INTRODUCTION TO CARDIAC ARRHYTHMIAS

There are pacemaker cells in the heart that fire when they reach the threshold. This firing is related to fluxes of sodium and calcium—two positive ions that leak into the cell and trigger the excitation. So one way to treat too much excitability is to interfere with this process. Another thing is conduction—one property of the action potential is that it is conducted through pathways (ie. the conduction from SA to AV node). You can play with this conduction to help with problems here.


DISTURBANCES OF CARDIAC RHYTHM


--Stimulus for heartbeat occurs in the SA node, (= pacemaker) and is spread as a wave of electrical discharge. When the impulse reaches the AV node it is momentarily delayed before passing down the bundle of His, purkinje fibers, and triggers ventricular contraction. The relationship of the EKG is as follows: P-R represents the atrial contraction; transmission of signal through the septum and walls of ventricles results in the QRS complex; ventricular systole occurs between the peak of the QRS complex and end of the T wave; diastole occurs during the T-P interval.

• The first heart sound occurs at the beginning of ventricular systole, caused by the closing of the AV and opening of the semilunar valves, and perhaps by muscular contraction.
  
• The second sound marks the beginning of ventricular diastole, it is caused by the closing of the semilunar valves.
  
• The faint third sound can sometimes be heard in early diastole.
  
• The fourth sound is from atrial contraction and immediately precedes the first heart sound.

Premature beats are occasioned by an abnormal stimulus arising in a ventricle, atrium, or the AV node. In atrial premature beats, the stimulus usually occurs in early diastole and spreads through the atria causing the ventricles to respond in a normal manner (usually). The normal atrial rhythm is disturbed. When the premature contraction takes place the preceded period of diastole is shortened and the regular rhythm interrupted.


Premature beats also may originate in the ventricles. Following the normal sinus beat, an impulse arises in one of the ventricles causing contraction, usually the ventricular impulse does not disturb the atrial rhythm and an EKG is usually needed for diagnosis. There is a compensatory pause after the premature beat, and is one way of distinguishing ventricular from atrial premature beats.


When the irregularity occurs infrequently, premature beats are not difficult to recognize, but in rapid succession may be mistaken for atrial fibrillation. Occasional premature beats are commonplace and are often experienced by normal people.


The mechanism of paroxysmal tachycardia is not known with certainty—usually the impulses arise in the atria, and the ventricles usually respond in a 1:1 ratio. The period of tachycardia usually begins and ends abruptly. On occasion, paroxysmal tachycardia may occur in the ventricles and although it may occur in healthy subjects it is usually indicative of heart disease. Only an EKG can distinguish atrial and ventricular tachycardia. Paroxysmal atrial tachycardia can also be recognized by certain conditions, the rate is usually between 120-200/ minute, the rhythm is regular and not effected by exercise. The beginning and ending of a paroxysm are abrupt. Pressure on the carotid sinus may cause the beat to change to normal. An occasional attack of paroxysmal tachycardia does not indicate heart disease, significance is determined by the underlying cardiac condition. The patient with ventricular tachycardia however has a disturbance which indicates heart disease.


Atrial flutter is a regular rapid beat varying from 250-350/ minute… though rapid the impulses are regular. The impulses delivered to the AV node are so rapid that the node cant respond to each impulse, commonly a 2:1 or higher degree of block occurs (each 2 pulses = 1 ventricular contraction).


In atrial fibrillation, the rate may vary from 300-500 and may be higher. Unlike flutter, the rate is irregular and there is no coordinated ventricular contraction. Ventricles respond irregularly, and on the EKG, P waves are replaced by a wavy baseline. The mechanism of atrial flutter and fibrillation is controversial. In atrial flutter, the ventricular rate is usually regular, and if it is not it will become so after exercise—this is not true of fibrillation. The diagnosis is best made by EKG.


The patient with atrial fibrillation or flutter will almost always have an underlying heart disease which determines the patients limitations. Fibrillations can also occur in the ventricles, and results in death.


When an impulse from the atrium isn’t transmitted at the normal rate by the AV node, there is heart block. The most common block is a simple delay in conduction, which becomes apparent on the EKG as a P-R interval that is increased in time. Another type of block is one in which there is a progressive delay in each ventricular response resulting in a periodic omission of ventricular contraction. If a high grade of block persists with a high degree of standstill for several seconds, a pacemaker can help. In complete AV block, the atrial and ventricular contractions are totally unrelated, and will sometimes coincide causing a booming heart sound that can allow identification by oscillation. Heart block of any degree is usually evidence of a disease and in most adults indicates a cardiac lesion involving the conduction tissue.

Category: Pharmacology Notes