Electrocardiogram Interpretation for Respiratory Therapy

Interpreting the Electrocardiogram

• Electrocardiogram (ECG) is a key diagnostic tool obtained by respiratory therapists (RTs)
• RTs can recognize and respond to dangerous cardiac events/arrhythmias
• This chapter covers cardiac physiology, lead placement, ECG interpretation, and key treatments for dysrhythmias

Key Terms

• Atrial Kick: Contraction of the atria pushing blood into the ventricles
• Automaticity: Cardiac cells depolarizing without stimulation
• Depolarization: Influx of sodium into cardiac cells, causing contraction
• Dysrhythmias: Abnormal heart rhythms
• Ectopic Beat: Heartbeat originating outside the sinoatrial (SA) node
• Ectopic Foci: Sites outside the SA node that trigger heartbeats
• Electrocardiogram: Diagnostic tool measuring heart's electrical activity
• Impulse-Conducting System: Pathway for electrical signals in the heart
• Repolarization: Return of cardiac cells to resting state
• S-T Elevation Myocardial Infarction (STEMI): Heart attack with specific ECG pattern

Chapter Objectives

• Describe indications/limitations of the electrocardiogram
• Discuss the electrophysiology of cardiac cells
• Describe how the cardiac impulse is initiated/conducted
• Review the anatomical components of the impulse-conducting system
• Discuss steps in interpreting an ECG
• Review features of a normal ECG tracing
• Compare/contrast features of various cardiac arrhythmias
• Review major treatment alternatives of major cardiac arrhythmias

Chapter Outline

• Basic Principles of Electrophysiology
• Impulse-Conducting System
• Electrocardiogram Procedural Summary
• Basic Electrocardiographic Waves
• Interpreting the Electrocardiogram
• Pulseless Electrical Activity

Clinical Use of ECGs

• ECGs use a 12-lead system for comprehensive electrical activity assessment
• A 3-lead system is usually used for telemetry
• ECGs are inexpensive, noninvasive, and easy to obtain, making them popular
• Used mainly for patients with signs/symptoms of cardiac disease
• Common symptoms include chest pain, shortness of breath, palpitations, weakness, lethargy, or syncope
• ECGs detect existing/previous abnormalities like myocardial infarction (MI)
• Resting ECGs have limited value in predicting future heart problems
• They cannot directly identify abnormalities like valvular defects

Basic Principles of Electrophysiology

• Heart muscle cells are stimulated/paced via cardiac impulse-conducting system
• Impulse-conducting system cells stimulate the heart independently of the nervous system
• Autonomic nervous system normally is a major role in controlling heart function
• Cardiac muscle cells maintain an electrical imbalance across the cell membrane
• Positive charge outside, negative charge inside, called the "polarized" resting state
• Stimulation causes Na+ influx, leading to depolarization and contraction
• Depolarization is immediately followed by repolarization
• Rapid return to "polarized" state by reestablishing electrical imbalance

Impulse-Conducting System

• Three types of cardiac cells carry electrical signals: pacemaker cells, specialized conducting tissue, and atrial/ventricular muscle cells

• Automaticity is the ability to depolarize without stimulation, varying among cell types

• This system initiates the heartbeat, controls heart rate, and coordinates chamber contraction, enabling effective blood movement

• Defects cause inadequate cardiac output and tissue perfusion

• The SA node, in the upper right atrium, has the greatest automaticity and normally paces the heart

• Heartbeats originating outside the SA node are ectopic beats

• The autonomic nervous system controls the SA node

• Sympathetic stimulation (e.g., adrenergic bronchodilators) increases heart rate

• Parasympathetic activation slows heart rate

• The SA node's electrical impulse passes through atrial pathways, including Bachmann's bundle

• This triggers atrial depolarization and contraction

• The impulse moves to the AV node in the intraventricular septum

• The AV node is the "backup" pacemaker with the second-highest automaticity in healthy hearts

• If the SA node fails due to ischemia or medication, the AV node paces ventricular activity at 40-60 beats/min to maintain cardiac output

• AV node delays temporarily allowing ventricles to fill with blood, and limits ventricular stimulation rate in fast atrial rhythms

• Impulse then exits the AV node, enters the bundle of His, and rapidly travels to the bundle braches

• Bundle branches carry the impulse rapidly into right and left ventricles and terminate in Purkinje fibers

• Fibers stimulate myocardium contraction from apex to base, giving coordinated ventricular contraction

• Impulse moves fastest in Purkinje fibers

• Ventricles repolarize preparing for the coming impulse

Electrocardiogram Procedural Summary

• Respiratory therapists (RTs) may perform or troubleshoot ECGs in some organizations

• Familiarity with setup and performance is expected

• Procedure involves gathering portable ECG unit, lead wires, and electrodes

• Adhesive electrodes connect to the ECG unit via lead wires

• Wires are attached to electrodes before skin placement, to prevent skin pressure

• AHA provides guidelines for proper lead placement

• 12 leads are subdivided into two groups:

• 6 extremity (limb) leads

• Four electrodes placed on wrists/ankles

• These are bipolar, measuring electrical activity in two directions

• ECG varies electrode orientation to create six views to measure any electrical that is directed up, down, left or right

• 6 chest (precordial) leads.

• These are unipolar, measuring electrical activity in only one direction

• Placed across the chest in a horizontal plane

• V1 in the fourth intercostal space, right of sternum

• V2 in the fourth intercostal space, left of sternum

• V6 at fifth intercostal space at left midaxillary line

• Each chest lead views electrical activity moving anteriorly or posteriorly

Basic Electrocardiographic Waves

• Wave of depolarization in the atria is the P wave

• It is normally no more than 2.5 mm high and 3 mm long

• Atrial hypertrophy may cause the P wave to enlarge in height and length

• Atrial repolarization not seen because ventricles activation obscures tracing

• Ventricular depolarization is the QRS complex

• Normally larger than P wave because ventricles have greater muscle mass

• QRS complex is not wider than 3mm (0.12s due to rapid impulse movement through ventricle

• These are some of the waves contained within a QRS complex:

• If the first wave of the complex is negative (downward), labeled the Q wave

• Initial positive (upward) deflection labeled the R wave

• Next negative deflection after R wave labeled the S wave

• All three components may not be present

• Ventricular repolarization after depolarization is then shown with the T wave

• Typical PR interval is 0.12 to 0.20 seconds and QRS segment is less than 0.12 seconds

• The PR interval measures time between atrial and ventricular depolarization

• Represents time for impulse to move from SA node across atria to AV node, before passing to ventricles

• This period is usually no longer than 0.20 seconds, longer suggest a impulse abnormally delayed at the AV node and a "block" is occurring as a result of a defect

• The ST segment measures time from end of ventricular depolarization to start of ventricular repolarization and is normally isoelectric and is a flat line that is not above or below the baseline

• Certain disorders can cause the configuration of the ST segment to become abnormal leading to elevated or depressed ST, usually resulting from cardiac ischemia and MI,

• Elevated ST segment is often associated with certain types of MIs

Electrocardiographic Paper and Measurements

• Grid-like boxes define time horizontally and voltage vertically
• Dark lines indicate larger 5 x 5 mm boxes, lighter lines indicate smaller 1 x 1 mm boxes
• Paper typically moves through ECG at 25 mm/s
• Large box represents 0.20 second and small box represents 0.04 second horizontally
• ECG is calibrated at 1 mV upward deflection of 10 small/ 2 large boxes vertically because measure the voltage occurring during depolarization

Interpreting the Electrocardiogram

• Step 1: Identify atrial and ventricular rates
• Normally, the two rates are the same but may differ if defect is present
• Estimate heart rate by counting QRS complexes (ventricular rate) or number of P wave (atrial rate) in 6 seconds (30 large boxes) then multiplying by 10
• Or with a regular rate complex can obtain number then divide that number by 300
• Step 2: Measure the PR Interval
• Measure start of P wave and start of the QRS complex, normally less than 0.2 seconds and consistently the same for each complex
• Intervals longer or vary from one complex to the next shows abnormality in impulse-conducting system
• Step 3: Evaluate the QRS complex
• Complex is shorter than 0.12 seconds and wider than 3 boxes, possible issue with the impulse-conducting system with in the ventricles, which can be associated will decrease in blood pressure and cardiac output
• Step 4: Evaluate the T Wave
• It needs to be normally upright and rounded, inverted can mean ischemia of the heart
• Abnormal configuration can result in electrolyte abnormalities such a hyperkalemia also
• Step 5: Evaluate the ST segment
• Should be flat at no more that 1 mm above/below baseline, either elevation or depression can mean serious issues particularly with myocardium perfusion
• Step 6: Identify the R-R interval
• Distance is measured between successive QRS complex and there have to be little variation in the R-R interval
• Variation between R-R intervals shows abnormal conduction like a "bundle branch block." exist
• Step 7: Identify the mean QRS axis
• Axis is identified through limb lead with the largest voltage; and if its a positive QRS complex complex the the axis is very close to the hexaxial lead that has the voltage and vice versa for if negative

Axis evaluation

• Determines the direction of current flow during ventricular depolarization
• Helps when hypertrophy is sususpected
• Mean QRS axis points leftward and downward, between 0 and +90 degrees on axis in frontal plane.
• The normal QRS axis helps with the position of the heart and compare with the mass of the ventricles

Recognizing Arrhythmias

• Normal Sinus Rhythm
• Begins with an upright P wave that is identical from one complex to the next with a consistent PR interval normally lasting 0.12 to 0.20 seconds and a QRS complex that is identical and no longer than 0.12
• The segment a has a flat ST and R-R interval is regular and do not vary for up to 0.12 second, an adult may obtain a 60-100 beat/min
• Sinus Tach
• Rates exceeds 100 beats/min in resting adults, with the P wave is before QRS complex and Intervals and shapes are normal in the ECG
• Common cause from Anxiety, pain, fever, hypovolemia, or hypoxemia and be treated with oxygen or reducers
• Sinus Bradycardia
• Rate is less than 60 beats/min compared to normal however, may be cause the the patient's blood pressure drop from a SA node abnormality, can be treated with stimulating that rate
• Sinus Arrythmia
  • Irregular spacing in comparison to complexes through a consistent period, may occur with medications but can may have no need of treatment
• First-degree heart block
  • interval for the PR is longer than point 0.2second as normally the QRS complex is preceded through the P wave same interval in its entirety, if the the passing is slower you can be treated with blockers to ensure the blood pressure
• Second-degree heart block
  • Two separate kinds; Type I (Wenckebach or Mobitz type I) may delay in the block conduction AV node
  • and be recognized progressively till P wave is dropping with the with cycle repeating, if MI or ischemia is present which lead to mobitz II
• Atrial flutter
  • Rapid depolar the atria is caused resulting the ectopic cause, numerous P are for every segment however, QR interval may be regular resulting from heart/ diseases/ etc.
• Third-degree: heart block
  • Indicates the conduction completely blocked and a and causes issues but if ventricles and atria have proper source will be considered if drug or MI happens
• Atrial fibrillation
  • When the muscle of the atria quivers not result contraction as determined with erratic behavior

Premature Ventricular Contractions

• Can occur when SA node is diseased causing excitement from the ventricles
• PVC's are often unique/bizarre with the QS complex
• The widening and ectopic means its outside with their the normal channels

Ventricular Tachycardia

• A run is of three in conjunction with VT easily recognizable as that the QRS complex does not have the initial segment
• When the complex are present the morphology will be monomorphic or polymorphism
  • Varies for rate from within 100 to 250 however, lasts 30 seconds for sustainablility