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Practice educator

ECGs 2: analysis of a rhythm strip

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The second article in a three-part series on electrocardiograms looks at rhythm strip analysis

Citation: Higgins D (2011) ECGs 2: analysis of a rhythm strip. Nursing Times; 107: 28, early online publication.

Author: Dan Higgins is senior charge nurse, critical care, University Hospitals Birmingham Foundation Trust.


Patients who are acutely ill, or at risk of becoming so, are not always nursed in critical care. Developing skills in basic monitoring and observation enables nurses to care for them in a safe and efficient manner.

Continual electrocardiogram rhythm monitoring can alert nurses to changes in a patient’s condition and aid diagnostic decisions. It should be used to identify simple cardiac rhythm abnormalities only, not for making specific diagnoses (Jevon, 2009).

The first article in this three-part series explored indications for ECG monitoring, common problems and nursing responsibilities when caring for monitored patients. This second part discusses the components of a normal ECG rhythm, a systematic six-step approach to rhythm analysis and actions to take if abnormality is seen.

Normal sinus rhythm

The electrical impulses normally initiated from the sinoatrial node of the heart are transmitted through the atrial muscle mass via the atrioventricular node. They then travel down the bundle of His and into the right and left ventricles via the Purkinje fibres. The electrical impulses are recorded using skin electrodes. In normal conduction, the following components are observed:

  • A P wave, associated with atrial contraction;
  • A QRS complex, associated with ventricular contraction;
  • A T wave, associated with ventricular relaxation (see Fig 1 on article PDF).

The time taken from the start of atrial contraction (beginning of the P wave) to the start of ventricular contraction (beginning of the QRS complex) is known as the P-R interval. This should take between 0.12 and 0.2 seconds. Conduction through the ventricles can be measured by looking at the width of the QRS complex, which should be less than 0.12 seconds.

These times can be measured on paper rhythm strips, printed from cardiac monitors. Standard ECG paper is composed of small squares; when the monitor is calibrated to a paper speed of 25mm per second, each of these represents 0.04 seconds. The normal P-R interval is less than five small squares and the QRS complex is less than three small squares. Normal sinus rhythm is shown in Fig 2 (on article PDF).

How to read a rhythm strip

A simple rhythm analysis can be carried out by observing the monitor display. Ideally, a paper strip that allows for the calculation of the PR interval and QRS width should be printed.

Precise identification of complex rhythm abnormalities requires experience. However, adopting a six-step approach to rhythm strip analysis, as outlined by the Resuscitation Council (UK) (2011), should enable nurses to identify a rhythm in enough detail to decide on the most appropriate action (Box 1).

Box 1. Six-step approach to rhythm analysis

  • Is there any electrical activity present?
  • What is the ventricular (QRS) rate?
  • Is the QRS rhythm regular or irregular?
  • Is the QRS width normal or broad/prolonged?
  • Is atrial activity (P waves) present? 
  • How is atrial activity related to ventricular activity?

Source: Resuscitation Council (UK) (2011)

Is any electrical activity present?

After physically assessing the patient, look at the monitor to check if any electrical activity is present. If the patient is breathing and has a pulse but a broken line or flatline is present, it is likely that a monitoring lead has become disconnected. Check all leads are firmly connected, as well as the electrode contacts, and reassess the rhythm.

What is the ventricular rate?

Most cardiac monitors display a heart rate but this should not be relied on as it may be inaccurate. If a rhythm strip is printed, the ventricular rate can be calculated by counting the number of cardiac cycles represented by each QRS complex in six seconds (30 large squares), and multiplying this by 10 (RCUK, 2011).

If the rhythm is irregular, calculating the heart rate using this technique can be difficult. A ventricular rate of more than 100 beats per minute is considered tachycardic, while one of less than 60 beats per minute is bradycardic.

Is the QRS rhythm regular or irregular?

A regular rhythm will have the same number of small squares between each QRS complex. Regularity can be assessed by looking at the monitor display or rhythm strip, but it can be difficult to detect in some tachyarrhythmias.

A rhythm strip should be printed and the distance between QRS complexes marked on another piece of paper. The marked paper can be moved along the rhythm to see if the distances are the same.

Sinus rhythm, bradycardia and tachycardia are regular rhythms. It is important to note that some potentially dangerous rhythms can be regular, for example complete atrioventricular block or atrial flutter.

Is the QRS width normal or broad?

This is assessed by measuring the number of small squares in the QRS complex. The normal interval is less than three small squares (0.12 seconds). If the interval is more than this, it is a broad complex rhythm; if it is less than two small squares, it is a narrow complex rhythm.

A broad QRS suggests the rhythm originates from below the atria and is a ventricular rhythm, while a narrow complex suggests it originates within the atria or the atrioventricular junction.

Is atrial activity present?

Atrial activity is represented by P waves, and each P wave should be the same shape. Use the shape (morphology) of P waves to assess the rate/regularity of atrial activity. In some abnormal rhythms, P wave morphology may be altered or obscured by ECG components, for example supraventricular or ventricular tachycardias. The best lead for observing atrial activity is lead II.

How does atrial activity relate to ventricular activity?

The interval between the P wave and each QRS complex is usually less than 0.2 seconds and should be consistent. The PR interval should be calculated and consistency assessed throughout the rhythm strip.

In atrioventricular blocks, the PR interval may be altered or there may be no relationship between the P waves and the QRS complex.

ECG rhythm abnormality

If rhythm abnormality is suspected, the first and most important step is a comprehensive patient assessment using the airway, breathing, circulation, disability, exposure (ABCDE) framework (RCUK, 2010). Emergency resuscitation should be started if necessary.

Adverse signs and symptoms associated with cardiac arrhythmias include:

  • Shock;
  • Syncope/fainting, loss of consciousness;
  • Myocardial ischaemia;
  • Heart failure.

Any of these signify an emergency.

If a patient has an arrythmia but their airway, breathing and circulation are normal, further investigations should be undertaken, such as rhythm analysis, a diagnostic 12-lead ECG, biochemical blood analysis and arterial blood gas analysis.

Experienced help should be sought as the rhythm may deteriorate or change further. The patient should be observed and physiological track and trigger systems should be used alongside a graded response strategy, in which scores suggesting acute deterioration require certain actions, such as urgent medical review (National Institute for Health and Clinical Excellence, 2007).

All aspects of ECG monitoring care should be accurately documented (Box 2).

Box 2. Documentation

All aspects of ECG monitoring care should be accurately documented, including the following:

  • The time monitoring started and finished
  • The indications for monitoring
  • Any significant patient events that occur during the monitoring process, such as shortness of breath
  • Any actions taken or treatments administered

Source: Nursing and Midwifery Council (2008)


ECG monitoring is a non-invasive technique that can be used with clinical assessment skills to alert nurses to changes in a patient’s condition. A structured approach to rhythm analysis defines any rhythm in enough detail to allow the most appropriate treatment decision to be made. If there is any change or abnormality in ECG, the patient should be assessed using the ABCDE framework and treated accordingly. This may involve resuscitation or further assessment and investigations.

Key points

  • A structured approach to rhythm analysis allows for the most appropriate treatment decision to be made
  • In normal sinus rhythm, a P wave is followed by a QRS complex, which is followed by a T wave
  • The heart rate can be calculated by counting the number of cardiac cycles in six seconds, then multiplying it by 10
  • If a change or abnormality is detected in an ECG, the patient should be assessed using the airway, breathing, circulation, disability, exposure (ABCDE) framework and treated accordingly
  • If a patient has an arrhythmia but their airway, breathing and circulation are not compromised, further investigations, should be carried out
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