Analysis and Interpretation of the Electrocardiogram

A Self-Directed Learning Module 

Technical Skills Program 

Queen’s University 

Department of Emergency Medicine 

  

 

Introduction

The electrocardiogram (ECG) is one of the most useful diagnostic tests in emergency medicine. It is an easy and inexpensive test that is used routinely in the assessment of patients with chest pain. The ECG is the cornerstone for making the diagnosis of cardiac ischemia and is used for making decisions about eligibility for thrombolytic therapy. 

To avoid misinterpreting the ECG, the clinician must have a systematic approach. This module is designed to guide the learner through a stepwise approach to ECG interpretation. Specific examples of a variety of abnormal ECG's are included at the end along with a brief quiz. 

Objectives

By the end of this learning module you should have a systematic approach to interpreting the ECG and be able to identify common ECG abnormalities. 

The 12 lead ECG

The 12 lead ECG is made up of the three standard limb leads (I, II and III), the augmented limb leads (aVR, aVL and aVF) and the six precordial leads (V1, V2, V3, V4, V5 and V6). 

 

Waves and complexes

 

Intervals and segments

PR Interval: From the start of the P wave to the start of the QRS complex

PR Segment: From the end of the P wave to the start of the QRS complex

J Point: The junction between the QRS complex and the ST segment

QT Interval: From the start of the QRS complex to the end of the T wave

QRS Interval: From the start to the end of the QRS complex 

ST Segment: From the end of the QRS complex (J point) to the start of the T wave 

 

Normal values

Heart rate 60 - 100 bpm 

PR interval 0.12 - 0.20 s 

QRS interval ≤ 0.12 s 

QT interval < half RR interval (males < 0.40 s; females < 0.44 s) 

P wave amplitude (in lead II) ≤ 3 mV (mm)

P wave terminal negative deflection (in lead V1) ≤ 1 mV (mm)

Q wave < 0.04 s (1 mm) and < 1/3 of R wave amplitude in the same lead 

Approach to the ECG

Developing a systematic approach to the interpretation of the ECG is a critical skill for all clinicians. The following outlines one such approach. 

Step 1: Determine the heart rate

There are a number of strategies for determining the heart rate. A simple, quick technique is to find a QRS complex that falls on a major vertical grid-line (1), then count the number of large squares to the next QRS complex (2). Dividing this number into 300 gives you the heart rate. In the ECG below, there are 2 large squares between QRS complexes. 300/2 gives a heart rate of 150 beats per minute. 

 

Step 2: Measure important intervals

The measurement of important electrocardiographic intervals usually includes the PR interval, the QRS interval and the QT interval. At a standard paper speed of 25 mm/second, the width of each small square (1mm) represents 0.04 seconds. One large square (5mm) represents 0.2 seconds. 

 

Step 3a: Calculate the electrical axis

The mean QRS axis refers to the average orientation of the heart's electrical activity. In most cases, an approximation of the axis will be sufficient for the ECG interpretation. There are many different approaches to axis determination, but this discussion will be limited to a simple technique which uses the leads I and aVF to calculate an approximate axis. 

 

  Lead 1 Lead aVF Description Interpretation Axis 

ECG#1 Leads I and aVF equally positive. The axis will be midway between 0° and 90°. Normal axis ~ 40°-50°  

   

ECG#2 Leads I and aVF both positive. Lead I more positive than aVF. The axis will therefore be oriented more toward 0°. Normal axis ~ 20° - 40°  

   

ECG#3 Lead I positive. Lead aVF almost equiphasic. Therefore, the axis will be approaching 0°. (Note: when a lead is equiphasic, the axis will be 90° to that lead.) Normal axis ~ 0°  

   

ECG#4 Lead I positive. Lead aVF negative.The axis will be oriented negatively past 0°. Left axis deviation ~ -30°  

   

ECG#5 Lead I negative. Lead aVF positive. The axis will be oriented positively past 90°. Right axis deviation ~ -120°  

   

ECG#6 Both leads I and aVF negative. The axis will be oriented between -90° and -180°. Indeterminate axis ~ -135°  

   

• Recall that the axis can be considered in terms of four quadrants, with lead I oriented at 0°, and aVF oriented at +90°. An ECG with the QRS axis oriented to the quadrant between 0° and 90° is said to be normal.

• An ECG with the QRS axis oriented to the quadrant between -1° and -90° is said to have left axis deviation.

• An ECG with the QRS oriented to the quadrant between +91° and 180° is said to have right axis deviation.

• An ECG with the QRS oriented to the quadrant between -91° and -180° is said to have an indeterminate axis because one cannot tell if it represents right or left axis deviation.

Step 3b: Calculate the electrical axis

The mean QRS axis is oriented towards the lead with the greatest net QRS deflection. To calculate the net QRS deflection, add up the number of small squares that correspond to the height of the R wave (positive deflection), and subtract the number of small squares that correspond to the height of the Q and S waves (negative deflection). 

  

  In actual fact, the net QRS deflection can be approximated without resorting to counting squares. In the example shown here, one can easily see that the net deflection is slightly more positive than negative. 

  

Step 3c: Calculate the electrical axis

Approximate the net QRS deflection for leads I and aVF. Remember that the mean QRS axis will be oriented towards the lead with the greatest positive net QRS deflection. If the net deflection is positive for both, the axis lies between leads I and aVF (0-90°) and is therefore normal. 

 

Step 4: Evaluate the cardiac rhythm

If the rhythm is regular, the RR interval should be constant throughout the ECG. This can be checked using calipers, or more simply by marking on a piece of paper the distance between two R waves, and comparing this distance between pairs of QRS complexes on the ECG. Next, check to see if a P wave is present before each of the QRS complexes. 

 

Step 5: Inspect P waves for atrial enlargement

The P waves in leads I, II, III and V1 should be inspected for evidence of right or left atrial enlargement. Usually, lead II will have the clearest P wave. 

• P wave amplitude should not exceed 3 small squares (3 mm or 0.3mV). If it does, this represents right atrial enlargement.

• In lead V1, the terminal negative deflection of the P wave represents left atrial depolarization and should not exceed 1 mm (0.1mV). If it does, this is indicative of left atrial enlargement. 

   

 

Step 6: Inspect QRS complexes for ventricular hypertrophy or low voltage

In the setting of Left Ventricular Hypertrophy (LVH), the left ventricle enlarges and so the leads oriented to the left ventricle (V5, V6, aVL) will "see" more electrical activity moving towards them. As well, the leads oriented away from the left ventricle (V1, V2) will "see" more activity moving away from them. In LVH therefore, leads V5, V6 and aVL will have tall R waves, while leads V1 and V2 will have deep S waves. (The arrow in the diagram on the right shows the direction of the net electrical activity in LVH.)  

V1 or V2 

  V5, V6 or aVL 

  The voltage criteria for LVH are satisfied if the sum of the amplitude of the deepest S wave in V1 or V2, and the amplitude of the tallest R wave in V5 or V6, is equal to or greater than 35 mm (3.5 mV). The voltage criteria are also satisfied if the amplitude of the R wave in lead aVL is equal to or greater than 12 mm (1.2mV). 

Step 7a: Inspect QRS complexes for bundle branch block or fascicular block

The normal QRS interval is 0.12 seconds (3 mm or 3 small squares) on the ECG. To correctly determine the QRS interval, use the lead with the widest QRS complex. If the QRS complex is less than or equal to 0.12 seconds, then no further analysis is necessary. If it is greater than 0.12 seconds, then you should try to determine the reason for the abnormally long QRS interval. 

A simple approach is to consider the following three possible causes for QRS widening: 

 

The type of bundle branch block can usually be determined from the examination of three key leads: I, V1 and V6. 

Step 7b: Inspect QRS complexes for bundle branch block or fascicular block

In the normal heart, at the beginning of ventricular depolarization, the QRS axis is oriented to the right because of left-to-right depolarization of the septum. This produces a small R wave in lead V1. Immediately following septal depolarization, the left and right ventricles depolarize. The size of the left ventricle results in a predominantly leftward axis for the remainder of the QRS complex. 

 

Step 7c: Inspect QRS complexes for bundle branch block or fascicular block

In the setting of RBBB, the initial part of the ECG is unchanged because septal depolarization and depolarization of the left ventricle are unaffected. However, the right ventricle depolarizes in a delayed and slow fashion. This results in a widening of the terminal part of the QRS complex and orientation of the axis of the terminal part of the QRS complex to the right. 

 

Step 7d: Inspect QRS complexes for bundle branch block or fascicular block

In the setting of LBBB, the septum is activated in a right to left direction, and then there is depolarization of the right and left ventricles through the right bundle. The result is that the QRS axis has a predominantly left orientation throughout and is wide secondary to the slow activation of the left ventricle. 

 

Step 7e: Inspect QRS complexes for bundle branch block or fascicular block

If the ECG cannot be characterized as a typical RBBB or a typical LBBB, then it can be categorized as an intraventricular conduction delay. This will not be addressed in any more detail at this time. 

Step 8: Assess Q waves and determine significance

The Q waves should be assessed and their significance determined, particularly in regard to the diagnosis of myocardial infarction. Small Q waves are commonly a normal finding in the inferior leads III and aVF, and in the anterolateral leads aVL, I, V5 and V6. Q waves of 0.04 seconds (1 mm) duration and greater than one third the R wave's amplitude in the same lead may be pathological. 

 

The pathological Q waves seen in V1 - V6 indicate that this patient has had an anterior MI in the past. This patient also has evidence of an acute inferior MI as shown by the ST segment elevation in leads III and aVF. 

Step 9: Assess ST segments and T waves

Assess the ST segment for the presence of elevations or depressions, together with T wave abnormalities. ST elevation can indicate the presence of conditions such as acute myocardial injury, Prinzmetal's (variant) angina, pericarditis, ventricular aneurysm or myocardial ischemia. 

 

This ECG is from a patient with an acute inferior MI. Note the ST elevation in the inferior leads (II, III and aVF). The ECG also shows ST depression in leads V1, V2 and V3 - likely a result of reciprocal changes associated with the MI. 

Step 10: Measure QT interval for specific diagnoses

The QT interval can be prolonged secondary to metabolic disorders and drug effects. It must be corrected for heart rate since it is rate dependent. The corrected QT interval is calculated using the following formula: 

• QTI corrected = (QTI observed) / (square root of RR interval)

The QTI corrected is often reported with computerized ECG interpretation. 

 

This ECG is from a male patient with familial prolonged QT syndrome. The QTI corrected is approximately 0.52 seconds. Normal QTI corrected: 0.40 seconds for males; 0.44 seconds for females. 

ECG index

The following represent common ECG findings that the clinician should be familiar with. 

Normal ECG

 

A normal ECG is illustrated above. Note that the heart is beating in a regular sinus rhythm between 60 - 100 beats per minute (specifically 82 bpm). All the important intervals on this recording are within normal ranges. 

1.  P wave: 

• upright in leads I, aVF and V3 - V6

• normal duration of less than or equal to 0.11 seconds

• polarity is positive in leads I, II, aVF and V4 - V6; diphasic in leads V1 and V3; negative in aVR

• shape is generally smooth, not notched or peaked

2. PR interval: 

• Normally between 0.12 and 0.20 seconds.

3. QRS complex: 

• Duration less than or equal to 0.12 seconds, amplitude greater than 0.5 mV in at least one standard lead, and greater than 1.0 mV in at least one precordial lead. Upper limit of normal amplitude is 2.5 - 3.0 mV.

• small septal Q waves in I, aVL, V5 and V6 (duration less than or equal to 0.04 seconds; amplitude less than 1/3 of the amplitude of the R wave in the same lead).

• represented by a positive deflection with a large, upright R in leads I, II, V4 - V6 and a negative deflection with a large, deep S in aVR, V1 and V2

• in general, proceeding from V1 to V6, the R waves get taller while the S waves get smaller. At V3 or V4, these waves are usually equal. This is called the transitional zone.

4. ST segment: 

• isoelectric, slanting upwards to the T wave in the normal ECG

• can be slightly elevated (up to 2.0 mm in some precordial leads)

• never normally depressed greater than 0.5 mm in any lead 

5. T wave: 

• T wave deflection should be in the same direction as the QRS complex in at least 5 of the 6 limb leads

• normally rounded and asymmetrical, with a more gradual ascent than descent

• should be upright in leads V2 - V6, inverted in aVR

• amplitude of at least 0.2 mV in leads V3 and V4 and at least 0.1 mV in leads V5 and V6

• isolated T wave inversion in an asymptomatic adult is generally a normal variant

6. QT interval: 

• Durations normally less than or equal to 0.40 seconds for males and 0.44 seconds for females. 

Acute anterolateral MI

Acute anterolateral MI is recongnized by ST segment elevation in leads I, aVL and the precordial leads overlying the anterior and lateral surfaces of the heart (V3 - V6). Generally speaking, the more significant the ST elevation , the more severe the infarction. There is also a loss of general R wave progression across the precordial leads and there may be symmetric T wave inversion as well. Anterolateral myocardial infarctions frequently are caused by occlusion of the proximal left anterior descending coronary artery, or combined occlusions of the LAD together with the right coronary artery or left circumflex artery. Arrythmias which commonly preclude the diagnosis of anterolateral MI on ECG and therefore possibly identify high risk patients include right and left bundle branch blocks, hemiblocks and type II second degree atrioventricular conduction blocks. 

 

Acute inferior MI

Leads II, III and aVF reflect electrocardiogram changes associated with acute infarction of the inferior aspect of the heart. ST elevation, developing Q waves and T wave inversion may all be present depending on the timing of the ECG relative to the onset of myocardial infarction. Most frequently, inferior MI results from occlusion of the right coronary artery. Conduction abnormalities which may alert the physician to patients at risk include second degree AV block and complete heart block together with junctional escape beats. Note that the patient below is also suffering from a concurrent posterior wall infarction as eveidenced by ST depression in leads V1 and V2. 

 

Acute posterior MI

When examining the ECG from a patient with a suspected posterior MI, it is important to remember that because the endocardial surface of the posterior wall faces the precordial leads, changes resulting from the infarction will be reversed on the ECG. Therefore, ST segments in leads overlying the posterior region of the heart (V1 and V2) are initially horizontally depressed. As the infarction evolves, lead V1 demonstrates an R wave (which in fact represents a Q wave in reverse). Note that the patient below is also suffering from an inferior wall myocardial infarction as evidenced by ST elevation in leads II, III and aVF. 

 

Acute right ventricular MI

In patients presenting with acute right ventricular MI, abnormalities in the standard 12 lead ECG are restricted to ST elevation greater than or equal to 1 mm in lead aVR. Although isolated right ventricular MI is usually seen in patients suffering from chronic lung disease together with right ventricular hypertrophy, it can occur in patients suffering a transmural infarction of the inferior-posterior wall which extends to involve the right ventricular wall as well. Right ventricular MI is most commonly caused by obstruction of the proximal right coronary artery and is frequently associated with right bundle branch block. Furthermore, only 5% - 10% of patients suffer from hemodynamic symptoms.