Normal features and basic measurements. Flashcards
What is an ECG?
A graphical representation of the heart’s electrical activity that occurs over time.
Cardiac depolarisation and repolarisation generates electrical currents which spread throughout the body - measured by electrodes placed on the body surface.
The ECG is recorded at a set speed, and the voltages are calibrated.
A standardised recording speed allows appreciation of heart rates and cardiac intervals and comparisons to be made between ECGs on different occasions.
What determines the direction of waveforms?
Polarisation dictates the direction of the trace on the ECG.
A wave of polarisation travelling toward a positive electrode results in a positive deflection on the ECG.
A wave of polarisation travelling away from a positive electrode results in a negative deflection on the trace.
A wave of depolarisation travelling at a right angle to a positive electrode results in a biphasic deflection on the trace.
What are standard ECG graph paper measurements?
25mm/s (standard paper speed)
1 small square = 0.04 s/ 40ms (time)
0.1mV/1mm (amplitude)
10mm/mV (standard paper amplitude)
5 small squares = 0.2s /200ms
0.5mV/5mm (amplitude)
What are the different waves that comprise the ECG?
ECG represent the sequence of depolarization and repolarization of the atria and ventricles
What does the P wave represent?
Atrial depolarisation (atrial contraction)
What does PR interval represent?
Time between the onset of atrial depolarisation and the onset of ventricular depolarisation
What does QRS represent?
Ventricular depolarisation (Ventricular contraction)
What does the T wave represent?
Ventricular repolarisation
How to calculate heart rate?
Heart Rate = 1500
No. small sq between R-R interval
Or count number of QRS complexes across 10 second rhythm strip and x 6
Normal P wave amplitude and duration?
< 2.5mm/0.25mV in height (2.5 small squares)
0.08-0.10 seconds duration (2-2.5 small squares)
Normal PR interval
Between 0.12 - 0.2 secs (3-5 small squares)
Normal QRS complex
Between 0.08 - 0.12 secs (2-3 small squares)
Normal QTc interval
QTc is prolonged if > 440ms in men or > 460ms in women
The P wave
The P wave represents the depolarisation of the atria
The impulse originates from the SA node
Normal duration: 0.08 – 0.10 s (80 – 100ms)
Normal amplitude: < 0.25mV
The PR interval
The PR interval represents the length of time it takes for the signal to travel from the SA node to the AV node.
Represents the amount of time the signal is ‘held’ at the AV node.
Why is there a short delay at the AV node?
Normal PR interval: 0.12 – 0.2 s (120 – 200ms)
It is measured from the beginning of the P wave to the beginning of the QRS complex
The QRS complex
The QRS complex represents the depolarisation of the ventricles.
Normal duration: 0.08 – 0.12 s (80 – 120 ms).
The direction of the QRS depends upon the direction of the electrical current in relation to the position of the ECG electrode.
Current flow towards a lead = waveform deflects upwards
Current flow away from a lead = waveform deflects downwards
Current flow perpendicular to a lead = waveform deflects in both directions (biphasic)
What do the Q wave, R wave and S wave represent?
- An initial negative wave
= Q wave - An initial positive wave
= R wave - A negative wave following a positive
= S wave
QT interval
The QT interval represents the amount of time it takes the ventricles to depolarise and repolarise (recovery to the normal resting state).
The duration of the QT interval should vary depending on the heart rate. This is then corrected for heart and called the QTc.
The faster the heart rate, the shorter the QT interval should be.
QTc is prolonged if > 440ms in men or > 460ms in women
How to Measure QTc
QT Interval = Measure from the start of the QRS complex to the end of the T wave.
RR Interval = Measure between the two preceding consecutive R waves
QTc = QT/square root RR
The ST segment
The ST segment represents the transient period where no electrical impulses can travel through the myocardium.
It is measured from the end of the QRS complex to the start of the T wave.
It is useful in the diagnosis of myocardial infarction and ischaemia.
It should be no more than 1mm above or below the isoelectric line.
The T wave
The T wave represents ventricular repolarisation.
T wave shape and orientation varies from lead to lead.
There is no clearly defined range for T wave amplitude.
As a general guide, the T wave should be no more than half the size of the preceding QRS.
Sinus Rhythm
Sinus Rhythm :-
Atrial impulse originating from SA node.
P wave preceding each QRS.
HR between 60 - 100 bpm.
Regular rhythm
What is sinus bradycardia?
SR
HR <60bpm
Regular
P wave before each QRS, identical
PR 0.12 to 0.20s
QRS <0.12s
What is sinus tachycardia?
SR
HR >100bpm
Regular
P wave before each QRS, identical
PR 0.12 to 0.20s
QRS <0.12s
What is sinus arrhythmia?
Heart rate usually 60-100bpm
Rhythm - irregular
P wave before each QRS, identical
PR 0.12 to 0.20s
QRS <0.12s
Common in children
Varies with respiration
Normal QRS morphologies
- inferior leads, usually no negative S waves
