Electrocardiography Flashcards
What are the 3 unique properties of cardiac muscle cells
- Automaticity: ability to discharge an electrical stimulus w/o stimulation from a nerve
- Rhythmicity: regularity of pacemaking activity
- Conductivity: the ability to spread impulses to adjoining cells very quickly
ECG represents electrical impulses of the heart and reflects activity of 4 types of myocytes
- Typical working myocytes: respond to electrical stimulus to contract & pump the blood
- Nodal: have the highest rate of rhythmicity but slow impulse conduction rates
- Transitional: conduct impulses 2x as fast as nodal cells
- Purkinji cells: have a low rate of rhythmicity yet a high rate of conductivity
What occurs when the electrical impulse is generated
- Potassium (K+) is prominent on the inside of the cell & sodium (Na+) on the outside
- Sodium ions flow inward (fast channel)
- Potassium ions start to flow outward (slow channel)
- As the cell becomes positive on the inside/interior the myocardial cells are stimulated to contract
Define depolarization
- Depolarization: electrical stimulation of specialized cells that causes contraction
Define repolarization
- Repolarization: cells return to a negative interior and a positive exterior, and muscle relaxation occurs.
- Begins when the potassium ion flow outward exceeds the sodium flow inward
Describe sympathetic and parasympathetic impact on the heart
- Sympathetic: increases HR, conduction velocity throughout AV node, contractility, & irritability; norepinephrine and epinephrine release
- Parasympathetic: general inhibitor on the rate of impulse formation & conduction velocity; acetylcholine release; Vagus nerve
Conduction system pathway through the heart
- Electrical pulse initiated in the SA node in right atrium -> internodal pathways
- Travels to the left atrium via the Bachmann bundle
- P wave on ECG = atrial depolarization
- Travels to AV node; atrial kick = PR interval on ECG
- Travels to His bundle and bundle branches
- Travels to Purkinje fibers, stimulating contraction; ventricular depolarization = QRS complex on ECG
- Repolarization= ST segment phase of ECG
Describe the 3 internal tracts
- Bachman (b/w R & L atrium, may be implicated in AFib)
- Wenkebach (middle)
- Thorel (posterior)
A single-lead tracing (rhythm strip) is assessed for ______________, _________, and presence of ______________.
- Heart rate
- Rhythm
- Arrhythmias
If hypertrophy, ischemia, or infarction is suspected, a ________ ECG should be obtained.
- 12 lead
Twelve-lead ECG assesses the following elements
- Heart rate
- Heart rhythm
- Hypertrophy
- Ischemia and/or infarction
Heart rate on ECG is determined by
- Six-second tracing
- R wave measurement
- Counting boxes
Describe how to determine HR when counting boxes on ECG
- Find an R wave
- Count off 300, 150, 100, 75, 60, & 50 for each black line that follows until the next R wave falls
- Take the difference between the two numbers the 2nd R wave follows between and divide by 5
How can you determine heart rate using the QRS Complexes
- On a 6 second strip
- Count the number of QRS Complexes shown
- Multiply by 10
- Gives you the BPM
Single-lead monitoring can accurately assess only ____ and ______.
It cannot diagnose _________ (ST-segment changes) and __________ (Q waves).
- Rate and rhythm
- Ischemia and infarction
Waveforms that represent depolarization of the myocardium are labeled
- P
- QRS
- T
What questions should you ask yourself when assessing rhythm on ECG
- Is there a P wave before each QRS Complex?
- Are the QRS Complexes regular or irregular?
- Best to check lead II on 12-lead ECG
What is the systematic approach to assess rate and rhythm
- Evaluate the P wave. (Is it normal and upright, and is there a P wave before every QRS? Do all the P waves look alike?)
Evaluate the PR interval. (Normal duration is 0.12–0.20 seconds.) - Evaluate the QRS complex. (Do all QRS complexes look alike?)
- Evaluate the QRS interval. (Normal duration is 0.06–0.10 seconds. >0.12 seconds is abnormal)
- Evaluate the QT interval. (Normal is <0.45 sec, if prolonged risk for arrhythmia is increased)
- Evaluate the T wave. (Is it upright and normal in appearance?)
- Evaluate the R to R wave interval. (Is it regular?)
- Evaluate the heart rate (6-second strip if regular rhythm; normal rate is 60–100 beats per minute).
- Observe the patient and evaluate any symptoms. (Do the observation, symptoms, or both correlate with the arrhythmia?)
How to assess intervals on ECG
- QRS interval should be <.120 (more than 3 small boxes is concerning)
- QTc should be <500
- > 500 is risk for spontaneous torsades
Normal sinus rhythm (NSR) is normal cardiac rhythm indicated by the following
- All P waves upright, normal, identical
- P wave exists before every QRS complex
- PR interval is between 0.12 and 0.2 second
- QRS complexes are identical
- QRS duration is between 0.06 to 0.10 second
- RR interval is regular
- Heart rate is between 60 and 100 bpm
Sinus bradycardia differs from NSR only by heart rate
- HR <60 bpm
- Individuals often asymptomatic
- Common in athletes & individuals taking β-blocking medications
Sinus tachycardia differs from NSR only by rate
- HR >100 bpm
- Usually benign
- Possible causes: exercise, anxiety, hypovolemia, anemia, fever or infection, medications (stimulants), and low cardiac output
Causes of bradycardia use the acronym DIE
- Drugs
- Ischemia
- Electrolytes
What are the 5 common causes of tachycardia
- Decreased O2 delivery; anemia, hypoxia, PE
- Infection
- Decreased volume: bleeding, dehydration
- Drugs: stimulants or alcohol (ETOH) withdrawal
- Adrenaline: pain, anxiety, nerves
Define Sinus arrhythmia
- Irregularity in rhythm; impulse is initiated by the SA node, but with a phasic quickening and slowing of the impulse formation
Signs one ECG of a sinus arrhythmia
- RR interval varies throughout
- HR b/w 40-100 bpm
- Respiratory type: related to respiratory cycle; rate increases with inspiration & decreases with expiration
- Non-respiratory type: may occur with infection, medication (toxicity associated with digoxin and morphine), and fever
When does an artifact appear on ECG
- Occurs when we are moving
Sinus pause or block occurs when
- When the SA node fails to initiate an impulse, usually for only one cycle
- Pause will be more than 2x the previous cardiac cycle of the underlying rhythm
- Overall HR = 70
Define Wandering atrial pacemaker
- Pacemaking in wandering pacemaker shifts from focus to focus, leading to irregular rhythm with an inconsistent pattern
Signs on ECG of a Wandering atrial pacemaker
- P waves are present but vary in configuration; each P-wave may look different
- PR intervals may vary but are usually within normal width
- QRS complexes are identical
- RR intervals vary
- HR is usually <100 bpm
Define premature atrial complex
- Defined as an ectopic focus in either atria that initiates an impulse before the next impulse is initiated by the SA node
- Often a pause follows the premature atrial complex but it may be compensatory
Define atrial tachycardia
- 3 or more premature atrial complexes in a row
- HR usually greater than 100 bpm; may be as fast as 200 bpm
Define paroxysmal (supraventricular) atrial tachycardia
- Sudden onset of atrial tachycardia or repetitive firing from an atrial focus
- Underlying rhythm is usually NSR, followed by an episodic burst of atrial tachycardia that eventually returns to sinus rhythm
- Patient may report “all of the sudden, my heart went racing”
Describe atrial flutter
- Usually there is more than one P wave for every QRS complex
- RR intervals may vary depending on the atrial firing and number of P waves before each QRS complex
- Rapid succession of atrial depolarization caused by an ectopic focus in the atria that depolarizes at a rate of 250 to 350 times per minute
- P waves are called flutter waves and look identical to one another, with a “sawtooth” pattern
Describe Atrial fibrillation (A-fib)
- An erratic quivering or twitching of the atrial muscle caused by multiple ectopic foci in the atria that emit electrical impulses constantly
- No true P waves and totally irregular rhythm
- RR interval is characteristically defined as irregularly irregular
- Rate varies but is called ventricular response.
- If ventricular response is below 100, atrial fibrillation is considered controlled atrial fibrillation.
- If ventricular response is more than 100, atrial fibrillation is considered uncontrolled. - If ventricular response is more than 120, this is considered rapid ventricular response (RVR) atrial fibrillation.
Define premature junctional or nodal complexes arrhythmia
- Premature impulses that arise from AV node or junctional tissue
- AV node becomes irritated & initiates an early beat
- Similar to premature atrial complexes, except an inverted, an absent, or a retrograde (wave that follows the QRS) P-wave is present
Define junctional (or nodal) rhythm
- AV junction takes over as pacemaker of heart
- Rate b/w 40-60 bpm
- Absence of P-waves before the QRS complex, but a retrograde P-wave may be identified
- RR intervals are regular
- If HR is too slow, symptoms of cardiac output decompensation can occur
Describe nodal (junctional) tachycardia
- AV junctional tissue acts as the pacemaker, but rate of discharge is accelerated.
- P waves are absent, but retrograde P wave may be present
- The RR interval is regular..
- Rate is usually greater than 100 beats per minute.
Describe a first-degree atrioventricular heart block
- Impulse may be initiated in SA node but delayed on way to AV node, or initiated in the AV node itself, and the AV conduction time is prolonged.
- PR interval is prolonged (>0.20 second/more than one big box)
Describe a Second-degree atrioventricular block, type I (Wenckebach or Mobitz I heart block)
- A benign, transient disturbance occurring high in the AV junction, preventing conduction of some impulses through the AV node.
- Typical appearance: a progressive prolongation of the PR interval until finally one impulse is not conducted through to the ventricles (no QRS complex following a P wave).
- The cycle then repeats itself.
- Usually asymptomatic and requires no intervention in asymptomatic patients.
Describe a Second-degree atrioventricular block, type II (Mobitz II)
- A ratio of P waves to QRS complexes that is greater than 1:1 and may vary from 2 to 4 P waves for every QRS complex
- Nonconduction of an impulse to the ventricles without a change in the PR interval
- Site of the block usually below the bundle of His and may be a bilateral bundle branch block
- Danger of progressing to complete heart block (third-degree AV block), a life-threatening condition
- Transvenous pacemaker is usually indicated even in patients without symptoms.
- The heart rate is usually below 100 and may be below 60 beats per minute.
Describe a Third-degree atrioventricular block
- Complete block in which all impulses that are initiated above the ventricle are not conducted to the ventricle
- No relation between P waves and QRS complex because the atria are firing at their own inherent rate
- Usual causes: acute myocardial infarction, digoxin toxicity, or degeneration of the conduction system
- QRS duration may be wider than 0.10 seconds if the latent pacemaker is in the ventricles.
- The heart rate depends on the latent ventricular pacemaker and may range from 30 to 50 beats per minute
- Requires permanent pacemaker insertion, with atropine and isoproterenol injection or infusion in acute situation
- Considered a medical emergency
The characteristic ECG findings for right bundle branch block are as follows
- QRS duration is greater than or equal to 120 milliseconds
- In lead V1 and V2, there is an RSR` (“bunny ears”) in leads V1 and V2
- In Leads 1 and V6, the S wave is of greater duration than the R wave, or the S wave is greater than 40 milliseconds
- In Leads V5 and V6, there is a normal R wave peak time
- In Lead V1, the R wave peak time is greater than 50 milliseconds
Describe premature ventricular complexes (PVCs) arrhythmias
- Ectopic focus originates an impulse from somewhere in one of the ventricles
- Easily recognized on ECG—impulse originates in muscle of heart, and conduction is very slow
- Absence of P waves in premature beat, with all other beats usually of sinus rhythm
- QRS complex of premature beat is wide and bizarre
- QRS duration of early beat is >0.10 second
ST segment and T wave often slope in opposite direction from normal complexes - PVC is followed by a compensatory pause
PVCs are considered serious or possibly life-threatening when they
- Are paired together
- Are multifocal in origin
- Are more frequent than six per minute
- Land directly on the T wave
- Are present in triplets (considered to be V Tach) or more
Describe ventricular tachycardia (V-tachy)
- Series of three or more PVCs in a row.
- P waves are absent.
- QRS complexes are wide and bizarre.
- Ventricular rate is 100 to 250 bpm.
- Can be precursor to ventricular fibrillation.
- Medical emergency: indicates increased irritability, with greatly diminished cardiac output and blood pressure
- Can progress to ventricular fibrillation and death
Define/describe Torsade de pointes
- Unique configuration of ventricular tachycardia called the “twisting of the points”
- Associated with prolonged Q-T interval (>0.5 second)
- Identified only in individuals receiving antiarrhythmic therapy and for whom the medication is toxic.
- Treatment is usually cardioversion.
Describe ventricular fibrillation (V-fib)
- Erratic quivering of ventricular muscle, resulting in no cardiac output
- Usually the sequel to ventricular tachycardia
- ECG shows grossly irregular up and down fluctuations of the baseline in an irregular zigzag pattern
The three standard limb leads are defined as follows
- Lead I is created by making the left arm positive and the right arm negative. Its angle of orientation is 0°.
- Lead II is created by making the legs positive and the right arm negative. Its angle of orientation is 60°.
- Lead III is created by making the legs positive and the left arm negative. Its angle of orientation is 120°
The three augmented limb leads are defined as follows
- Lead aVL is created by making the left arm positive and the other limbs negative. Its angle of orientation is −30°.
- Lead aVR is created by making the right arm positive and the other limbs negative. Its angle of orientation is −150°.
- Lead aVF is created by making the legs positive and the other limbs negative. Its angle of orientation is +90°, that is, toward the feet.
Where are each of the precordial leads placed
- V1 is placed in the fourth intercostal space to the right of the sternum.
- V2 is placed in the fourth intercostal space to the left of the sternum.
- V3 is placed between V2 and V4.
- V4 is placed in the fifth intercostal space in the midclavicular line.
- V5 is placed between V4 and V6.
- V6 is placed in the fifth intercostal space in the midaxillary line.
What region of the heart is each lead placed
- V2, V3, V4 = Anterior
- I, aVL, V5, V6 = Left lateral
- II, II, aVF = Inferior
- aVR, V1 = Right ventricular
What vascular territories are each of the leads placed
- Right coronary artery: II, III, aVF (may lead to blocks & bradycardia
- Left anterior descending: V1-V6
- Circumflex: I, aVL
What do you see on ECG with right ventricular hypertrophy
- V1 has a large R-wave and an S-wave smaller than the R-wave
- R-wave becomes progressively smaller in the successive chest leads (V2, V3, V4, V5)
What do you see on ECG with left ventricular hypertrophy
- A deep S-wave occurs in V1 and a large R-wave in V5
- If, when the depth of the S-wave in V1 is added to the height of the R-wave in V5 the resulting number is greater than 35, then L ventricular hypertrophy is present
Define ischemia, infarction, and injury
- Ischemia—reduced blood flow to myocardium from occlusion of coronary arteries
- Infarction—cell death resulting from complete occlusion of coronary artery
- Injury—indicates acuteness of the infarction
Ischemia is demonstrated on 12-lead ECG with T wave __________ or ST segment ___________
- Inversion and depression
- T wave is sensitive indication of changes in repolarization activity within the ventricles
During an acute STEMI, the EKG evolves through three stages
- T-wave peaking followed by T-wave inversion (A and B, below)
- ST-segment elevation (C)
- The appearance of new Q waves (D)
T waves of myocardial ischemia are inverted ________________, whereas in most other circumstances, they are ____________, with a gentle ________slope and rapid ________slope
- Symmetrically and asymmetric
- Downslope and upslope
When a region of myocardium dies it becomes electrically silent directing all electrical forces away from the area of infarction, therefore, an electrode overlying the infarct will record
- A deep negative deflection (Q-wave)
- aVR should not be considered when using Q-waves to look for possible infarction due to its orientation on the frontal plane
How to identify a pathologic Q-wave on ECG
- > 40ms (1 mm) wide (one small box)
- > 2mm deep (2 small boxes)
- > 25% of depth of QRS complex
- Any Q-wave in leads V1-3
- Present in 2 contiguous leads
A significant Q-wave is defined as a minimum of
- One small square wide and one-third the height of the QRS
What ECG findings may indicate a subendocardial injury (signs of ischemia)
- T-wave inversion
- ST segment depression with no T-wave
Describe a subendocardial infarction
- An acute injury to the myocardial wall that does not extend through the full thickness of the ventricular wall; injury is only to the subendocardium
- ECG shows ST-segment depression while the patient is at rest in the presence of chest pain or of suspected coronary ischemia.
- Indicates that a transmural (also called STEMI or Q-wave) infarction could be pending
Describe ST segment elevation
- Elevation of the ST segment above the baseline when following part of an R wave indicates acute transmural injury.
- In the presence of acute infarction, the ST segment elevates and then later returns to baseline (within 24–48 hours).
- ST-segment elevation may also occur in the presence of a ventricular aneurysm (ballooning out of the ventricular wall).
Describe normal, right, and left axis deviations
- Normal = upward QRS in I and aVF
- R axis deviation = downward QRS in I and upward QRS in aVF
- L axis deviation = upward QRS in II and downward QRS in aVF
What are the 2 main/serious causes of right axis deviation
- Right ventricular hypertrophy
- Lateral wall myocardial infarction
What are the 2 main/serious causes of left axis deviation
- Left ventricular hypertrophy
- Inferior wall myocardial infarction
Define acute pericarditis
- Inflammation of the pericardial sac
- Often a complication following myocardial infarction and/or open heart surgery
- Pericardial pain is intense and can mimic angina
What ECG changes can you see with acute pericarditis
- Widespread distribution of ST-T changes without reciprocal depression distinguishes acute pericarditis from early myocardial infarction
