ECG interpretation: the basics


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Presentation to participants undertaking the: Critical Care Transition Program at ACT Health, 2008

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  • I have recently produced a site on healthcare resources.

    One of these resources is an ECG Interpretation and Rhythm Recognition resource, which is available both as a e-book and also as a paperback.

    I truly believe that this resource would benefit your readers / users.

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    The url to the specific ECG Interpretation and Rhythm Recognition resource is

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    Jamie BIsson
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  • P-R Interval Location: From beginning of atrial depolarisation P wave to beginning of ventricular depolarisation QRS complex. Height: not measured. Shape: not measured. Duration: 0.12 to 0.20secs . Significance: Evidence of impulse formation or conduction problems: eg: 1st degree heart block > 0.20 secs. WPW syndrome < 0.12secs (impulse arrives early to the ventricles (delta wave slurring the QRS complex).
  • QRS Complex Location: Follows PR interval Shape: 3 waves, differs between leads. Height: not < than 6mm in V1 and V6 8mm in V2 and V5 10mm in V3 and V4 Should not be > than 25 – 30mm in any of these leads. Bipolar limb leads: when adding +ve and -ve components, should be > 6mm. Duration: 0.05 – 0.10 secs. Significance: Ventricular myocardial cell activity Widened may indicate bundle branch block, PVCs, idioventricular rhythms, ventricular tachycardia. ST Segment Location: links the QRS complex with the T wave, extending from end of S wave to beginning of T wave. Shape: isoelectric PR segment is the baseline for which ST segment deviations are judged. Duration: not measured. Significance: May indicate myocardial damage. Elevation: myocardial injury Depression: Myocardial ischemia. Changes: Pericarditis, LV hypertrophy, PE, elect imbalance
  • T-Wave The normal T wave is usually in the same direction as the QRS except in the right precordial leads. In the normal ECG the T wave is always upright in leads I, II, V3-6, and always inverted in lead aVR. Location: follows ST segment. Shape: Upright in most leads, usually same direction as QRS complex, round + asymmetrical. Normal to have isolated inversion of T wave in L111, aVR & V1. Height: no > than 5mm in bipolar leads and < 10mm in precordial leads. Duration: not measured. Significance: Ventricular repolarisation. Inverted or flattened T waves: ? myocardial ischemia or hypokalemia. Peaked T waves: hyperkalemia. Notched: normal in children, may indicate pericarditis in adults.
  • Atrial activity is poorly defined; may see course or fine undulations or no atrial activity at all. If atrial activity is seen, it resembles an old saw (when compared to atrial flutter that often resembles a new saw ). Ventricular response is irregularly irregular and may be fast (HR >100 bpm, indicates inadequate rate control), moderate (HR = 60-100 bpm), or slow (HR <60 bpm, indicates excessive rate control, AV node disease, or drug toxicity). A regular ventricular response with A-fib usually indicates complete AV block with an escape or accelerated ectopic pacemaker originating in the AV junction or ventricles (i.e., must consider digoxin toxicity or AV node disease). The differential diagnosis includes atrial flutter with an irregular ventricular response and multifocal atrial tachycardia (MAT), which is usually irregularly irregular. The differential diagnosis may be hard to make from a single lead rhythm strip; the 12-lead ECG is best for differentiating these three arrhythmias. Causes Ischemic heart disease Hypertension Rheumatic HD Valvular HD Diabetes Alcohol abuse Thyroid disorders Treatment Control ventricular response Synchronized cardioversion if short term problem ( less than 48 hours) Drug therapy to gain control of ventricular rate
  • Causes Heart failure Mitral valve disease Hyperthyroidism Pericardial disease Arterial hypoxia AMI Treatment Treat underlying cause Synchronized cardioversion Drug therapy digoxin, calcium channel blockers Def: supraventricular atrial rate 250-400 beats/minute from a single atrial focus P-waves lost in rapid atrial rate Saw toothed ECG flutter waves Regular atrial activity with a "clean" saw-tooth appearance in leads II, III, aVF, and usually discrete 'P' waves in lead V1. The atrial rate is usually about 300/min, but may be as slow as 150-200/min or as fast as 400-450/min. Untreated A-flutter often presents with a 2:1 A-V conduction ratio. This is the most commonly missed supraventricular tachycardia because the flutter waves are often difficult to find when there is 2:1 ratio. Therefore, always think "atrial flutter with 2:1 block" whenever there is a regular supraventricular tachycardia @ ~150 bpm! (You won't miss it if you look for it in a 12-lead ECG) The ventricular response may be 2:1, 3:1 (rare), 4:1, or irregular depending upon the AV conduction properties and AV node slowing drugs on board (e.g., digoxin, beta blockers).
  • Treatment Non-Pharmacological Valsalva Maneuver  conversion in 20% Carotid Sinus Massage  conversion in 10% Elective Cardioversion Pharmacological Adenosine  rapid onset, reversion 94% Verapamil  slower onset, reversion 91%, documented cases of pt becoming unstable haemodynamically Causes Normal physiological response to fever, exercise, stress, fear. Hyperthyroidism Anaemia Pulmonary embolism Sepsis Drug induced
  • Unifocal, multi-focal, grouped (cuplet, triplet or salvo [4-6 self limiting VT]
  • These leads provide additional vector views of cardiac depolarisation in the frontal plane. Unlike leads I, II, III, the augmented leads utilize a central negative terminal. This virtual "electrode" is calculated by the ECG computer to measure vectors originating roughly at the centre of the heart.
  • QRS complex is measured due to larger muscle mass / electrical activity
  • Twice a P-P (regular spacing)
  • Type II AV block (mobitz) is almost always located in the bundle branches, which means that the QRS duration is wide indicating complete block of one bundle; the nonconducted P wave is blocked in the other bundle. In Type II block several consecutive P waves may be blocked as illustrated below:
  • ECG interpretation: the basics

    1. 1. ECG INTERPRETATION: the basics Jamie Ranse Critical Care Education Coordinator Staff Development Unit ACT Health
    2. 2. Overview <ul><li>Conduction Pathways </li></ul><ul><li>Systematic Interpretation </li></ul><ul><li>Common abnormalities in Critical Care </li></ul><ul><ul><li>Supraventricular arrhythmias </li></ul></ul><ul><ul><li>Ventricular arrhythmias </li></ul></ul>
    3. 3. Conduction Pathways
    4. 4. <ul><li>P wave = atrial depolarisation. </li></ul><ul><li>PR Interval = impulse from atria to ventricles. </li></ul><ul><li>QRS complex = ventricular depolarisation. </li></ul><ul><li>ST segment = isoelectric - part of repolarisation. </li></ul><ul><li>T wave = usually same direction as QRS - ventricular repolarisation. </li></ul><ul><li>QT Interval = This interval spans the onset of depolarisation to the completion of repolarization of the ventricles. </li></ul>Conduction Pathways
    5. 5. Interpretation
    6. 6. <ul><li>Rate = Number of P’s (atrial) R’s (ventricular) per minute (6 second [30 squares] X 10 = minute rate). </li></ul><ul><li>Rhythm = Regular or irregular. Map P-P and R-R intervals. </li></ul>Interpretation P rate: 8 x 10 = 80 R rate: 8 x 10 = 80
    7. 7. <ul><li>3. P wave = present, 1 per QRS, shape, duration, voltage. </li></ul><ul><li>4. P-R interval = length (0.12 - 0.2 sec = <1 big square), isoelectric. </li></ul>Interpretation
    8. 8. <ul><li>5. QRS = duration (0.06 - 0.10 ) , voltage, q or Q waves </li></ul><ul><li>6. ST Segment = shape, isoelectric with PR segment </li></ul>Interpretation
    9. 9. <ul><li>7. T wave = shape, direction </li></ul><ul><li>8. QT interval = length (R-R/2 or QT c <0.40 sec) </li></ul>Interpretation
    10. 10. Abnormalities: Supraventricular arrhythmias <ul><li>Atrial Fibrillation </li></ul><ul><li>Atrial Flutter </li></ul><ul><li>Supraventricular Tachycardia (SVT) </li></ul><ul><li>Premature Ventricular Complexes (PACs) </li></ul>Abnormalities: Ventricular arrhythmias
    11. 11. Conduction Pathways Supraventricular Narrow QRS complex Ventricular Wide QRS complex
    12. 12. Abnormalities: atrial fibrillation Rhythm: Irregular Rate: A: 350 – 650; V: varies P: poorly defined P-R: N/A QRS: narrow complex S-T: normal T: normal Q-T: normal
    13. 13. Abnormalities: atrial flutter Rhythm: Regular / Irregular Rate: A: 220 – 430; V: <300 (2:1, 3:1 or sometimes 4:1) P: Saw toothed appearance P-R: N/A QRS: narrow complex S-T: normal T: normal Q-T: normal
    14. 14. Abnormalities: supraventricular tachycardia (SVT) <ul><li>Rhythm: Regular </li></ul><ul><li>Rate: >100 </li></ul><ul><li>P: not visible </li></ul><ul><li>P-R: not defined </li></ul><ul><li>QRS: narrow complex </li></ul><ul><li>S-T: depression (sometimes) </li></ul><ul><li>T: normal </li></ul><ul><li>Q-T: prolonged (sometimes) </li></ul>
    15. 15. Abnormalities: premature ventricular complexes
    16. 16. Examples
    17. 17. Examples
    18. 18. ECG INTERPRETATION: 12 Lead
    19. 19. Overview <ul><li>Lead Placement </li></ul><ul><li>Axis </li></ul><ul><li>Common abnormalities in Critical Care </li></ul><ul><ul><li>Heart block </li></ul></ul><ul><ul><li>Bundle branch blocks </li></ul></ul><ul><ul><li>Life threatening arrhythmias </li></ul></ul>
    20. 20. Lead Placement <ul><li>V1 = 4th ICS right sternum </li></ul><ul><li>V2 = 4th ICS left sternum </li></ul><ul><li>V3 = midway between V2 and V4 </li></ul><ul><li>V4 = 5th ICS midclavicular </li></ul><ul><li>V5 = between V4 and V6 anterior auxiliary line </li></ul><ul><li>V6 = midauxillary line lateral to V4 and V5 </li></ul>
    21. 21. Lead Placement <ul><li>Electrical activity towards = ↑ </li></ul><ul><li>Electrical activity away = ↓ </li></ul>
    22. 22. Lead Placement
    23. 23. Axis <ul><li>The direction of an ECG waveform in the frontal plane measured in degrees </li></ul><ul><li>Represents the flow of the majority of electrical activity </li></ul><ul><li>Normally the QRS complex is measured </li></ul>
    24. 24. Axis <ul><li>Each lead has its own axis </li></ul>
    25. 25. Lead Placement <ul><li>Standard Leads (bipolar) </li></ul><ul><li>I - lateral wall </li></ul><ul><li>II - inferior wall </li></ul><ul><li>III - inferior wall </li></ul><ul><li>Augmented leads (unipolar) </li></ul><ul><li>aVR - no mans land </li></ul><ul><li>aVL - lateral wall </li></ul><ul><li>aVF - inferior wall </li></ul><ul><li>Chest Leads (unipolar) </li></ul><ul><li>V1 - septal wall </li></ul><ul><li>V2 - septal wall </li></ul><ul><li>V3 - anterior wall </li></ul><ul><li>V4 - anterior wall </li></ul><ul><li>V5 - lateral wall </li></ul><ul><li>V6 - lateral wall </li></ul>
    26. 26. Lead Placement No-mans land, inferior , lateral , anterior , septal ,
    27. 27. Abnormalities: bundle branch blocks <ul><li>QRS widened, greater than 0.12 secs </li></ul><ul><li>Change in axis </li></ul><ul><li>Difficult to interpret ECG </li></ul><ul><li>Right or Left </li></ul><ul><li>Normal P wave </li></ul><ul><li>Followed by a T wave </li></ul>
    28. 28. Abnormalities: right bundle branch blocks <ul><li>Indicates conduction problems in the right side of the heart </li></ul><ul><li>May be normal in healthy people </li></ul><ul><li>R wave in V1, ie two R waves in V1 </li></ul><ul><li>Q wave in V6 </li></ul><ul><li>Lead V1 cats ears </li></ul>
    29. 29. Abnormalities: left bundle branch blocks <ul><li>Always indicates heart disease, usually of the left side of the heart </li></ul><ul><li>Hard to interpret an ECG with LBBB </li></ul><ul><li>Lead V1 Q wave and an S wave </li></ul><ul><li>Lead V6 an R wave followed by another R wave </li></ul><ul><li>Lead V6 Rabbit ears </li></ul>
    30. 30. Abnormalities: heart block <ul><li>SA block (exit block) </li></ul><ul><li>1 st degree AV block </li></ul><ul><li>2 nd degree AV block </li></ul><ul><ul><li>Wenckeback (type I) </li></ul></ul><ul><ul><li>Mobitz (type II) </li></ul></ul><ul><li>3 rd degree AV block </li></ul>
    31. 31. Abnormalities: heart block – SA block
    32. 32. Abnormalities: heart block – 1 st degree AV
    33. 33. Abnormalities: heart block – 2 nd degree AV <ul><li>Wenkeback </li></ul><ul><li>Mobitz </li></ul>
    34. 34. Abnormalities: heart block – 3 rd degree AV
    35. 35. Abnormalities: life threatening arrhythmias <ul><li>Ventricular Tachycardia </li></ul><ul><li>Ventricular Fibrillation </li></ul><ul><li>Asystole </li></ul>
    36. 36. Abnormalities: life threatening arrhythmias - VT
    37. 37. Abnormalities: life threatening arrhythmias - VF
    38. 38. Abnormalities: life threatening arrhythmias – Asystole
    39. 39. Examples
    40. 40. Examples