Advanced ECG’s for MLA’s
Cathie Cousins, RN, BScN, CCN(C)
May 13, 2006 Cathie Cousins,RN BScN CCN(C) 2
Objectives
1. To review Basic Concepts for the 12-Lead ECG
To discuss the following on the 12-Lead ECG2. Bradycardia3. Tachycardia4. Ventricular Ectopy5. ST and T wave changes6. Pacemakers
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1. Basic Concepts
• The heart is a pump with an electrical conduction system
• 2 basic types of cardiac cells in the heart
• Myocardial cells or “muscle” cells
• Specialized cells of the conduction system or “pacemaker” cells
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Electrical Axes and Vectors
• Each of the 12 leads on the ECG has a different pattern because each lead views the hearts electrical axis from a different position
• Atrial and ventricular depolarization and repolarization generate an electric current known as an electrical axis or vector (different from the axis of a lead)
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• Average of all the ventricular vectors points to the left and downward
• Knowing the electrical axis of the heart enables us to determine the normal pattern of each lead and the cause for altered patterns in each lead
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Rate
• Both the atrial and ventricular rates should be measured
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The Grid Method for Rate
• Uses the distance between 2 sequential complexes on the ECG
• Each small square represents 0.04 seconds
- 1500 small squares in 1 minute
- 300 large squares in 1 minute• Count the large squares between P waves for
atrial rate and R waves for ventricular rate• 300 ÷ number of large squares = number of
beats/min
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Quick Tips• 300 ÷ 5 large squares = 60 bpm • 5 or > large squares per minute = Bradycardia
• 300 ÷ 3 large squares = 100 bpm• 3 or > large squares per minute = Tachycardia
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2. Bradycardia
• Bradycardia is a heart rate < 60/min
• Bradycardia can be due a slow sinus rate, the origin of the rhythm or an AV block:
- Sinus Bradycardia - Junctional Rhythm - Idioventricular Rhythm - 2° AV Block Type I - 2° AV Block Type II - 3° AV Block
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Sinus Bradycardia
• Sinus node is pacing at a rate < 60/min• P wave, QRS normal
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Junctional Rhythm
• Sinus node and atria fail to pace the heart.
AV junction paces at → 40-60/min• No P wave or PR interval < 0.12, QRS normal
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Idioventricular Rhythm
• Sinus node, atria, and AV junction fail to pace. Ectopic pacemaker in the ventricles paces at → 20-40/min
• No P wave, QRS wide, ST & T waves often abnormal
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AV Blocks• 2° Type I and 2°Type II AV Blocks, sinus node
paces the heart • Not ever P wave results in QRS,
QRS normal or wide
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• 3° AV Block, sinus node paces the heart• P waves do not result in QRS
AV junction paces, QRS normal
Ventricles pace, QRS wide
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3. Tachycardia
• Tachycardia is a heart rate > 100/min
• Tachycardia can be due to:
- Sinus Tachycardia
- Supraventricular Tachycardia
- Ventricular Tachycardia
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Sinus Tachycardia
• Sinus node is pacing at a rate > 100/min
• P wave, QRS normal
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Superventricular Tachycardia• Ectopic focus in atria or AV junction paces the heart
or Abnormal conduction thru AV node
or Accessory pathway• P wave or no P wave, QRS narrow or wide,
rate > 150/min
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Ventricular Tachycardia
• Ectopic pacemaker in ventricles paces the heart• No P wave, QRS wide and bizarre
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4. Premature Ventricular Contractions
QRS Duration
• QRS duration - depolarization of right and left ventricles, from the endocardium to epicardium
• Normal QRS duration - 0.06-0.10 sec
• QRS duration > 0.10 sec, a conduction delay exists in the bundle branches, Purkinjie network or ventricular myocardium, or ventricular ectopic conduction exists
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• PVC’s, premature ventricular complexes:
the premature beat originates in an ectopic
focus in one ventricle, it depolarizes that
ventricle, then the other
• No P wave, QRS wide & bizarre, ST often abnormal, T wave often opposite the rhythm
• Multifocal PVC’s come from more than one ectopic focus, each foci has a different shape
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• 1 PVC = a PVC• 2 PVC’s = couplet• 3 PVC’s = triplet• 4 PVC’s = ventricular tachycardia
• Every 2nd PVC = bigeminy• Every 3rd PVC = trigeminy
• Bigeminy or trigeminy can refer to any ectopic beat so clarify -
eg. bigeminal PVC’s or bigeminal PAC’s, etc.
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5a. ST Segments
• ST segment = end of ventricular repolarization + early part of ventricular repolarization
• ST segment normally isoelectric
• Ischemic + injured myocardial cells altered
membrane potentials, this allows a current to flow as seen in ST elevation + depression
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Measuring ST Segments
• ST measurement = vertical difference between the isoelectric line + end of QRS complex, the “J” point”
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ST Segment Elevation
• ST segment elevation = >1 mm (>0.1 mV) above baseline after the J point
• ST segment elevation due to severe injury temporary until ischemia resolved or injured heart tissue heals or dies
• ST segments elevate in leads facing the injury
• ST segments depress in leads opposite (reciprocal ) leads
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Types of ST Elevation in AMI
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Other Common Causes of ST Segment Elevation
• Coronary artery vasospasm• Acute pericarditis• Ventricular aneursym• Hyperkalemia• Non-specific ST-T wave changes
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ST Segment Depression
• ST segment depression = > 1 mm below baseline after the J point
• ST segment depression due to severe ischemia temporary until ischemia resolved or heart tissue heals
• ST segments depress in leads facing the ischemia
• ST segments elevate in opposite (reciprocal) leads
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Types of ST Depression in AMI
Different types of
ST depression in AMI:
- downsloping
- horizontal
- upsloping
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Other Common Causes of ST Segment Depression
• Left and right ventricular hypertrophy• Left and right bundle branch block• Digitalis in therapeutic and toxic doses
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Acute MI Facing Leads Opposite Leads
Anterior
Septal V1-V2 None
Anterior V3-V4 None
Lateral I, aVL, & V5 or V6 II, III, & aVF
Inferior II, III, & aVF I & aVL
Posterior V7,V8, V9 on 18 lead V1-V4
Right Ventricle V4R, V5R, V6R on 18 lead None
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5b. T waves
• A T wave represents ventricular depolarization
• T waves normally upright, rounded, and slightly asymmetrical. Normally negative in aVR.
• Normally 1/8 to 2/3 the height of the QRS complex
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Abnormal T Waves in AMI
• Normal Heart - positive T wave
• Subendocardial Ischemia - symmetrically positive tall, peaked T wave
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• Subepicardial
Ischemia -
symmetrically
negative deep T wave
• Late phases in AMI -
deeply inverted
T waves with
abnormal Q waves
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6. Pacemakers
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The 3 Functions of Pacing
1. Sensing – the ability of the pacemaker to recognize the patient’s intrinsic heartbeat
2. Pacing – the pacemaker produces a stimulus either when the sensing circuit does not detect an intrinsic heartbeat or at a predetermined time interval
3. Capturing – the depolarization of the
myocardium in response to pacing
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Pacemaker Codes
• I Chamber(s) paced
• II Chamber(s) sensed
• III Response to sensing
• IV Programmable function(s)
• V Antitachyarrhythmia function(s)
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Pacing Leads Sites - Permanent
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Pacing Leads Sites - Temporary
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Pacemaker Sites - Temporary
Transcutaneous
– External Pacing
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Pacemaker Strip 1
1. Sensing 2. Pacing3. Capturing
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Pacemaker Strip 2
1. Sensing
2. Pacing
3. Capturing
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Thank You
• Remember: It is the team that assists the patient in achieving wellness.
• Thank you and enjoy the exciting world of 12 Lead ECG’s.
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