How to Read an EKG Rhythm Strip: A Nurse's Step-by-Step Guide

Learning how to read an EKG rhythm strip is one of the most useful bedside skills a nurse can build. It pays off every shift on Med-Surg, Telemetry, the ER, and the ICU. If rhythm strips are new to you, this is the perfect place to start.

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What Is a Rhythm Strip?

EKG (or ECG) stands for electrocardiography — the electrical activity of the heart traced onto paper or a monitor. A rhythm strip is at least a 6-second tracing showing one or two leads.

Leads are simply different "views" of the heart. A full 12-lead EKG captures 12 views, but most portable and bedside monitors only track 3 to 5 leads at a time. The good news: a single rhythm strip is much easier to interpret than a 12-lead, and most strips are read from Lead II, which gives a clean view of the conduction system.

The goal of reading a rhythm strip is to determine the rate and rhythm. That is ideal for spotting the baseline rhythm plus any arrhythmias or ectopy (like a premature beat). A 12-lead also assesses rate and rhythm, but adds a near-360° view that can reveal ischemia or infarction, conduction delays, and chamber enlargement.

Reading the Graph Paper

A rhythm tracing is printed on standardized graph paper, usually as a 6-second strip. Thick black lines mark every 3 seconds, so the distance between three black lines equals 6 seconds. That makes rate estimation easy for irregular rhythms: count the complexes in a 6-second strip and multiply by 10.

The paper is built from small boxes and large boxes — five small boxes make one large box. Those boxes are how you measure time and decide whether a rhythm is regular.

The PQRST Explained

So much for the paper — what about the actual tracing? An electrical tracing is made of waves, complexes, and intervals, and each one represents a specific event in the heart's conduction. Learning the PQRST is the key to interpreting any strip.

Normal Sinus Rhythm (NSR)

Before you can recognize an arrhythmia, you need to know what normal sinus rhythm looks like — it is the gold standard you compare every strip against. The "sinus" means the signal originates from the SA node, and its presence tells you the conduction system is doing its job (though certain blocks can still hide inside it).

The Systematic Approach

Always read a strip the same way every time so you never miss anything. Run through these six questions in order:

Other Sinus Rhythms

These are variations that are still driven by the SA node (plus paced rhythms, which replace NSR once a pacemaker is placed).

Sinus Bradycardia (SB)

Same as NSR, but the rate is under 60 bpm. It can be perfectly normal in well-conditioned athletes, in patients on beta-blockers, and during sleep.

Sinus Tachycardia (ST)

Same as NSR, but the rate is over 100 bpm (usually under 150 at rest). At rest it often signals an underlying cause — anxiety, certain drugs, sepsis, dehydration, or volume loss. You don't treat the ST itself; you treat the underlying cause (for example, give fluids for volume depletion).

Paced Rhythm

Paced rhythms look different depending on lead placement. An atrial lead produces an NSR-like tracing with a pacer spike before the P wave. A ventricular lead looks like a slow VTACH with a pacer spike before the QRS. You can also see both. Some monitors only show the spike when that function is enabled — so if you see a very slow, wide rhythm, ask whether the patient has a pacemaker.

Heart Blocks

A heart block is a delay or blockage in transmitting the signal from the atria to the ventricles, often with a junctional or ventricular escape rhythm.

First-Degree AV Block

Generally no big deal — common with age and beta-blockers. The PR interval is consistently longer than 200 ms, but every P still conducts.

Second-Degree Type I (Wenckebach / Mobitz I)

A progressively lengthening PR interval that eventually leads to a dropped QRS, then the cycle resets.

Second-Degree Type II (Mobitz II)

A consistent PR interval with randomly dropped QRS complexes. More dangerous than Type I because it can progress to complete block.

Third-Degree (Complete Heart Block)

Complete dissociation of the atria and ventricles — the P waves and QRS complexes march to their own beat with no relationship between them.

Atrial Arrhythmias

Atrial Fibrillation (AF)

A very common arrhythmia you will absolutely see in the hospital. AF can be new-onset, with RVR, paroxysmal, or chronic. It is an irregularly irregular rhythm — there is no pattern to the QRS spacing. Both atria are quivering ("fibrillating"), which produces fast ventricular rates and poor atrial blood flow that predisposes to clots. That's why these patients get rate control (metoprolol, Cardizem) and usually anticoagulation (heparin, Eliquis, and so on).

On the strip: no P waves, a fibrillatory baseline, and narrow QRS complexes that never map out. Rates over 100 bpm are termed AF with RVR.

Atrial Flutter

Similar to AF and treated much the same, but instead of fibrillating, the atria are "fluttering" — usually from a reentrant loop near the AV node. You'll see classic saw-tooth flutter waves (F waves). The AV node conducts only a fraction of them, commonly 2:1 for a ventricular rate around 150 (also 3:1 near 100, 4:1 near 75, or variable). Flutter is usually regular.

Supraventricular Tachycardia (SVT)

An umbrella term for any fast tachycardia originating above the ventricles, but clinically it usually means AV nodal reentrant tachycardia (AVNRT) — a reentrant loop in or near the AV node. A PAC or PVC at the wrong moment can send the signal racing around that loop, producing very fast rates (roughly 140 to 220 bpm). The faster it goes, the more symptomatic the patient. On the strip: regular, narrow QRS, usually no visible P waves, often with ST depression. Treatment starts with vagal maneuvers and often adenosine or Cardizem.

Ventricular & Lethal Rhythms

Ventricular Tachycardia (VTACH / VT)

A fast tachyarrhythmia originating in the ventricles, often from coronary disease such as a prior or current MI. It is regular, 100 to 250+ bpm, with a wide QRS (>140 ms). P waves are usually absent, though AV dissociation appears in a majority of cases. The patient may or may not have a pulse — either way, VT is serious.

Ventricular Fibrillation (VF / VFIB)

A deadly arrhythmia with no pulse — the patient is coding. Like AF, but it is the ventricles fibrillating. Coronary artery disease and severe electrolyte abnormalities are common causes. It is irregular with no pattern, showing coarse or fine fibrillation, and will degenerate into asystole if not defibrillated.

Asystole

The absence of cardiac activity — essentially a flat, wavy line, sometimes with occasional P waves early on. The patient is in cardiac arrest. Follow ACLS.

Pulseless Electrical Activity (PEA)

The monitor shows an organized rhythm (often NSR or SB), but there is no mechanical contraction and no pulse. The patient is unresponsive and pulseless — this is a code blue. Follow ACLS.