6. The ECG - calculating heart rate and intervals Flashcards
(19 cards)
What is normal ECG paper speed?
25 mm/sec
How many small and large squares are there in a second?
25 small squares = 1 sec
5 large squares = 1 sec
How long does 1 small or large square last?
1 small square = 0.04 sec (40 ms)
1 large square = 0.2 sec (200 ms)
How many large sqaures are there in 6 seconds and 1 min?
6 secs = 30 large squares
1 min = 300 large squares
How is heart rate (bpm) calculated for regular and irregular rhythms?
Regular
- 300/no. of squares of R-R interval
Irregular
- no. of QRS in 6sec x 10
What is the normal range for the PR interval?
0.12 - 0.20 secs (3-5 small boxes)
What is the normal range for the QRS interval (width of QRS complex)?
< 0.12 secs (<3 small boxes)
What does the QT interval represent?
Ventricular depolarisation + repolarisation
Why is QT interval usually corrected for changes in heart rate?
QT interval gets shorter with increasing heart rate (shorter diastole) - can mask problems so must be corrected for HR to assess this accurately
What is the upper limit for corrected QT interval?
- 45 secs (11-12 small boxes) in adult males
0. 47 secs (11-12 small boxes) in adult females
What is sinus rhythm?
Normal rhythm of depolarisation initiated by sinoatrial node (as has fastest rate of depolarisation)
How is normality of sinus rhythm assessed?
1 - is the rhythm regular? (use rhythm strip and paper)
2 - is heart rate normal (60-100 bpm)?
3 - are P waves present and preceding every QRS?
4- are P waves upright in leads I and II?
5 - is PR interval normal (0.12-0.20 secs/3-5 small boxes)?
6 - is QRS width normal (<0.12 secs/<3 small boxes)?
What is a sinus rhythm with rate <60bpm or >100bpm called?
sinus bradycardia or sinus tachycardia
What does a shorter R-R interval indicate?
faster heart rate
What does a wider QRS complex indicate?
ventricular depolarisations that aren’t initiated by normal conductance mechanism
What does a longer P-R interval indicate?
slow conduction from atria to ventricles (1st degree heart block)
What does a raised or depressed ST segment indicate?
myocardial infarction or ischaemia (should be isoelectric)
What does a prolonged QT interval indicate?
prolonged repolarisation of ventricles, which can lead to arrhythmias as occur in long QT syndrome
What are the most common causes of 1st degree heart block?
- AV nodal disease
- enhanced vagal tone (e.g. in athletes)
- myocarditis (e.g. from Lyme disease)
- acute MI (esp. when inferior)
- electrolyte imbalances (hypokalaemia)
- medication
