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Electrophysiology
Cardiac Physiology Review
Cardiac Cycle
• The heart is 2 pumps that work together– right (pulmonary) and left (systemic) half
• Repetitive, sequential contraction (systole) and relaxation (diastole) of heart chambers
Cardiac Cycle
• Blood moves through circulatory system from areas of higher lower pressure– Contraction of heart produces the pressure
Cardiac Physiology
The Cardiac Cycle– Diastole
Relaxation/filling phase
Coronary arteries fill
– SystoleEjection fraction
Normally, 2/3 of ventricular volume is ejected
Bledsoe/Porter/Cherry, Essentials of Paramedic Care, Second Edition Update
© 2011 by Pearson Education, Inc. Upper Saddle River, New Jersey
Diastole
Bledsoe/Porter/Cherry, Essentials of Paramedic Care, Second Edition Update
© 2011 by Pearson Education, Inc. Upper Saddle River, New Jersey
Systole
Cardiac Physiology
• Stroke volumeAmount of blood ejected in 1 cycle
Dependent onPreload
Cardiac contractility
Afterload
Preload
• Affected by venous blood pressure and the rate of venous return
• Related to the ventricular end-diastolic volume (EDV)– a higher EDV implies a higher preload
• Amount of stretch to RV/LV due to end diastolic pressures
Contractility
• Intrinsic ability of the heart to contract independent of preload and afterload
• Contractility is synonymous with inotropy
Afterload
• Maximum tension of the myocardium mass at end of systole– Tension or stress developed in the wall of the left
ventricle during ejection (systole)
• Dilated LV has a higher afterload• Conversely, a hypertrophied LV has a lower
afterload
Starling’s Law
The more the myocardial muscle is stretched, the greater its force of contraction will be
The more diastolic volume, the greater the cardiac output.
Cardiac Output
Volume of blood that the heart pumps in 1 minute
Stroke volume (mL) x heart rate (bpm) = cardiac output (mL/min)
SV x HR = CO
Endocrine Role
Gail Walraven, Basic Arrhythmias, Seventh Edition©2011 by Pearson Education, Inc., Upper Saddle River, NJ
The Heart is an Endocrine Organ
• Hormones are secreted by the heart in response to hemodynamic stress
• Effects Diuresis (loss of water), natriuresis (loss of sodium), and vasodilation
• These hormones are referred to as natriuretic peptides:
Atrial natriuretic peptide (ANP)
Brain natriuretic peptide (BNP)
Natriuretic peptides
• Atrial natriuretic peptide (ANP) Released by atrial muscle cells in response to atrial distension and sympathetic stimulation
Counters renin-angiotensin-aldosterone system
• Brain natriuretic peptide (BNP)Secreted principally by the ventricles of the heart in response to excessive stretching of myocytes
Results in decreased central venous pressure (CVP), cardiac output, and blood pressure
BNP levels are elevated in congestive heart failure
Nervous System Control
Gail Walraven, Basic Arrhythmias, Seventh Edition©2011 by Pearson Education, Inc., Upper Saddle River, NJ
Nervous System ControlElectrolytes
Sympathetic
ParasympatheticAutonomic Control of the Heart – Chronotropy
• Rate
– Inotropy• Contractility
– Dromotropy• Conductivity
Autonomic Nervous System
Cardiac Plexus
• Formed by the cardiac nerves derived from the cervical ganglia of the sympathetic trunk & the cardiac branches of the vagus & laryngeal nerves
Function of the Heart & Control of Heartbeat
•Contracts spontaneously• does not need nervous stimulation to contract
• Motor nerves that supply the human heart modulate HR
• Sympathetic motor impulses ↑ HR• T3-T4
• GO UP TO THE NECK, AND COME BACK DOWN TO THE HEART
• Parasympathetic motor impulses ↓ HR• VAGUS NERVE (X)
25
Electrolytes!
Sodium & Potassium
Sodium (Na+)Plays a major role in depolarizing
Greater concentration outside cell Must be actively pumped in
Sodium-Potassium Pump = active transport **requires ENERGY**
Triggered by depolarization propagation
Potassium (K+) Influences repolarization
Greater concentration inside cells
The Sodium Potassium Pump
• Depolarization caused Ca++ to enter cell and enables transmitter molecule to be released– Which excites
neighboring cells
Electrolytes
Chloride (Cl–) Transmission of impulses (wave of depolarization)
Magnesium (Mg2+)
Intracellular magnesium is correlated with intracellular K+
essential for nucleic acids, enzyme function (esp. synthesis of ATP)
Cardiac Muscle
33
Characteristics of Cardiac Muscle
1. Cardiac muscle = intermediate • between skeletal & smooth
muscle• Excitatory and conductive
fibers
2. Cardiac muscle = uninucleate
3. Intercalated discs
Intercalated discsWhen one cell becomes excited, the action potential spreads rapidly across the entire group of cells
SyncytiumWork together
Synctia
The heart has two syncytia:– Atrial syncytium
Contracts from superior to inferior
– Ventricular syncytiumContracts from inferior to superior
The only way an impulse can be conducted from the atria to the ventricles is through the atrioventricular (AV) bundle.
Cardiac Muscle 1000X
intercalated disc
striations
short branching cells; intercalated discs at cell junctions
nucleus
Initiation of Electrical Flow
• Polarization– “Ready” state
• Depolarization– “Discharge” state
• Repolarization– “Recovery” state
Depolarization of Cardiocytes
• Resting PotentialInside of the cell is more negatively charged than the outside
• Action PotentialInflux of sodium changes the membrane polarity
Repolarization
• Cell membrane remains permeable to sodium for only a fraction of a second
• Sodium actively pumped outside the cell• Returns to polarized state
Properties of Conduction System
1. Excitability
2. Conductivity• Dromotropy
3. Automaticity
4. Contractility• Inotropy
Bledsoe et al., Paramedic Care: Principles & Practice, Volume 3: Medical Emergencies, 3rd Ed.
© 2009 by Pearson Education, Inc. Upper Saddle River, NJ
3 A-V bundle path shown with blue
1
2 3 A-V bundle path shown with blue arrows
Conduction System
Each component of the conductive system has its own intrinsic rate of self-excitation
– SA node = 60–100 bpm– AV node = 40–60 bpm– Purkinje system (Ventricles)
= 20–40 bpm
• What happens if SA node stops firing???
Pacemaker site with the fastest rate will
generally control the heart
Irritability
• A site along the conduction pathway becomes irritable and speeds up
• Overrides higher pacemaking sites for control of the heart
Escape Mechanism
• The normal pacemaker slows down or fails
• Lower pacing site assumes pacemaking responsibility– This is how you get escape beats,
junctional and ventricular rhythms