STEP 1. Is the rhythm regular or irregular?
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:
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
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.
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:
Key Takeaway
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STEP 1. Analyze the P wave morphology.
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
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
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.
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 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:
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.
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:
Asymmetric T wave negativity often suggests ischemia, but a variety of differential diagnoses are possible. The differential includes, but is not limited to:
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.
The fusion of the U and T waves indicates hypokalemia or long QT syndrome.