Heart blocks Flashcards
What is a first degree heart block
A slowing of conduction through the AV node/to the AVN from the SAN
How does a first degree heart block identify itself on an ECG
Appears as an unusually long PR interval.
What is another name for a Mobitz type I block
Wenckebach block - The PR interval gradually increases in length until the AV node fails to fire so there is no ventricular contraction - usually every third of fourth beat
What is a Mobitz type II block
The PR interval is normal - however every nth ventricular depolarization is missing - P wave returns at the expected time after missing the ventricular contraction
What occurs in a third degree heart block
No impulses are conducted through a given area in either direction
How is the cardiac cycle effected by a complete heart block
SAN and AVN now beat under the control of their own pacemakers - No consistent PR interval
AVN natural pacemakers are the Purkinje fibre cells - notoriously slow and unreliable so cardiac output falls, as well as blood pressure
How does complete heart block show on an ECG
Regularly spaced P intervals - but irregular spacing of QRS and T waves which have a low frequency and no relationship to the P wave
What is the cannon wave
3rd degree block - Atrial contraction against a closed tricuspid valve generates a pressure wave in the jugular vein
What is a bundle branch block
Disease often in the His-Purkinje fibre system - when heart rate exceeds a critical level these fibrous cells are unable to keep up - causing depolarization to travel slowly from one myocyte to the next in ventricular depolarization - prolonging of the QRS complex
How does circus movement develop
Due to a unidirectional conduction block in a myocardial cell - impulses being conducted down fibres split at a fork in the road - if one fork has a unidirectional block the excitation travels no further. On the other fork however - once the impulse reaches the ventricular muscle it travels back up the blocked fork and travels retrogradely through the region of the unidirectional block - Enough time has gone by for the refractory period of the initial excitatory cells to pass - so they can fire APs once again
How does circus movement cause arrhythmias
Frequency of re-entry will outpace the SAN pacemaker - so is responsible for a range of atria and ventricular tachyarrhythmias.
What is the cause of Wolff-Parkinson-White syndrome
Bundles of Kent - transmitting fibres from the SAN straight to the ventricle bypassing the AVN block
How is WPWS seen on an ECG
Normal P interval - short PR interval - Prolonged QRS interval - early excitation of the ventricles presents itself as a delta wave at the beginning of the QRS complex
Also establishes a transient refractory block causing re-entry - tachycardia
