously that is used as a measure of direction of ventricular depolarization

Question 26

how is an ECG used to evaluate structural cardiac changes?

Answer 26

MEA measurement - can only be determined from a 6-lead ECG

useful in small animals only to diagnose conduction abnormalities, determining the origin of arrhythmias, & assessing for ventricular hypertrophy

Question 27

normal MEA points towards what aspect of the heart? why?

Answer 27

left ventricle - it is the largest chamber

Question 28

what does it mean if there is MEA deviation?

Answer 28

right ventricular hypertrophy - heartworm disease, pulmonary hypertension

abnormality in intraventricular conduction system - bundle branch block

Question 29

why can you never make a diagnosis of cardiac structural disease off of an ECG alone?

Answer 29

voltage changes are not specific

always need further investigation - thoracic radiographs & echocardiograms

Question 30

why can you not use ECG to detect structural cardiac disease in large animals? what can you use ECG for?

Answer 30

the individual variation between ECG appearance is very common - normal to see significant differences in the appearance/polarity of the QRS between different individuals

useful for characterizing rhythm disturbances

Question 31

how do you calculate a HR from an ECG with a paper speed of 25 mm/s?

Answer 31

1500/# of RR complexes over 6 seconds

at 25 mm/s, 15 large squares = 3 seconds

Question 32

how do you calculate a HR from an ECG with a paper speed of 50 mm/s?

Answer 32

3000/# of RR complexes over 6 seconds

at 50 mm/s, 5 large squares = 1 second

Question 33

what does paroxysmal mean?

Answer 33

burst of abnormal rhythm that can last for a few beats or up to a few hours

Question 34

what classifies a heart rhythm as regular?

Answer 34

less than 10% variation in RR interval

Question 35

what classifies a heart rhythm as regularly irregular?

Answer 35

more than 10% variation in RR interval but the pattern repeats or is predictable

Question 36

what classifies a heart rhythm as irregularly irregular?

Answer 36

variable RR interval with no pattern/predictability

Question 37

what main questions should you consider when looking at an ECG?

Answer 37

1. what is the heart rate? does it match what is auscultated?
2. is there a P wave for every QRS?
3. is there a QRS for every P wave?
4. are the PP & RR intervals the same? are the PP & RR intervals consistent between each complex?
5. what does the QRS complex look like?

Question 38

what does a tall, skinny/ narrow QRS complex mean?

Answer 38

electrical impulse irregularity originates from the supraventricular region (sinoatrial node, atrial myocardium, atrioventricular node) & is conducted through the normal pathway