Matters Of The Heart-Normal and Abnormal Heart Rhythyms (Part 1)

ATRIAL RHYTHMS-

Normal Sinus Rhythm

The impulse for this rhythm originates from a cluster of myocytes and nerve cells called the SA Node (the 'pacer' in a healthy heart). 
On an EKG strip, it appears with the normal pattern of PQRST complexes with a consistant rate and rhythm (with equal distance between identical waves of neighboring complexes).

The rate for this rhythm is 60-100 PQRST 'heartbeats' per minute.

Sinus Arrhythmia

The impulse for this rhythm also originates from the SA Node. However, the SA Node paces at a faster rate when the patient is taking a breath (respiratory inspiration), and paces at a slower rate when the patient is releasing a breath (respiratory expiration). This is because taking a breath stimulates the the SA Node via the sympathetic nervous system, and  releasing a breath stimulates the parasympathetic inhibition of the pacing impulse.

This mechanism is not inherently pathologic, in fact, it functions in most human beings. The difference in heart rate during inspiration and heart rate during expiration is called 'Heart Rate Variability' and is an indicator of resiliency after Myocardial Infarct.

Sometimes the SA Node doesn't work, this could be because it's not getting enough oxygen or because the tissue that makes up the SA Node has died. When this happens, we have what is called a

Wandering Pacemaker

A wandering pacemaker is a clump of atrial cells (atrial foci) that generate an electrical impulse. Often there are several sites within the atria that act as foci and can generate an electric impulse, this will be reflected in the shape of the 'P' waves on the EKG strip.

Because the pacer generating a given heartbeat comes from a different location compared to different heartbeat, the PQRST complexes are often dissimillar in appearance. Their rhythm is irregular,  the rate is usually 60-80 beats per minute (less then 100 bpm).

Multifocal Atrial Tachycardia (MAT)

This rhythm is similar to that of a wandering pacemaker, except that it's fast! Three or more atrial foci are involved in this rhythm and the rate is over 100 bpm. It is often seen in patients with Chronic Obstructive Pulmonary Disease (COPD) and in heart disease patients who have digitalis toxicity (too much digoxin (a heart disease medication) in their system).

Atrial Fibrillation

In this rhythm, multiple atrial foci are firing all at the same time: this appears as a bunch of a bunch of small spiked deflections of various ampletude instead of 'P' Waves on a 'PQRST' complex.
Despite the multiple 'pacer' impulses, these impulses only occassionally are able to travel through the AV Node to initate ventricular contraction. This causes the ventricular rate to also be irregular.

In this rhythm, the atria does not depolarize or contract syncronously, this increases the risk that blood will become stagnant and not move from one compartment to another. This is dangerious because it contributes to clot formation which can travel to another part of the body and cut off circulation to parts of the body (causing severe acute problems such as occlusive stroke, heart attack, or pulmonary embolism).