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.

http://en.wikipedia.org/wiki/File:Starling_RAP_combined.svg

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)

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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

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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

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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