electrocardiograms (ECG) Flashcards
define electrocardiogram (ECG)
a measure of the currents
generated in the EXTRACELLULAR FLUID by the changes occurring simultaneously in many cardiac cells
what is on y-axis of an ECG
voltage
what is on x-axis of ECG
time
define electrode
a conductive pad that is attached to the skin and enables recording of electrical currents
define lead
a plane in which you are looking at the heart
the resulting measurements of an ECG are referred to as leads
what is the P wave
first positive deflection on the ECG
represents atrial depolarisation
occurs when SA node creates an action potential that depolarizes the atria
seen in every lead apart from aVR
what is the QRS complex
ventricular depolarisation
still called QRS even if Q and/or S are
missing depending on what lead you are looking at
what is the T wave
ventricular repolarisation
occurs after the QRS complex
how many leads in a normal ECG
12
define segment
a period of isoelectric neutrality
define interval
just a region including magnitude
what is the PR segment
the flat line between the end of the P-wave and the start of the QRS complex
reflects the time delay between atrial and ventricular activation.
also serves as the baseline
what is the ST segment
plateau phase of ventricular repolarization
interval between depolarisation & repolarisation
what is the PR interval
atrioventricular conduction time
time taken for atria to depolarise and electrical activation to get
through AV node
what is the QT interval
total ventricular contraction during systole.
the time from the start of the Q wave to the end of the T wave, time taken for ventricular depolarisation and repolarisation.
which lead is usually focused on when interpreting an ECG and why
lead II (2)
gives a good view of the P wave
what is tachycardia
increased heart rate
what is bradycardia
decreased heart rate
what is dextrocardia
heart on right side of chest instead of left
what is acute anterolateral myocardial infarction
ST segments are raised in anterior. (V3- V4) and lateral (V5-V6) leads
what is acute inferior MI
ST segments are raids in inferior (II, III, aVF) leads
is atrial repolarisation usually evident on an ECG
no
because it occurs at the
same time as the QRS complex so is hidden
describe electrical impulses in the heart
they move in 3 dimensions
ECG only measure voltage in 1 dimension
- If an impulse is towards the electrode it looks big
- If an impulse is away from the electrode it looks small or even negative
compare impulses from atria to impulses from ventricles
the impulse from the atria is smaller since the atria are smaller than the ventricles thus less myocytes
what are the 12 leads used in an ECG
6 limb leads
6 chest leads
describe the 6 limb leads used in an ECG
3 bipolar - I, II, III
3 unipolar (augmented) - aVR, aVL, aVF
describe the 6 chest leads used in an ECG
all unipolar - V1, V2, V3, V4, V5, V6
what are the bipolar limb leads and where are they
I, II, III
form a triangle between electrodes on the wrists and left leg (right leg is a ground electrode)
the negative poles are REFERENCE
electrodes
the positive poles are RECORDING electrodes
what are the unipolar (augmented) leads and where are they
aVR, aVL & aVF
bisect the angles of the triangle by combining two electrodes as reference
e.g. for lead aVL, the right wrist & foot are combined as the negative pole, thus creating a reference point along the line between them, pointing toward the recording electrode on the left wrist
what are the chest leads
precordial leads
V1, V2, V3, V4, V5, V6
where do you place lead V1
4th IC space to right of sternal border
where do you place lead V2
4th IC space to the left of sternal border
where do you place lead V3
midway between V2 and V4
where do you place lead V4
5th IC space, mid-clavicular line
where do you place lead V5
anterior axillary line at same level as V4
where do you place lead V6
midaxillary line at same level as V4 and V5
what does an ECG graph show
the graph shows changes in voltage over time
what does each small square on an ECG represent
40ms
what does each big square on an ECG represent
0.2s
describe P waves in a normal ECG
POSITIVE in EVERY LEAD (apart from the aVR)
describe T waves in a normal ECG
POSITIVE in EVERY LEAD (apart from the aVR & sometimes the V1
and V2 depending on trace)
