How to Read ECG Strip

Medical professionals read an ECG strip which is an integral part of medical practice. An ECG strip records the heart’s electrical activity and provides insights into the patient's heart rhythm and rate. Interpreting an ECG helps us to know about the heart conditions, from arrhythmias to heart blockages, ensuring favorable patient outcomes.

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Reading an ECG strip requires skill, practice, and knowledge. This blog will guide us through the step-by-step process of ECG interpretation, heart rate, rhythm, and waveforms giving us confidence in our ability to identify abnormalities and understand what they signify about the heart’s condition.

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Before picking to interpret an ECG, confirm patient details like:

• Name and date of birth.

• Date and time the ECG was performed.

• Calibration (25 mm/second and 10 mm/mV).

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ECG Strip and its Uses

An ECG is a simple, non-invasive test done by doctors for a variety of reasons.

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The reasons are to:

• Monitor heart function: It helps doctors assess the well-being of the heart.

• Detect abnormal rhythms: It identifies irregular heartbeatsindicating underlying conditions.

• Diagnose heart attacks: It helps to identify signs of a heart attack or reduced blood flow. 

• Track heart treatment progress: It monitors treatment, medications, or pacemakers to track how they affect heart function.

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Normal Sinus Rhythm

The normal sinus rhythm (NSR) is a consistent pattern of heartbeats indicating healthy heart function. NSR is caused by electrical signals from the sinus node. The sinus node is located in the right atrium and is the natural pacemaker of the heart. These signals travel through the heart valves and cause contraction and expansion to facilitate blood flow.

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Role of the sinus node

The sinus node, also known as the sinoatrial (SA) node, is the site of origin of electrical stimuli 60 to 100 times a minute. Under normal conditions, it activates the atria, and the stimulus is passed through conduction pathways to make the ventricles contract and pump blood.

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The SA node causes the contraction of the atria. These signals are then transmitted through the atrioventricular nodes to the lower heart chambers, making them contract to pump the blood.

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Normal sinus rhythm exhibits the following features

• The regular rhythm ranges between 60 and 100 beats per minute.

• Normal P-wave morphology.

• Narrow QRS complexes (<100 ms wide).

• Constant PR interval.

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Components in an ECG

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Understanding paper speed

The rate at which the ECG machine produces a tracing is the paper output speed. The standard output is 25 mm per second. If a different paper speed is used, then calculations need to be modified.

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The standard paper speed is 25 mm per second

• One small square (1 mm) = 0.04 seconds or 40 milliseconds.

• 5 small squares (5 mm) = one large square =  0.2 s or 200 ms.

• 5 large square = 1 second.

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Standard paper speed= 25mm/sec. The rhythm strip has.

250 small squares = 50 large squares = 10 seconds.

We need to understand the rhythm strip recorded for one minute will be comprised of

1500 small squares = 300 large squares = 1minute.

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Steps to read an ECG strip 

A step-by-step approach helps to ensure that no information is overlooked.

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Step 1: Determine the heart rate

The normal heart rate is 60- 100 bpm for a healthy adult.

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• Divide 1500 by the number of small squares between consecutive R waves. It is used for fast heart rates.

• Divide 300 by the number of large boxes. Enclosed between the two R waves.

Step 2: Evaluate the rhythm

Determine if the heart rhythm is regular or irregular by assessing the RR interval.

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• Regular rhythm Shows consistent RR intervals.

• Irregular rhythm shows variable RR intervals. It can be due to arrhythmias like atrial fibrillation or ectopic beats.

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Step 3: Analyse P waves

P waves represent atrial depolarization. Check if they are present and uniform.

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• Sinus rhythm P wave should be consistent in shape and precede each QRS complex.

• Abnormal P waves indicate atrial abnormality, such as atrial flutter or fibrillation.

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Step 4: Assess PR interval

PR interval is the time between the onset of atrial and ventricular depolarization.

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• Normal range. 0.12 -0.20 seconds.

• Abnormal PR intervals. A prolonged PR interval may indicate a heart block, while a shortened interval suggests pre-excitation syndrome like Wolf Parkinson White syndrome.

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Step 5: Examine the QRS complex

QRS complex shows ventricular depolarization.

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• The normal range is less than 0.12 seconds.

• A wide QRS complex indicates bundle branch blocks or ventricular rhythms.

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Step 6: Evaluate ST segment

ST segment should be level with the baseline.

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• ST elevation indicates myocardial infarction or heart attack.

• ST depression is associated with ischemia or strain.

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Inspect T wave

T waves represent ventricular repolarization

Inverted T waves: Suggest ischemia or old myocardial infarction

Tall, Peaked T waves: indicates hyperkalemia

Step 7: Measure QT Interval

QT interval represents ventricular depolarization and repolarization.

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• The normal range is less than 0.44 s.

• Prolonged QT intervals are associated with arrhythmias.

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Special consideration in ECG interpretation

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Age and ECG interpretation

• Pediatric ECG: Children have a high heart rate with sinus arrhythmia which is considered normal.

• Elderly patients. They exhibit slow heart rate and frequent arrhythmias 

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Medications

• Beta-blockers and digoxin are common drugs that affect heart rate and rhythm.

• Electrolyte imbalances also affect ECG.

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Heart blocks

Heart blocks happen when electrical signals are blocked or delayed, causing dizziness, fatigue, or syncope. ECG interpretation and effective patient care depend on identifying different types of heart blocks that requires urgent management.

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• First-degree AB block: Have prolonged PR intervals>0.2s and are usually benign.

• Second-degree AV block (type I): ): The PR interval gets longer with each beat until a QR complex is missing.

• Second-degree AV block (type II): The PR interval stays the same, but some QRS complexes are absent.

• Third-degree (complete) AV block: Atria and ventricles beat independently, resulting in a slow ventricular rate.

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10 Steps for Rhythm Identification

Consistently following a process and analyzing a patient's ECG helps to identify the ECG rhythm.

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1. Is the ECG rhythm regular or irregular?

RR interval is the distance between two consecutive RR waves. If this interval is consistent, the rhythm is considered regular. On the other hand, if this varies, it indicates an irregular rhythm and suggests conditions like fibrillation or other arrhythmias.

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2. Calculate the heart rate

The method to calculate the heart rate has been explained above. The calculations help to understand the speed at which the heart is beating. It is important for assessing cardiac health.

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If the heart rate is lower than 60 bpm, it is bradycardia; above 100, it is tachycardia. Heart Rates above 150 are dangerous and signify supraventricular tachycardia or ventricular tachycardia.

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3. Find the P waves

P waves should be identified on the ECG strip. Each P wave should precede a QRS complex; normal P waves are smooth and rounded. Indicating proper atrial contraction. Absent or irregular P waves suggest atrial fibrillation or other arrhythmias.

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4. Measure the PR interval

The time taken by the impulse to travel from the atria to the ventricles is the PR interval. The distance from the beginning of the P wave to the start of the QRS complex. Normal PR interval ranges from 0.1 to 0.20 seconds. Prolonged intervals suggest first-degree AV block, while short intervals suggest pre-excitation syndrome.

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5. Measure the QRS segment

Counting the small squares covered by the QRS complex helps to determine its width. It represents ventricular depolarization and should be less than 0.12 seconds. A wide QRS complex indicates. Bundle, branch block or ventricular arrhythmias, while a narrow complex suggests normal conduction.

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6. Observe the T-wave

Ventricular contraction is visible in the T wave. The T wave should be upright and smooth in most ECG leads. Abnormalities of T wave inversion, flattening, or peaking conveys ischemia or electrolyte imbalance.

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7. Note any ectopic beats

Ectopic beats or extra beats occur when electrical impulses are formed outside the conduction pathway. Identifying premature atrial contractions (PAC) or premature ventricular contraction (PVC)is important to assess heart health.

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8. Determine the origin

Identify in the ECG from where the rhythm is originating. The point of origin has characteristic features.

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• Sinus: 60 to 100 beats per minute, Regular rhythm, P waves upright, round and present before each QRS segment. Normal PR interval. Normal QRS duration.

• Atrial: Rhythm can be irregular or regular; Normal QRS segment but P waves premature and of different shape, flattened, notched, peaked, inverted, or hidden.

• Junctional: Look for a junctional type of P wave inverted before, during, or after the QRS segment that is normal in duration.

• Ventricular: Wide QRS segment and no P waves since the impulse originates below the SA node.

• Paced rhythm: Low voltage spikes before the QRS complex.

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9. Correctly identify the rhythm

Based on the information gathered, classify the heart rhythm as normal or abnormal. Rhythm identification is essential for diagnosing the heart condition. By recognizing the type of rhythm, we can develop an effective treatment plan for the patient.

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10. Stay current on ECG

Staying updated requires attending regular workshops, reading current literature, and practicing with different ECG strips. Upgrading skills and ongoing learning ensure that we are well-equipped for better patient care.

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Conclusion

To read an ECG strip, we require.

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• Knowledge of the characteristics of ECG paper.

• Steps for interpreting rhythm.

• Tips for recognizing different types of heart blocks.

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By understanding these elements, we can analyze cardiac rhythms and identify issues. Practice makes a man perfect, and it is here Gauze enhances learning; it offers tools and practice materials to support us in ECG interpretation and improving patient care.