Week 7 - The Electrocardiogram Flashcards
Which two planes is the heart inspected in during an ECG, via placement of the electrodes?
- Frontal
2. Horizontal
What does the amplitude of the signal observed on a ECG depend on?
- How much muscle is depolarising
2. How directly towards the electrode the excitation is moving
Name the confounders that can influence the quality of a ECG reading.
- Lead misplacement
- Muscle contraction: shivering, movement, talking, coughing
- Interference, e.g. alternating current
- Poor electrode contact: sweat, cable pull, hair
Give the equations for calculating heart rate from a regular and irregular heart rhythm.
- Regular: 300/ Number of large squares of the R-R interval
2. Irregular: Number of QRS complexes in 6 seconds x 10
State the normal ranges for the following intervals:
- PR interval
- QRS interval
- QT interval
- 0.12 - 0.20 seconds - 3 to 5 small boxes
- <0.12 seconds: less than 3 small boxes
- Upper limit of corrected QT interval (varies with heart rate):
- 45 seconds: adult male
- 47 seconds: adult female
What does the PR interval estimate?
Conduction in the AV node and Bundle of His
What do narrow and broad QRS complexes indicate?
Narrow QRS Complexes: Rhythm originating in atria/AVN
Broad QRS complexes: Rhythm originating in ventricles (or bundle branch block is present)
Outline the features of the sinus rhythm.
- Depolarisation initiated by the sinus node
- Rate: 60-100 bpm
- P is upright: Leads I and II
- Normal PR interval
- Every p wave followed by a QRS
- Every QRS preceded by a P wave
- Normal QRS width
Define sinus bradycardia and tachycardia.
Sinus rhythm with rate lesser than 60/minute: Sinus bradycardia
Sinus tachycardia: Sinus rhythm with rate greater than 100/minute
Outline the features of atrial fibrillation.
- Impulses have random, chaotic pathways in atria
- Baseline on an ECG: irregular, ventricular response: irregular
- Multiple abnormal atrial pacemakers discharging randomly
- Chaotic atrial depolarisation leads to loss of normal atria contraction (atria quivering not contracting)
- Impulses conducted to ventricles irregularly
- Pulse and heart rate irregularly irregular
- P wave absent
- QRS complexes irregularly irregular
What does the PR interval indicate?
Time taken for conduction of impulse to the ventricles
Outline the features of a first degree heart block.
- Normal P wave
- Slow conduction in AV node or His bundle due to ischaemia or degenerative change
- PR interval prolonged: >5 small squares
- Normal QRS
Define ventricular tachycardia.
A run of three or more consecutive ventricular ectopics
Outline the features of a second degree heart block (Type 1).
- Mobitz Type 1 heart block, Wenkebach phenomenon
- Progressive lengthening of PR interval, until one P is not conducted - this allows time for AVN to recover
- Cycle begins again
Outline the features of a second degree heart block (Type 2).
- Normal PR interval
- Sudden lack of conduction of a beat: dropped QRS
- High risk of progression to a complete heart block
- Requires insertion of a pacemaker
Outline the features of a complete AV block (third degree heart block).
- No relationship between P waves and QRS complexes
- P waves: normal rate, but not conducted to ventricles
- Ventricular pacemaker takes over: ventricular escape rhythm
- Rate is very slow: 30-40 beats per minute, usually wide QRS complexes, HR too slow to maintain BP and perfusion
- Requires urgent insertion of a pacemaker
Define ectopic foci.
Abnormal pacemaker sites within the heart muscle that display automaticity
Outline the events occurring during the P, Q, R, S and T waves of the ECG.
P: Atrial depolarisation Q: Septal depolarisation spreading to ventricle R: Main ventricular depolarisation S: End ventricular depolarisation T: Ventricular Repolarisation
