ECG Part 1 - Obtaining & Reading an ECG Flashcards
(38 cards)
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
