Cardiac Electrophysiology

• Premature atrial contraction was noted on strip 13
• Sinus arrhythmia not on exam

Objectives for Understanding ECGs

• Draw and explain the P wave, QRS complex, T wave, and U wave
• Explain where the PR and ST segments are
• Define the absolute and relative refractory periods and the implications of each
• Be able to label, on a rhythm strip, all the waves and complexes

Key Waveforms and Intervals

• P wave signifies atrial depolarization
• PR interval shows conduction of impulse from SA node to AV node to ventricles
• QRS complex indicates ventricular depolarization, atrial repolarization is hidden
• Q wave is a negative deflection that may or may not be present
• R wave is a positive deflection that may be singular of multiple that is labeled R'
• S wave is a negative deflection labelled QS if no R wave is present
• QT interval shows ventricular depolarization and repolarization
• ST segment signifies completion of ventricular depolarization, beginning of ventricular repolarization
• T wave represents ventricular repolarization ("resting phase")
• U wave happens after the T wave, its not always visible but can be due to purkinje repo

Refractory Periods

• Refractoriness is that cells need to recover before responding to a stimulus
• The refractory period is longer than the contraction itself

Absolute Refractory Period

• AKA effective refractory period.
• A time where cells cannot respond to another stimulus
• Myocardial mechanical cells cannot contract
• The electrical conduction system cannot conduct an electrical impulse
• Sustained tetanic contractions cannot be provoked in cardiac muscle
• Phases 0-3 of the cardiac action potential is when the QRS complex starts until the peak of the T wave

Relative Refractory Period

• AKA vulnerable period
• Cardiac cells can respond (depo) to a stronger stimulus.
• This phase is the downslope of the T wave.

Supernormal Period

• A weak stimulus can cause cardiac cells to depolarize.
• Dysrhythmias can develop
• End of the T wave.

Pacemakers of the Heart

• The pacemaker of the heart is what defines its rate and rhythm
• Steps for tracking a cardiac impulse:
• Sinus node → Atria → AV node → Bundle of His → Bundle Branches → Purkinje fibers → Ventricles
• SA node or sinoatrial node is located in the right atrial wall, just inferior to superior vena cava opening and its the primary pacemaker and impulse goes to atria, rate of 60-100 bpm can be expected
• AV junction is the floor of right atrium behind tricuspid valve and near opening of coronary sinus, impulse comes from SA node, delays and relays to bundle of His so atria can contract before ventricles, rate of 40-60 bpm is expected
• Bundle of His is from superior interventricular septum, receives impulse from AV node and relays to r and l bundle branches, has a rate of 40-60 bpm
• Bundle is from the interventricular septum and and receives impulse from BB
• Purkinje fibers is from ventricular myocardium, receives impulse from BB and relays to ventricular myocardium, rate of 20-40 bpm is expected

Arrhythmia

• Arrhythmia if from 3 things
• Enhanced Automaticity
• Triggered Activity
• Reentry
• Enhanced Automaticity: Abnormal condition in which cardiac cells begin to depolarize spontaneously or a pacemaker site other than the SA node increases its firing rate

Enhanced Automaticity Possible Causes

• Catecholamines (epinephrine)
• Administration of atropine sulfate
• Digitalis toxicity
• Acidosis
• Alkalosis
• Hypoxia
• Myocardial ischemia or infarction
• Electrolyte disturbances like, Hypokalemia, hyperkalemia or hypocalcemia

Triggered Activity + Reentry

• If an area with delayed impulse is relatively refractory, depolarization may occur
• Can lead to single premature beat, repetitive electrical impulses, and short periods of fast HR
• Some causes are hyperkalemia and certain antiarrhythmics

Escape

• SA node slows or fails to initiate depo, and a lower site spontaneously produces impulses (assumes pacing responsibility)
• Protective mechanisms to maintain cardiac output
• Originate in AV junction or ventricles Examples:
• Junctional escape beats, junctional rhythm, idioventricular rhythm (AKA ventricular escape rhythm), or ventricular escape beats

Conduction Disturbances

• Disturbances may occur because there is Trauma, Drug toxicity, Electrolyte disturbances, Myocardial ischemia, Infarction
• Conduction may be too rapid or slow
• AV blocks

ECG Indicators

• Absent/abnormally shaped or prolonged QRS complex/interval
• Absent, multiple in number, or abnormally shaped P wave
• Abnormally shortened or prolonged PR intervals
• Abnormally fast/slow or irregular/interrupted rhythm HR

Determining HR

• Count HR on 6 second strip x 10 = HR
• Count number of little blocks between QRS complexes and divide by 1500

Electrodes

• Small adhesive patches with conducting gel, pick up electrical impulses, send them through lead wires on an ECG/cardiac monitor
• Paper, plastic, or metal device containing conductive media, applied to the skin at specific locations
• Types are:
• Metal disk
• Metal suction cup
• Disposable disk - Adhesive ring w/ conductive substance in the center

Electrode Application Protocol

• Remove oil + dead cells first
• Avoid alcohol swabs, use brisk dry rub

EKG Paper Delineations

• Each small block is 0.04 sec per box, with 1500 small boxes per min
• Each big block is 5 small boxes or 0.20 sec per box, with 300 big boxes per min

Intervals on a Rhythm Strip

• PR interval: Normal range is 0.12-0.20 sec or 3-5 small blocks
• QRS complex: Normal is <0.12 sec (usually 1-2.5 blocks)
• QT interval: Normal varies with HR + sex, usually around 0.40, women tend to have shorter QT intervals
• QT interval can be prolonged by antidysrhythmics, causing dysrhythmias

Sinus Rhythm Characteristics

• QRS complex/interval: Narrow complexes w/ uniform shape and regular spacing (<0.12 seconds)
• P wave: Upright and rounded, married to QRS
• PR interval: 0.12-0.20 sec, constant from beat to beat
• HR: 60-100

Escape vs Usurpation

• Escape happens when the SA node slows down or fails to initiate depo, and a lower site spontaneously produces impulses
• Usurpation happens when you start producing impulses automatically and have electrical impulses pacing the heat

Bipolar Leads and Limbs

AKA standard limb leads, positive and negative electrodes which record the potential difference between 2 selected electrodes

• Lead I measures diff between left arm (+) and right arm (-)
• Lead II measures difference between left leg (+) and right arm (-)
• Lead III measures difference between left leg (+) and left arm (-)
• Limb leads use roman numerals, while precordial leads use arabic numerals

Precordial Leads

• Six unipolar leads that see the heart from the horizontal plan
• All are positive
• V1 is to the Right of sternum
• V2 is to the Left of sternum
• V3 is in Between V2 and V4
• V4 is the 5th intercostal, mid-clavicle
• V5 is the Anterior axillary line
• V6 is past that

Common Hospital Monitoring Leads

• Lead II: Positive electrode left abdomen, negative electrode right shoulder, ground left shoulder
• MCL₁: Positive electrode 4th ICS RSB, negative electrode left shoulder, ground electrode right shoulder

Electrocardiographic Truths

• Positive QRS indicates impulse traveling towards the positive electrode
• Negative QRS indicates impulse traveling away from the positive electrode
• Isoelectric QRS indicates impulse traveling perpendicular to the positive electrode (no electrical activity)
• A Flat line "written" means there is no impulse at all

Leads and Deflections

• Lead II shows postive direction
• Lead V1 is most Negative
• Lead V6 is Mostly positive

Cardiac Cells

• Myocardial Cells are mechanical and responsible for contractility and relaxation, they do not create electrical impulses independently
• Pacemaker Cells are specialized in the electrical conduction system and spontaneously (automaticity) generate and conduct electrical impulses
• Electrolytes include, Na+, K+, Ca++, and CI-
• Polarization inside a cell is more negative during rest
• Depolarization occurs when the pacemaker fires and the cell membrane becomes permeable to Na+ and K+ and becomes positive
• Repolarization restores normal (negative charge) and is Permeable to K+
• Relaxation (mechanical), ready to be stimulated again, Proceeds from epicardium → endocardium

Action Potential Phases

• Phase 0: Rapid Depolarization, Na+ rapidly enters the cell, Cell depolarizes, and cardiac contraction begins
• Phase 1: Early Rapid Repolarization, Na+ channels close, Na+ slowly and Cl- enter cell, and K+ leaves cell
• Phase 2: Plateau, Ca++ slowly enters cell, Cardiac contraction period increased, CCBs (-pines) slow Ca++ movement, hypocalcemia + meds shorten ST
• Phase 3: Final Rapid Repolarization, Cell rapidly completes repolarization and becomes more negative and more sensitive to stimuli, blocked K+ results in no AP
• Phase 4: Resting membrane Potential, Ready for stimulus to reactivate

Cardiac Vocabulary

Terms:

• Arrhythmia: dysrhythmia/abnormal heart rhythm
• No automaticity: Contraction in a skeletal muscle not in the heart

Cardiac Properties

• Automaticity: Ability of cardiac cells to create an electrical impulse without stimulation
• Excitability: Ability of cardiac muscle cells to respond to outside stimulus
• Conductivity: Ability of a cardiac cell to receive an electrical impulse and conduct it to an adjoining cardiac cell
• Contractility: Ability of myocardial cells to shorten in response to an electrical impulse
  • Medications can help like digitalis, dopamine and epinephrine

Leads

• Records the heart's electrical activity as a specific waveform and complexes
• Monitors HR
• Evaluates effects of heart conditions or injury
• Evaluates pacemaker function
• Evaluates response to antiarrhythmics
• Obtains baseline reading

Explore cardiac electrophysiology concepts, from ECG interpretation (P wave, QRS complex, ST segment, T wave) to refractory periods, escape rhythms, and the AV junction's role. Discover how electrolyte imbalances and medications impact heart rate and rhythm.