Folks are often concerned when they are told that their own heart beat is "out of rhythm." To allay some fears about ”irregular” heartbeats, I’d like to start at the beginning with the SA node.
First of all, we all have a "respiratory sinus arrhythmia," which is a bit of a misnomer when one considers that it is quite normal, and the word arrhythmia can conjure up the thought that things are out of synch.
Respiratory Sinus Arrhythmia is Normal
Luckily, the heart’s conduction system is not some fixed mechanical thing. It responds to the body, to the mind (yes, stress can raise havoc with your heart’s rhythm, too) and to the environment.
The sinus node does not fire like an automaton—if it does, then heart rate variability (HRV) is low, and that is actually a risk factor for sudden cardiac death.
Instead, the SA node is influenced by the breath—we call that a respiratory variation in the heart beat. The beat increases slightly as you inhale, and decreases slightly as you exhale, which is why conscious breathing exercises are so good for your heart.
Other SA Node Factors
- Activity level
- The foods you eat (caffeine, sugar, chocolate, alcohol, etc)
- The chemicals you ingest (food dyes, medication, etc)
- The world around you (altitude, electric smog, etc.)
Respiratory sinus arrhythmia is a relatively gradual slowing and speeding of the heart, like your foot on the gas pedal on normal traffic.
Premature Atrial Contractions (PACs)
As for PACs, this is an abbreviation for a premature atrial contraction generated by the SA node, or perhaps another electrical cell in the atria outside the usual conduction system.
Think of the atrium as a receiving room in a building. Well, the two atria (plural in Latin terms for atrium) are the upper receiving chambers of the heart. The atria receive the oxygen depleted blood as it returns to the heart via the vena cava—very large veins.
The right atrium ejects the blood out through the pulmonary artery to the lungs to pick up more oxygen. The left atrium “receives” the freshly oxygenated blood from the lungs, and sends it along to the left ventricle to be pumped out through the aorta to the body.
A PAC is generated in the atria, so it looks like a normal heartbeat. The P wave may appear different if the PAC came from outside the SA node.
A PAC will cause the atrium (one of the two top chambers of the heart) to contract prematurely—before the next heartbeat was due.
As with all early beats, PACs can happen randomly, in couplets, in triplets or in foursomes. Four or more PACs in a row are described as a premature atrial tachycardia (PAT) or a supraventicular tach (SVT), as the beats are initiated from “above” the ventricles. A paroxysmal PAT or PSVT means that the burst of early beats was abrupt in onset, as well as in cessation.
Much like respiratory sinus arrhythmia, PACs are usually not a problem for healthy people unless they are frequent or sustained, in which case they may be perceived or cause symptoms. Some folks take beta blockers or calcium blocking medications if their PACs are troublesome. Avoiding the triggers listed above is also key.