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Rhythm Analysis

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Rhythm Analysis

STEP 1. Is the rhythm regular or irregular?

  • Regularity or irregularity of rhythm should be evaluated using calipers. 
    • Each R-R interval on a 6-second rhythm strip should be measured with calipers. 
    • Lead II is the lead most often referenced when evaluating R-R intervals for regularity of rhythm. 
  • The rhythm is regular if the R-R intervals are equidistant from one another. 
  • The rhythm is not regular if the R-R intervals are not equidistant from one another.

R-R interval measurement.

R-R Interval Measurement

A regular rhythm is typically present in patients with a sinus rhythm. However, a regular rhythm does not exclude the possibility of underlying disease. Some forms of supraventricular tachycardia, ventricular tachycardia, and escape rhythms present as a regular rhythm.

Atrial fibrillation is the most common cause of an irregular rhythm throughout the R-R intervals. 

STEP 2. Determine the presence of a P wave

If the P wave is present, then the patient has a sinus rhythm. If it is not a sinus rhythm, then there are three possibilities:

    • An abnormal or non-sinusoidal P wave depicts an atrial rhythm.
    • Absent P waves depict an atrioventricular junctional rhythm.
    • The P wave has been replaced by other atrial waves, such as occurs in atrial flutter or atrial fibrillation.

STEP 3. Determine the rate

The rate can be determined manually but is calculated automatically by advanced ECG systems.

Normal ECG timing, amplitude, and rate.

Normal ECG Timing, Amplitude, and Rate

STEP 4. Measure the PQ interval. 

The normal PQ interval is 120–200 milliseconds. The PQ interval helps determine if every P wave is conducted.

  • If not every P wave is conducted, the patient may have some form of second-degree AV block. 
  • If there are no P waves conducted, then the atria and ventricles are generating impulses independently from one another. The patient likely has a third-degree AV block or complete AV block. 
  • If the P waves are twisting around the QRS complex, then the patient may have some special form of AV dissociation.

STEP 5. Measure the QRS duration

The normal QRS duration is 90 milliseconds (0.09 seconds) or less. 

A QRS duration > 120 milliseconds (0.12 seconds) is considered prolonged. 

A prolonged QRS duration that exhibits a pattern of bundle branch block may be one of the following:

  • Supraventricular tachycardia with aberration
  • A rhythm originating from the ventricles with AV dissociation
    • If the rate is slow, the patient has a ventricular escape rhythm.
    • If the rate is normal, then the patient has an idioventricular rhythm.
    • If the rate is fast, then the patient has ventricular tachycardia.

Key Takeaway

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  • A regular rhythm presents with equidistant R-R intervals on a 6-second strip.
  • The presence of P waves indicates a sinus rhythm.
  • A prolonged PQ interval indicates an AV block.

Analyzing ECG Morphology


Related Video – OQQ: Foci vs Morphology – What’s the difference?


STEP 1. Analyze the P wave morphology. 

  • The normal P wave has a duration of 0.09 seconds. 
  • P wave duration > 0.11 seconds with a pronounced terminal negativity in lead V1 indicates left atrial enlargement.
  • P wave voltage > 2.5 mm in leads III and aVF indicates right atrial enlargement.
  • An increased P wave duration in lead V1 and an increased P wave voltage in leads III and aVF indicate the patient has bi-atrial enlargement.
  • An inverted P wave on lead I with a positive P wave in lead aVR indicate that there is a false poling of the upper leads (likely the erroneous interchange of the upper limb leads).

STEP 2. Analyze the QRS morphology. 

The mean frontal QRS axis (ÅQRSF) is determined as described in section I. The clinician then determines if there is a broad QRS configuration, if the QRS duration is prolonged, or if the QRS complex is widened. 

Presence of a QRS aberration:

If the QRS aberration is > 0.12 seconds, then the patient may have a right bundle branch block (RBBB). RBBB presents with the typical RSR pattern in leads V1–V3, as seen in Figure 2.

Right bundle branch block RSR pattern.

Right Bundle Branch Block RSR Pattern

If the QRS aberration is > 0.14 seconds, then the patient may have a left bundle branch block (LBBB). LBBB presents with a broad monophasic R wave in the lateral leads (I, AVL, and V5–V6), as seen in Figure

Broad monophasic R wave pattern.

Broad Monophasic R Wave Pattern

A QRS duration > 0.16 seconds is a rare finding and likely a BBB that is suspicious for severe hyperkalemia, as seen in Figure 4.

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Bilateral BBB typically presents with RBBB + LAFB or RBBB + LPFB. 

LAFB = left anterior fascicular block; LPFB = left posterior fascicular block

Atypical BBB configurations with a QRS duration > 0.14 seconds are suspicious for ventricular origin with atrioventricular dissociation.

STEP 3. Look for pathologic Q waves or QS segments

Old myocardial infarction presents with Q waves with symmetric negative T waves concurrent with a medical history of coronary artery disease or with an increased risk factor for coronary heart disease.

Myocardial infarction with Q and T waves.

Myocardial Infarction With Highlighted Q and T Waves

Atypical ECG findings of an old MI include asymmetric discordant T waves with a clinical history representing a low probability of coronary artery disease.     

Common differential diagnoses in the presence of Q or QS segment anomalies include:

  • Artifact
  • Situs inversus: a medical condition in which the heart position is mirrored; the normal left atrium and ventricle are on the right, and the right side of the heart is on the left. Instead of the main impulse traveling downward and to the left, the impulse travels downward and to the right. 
  • Left and/or right ventricular hypertrophy
  • Pre-excitation (QS segment in leads III, aVF)
  • Hypertrophic obstructive cardiomyopathy
  • LBBB (presence of QS in III, aVF, V1 to V4 with a duration of 0.14 seconds or longer)
  • Left anterior fascicular block and left posterior fascicular block
  • Presence of a delta wave
  • An intraventricular conduction disturbance
  • An old MI

STEP 4. Evaluate the ST segment. 

ST elevation usually indicates an acute MI. The diagnosis of MI is likely in patients with elevated cardiac markers or preexisting coronary artery disease.

ST depression in the presence of coronary artery disease indicates ischemia, left ventricular hypertrophy, or left ventricular overload. It can also be related to bundle branch blocks.

  • ST elevation can be a normal variant in cases of early repolarization, particularly in leads V2 and V3
  • The presence of ST elevation in leads I, II, and aVF is indicative of acute pericarditis.
  • The mirror imaging of ST depressions is seen in left ventricular hypertrophy or systolic left ventricular overload.

STEP 5. Evaluate T (and U) waves. 

Asymmetrically inverted T waves are normal in lead V1. They are also normal in leads aVF and III (and sometimes lead II). ÅQRSF is vertical. When there is a left ÅQRSF, inverted T waves are also normal in lead aVL. 

Pathologic negative T wave projection is indicative of: 

  • Left ventricular hypertrophy
  • Left ventricular overload
  • Preexcitation
  • Bundle branch block

Asymmetric T wave negativity often suggests ischemia, but a variety of differential diagnoses are possible. The differential includes, but is not limited to:

  • Late stage pericarditis 
  • Left ventricular hypertrophy 
  • Left ventricle overload 
  • Acute pancreatitis 
  • Drugs

High, symmetric T waves may indicate ischemia. They may also indicate a high serum potassium level. 

The negative deflection of the U wave is ischemic in nature.

STEP 6. Evaluate the QT Segment. 

  • A prolonged QT segment is indicative of long QT syndrome and hypocalcemia
  • Shortened QT segments represent hypercalcemia

The fusion of the U and T waves indicates hypokalemia or long QT syndrome.

Summary: Last 6 Steps in Rhythm Analysis: ECG Morphologies

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