ECG Part 1 - Obtaining & Reading an ECG Flashcards
what is an ecg?
graphic tracing of voltage fluctuations at the body surface that represents how action potentials are generated & distributed through cardiac myocytes
what is the standard electrode placement used for ECGs?
left forelimb - black
right forelimb - white
left hindlimb - read
what is einthoven’s triangle?
standard convention for interconnecting electrodes to record the ECG in 3 leads - heart sits in a triangle between the forelimbs & hindlimb
what does each lead represent in regards to voltage differences?
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
how does voltage deflection change with each lead?
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
what is the ideal set up for recording an ECG for a small animal patient?
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
how is base-apex set up used for ecgs on horses?
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
what is the ecg recording when you see a p wave?
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
what happens at the end of atrial depolarization that is too small to detect on ECG?
voltage difference returns to zero at the end of atrial depolarization & then atrioventricular node depolarization occurs but it is too small to detect
what is the ecg recording when you see a q wave?
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
what is the ecg recording when you see a r wave?
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
what is the ecg recording when you see an s wave?
s wave, negative, depolarization finishes & the left ventricle becomes slightly negative compared to the right
what is does the QRS complex represent on an ecg?
ventricular depolarization - r wave is the predominant deflection
what is the ecg recording when you see an t wave?
t wave, can be positive or negative - represents the repolarization of the ventricles
waves of repolarization doesn’t travel predictably & varies between animals
is atrial repolarization seen on ECG readings?
no - too small for the ECG to detect
what does the PR interval represent?
time between the start of atrial depolarization & ventricular depolarization
what does the QRS wave represent?
time of ventricular depolarization
what does the QT interval represent?
time from the beginning of ventricular depolarization to the end of ventricular repolarization - approximately the duration of ventricular action potential
what does the PP interval represent?
time between atrial depolarization - atrial rate
what does the RR interval represent?
time between ventricular depolarizations - ventricular rate
T/F: in a normal heart, the PP interval is equal to the RR interval
true
what does the vertical axis of the ECG measure? what is the standard calibration?
measures the strength of each electrical impulse
standard - 10 mm (10 small boxes) = 1 millivolt
what does the horizontal axis of the ECG measure? what are the standard calibrations?
measures timing of cardiac events
25 mm/s = 25 small boxes =1 sec
50 mm/s = 50 small boxes = 1 sec
when would you want a 50 mm/s speed over a 25 mm/s speed on an ECG?
you would want it for a patient with an abnormality associated with a higher heart rate such as a cat with tachyarrhythmias
what is MEA?
mean electrical activity - sum of all the waves of depolarization that occur simultaneously that is used as a measure of direction of ventricular depolarization
how is an ECG used to evaluate structural cardiac changes?
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
normal MEA points towards what aspect of the heart? why?
left ventricle - it is the largest chamber
what does it mean if there is MEA deviation?
right ventricular hypertrophy - heartworm disease, pulmonary hypertension
abnormality in intraventricular conduction system - bundle branch block
why can you never make a diagnosis of cardiac structural disease off of an ECG alone?
voltage changes are not specific
always need further investigation - thoracic radiographs & echocardiograms
why can you not use ECG to detect structural cardiac disease in large animals? what can you use ECG for?
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
how do you calculate a HR from an ECG with a paper speed of 25 mm/s?
1500/# of RR complexes over 6 seconds
at 25 mm/s, 15 large squares = 3 seconds
how do you calculate a HR from an ECG with a paper speed of 50 mm/s?
3000/# of RR complexes over 6 seconds
at 50 mm/s, 5 large squares = 1 second
what does paroxysmal mean?
burst of abnormal rhythm that can last for a few beats or up to a few hours
what classifies a heart rhythm as regular?
less than 10% variation in RR interval
what classifies a heart rhythm as regularly irregular?
more than 10% variation in RR interval but the pattern repeats or is predictable
what classifies a heart rhythm as irregularly irregular?
variable RR interval with no pattern/predictability
what main questions should you consider when looking at an ECG?
- what is the heart rate? does it match what is auscultated?
- is there a P wave for every QRS?
- is there a QRS for every P wave?
- are the PP & RR intervals the same? are the PP & RR intervals consistent between each complex?
- what does the QRS complex look like?
what does a tall, skinny/ narrow QRS complex mean?
electrical impulse irregularity originates from the supraventricular region (sinoatrial node, atrial myocardium, atrioventricular node) & is conducted through the normal pathway
