EKG and Cardiac Rhythm Review
EKG Overview
1. What an EKG Actually Shows
Definition: An electrocardiogram (EKG) records the electrical activity of the heart rather than the mechanical contraction.
Electrical Impulse Pathway: Starts at the SA node, spreads through the atria, passes through the AV node, and continues into the ventricles.
Components of EKG Tracing: Reflects depolarization and repolarization of myocardial cells:
P wave: Atria depolarize
PR interval: Impulse travels from SA node through AV node toward the ventricles
QRS complex: Ventricles depolarize
ST segment: Early ventricular recovery
T wave: Ventricles repolarize
QT interval: Total time from ventricular depolarization to ventricular repolarization
2. EKG Paper Basics
Importance: Understanding the EKG paper layout is crucial for measurement interpretation.
Standard Units: At standard paper speed:
1 small box horizontally: = 0.04 seconds
1 large box horizontally: = 0.20 seconds
1 small box vertically: = 0.1 mV
1 large box vertically: = 0.5 mV
Calibration Factors:
10 mm high: = 1 mV
0.2 sec long: = 1 large box
Significance of Measurements:
Height/Amplitude: Indicates the amount of muscle mass depolarizing
Width/Duration: Shows the conduction speed through the heart
3. Normal Intervals and Values
PR Interval: Measurement from the beginning of the P wave to the beginning of the QRS complex.
Normal Range: 0.12–0.20 seconds (3–5 small boxes)
QRS Complex: Reflects ventricular conduction.
Normal: Narrow
Wide QRS (> 0.10 sec): Suggests abnormal ventricular conduction (e.g., bundle branch blocks)
QT Interval: Measured from the start of the QRS to the end of the T wave.
Normal Range: 0.36–0.44 seconds (9–11 small boxes). A prolonged QT interval increases risks for dangerous arrhythmias.
T Wave Characteristics:
Normally follows the QRS
Typically points in the same direction as the QRS
Usually asymmetrical
Inverted T wave in multiple leads: Clinically significant.
4. How to Read an EKG Step by Step
Step 1: Look at the Rhythm
Questions to ask:
Is it regular or irregular?
Are the R-R intervals equal?
Are the P-P intervals equal?
Step 2: Find the Heart Rate
For regular rhythms:
300 Method: Count large boxes between two R waves:
1 large box = 300 bpm
2 large boxes = 150 bpm
3 large boxes = 100 bpm
4 large boxes = 75 bpm
5 large boxes = 60 bpm
6 large boxes = 50 bpm
1500 Method: Count small boxes between two R waves, then divide 1500 by that number:
For example, 17 small boxes gives 1500/17 = 88 bpm.
For irregular rhythms:
6-second Method: Count QRS complexes in 6 seconds and multiply by 10:
For example, 8 cycles gives 80 bpm.
Step 3: Check for P Waves
Questions to ask:
Is there a P wave before every QRS?
Is there a QRS after every P?
Do all the P waves look the same?
Step 4: Measure the PR Interval
Questions to ask:
Is it within normal limits (0.12–0.20 sec)?
Is it prolonged?
Is it progressively lengthening?
Is there no consistent PR relationship?
Step 5: Look at the QRS
Questions to ask:
Is it narrow or wide?
Is it consistent?
Does it show any special patterns like rSR′ in V1 (rabbit ears)?
Step 6: Evaluate ST Segment and T Wave
Questions to ask:
Is there any ST elevation or depression?
Is the T wave normal or inverted?
Step 7: Determine the Axis
Use leads I and aVF to assess the axis; this is a quick shortcut.
EKG Axis Deviation
5. Axis Deviation Made Simple
Definition: The axis indicates the overall direction of ventricular depolarization — the mean QRS vector in the frontal plane.
Hexaxial Reference System: Used to determine the axis, which can help identify conditions like ventricular hypertrophy, infarction, COPD, and electrical blocks.
Normal Axis Deviation: If QRS is upright in both lead I and lead aVF.
Normal Axis Range: Approximately 0° to +90°, with a small allowance of about –15° to +105°.
Left Axis Deviation (LAD): If QRS is positive in lead I and negative in lead aVF:
Nonpathologic LAD: 0 to –30°
Pathologic LAD: –30 to –90°
Possible Causes:
Left ventricular hypertrophy
Right or inferior MI
Left anterior hemiblock
Hyperkalemia
Wolff-Parkinson-White syndrome
Nonpathologic in athletic or endurance-trained hearts for mild LAD.
Right Axis Deviation (RAD): If QRS is negative in lead I and positive in lead aVF:
RAD is defined as: +105° to +180°.
Possible Causes:
Right ventricular hypertrophy
COPD
Anterolateral MI
Left posterior hemiblock
Pulmonary embolism
Notes: May be normal in children and tall, thin adults.
Memory Aid: In the presence of pulmonary disease, “everything RIGHT goes wrong” (right axis deviation, right atrial enlargement, right ventricular hypertrophy, right bundle branch block).
Normal Sinus Rhythm and Sinus Rhythms to Know
6. Normal Sinus Rhythm
Definition: Occurs when the SA node controls the heart normally.
Criteria:
One P wave for every QRS
Normal PR interval
Normal QRS
Heart rate: 60–100 bpm
Normal axis with upward deflections in lead I and aVF
Patient should be asymptomatic.
7. Sinus Rhythms You Need to Know
Sinus Bradycardia
Definition: Sinus rhythm with HR < 60 bpm.
Details:
Commonly seen in healthy, trained individuals (e.g., athletes) and reflects increased vagal tone.
Symptoms can include lightheadedness, syncope, hypotension, and decreased cardiac output.
May be observed with first-degree AV block, bundle branch blocks, ectopy, and chamber enlargements.
Symptoms of sinus bradycardia may relate to sick sinus syndrome.
Treatment: Atropine; Pacemaker implantation if necessary.
Mechanism of Atropine: Blocks parasympathetic influence, reducing vagal slowing on the heart, enabling the SA node to increase heart rate.
Sinus Tachycardia
Definition: Sinus rhythm with HR > 100 bpm.
Importance:
Usually due to decreased vagal tone or increased sympathetic tone.
Can lead to reduced ventricular filling time and decreased cardiac output.
May precipitate or worsen myocardial ischemia and angina.
Possible Causes:
Bronchodilators (e.g., albuterol)
Caffeine/pre-workout
Nicotine
Amphetamines
Antianxiety/antidepressant medications
SNRIs like Cymbalta
Stress/anxiety
Congestive heart failure.
Treatment: Address the underlying cause, with no specific medication universally prescribed for simple tachycardia.
Sinus Arrhythmia
Characteristics:
Generally still results in a 1:1 ratio of P:QRS.
Rhythm is irregular, often related to breathing (respiratory sinus arrhythmia).
Heart rate usually fluctuates between 60–100 bpm with irregular R-R intervals.
While it can appear odd, it is generally benign.
Chamber Enlargement / Hypertrophy
8. Chamber Enlargement / Hypertrophy
Changes in EKG: Changes in amplitude, shape, and duration of waves can indicate chamber enlargement.
Right Atrial Enlargement (RAE)
Other Names: P-pulmonale, cor pulmonale.
ECG Clues:
Tall, peaked P waves in lead II.
P wave amplitude > 2.5 mm.
Biphasic P in V1 with positive portion greater than negative portion.
Main Cause: Pulmonary hypertension.
Other Causes:
COPD
Tricuspid valve disease
Pulmonary embolism
Cystic fibrosis
Obstructive sleep apnea.
Left Atrial Enlargement (LAE)
ECG Clues:
Wide, notched, prolonged P wave.
P wave duration ≥ 0.12 sec in lead II.
Biphasic P in V1.
Causes:
Chronic hypertension
Mitral stenosis
Mitral insufficiency/prolapse/regurgitation
Heart failure
Atrial fibrillation.
Right Ventricular Hypertrophy (RVH)
Symptoms:
Dyspnea
Dizziness/syncope
Ankle/foot/leg edema
Frequent sinus tachycardia.
ECG Clues:
May show right axis deviation
Tall R waves in V1 and V2 ≥ 7 mm
R/S ratio ≥ 1 in V1 and V2
Deep S waves in left chest leads.
Common Causes:
COPD
Pulmonary valve stenosis
Pulmonary hypertension
Chronic mitral stenosis
Congenital lesions.
Left Ventricular Hypertrophy (LVH)
ECG Clues:
Taller than normal R waves in left chest leads.
Deeper than normal S waves in right chest leads.
S in V1 or V2 + R in V5 or V6 > 35 mm.
Causes:
Aortic stenosis/insufficiency
Increased left ventricular filling
Coronary artery disease
Genetics
Long-term mitral insufficiency.
Clinical Importance: Severe LVH can precede sudden cardiac death, but it may regress with controlled blood pressure.
AV Blocks
9. AV Blocks Overview
Overview of AV Block Diagnosis: Based on the relationship among P waves, PR intervals, and QRS complexes.
First-Degree AV Block
Features:
Sinus rhythm
PR interval > 0.20 sec
PR remains consistent
One P wave for each QRS.
Causes:
Increased parasympathetic tone
AV nodal disease
RCA coronary artery disease
Medications
Electrolyte disturbances.
Treatment: Usually none; atropine may be used in the presence of marked bradycardia.
Key Phrase: “Long PR, but every P still gets through.”
Second-Degree AV Block Type I (Mobitz I, Wenckebach)
Features:
PR intervals progressively elongate until one P wave doesn’t conduct, resulting in a dropped QRS.
Causes:
Medications
Ischemia of the right coronary artery
Enhanced vagal tone
Acute inferior wall MI.
Treatment: Usually benign and asymptomatic; atropine if symptomatic.
Key Phrase: “Longer, longer, longer, drop.”
Second-Degree AV Block Type II (Mobitz II)
Features:
Intermittent non-conducted P waves
No progressive PR prolongation
Sudden dropped QRS complexes.
Causes:
Structural conduction system damage
Anteroseptal MI
Fibrosis of the conduction system
Cardioactive medications.
Treatment: Pacemaker.
Key Phrase: “Some P waves fail, but the PRs that do conduct stay the same.”
Third-Degree AV Block (Complete Heart Block)
Features:
Complete absence of conduction through the AV junction
Atria and ventricles beat independently
Regular P-P intervals, regular R-R intervals
No consistent PR intervals; atrial rate faster than ventricular rate.
QRS can be normal width or wide.
Causes:
Enhanced vagal tone
Toxicity from cardioactive medications
Acute MI.
Treatment: Temporary pacemaker; stop offending medications if drug-induced; vagolytic agents like atropine may be considered.
Key Phrase: “P waves and QRS complexes are both regular, but they are not talking to each other.”
Bundle Branch Blocks
10. Bundle Branch Blocks Overview
Definition: These are conduction problems affecting the ventricles, primarily distinguished by a wide QRS complex.
Right Bundle Branch Block (RBBB)
Characteristics:
Right bundle is blocked, causing delayed depolarization of the right ventricle.
ECG Clues:
Wide QRS
rSR′ pattern in V1 (resembling rabbit ears)
Inverted T wave may appear in right chest leads
Wide S wave in lead I and V6.
Causes:
Can be nonpathologic
i.e., idiopathic aging changes, congenital defects (e.g., ASD), MI involving LAD.
Symptoms: May include bradycardia, syncope.
Memory Aid: RBBB = rabbit ears in V1.
Left Bundle Branch Block (LBBB)
Characteristics:
Left bundle is blocked, causing late depolarization of the left ventricle.
ECG Clues:
Wide QRS
Wide, often notched QRS in left chest leads
Secondary ST-T changes opposite QRS direction
Inverted T waves
May demonstrate left axis deviation.
Causes:
Aortic stenosis
Acute MI
Extensive CAD
LAD injury
Chronic hypertension
LVH
Conduction system disease.
Big Idea: LBBB often indicates more serious structural disease than RBBB, and it can be an early sign of underlying heart disease.
Medications and Treatments
11. Medications and Treatments
Atropine: Used for:
Symptomatic sinus bradycardia
First-degree AV block with marked bradycardia
Second-degree AV block type I if symptomatic bradycardia is present
Some third-degree block situations, acting as a vagolytic
Sinus node block/pause/arrest when notable bradycardia is evident.
Mechanism of Atropine: It blocks parasympathetic influence on the heart, thus counteracting vagal slowing which allows heart rate to increase.
Pacemaker**:
Indications:
Symptomatic sinus bradycardia / sick sinus syndrome
Mobitz II
Third-degree AV block
Sinus node block/pause/arrest lasting longer than 3 seconds or related to sick sinus syndrome.
Function of Pacemaker: Provides electrical impulses when the heart’s conduction system is too slow or unreliable.
Exam Strategy: If encountering a dangerous conduction block below the AV node or complete heart block on the exam, consider pacemaker as a potential solution.
Fix the Underlying Cause: Critical for addressing sinus tachycardia; examples include:
Stimulant use
Anxiety/stress
Bronchodilators
CHF
Key Exam Strategy: When faced with sinus tach, the foremost answer should involve identifying and correcting the underlying cause rather than pharmacologically managing the rhythm alone.
Rhythm-Strip Recognition Cheat Sheet
12. Rhythm-Strip Recognition Cheat Sheet
Normal Sinus Rhythm:
Regular
Rate: 60–100 bpm
1 P wave before each QRS
Normal PR
Normal QRS.
Sinus Bradycardia:
Same as sinus rhythm
Rate < 60 bpm.
Sinus Tachycardia:
Same as normal sinus rhythm
Rate > 100 bpm.
Sinus Arrhythmia:
Sinus rhythm pattern
Irregular R-R spacing, often related to respiration.
First-Degree AV Block:
Every P has a QRS
PR is prolonged and constant.
Mobitz I:
PR progressively increases
One beat is dropped.
Mobitz II:
PR remains constant
Random-looking dropped QRS.
Third-Degree Block:
Regular P waves
Regular QRS
No correlation between the two.
RBBB:
Wide QRS
rSR′ in V1
Wide S in I and V6.
LBBB:
Wide QRS
Notched/wide pattern in left chest leads
Discordant ST-T changes.
Summary of Key Points
13. Key Verbal Summary for Strip Reading
A polished response during an examination might state:
"First, I assess the rhythm for regularity.
Then, I calculate the heart rate using either the 1500 method for regular rhythms or the 6-second method for irregular rhythms.
I’ll look for a P wave preceding every QRS and a corresponding QRS following every P.
Next, I will measure the PR interval for normalcy, prolongation, progressive change, or inconsistency.
Following that, I’ll inspect the QRS for width and morphology to determine if conduction is normal or indicative of a bundle branch block.
Lastly, I’ll analyze the ST segment and T wave for signs of ischemia or repolarization shifts, and finally assess the mean QRS axis utilizing lead I and aVF."
14. High-Yield Compare-and-Contrast Section
Mobitz I vs Mobitz II
Mobitz I:
Progressive PR prolongation;
Typically benign;
Atropine if symptomatic bradycardia presents.
Mobitz II:
No progressive PR lengthening;
Dropped QRS complexes;
Potential threat to life;
Requires pacemaker.
RBBB vs LBBB
RBBB:
Delayed depolarization of the right ventricle;
rSR′ in V1;
Wide S wave in I/V6;
Generally less serious.
LBBB:
Delayed depolarization of the left ventricle;
Wide/notched left-sided complexes;
Opposite-direction ST-T changes;
More closely associated with heart disease.
Right Atrial Enlargement (RAE) vs Left Atrial Enlargement (LAE)
RAE:
Tall peaked P waves;
Associated with pulmonary disease.
LAE:
Wide notched P waves;
Indicative of left-sided pressure/volume overload.
15. Fast Memorization Section
Key Numbers to Memorize:
Small Box Horizontal: 0.04 sec.
Large Box Horizontal: 0.20 sec.
PR Interval: 0.12–0.20 sec.
QT Interval: 0.36–0.44 sec.
Normal Sinus Rhythm: 60–100 bpm.
Sinus Bradycardia: <60 bpm.
Sinus Tachycardia: >100 bpm.
Axis Determination:
Lead I up + aVF up = Normal axis.
Lead I up + aVF down = Left axis deviation (LAD).
Lead I down + aVF up = Right axis deviation (RAD).
AV Block Summary:
Long PR interval only = First-degree block.
Progressive lengthening leading to a drop = Mobitz I.
Fixed PR with dropped beats = Mobitz II.
P and QRS are independent = Third-degree block.
Treatment recommendations:
Symptomatic Bradycardia: Atropine.
Mobitz II: Requires pacemaker.
Third-degree AV block: Temporary pacemaker, sometimes atropine/vagolytic support.
Sinus Tachycardia: Address the underlying cause.
16. Best Study Tips Before Monday
Focus on this order:
Learn the EKG box and interval values thoroughly.
Practice the step-by-step sequences for reading EKGs.
Memorize the key sinus rhythms (NSR, bradycardia, tachycardia, Mobitz I, Mobitz II, third-degree block, RBBB, LBBB).
Understand and commit to memory the axis determination using leads I and aVF.
Memorize the connections between treatments (atrophine vs pacemaker vs underlying cause).