Conduction disturbances can occur at the level of the sinoatrial (SA) node, the atrioventricular (AV) node and the bundle branch system.
|Atrial rate||< 60 bpm|
|Effect of adenosine||slows down|
|Example ECG: Sinusbradycardia of around 40 bpm.|
In sinusbradycardia the sinus node fires at a slow (<60 bpm) rate. This can be caused by e.g.: medication (beta-blockers), ischemia (typically inferior myocardial infarction with involvement of the sinus node artery), hypothermia and hypothyroidism. In an athlete a resting heart rate of < 60 bpm often is normal.
During sinus arrest the normal pacemaker cells in the sinus node stop firing. Often an ectopic pacemaker (sometimes from within the sinus node) takes over, but the new rate varies slightly from the old one.
During asystole there is no cardiac activity. When prolonged this results in immediate death. Again, asystole is a very unlikely diagnosis in a concious patient and a technical error should be checked first (loose wire, very low gain). Although often called a 'flatliner', the baseline in asystole is often shifting up and down a little and in fact if it isn't (and looks like it is often depicted in movies) a loose wire is the most likely cause.
Sick Sinus Syndrome
In sick sinus syndrome there is a malfunctioning sinus node. Several arrhythmias can result from this:
- Symptomatic slow sinusbradycardia in the absence of medication
- Sinusarrest or exit block
- Combinations of sinoatrial and atrioventricular conductions disturbances
- Brady-tachycardia syndrome; typically there is sinusbradycardia, sinusarrest or SA-block which is alternated by periods with fast or (ir)regular atrial arrhythmias (atrial fibrillation, atrial flutter, atrial tachyardia or sinustachycardia).
Sino-atrial exit block
to be filled in
In atrioventricular block the conduction between atria and ventricles is disturbed leading to an incread PQ interval or to drop out of QRS complexes: atrial activity that is not followed by ventricular activity. Three degrees of block can be distinguished.
1st degree AV block
In first degree AV block there is a prolongation of PQ duration (PQ time > 0.20 sec). Still every p-wave is being followed by a QRS complex.
First degree AV block is present in 16% of >90-year olds  and is mostly caused by a degeneration of the conduction system. First degree AV block is relatively harmless.
2nd degree AV block
In second degree AV block not all p-waves are being followed by QRS complexes: beat dropout occurs. Second degree AV block can be categorized in 3 types:
Second degree AV block type I(Wenckenbach)
In second degree AV block type I, the PQ interval prolongs from beat to beat up until the drop-out of one QRS complex. The characteristics of a Wenkebach block:
- QRS complexes cluster (e.g. a 5:4 block or 4:3 block)
- The PQ interval prolongs every consecutive beat
- The PQ interval that follows upon a dropped beat is the shortes.
- The RR interval shortens (!) every consecutive beat.
- The amount of block decreases during exercise (e.g. a 4:3 block improves into a 6:5 block)
The conduction disturbance in a type I block originates in the AV node. Isolated second degree AV block type I is relatively benign and not a pacemaker indication.
Second degree AV block type II(Mobitz)
In second degree AV block type II, beats are dropped irregularly without PQ interval prolongation. As the drop out of beats is irregular, no clustering of QRS complexes can be seen as in second degree block type I. Second degree AV block type II marks the starting of trouble and is a class I pacemaker indication.  The cause of second degree AV block type II can be found distally from the AV node: in the HIS bundle or in the bundle branches or Purkinje fibers.
An important differential diagnosis of second degree AV block type II is a atrial extrasystoly with compensatory pause. This diagnosis is much more common and harmless.
High grade AV block
High grade AV block is defined as two or more p-waves not followed by QRS complexes.
3rd degree AV block
Third degree AV block is synonymous to total block: absence of atrioventricular conduction. P-waves and QRS complexes have no temporal relationship. The ventricular rhythm can nodal, idioventricular or absent. Absent ventricular rhythm results in asystole and death.
During third degree AV block the blood supply to the brain can insufficient, leading to loss of consciousness. Adams Stokes (or Stokes-Adams) attacks (often misspelled as Adam Stokes) attacks arte attacks of syncope or pre-syncope in the setting of third degree AV block.