Matters Of The Heart-Escape and Premature Beats/Rhythms

Escape Beats and Rhythms

So remember back when I was talking about how each of the myocites has a property called automaticity...which means it can generate an electrical impulse and become a pacer foci for the other myocardial cells to respond to?
This is supposed to act as a safeguard it keeps the heart beating even when the normal pacing is disturbed. But sometimes this pacing activity persists even after normal SA node pacing has resumed.

These automaticity foci are said to escape the normal SA node conduction. When this happens for only one or a few beats (because the SA node had resumed pacing activity) it is called an "escape beat". When it happens for an extended period of time, it is called an escape rhythm.

For instance, lets say a patient's SA node.has stopped pacing activity and the patient had gone into Sinus Arrest. After a short pause, an automaticity foci in the atria picks up pacing activity and the rhythm becomes an atrial escape rhythm. On an EKG strip, this rhythm looks very similar to the PQRST waveform (but the P waves might be of a slightly different shape), with a rate of 60-80 bpm.

If Sinus Arrest occurs and no automaticity foci pick up pacing activity, an automaticity foci in the atrio-ventricular junction will start to pace depolarization. This is called a junctional escape rhythm. On an EKG strip thse will be noted to pace slower then atrial foci (40-60bpm)  and because of the retrograde dispursement of the depolarization, P waves may not be visable. If P waves are visable, they will be inverted and may be before or after the QRS complex. 

If a ventricular focus is not stimulated from a pacer from the atria or the AV junction, it's own pacers will escape and an a ventricular escape rhythm will be seen on an EKG strip. This type of rhythm is slow (the ventricles only pace 20-40bpm). The depolarization wave goes backward, so QRS complexes are wide, and the P waves are inverted.

Premature Beats-

If the heart is under stress from certian drugs, hormones, sympathetic simulation, a deficit of certian elctrolytes, or lack of oxygen  (even if normal SA node pacing and conduction are intact) the foci may become irritable and produce an extra heartbeat. This is called  premature beat and can occur at any of the three levels of the heart: Premature Atrial Beats, Premature Junctional Beats, Premature Ventricular Beats. On an EKG strip, these types of pacing activity will appear as an "early" PQRST-like complex that may disrupt another waveform whose appearance will vary depending on the site of origin:
Altered P waves=Premature atrial beat
Absent or retrograde P waves=premature junctional beat
Retrograde P waves and wide QRS complex=Premature Ventriclar Beat...more commonly called a Premature Ventricular Complex (PVC).

Sometimes multiple 'premature' impulses occur in a series, such as in bumigeminy (every other beat is premature) or trigeminy (every third beat is premature). If stressors are unrelieved this may progress to a run of premature beats, and eventually (if it continues to be unrelieved) it may convert to a dangerous heart rhythm like ventricular tachycardia.