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Chapter 18: Interpreting the Electrocardiogram

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Learning Objectives

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• Describe indications for and limitations of the electrocardiogram (ECG).

• Discuss the electrophysiology of cardiac cells.

• Describe how the cardiac impulse is conducted.

• Review the electrocardiogram equipment set-up.

• Discuss steps involved in interpreting an ECG.

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• Review the features of a normal ECG tracing.

• Compare and contrast the features of various cardiac arrhythmias.

• Review major treatment alternatives for major cardiac arrhythmias.

• Examine wearable and remote ECG monitoring systems.

The Electrocardiogram

• An electrocardiogram (ECG) is a widely utilized diagnostic tool due to its characteristics:
    - Inexpensive
    - Noninvasive
    - Easy to obtain

• Primarily employed to assess patients suspected of acute myocardial conditions.

• Also used as a health screening tool for patients over the age of 40.

• Limitations:
    - Cannot predict future heart attacks.
    - Cannot detect structural defects, e.g., valve stenosis.

The Electrocardiogram Equipment

• Important diagnostic tool often obtained by respiratory therapists (RTs).

• RTs play a crucial role in recognizing and responding to life-threatening cardiac events and arrhythmias.

• ECG can be performed using:
    - A 12-lead system:
      - Provides greater diagnostic value.
    - A 3-lead system:
      - Commonly used for telemetry.

Basic Principles of Electrophysiology

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• The impulse-conducting system delivers electrical stimuli to activate and pace the myocardium.

• Cardiac cells are polarized with:
    - Positive charge outside the cell.
    - Negative charge inside the cell.

• Upon stimulation, cardiac cells undergo depolarization as sodium ions rush into the cells, leading to:
    - Brief muscle contraction (depolarization).

• Repolarization occurs to restore electrical imbalance across the cell membrane.

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• The impulse-conducting system comprises three distinct types of cardiac cells capable of electrical excitation:
    - Pacemaker cells (e.g., sinoatrial node)
    - Specialized rapidly conducting tissue (e.g., Purkinje fibers)
    - Atrial and ventricular muscle cells

• All three cell types possess intrinsic ability to spontaneously depolarize (known as automaticity).

Impulse-Conducting System

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• The system is responsible for:
    - Initiating the heartbeat
    - Controlling heart rate
    - Coordinating contraction of heart chambers

• Defects in the system may lead to:
    - Inadequate cardiac output
    - Decreased tissue perfusion

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• The sinoatrial (SA) node typically exhibits the highest degree of automaticity and serves as the primary pacemaker of the heart.

• The atrioventricular (AV) node serves as a backup pacemaker in the event that the SA node fails.

• After leaving the AV node, impulses traverse through:
    - Bundle of His
    - Bundle branches
    - Purkinje fibers

ECG Procedural Summary

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• A physician's order is required for a 12-lead ECG.

• The procedure requires:
    - A portable ECG unit
    - Lead wires
    - Electrodes

• Placement of 12 leads is divided into two groups:
    - 6 extremity (limb) leads
    - 6 chest (precordial) leads

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• Limb leads are bipolar and measure heart activity in terms of directionality (up, down, left, right):
    - Leads I, II, III, aVR, aVL, and aVF

• Chest leads (precordial) are unipolar and assess cardiac activity moving anteriorly or posteriorly:
    - Leads V1, V2, V3, V4, V5, and V6

• An ECG assists in diagnosing heart conditions such as myocardial infarction and cardiac ischemia.

Basic ECG Waves

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• Atrial depolarization is visualized as the P wave.
    - Normal P wave height is no greater than 2.5 mm and length no longer than 3 mm.

• Atrial repolarization is generally not observable on an ECG tracing as it is obscured by concurrent ventricular electrical activity.

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• Ventricular depolarization is represented by the QRS complex.
    - The QRS complex is larger than the P wave due to the greater muscle mass of the ventricles compared to the atria.
    - Normal QRS complex width is not more than 3 mm (0.12 seconds).

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• If the initial wave of the QRS complex is negative (downward), it is designated as the Q wave.

• The first positive (upward) deflection is known as the R wave.

• The subsequent negative deflection following the R wave is labeled the S wave.
    - Not all QRS complexes display all three components.

• The wave of ventricular repolarization appears as the T wave.

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• The PR interval:
    - Represents the duration from the start of atrial contraction to the start of ventricular contraction. - Typically not greater than 0.20 seconds.
      - Long PR intervals indicate a first-degree heart block.

• The ST segment:
    - Represents the time from the end of ventricular depolarization to the start of ventricular repolarization; it is normally isoelectric.
      - Elevated or depressed ST segments can indicate cardiac ischemia.

ECG Paper and Measurement

• The grid-like structure of the ECG paper allows for measurement:
    - The horizontal axis (x-axis) denotes time.
    - The vertical axis (y-axis) denotes voltage.

• Measurement details:
    - A large box (5 mm × 5 mm) corresponds to 0.20 seconds.
    - A small box (1 mm × 1 mm) corresponds to 0.04 seconds.
    - An upward deflection of 1 mV will result in deflection of 10 boxes, which indicates depolarization of the ventricles.

Interpreting the Electrocardiogram

• To interpret an ECG, one must:
    - Identify atrial and ventricular rates.
    - Measure the PR interval.
    - Evaluate the QRS complex.
    - Evaluate the T wave.
    - Evaluate the ST segment.
    - Assess the R-R interval to evaluate the regularity of rhythm.
    - Identify the mean QRS axis.

Axis Evaluation

• Axis evaluation aids in determining the general direction of current flow during ventricular depolarization, which can help identify ventricular hypertrophy.
    - Normally, the mean axis ranges between 0 and +90 degrees.
    - Right-axis deviation (+90 to +180 degrees) typically indicates right ventricular hypertrophy.
    - Left-axis deviation (between +90 and -90 degrees) is typically associated with left ventricular hypertrophy.

Normal Sinus Rhythm

• Characterized by:
    - An upright P wave that is identical throughout the ECG strip.
    - A PR interval of less than 0.20 seconds.
    - Identical QRS complexes that do not exceed 0.12 seconds in duration.
    - A flat ST segment.
    - A regular R-R interval with a heart rate ranging from 60 to 100 beats per minute.

Sinus Tachycardia

• Identified when the heart rate exceeds 100 beats per minute at rest.
    - Each QRS complex is preceded by a P wave.
    - Common causes include acute illness, pain, anxiety, fever, hypovolemia, and hypoxemia.
    - Certain medications, such as bronchodilators, may also induce sinus tachycardia.
    - Treatment focuses on addressing the underlying cause of the increased heart rate.

Sinus Bradycardia

• Defined as a heart rate of fewer than 60 beats per minute.
    - Each QRS complex is preceded by a P wave.
    - PR interval and QRS components are considered normal.
    - Clinical significance arises only if it results in symptoms such as hypotension or syncope.
    - Atropine is an effective treatment.

Sinus Arrhythmia

• A prevalent arrhythmia characterized by irregular spacing between QRS complexes.
    - The R-R interval varies by more than 0.12 seconds.
    - Can occur due to effects of breathing or as a side effect of medications (e.g., digoxin).
    - Most cases are benign and do not require treatment.

First-Degree Heart Block

• Characterized by a PR interval that exceeds 0.20 seconds.
    - Each QRS complex is preceded by a P wave.
    - This is due to a delay in conduction from the SA node through the AV node or bundle of His.
    - Typically, R-R intervals are regular.
    - Commonly occurs after a myocardial infarction affecting the AV node or may be a consequence of specific medications (e.g., digoxin, beta blockers).
    - Generally, treatment is not necessary.

Second-Degree Heart Block

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• Divided into two types:
    - Type I (Wenckebach or Mobitz type I):
      - Recognized by progressively longer PR intervals until a P wave fails to conduct to the ventricles.
    - Type II (Mobitz type II):
      - Less common and indicates more serious issues (e.g., myocardial infarction).
      - Identified by a series of non-conducted P waves followed by a P wave that successfully conducts to the ventricles.

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• Treatment for Type I is typically unnecessary as it rarely affects cardiac output or causes symptoms.

• Type II requires treatment, often with medications such as atropine.
    - Because Type II block can advance to a third-degree heart block without warning, a pacemaker is indicated, even if the patient shows no symptoms.

Third-Degree Heart Block

• Occurs when there is a complete obstruction in the conduction system between the atria and ventricles.

• The atria and ventricles are paced independently, showing no relationship between P waves and QRS complexes.

• Both P-P intervals and R-R intervals remain regular, yet they remain uncorrelated.

• Treatment options include medications to accelerate ventricle pacing and the placement of a pacemaker.

Atrial Flutter

• Normal P waves are replaced with rapid atrial depolarization originating from an ectopic focus (ranging from 250 to 350 beats per minute).

• This manifests as a characteristic sawtooth pattern, with multiple P waves occurring for each QRS complex.

• Atrial flutter can be caused by various disorders including rheumatic heart disease, coronary artery disease, renal failure, stress, and hypoxemia.

• Treatment typically includes medication and cardioversion.

Atrial Fibrillation

• Results from a quivering motion of the atrial muscle, leading to an irregular waveform where no distinct P waves are observed.

• The ventricular rate may be variable and irregular.

• Atrial fibrillation diminishes cardiac output and can lead to thrombi formation within the atria due to stagnant blood.

• Most cases necessitate cardioversion as a therapeutic intervention.

Premature Ventricular Contractions (PVCs)

• Occur when ectopic beats originate in the ventricles.

• Common causes include hypoxia, electrolyte imbalances, and acid-base disturbance.

• PVCs show a wide QRS complex without a preceding P wave.

• Frequent PVCs necessitate treating the underlying cause; lidocaine can provide temporary relief in certain cases.

Ventricular Tachycardia (V-tach)

• Defined as a series of three or more consecutive PVCs.

• Easily recognized by a series of wide QRS complexes with absent preceding P waves.

• The ventricular rate generally ranges from 100 to 250 beats per minute.

• V-tach signifies a severe arrhythmia that can escalate to ventricular fibrillation if untreated.

• Treatment includes cardioversion and various medications.

Ventricular Fibrillation

• Recognized as the most life-threatening arrhythmia, defined by erratic quivering of the ventricular muscle mass, leading to an instantaneous drop in cardiac output to zero.

• The ECG reveals grossly irregular fluctuations with a zigzag appearance.

• Treatment protocols include rapid defibrillation, cardiopulmonary resuscitation (CPR), oxygen administration, and antiarrhythmic agents.

Pulseless Electrical Activity (PEA)

• An infrequent yet critical arrhythmia characterized by an ECG pattern that does not result in a palpable pulse.

• Generally precipitated by events such as tension pneumothorax, myocardial infarction, drug overdose, etc.

• Immediate emergency life support and reversal of the underlying cause are vital for treatment.

Wearables and Remote Monitoring

• Remote ECG monitoring has been utilized for several decades.

• Various smaller devices, which can be worn on patients' wrists, have emerged for monitoring heart rate and detecting selected abnormal heart rhythms.