ECG Interpretation for Student Nurses: Rate, Rhythm and What to Check First

An ECG becomes much easier when you stop seeing a page full of spikes and start seeing one electrical journey repeated across the paper. The impulse should begin in the atria, pause briefly at the atrioventricular node, travel through the ventricles and then allow the ventricles to recover. Your job is to check whether that journey happened in the expected order, at a sensible speed and in a consistent pattern.

This guide gives you a beginner-friendly system that you can use every time. It will not make you an advanced ECG practitioner in one sitting, but it should help you describe what you can see, recognise important patterns and know when to escalate.

Before you read the ECG, look at the patient

An ECG is one part of an assessment. If the person has chest pain, breathlessness, syncope, hypotension, reduced consciousness or other signs of deterioration, begin an ABCDE assessment, call for registered help and follow the local escalation process. Do not delay urgent care while trying to name a rhythm.

Confirm the patient’s identity, the date and time, the reason for recording the ECG, symptoms, observations and relevant medicines. Then check that the tracing belongs to the correct patient and compare it with previous ECGs where available.

First, understand the paper

Standard resting ECGs are usually recorded at 25 mm per second and 10 mm per millivolt. Check the printed calibration rather than assuming it is standard.

• One small horizontal square represents 0.04 seconds.
• One large square, made from five small squares, represents 0.20 seconds.
• Five large squares represent 1 second.
• Thirty large squares represent 6 seconds.

Look for artefact, wandering baseline, loose electrodes or excessive movement. Poor recording quality can imitate an arrhythmia. If the tracing does not fit the patient, check the patient, electrodes and cables and seek help before drawing conclusions.

What one heartbeat is showing you

Use the same electrical story for every complex:

• P wave: atrial depolarisation. In everyday terms, the atria have been electrically activated.
• PR interval: the time from the start of atrial activation to the start of ventricular activation. Much of the delay occurs through the atrioventricular node.
• QRS complex: ventricular depolarisation.
• ST segment: the normally near-flat interval between ventricular depolarisation and the main repolarisation represented by the T wave.
• T wave: ventricular repolarisation, when the ventricles electrically recover.
• QT interval: the total time from the start of ventricular depolarisation to the end of repolarisation.

The Bleepbook ECG sequence

Use this order every time: patient and paper, rate, rhythm, P waves, PR interval, QRS, QT, ST and T waves, then compare and escalate. A fixed sequence prevents you from staring at the most obvious abnormality and missing everything else.

1. Patient, paper and recording quality

Confirm identity, symptoms, observations, calibration and lead labels. A 12-lead ECG uses ten electrodes to create twelve electrical views. If electrodes are misplaced, the tracing may be misleading. See 12-lead ECG placement before interpreting a tracing that looks unexpected.

2. Calculate the rate

For a regular rhythm, count the large squares between two consecutive R waves and divide 300 by that number. Four large squares gives about 75 beats per minute. For greater precision, divide 1500 by the number of small squares.

For an irregular rhythm, count the QRS complexes in a 6-second strip and multiply by 10. This is an estimate, but it is more useful than applying the 300 rule to an irregular R-R interval.

Then describe the rate in context. In adults, a resting rate below 60 beats per minute is often described as bradycardia and above 100 as tachycardia, but the number alone does not tell you whether the patient is stable or what caused it.

3. Decide whether the rhythm is regular

Compare several R-R intervals, not just two. You can use paper or callipers according to local practice.

• Regular: the R-R intervals are approximately equal.
• Regularly irregular: there is a repeating variation.
• Irregularly irregular: there is no repeating pattern. Atrial fibrillation is a common cause, but do not diagnose it from irregularity alone.

4. Find the P waves

Ask four questions:

1. Can I see P waves?
2. Do they look similar to one another?
3. Is there one P wave before every QRS?
4. Does every P wave lead to a QRS?

In sinus rhythm, P waves should be present before each QRS and usually upright in lead II. If P waves are absent, hidden, replaced by flutter waves or unrelated to the QRS complexes, the rhythm needs closer review.

5. Measure the PR interval

Measure from the start of the P wave to the start of the QRS. The usual adult PR interval is 0.12 to 0.20 seconds, or three to five small squares. Then check whether it stays constant.

• A prolonged but constant PR interval may fit first-degree atrioventricular block.
• Progressive PR lengthening before a dropped QRS suggests Mobitz I second-degree block.
• Intermittent dropped QRS complexes with a constant PR in conducted beats may suggest Mobitz II and require urgent review.
• P waves and QRS complexes that have no consistent relationship suggest complete heart block.

These patterns require a full clinical assessment and experienced interpretation. As a student, describe what you see and escalate rather than attaching a confident diagnosis to an uncertain strip.

6. Look at the QRS complexes

Measure from the start to the end of the QRS. A QRS under 0.12 seconds, fewer than three small squares, is described as narrow. A QRS of 0.12 seconds or more is broad.

A narrow-complex tachycardia usually begins above the ventricles. A broad-complex tachycardia may be ventricular tachycardia (VT) or a supraventricular rhythm with abnormal ventricular conduction. In practice, a new regular broad-complex tachycardia should be regarded as possible VT and escalated immediately. Resuscitation Council UK advises that the safest approach is to treat broad-complex tachycardia as VT unless there is a previous specific diagnosis of supraventricular tachycardia with bundle branch block.

7. Check the QT and QTc

Measure the QT interval from the start of the QRS to the end of the T wave. Because the QT changes with heart rate, machines usually report a corrected value called QTc. QT assessment is easy to get wrong when the T-wave end is unclear or the rhythm is irregular. Use the machine value as a prompt, not a verdict, and ask a registered or experienced clinician to review suspected prolongation, particularly when medicines or electrolyte disturbance may be relevant.

8. Review the ST segments and T waves

Look across anatomically related leads, not at one isolated squiggle. Ask whether the ST segment is raised or depressed, whether T waves are inverted, unusually tall or different from a previous ECG, and whether the change fits the patient’s symptoms.

New or dynamic ST-T changes with chest pain, sweating, nausea, breathlessness or haemodynamic compromise require urgent escalation. Do not use a normal-looking ECG to dismiss concerning symptoms, and do not wait for diagnostic certainty before reporting possible myocardial ischaemia.

9. Add the wider 12-lead review

Once the rhythm sequence is reliable, formal interpretation also considers the electrical axis, R-wave progression, pathological Q waves, conduction patterns and changes in contiguous leads. These are important, but beginners learn more safely by first becoming consistent with rate, rhythm, P waves, intervals, QRS width and ST-T review. Use local training and supervised practice to build the next layer.

A worked example from start to finish

Imagine a clear tracing recorded at 25 mm/s and 10 mm/mV. The R waves are four large squares apart. The rhythm is regular. Each QRS is preceded by a similar upright P wave. The PR interval is four small squares and remains constant. The QRS is two small squares wide. There is no obvious new ST elevation or depression on your initial review.

1. Rate: 300 divided by 4 gives approximately 75 beats per minute.
2. Rhythm: regular.
3. P waves: one similar P wave before every QRS.
4. PR: four small squares equals 0.16 seconds, within the usual range.
5. QRS: two small squares equals 0.08 seconds, so it is narrow.
6. ST-T: no obvious abnormality on this initial review.

A sensible beginner’s description is: regular sinus rhythm at approximately 75 beats per minute, with a constant PR interval and narrow QRS complexes; no obvious acute ST-T change seen on initial review. You would still relate it to symptoms, compare previous ECGs and obtain the required registered or medical review.

Common rhythm patterns

The strips below are simplified teaching examples. Real tracings may be less tidy, and a rhythm strip does not replace assessment of the full 12-lead ECG and the patient.

Sinus rhythm, sinus bradycardia and sinus tachycardia

Sinus rhythms have one consistent P wave before every QRS and a consistent PR interval. The label changes according to rate: sinus bradycardia is slower than 60 beats per minute and sinus tachycardia is faster than 100. Always ask why. Pain, fever, hypovolaemia, anxiety, medicines, athletic conditioning and conduction disease are among many possible contexts.

Atrial fibrillation

The ventricular rhythm is irregularly irregular and consistent P waves are absent. The baseline may show fibrillatory activity. NICE recommends a 12-lead ECG to diagnose atrial fibrillation when an irregular pulse raises suspicion (NICE, 2021). Report the rhythm and the patient’s symptoms and observations; do not infer the duration or cause from the strip.

Atrial flutter

Atrial activity may look like repeating saw teeth, often most visible in the inferior leads. Ventricular conduction can be fixed, such as 2:1, or variable. A flutter rhythm can therefore appear regular or irregular.

Supraventricular tachycardia

SVT commonly appears as a fast, regular, narrow-complex rhythm. P waves may be hidden within the QRS or T waves. The ECG appearance alone does not tell you whether the patient is stable. Assess for adverse features and seek urgent registered and medical support.

Ventricular ectopic beats

A premature ventricular complex arrives early, is usually broad and has a different shape from the surrounding QRS complexes. It may be followed by a compensatory pause. Record the frequency and pattern, assess symptoms and observations, and consider the clinical context, including possible electrolyte disturbance or myocardial disease.

Complete heart block

The atria and ventricles beat independently. P waves continue at their own rate while QRS complexes follow a slower escape rhythm, so there is no stable PR relationship. This can cause profound bradycardia, syncope and haemodynamic compromise and requires urgent escalation.

Rhythms that demand immediate action

Ventricular tachycardia

VT is usually a fast, regular, broad-complex rhythm. The patient may have a pulse or may be in cardiac arrest. Check the patient immediately. A broad-complex tachycardia with adverse features needs emergency senior help; pulseless VT is a shockable cardiac-arrest rhythm.

Ventricular fibrillation

VF has no organised QRS complexes and no effective cardiac output. If the patient is unresponsive and not breathing normally, call the emergency response, start CPR and attach a defibrillator according to your training and local process.

Asystole

Asystole is the absence of meaningful ventricular electrical activity and is a non-shockable cardiac-arrest rhythm. Confirm the patient, check the leads and follow the cardiac-arrest algorithm. A loose lead can produce a flat line, but equipment checks must not delay resuscitation.

A quick decision pathway when you feel stuck

1. Is the patient stable? If not, escalate first.
2. Is there an organised QRS rhythm?
3. What is the ventricular rate?
4. Is it regular, regularly irregular or irregularly irregular?
5. Are P waves present, and what is their relationship to each QRS?
6. Is the PR interval constant?
7. Is the QRS narrow or broad?
8. Are there new ST-T changes across related leads?
9. Does the tracing fit the symptoms, observations and previous ECG?

Common beginner mistakes

• Reading the machine statement first: automated interpretation can be useful, but it can also be wrong. Complete your own structured review before comparing it.
• Calling every regular rhythm sinus: sinus rhythm requires an appropriate P wave before every QRS, not simply a regular rate.
• Using the regular-rate formula on AF: estimate an irregular rate over 6 seconds instead.
• Confusing artefact with arrhythmia: look at the patient, baseline, electrodes and other leads.
• Stopping after naming the rhythm: rate, intervals, QRS width and ST-T changes still matter.
• Ignoring change over time: a new difference from a previous ECG may be more important than an isolated textbook threshold.

What should a student nurse report?

Use specific observations rather than saying only that the ECG looks abnormal. A clear SBAR report might be:

Situation: “Mr Patel has new central chest discomfort and looks pale.”
Background: “He was admitted with pneumonia and has no known arrhythmia.”
Assessment: “His pulse is 142 and regular, blood pressure is 92/58, and the ECG shows a regular broad-complex tachycardia.”
Recommendation: “Please review him immediately. I am continuing ABCDE observations with my supervising nurse.”

Your role is to record accurately, recognise concerning features, relate the tracing to the person and escalate within your competence. The NMC Code requires prioritisation of people, effective practice, preservation of safety and professional conduct (NMC, 2018).

Check your understanding

1. Why is a regular rhythm not automatically sinus rhythm?
2. Which rate method would you use for an irregularly irregular rhythm?
3. What does a broad QRS tell you, and what does it not tell you?
4. What pattern suggests complete heart block?
5. What is your first action if the monitor appears to show VF?

References

National Institute for Health and Care Excellence (NICE) (2021) Atrial fibrillation: diagnosis and management. NG196. London: NICE.

Nursing and Midwifery Council (NMC) (2018) The Code: Professional standards of practice and behaviour for nurses, midwives and nursing associates. London: NMC.

Resuscitation Council UK (2025) Adult advanced life support guidelines. London: Resuscitation Council UK.

Resuscitation Council UK (n.d.) How to read a rhythm strip. London: Resuscitation Council UK.

Society for Cardiological Science and Technology (SCST) (n.d.) Revised standards for recording and reporting 12-lead electrocardiograms. London: SCST.