Introduction to Dysrhythmias

Core Concepts

What makes a rhythm "abnormal"

A dysrhythmia is described by four features:

1. Site of origin — where the impulse starts. Normal is the SA node; abnormal sites include the atria, the AV junction, and the ventricles.
2. Rate — too fast (tachycardia) or too slow (bradycardia), either globally or for that site's expected rate.
3. Regularity — regular, irregular, or irregularly irregular.
4. Conduction — how the impulse travels, including delays or blocks at the AV node or within the ventricles.

These features are read directly from the ECG: P waves tell you about atrial activity, the QRS tells you about ventricular activity, and their relationship tells you about conduction.

Mechanisms: three ways electricity goes wrong

Most dysrhythmias arise through one of three mechanisms:

• Altered automaticity — a cell fires too fast, too slow, or in the wrong place. Examples: the SA node firing at 140 beats/min in sinus tachycardia, or an irritated ventricular cell firing early (premature ventricular contraction).
• Triggered activity — an action potential fires again prematurely because of abnormal after-depolarizations during or just after repolarization. Digoxin toxicity and low potassium/magnesium are classic causes; this mechanism underlies many torsades de pointes episodes.
• Re-entry — an impulse travels down one pathway, loops back through a second pathway that has recovered from refractoriness, and re-excites tissue it already passed. Re-entry underlies many supraventricular tachycardias, atrial flutter, and ventricular tachycardia, and it is the mechanism antidysrhythmic drugs most often target by lengthening refractory periods.

Bradycardia vs. tachycardia: the first split

The first clinical split is rate. Bradycardias (slow rhythms) matter when the rate is too slow to perfuse the brain and organs, producing dizziness, syncope, or fatigue. Tachycardias (fast rhythms) matter because a very fast rate leaves too little diastolic time for ventricular filling and coronary perfusion, so cardiac output falls even though the heart is working hard. Within tachycardias, the next question is whether the QRS is narrow (supraventricular origin) or wide (ventricular origin or aberrant conduction) — a distinction that changes the differential and, ultimately, drug selection. Whether a specific dysrhythmia needs treatment at all depends on symptoms, hemodynamic stability, and the underlying condition — not on the ECG alone.

The ECG in dysrhythmia work

A systematic rhythm-strip approach:

1. Is there a P wave before every QRS? If yes, the SA node is driving the rhythm (sinus). If P waves are absent, buried, or unrelated to the QRS, the rhythm originates elsewhere.
2. Is the QRS narrow or wide? Narrow suggests a supraventricular origin (normal His-Purkinje conduction); wide suggests a ventricular origin or conduction block.
3. Is the rhythm regular? Atrial fibrillation is classically irregularly irregular; most others are regular.
4. What is the rate? Count QRS complexes over time and extrapolate to beats per minute.

This screen does not diagnose every rhythm, but it reliably separates the common ones and tells you where to look next.

When drugs come in: the role of antidysrhythmics

Antidysrhythmic drugs suppress abnormal rhythms by changing the electrical properties of cardiac cells: slowing automaticity, slowing conduction, or lengthening refractory periods. They do not "repair" the heart — they alter the electrical environment so the abnormal rhythm cannot sustain itself. Because every one of these effects can also be proarrhythmic, drug therapy is reserved for dysrhythmias that matter, is chosen based on the rhythm and the patient's condition, and is monitored with serial ECGs and electrolytes. Doses, indications, and monitoring schedules are outside the scope of this study guide — always verify against current references and the prescriber's orders.

Common Confusions