I. Structures
A. Heart has 4 chambers – RA, RV, LA, LV
1. RA & LA are reservoirs for blood being sent to the RV & LV
2. RV & LV are the main pumping chambers of the heart
B. Heart contains 4 valves
1. AV valves & 2 semilunar valves
2. Tricuspid valve is between the RA & RV (AV valve)
3. Bicuspid or Mitral valve is between the LA & LV (AV valve)
4. Pulmonic valve is between the RV & pulmonary artery (semilunar valve)
5. Aortic valve is between the LV & aorta (semilunar valve)
6. Valves open and close in response to pressure changes in the heart
7. Valves act as one way doors to keep blood moving forward
II. Circulation – it is important to understand blood flow through the heart in order to understand overall function of the heart and how changes in electrical activity affect peripheral blood flow.
A. Deoxygenated blood from the body returns to the heart via the superior and inferior vena cava ---- empties into the right atrium ---- through the tricuspid valve ---- into the right ventricle ---- through the pulmonic valve ---- into the pulmonary artery ---- lungs through the pulmonary circulation, contacting alveoli and exchanging gasses ---- to the pulmonary vein ---- into the left atrium ---- through the mitral valve (bicuspid valve) ---- into the left ventricle ---- through the aortic valve ---- into the aorta ----then to the capillary beds throughout the whole body for gas exchange.
B. Blood supply to the heart is provided by the right and left coronary arteries which arise from the aorta, just above and behind the aortic valve
III. Nervous system – Heart is supplied by 2 branches of the autonomic nervous system
A. Sympathetic nervous system (or Adrenergic)
1. Accelerates the heart
2. Two chemicals are influenced by the sympathetic system – epinephrine & norepinephrine
3. These chemicals increase heart rate, contractibility, automaticity, and AV conduction
B. Parasympathetic nervous system ( or Cholinergic)
1. Slows the heart
2. The vagus nerve is one of this systems nerves, when stimulated slows heart rate and AV conduction.
IV. Electrophysiology
A. Cardiac cells – two types, electrical and myocardial (``working")
1. Electrical cells
a) Make up the conduction system of the heart
b) Are distributed in an orderly fashion through the heart
c) Possess specific properties
(1) automaticity – the ability to spontaneously generate and discharge an electrical impulse
(2) excitability – the ability of the cell to respond to an electrical impulse
(3) conductivity – the ability to transmit an electrical impulse from one cell to the next
2. Myocardial cells
a) Make up the muscular walls of the atrium and ventricles of the heart
b) Possess specific properties
(1) contractility – the ability of the cell to shorten and lengthen its fibers
(2) extensibility – the ability of the cell to stretch
B. Depolarization and Repolarization
1. Cardiac cells at rest are considered polarized, meaning no electrical activity takes place
2. The cell membrane of the cardiac muscle cell separates different concentrations of ions, such as sodium, potassium, and calcium. This is called the resting potential
3. Electrical impulses are generated by automaticity of specialized cardiac cells
4. Once an electrical cell generates an electrical impulse, this electrical impulse causes the ions to cross the cell membrane and causes the action potential, also called
6. Depolarization with corresponding contraction of myocardial muscle moves as a wave through the heart
7. Repolarization is the return of the ions to their previous resting state, which corresponds with relaxation of the myocardial muscle 8. Depolarization and repolarization are electrical activities which cause muscular activity9. The action potential curve shows the electrical changes in the myocardial cell during the depolarization – repolarization cycle
10. This electrical activity is what is detected on ECG, not the muscular activity
C. Action Potential
1. The action potential curve consists of 5 phases, 0 to 4
2. The 5 phases:
a) Phase 4 – rest
(1) this is the cells resting phase
(2) the cell is ready to receive an electrical stimulus
b) Phase 0 – upstroke
(1) is characterized by a sharp, tall upstroke of the action potential
(2) the cell receives an impulse from a neighboring cell and depolarizes
(3) during this phase the cell depolarizes and begins to contract
c) Phase 1 – spike
(1) contraction is in process
(2) the cell begins an early, rapid, partial repolarization
d) Phase 2 – plateau
(1) contraction completes, and the cell begins relaxing
(2) this is a prolonged phase of slow repolarization
e) Phase 3 – downslope
(1) this is the final phase of rapid repolarization
(2) repolarization is complete by the end of phase 3
f) Phase 4 – rest
(1) return to the rest period
(2) the period between action potentials
3. Refractory and supernormal periods
a) Absolute refractory period
(1) a period in which no stimulus, no matter how strong, can cause another depolarization
(2) onset of phase 0 begin the absolute refractory period, and extends midway through phase 3
(3) begins with the onset of the Q wave and ends at about the peak of the T wave
b) Relative refractory period
(1) the cell has partially repolarized, so a very strong stimulus could cause a depolarization
(2) also called the vulnerable period of repolarization (a strong stimulus occurring during the vulnerable period may push aside the primary pacemaker and take over pacemaker control)
(3) occurs in the 2nd half of phase 3
(4) corresponds with the downslope of the T wave
c) Supernormal period
(1) near the end of the T wave, just before the cell returns to its resting potential
(2) is NOT a normal period in a healthy heart
(3) a period in which a stimulus weaker than normally required can cause a depolarization
(4) this is a short period at the very end of phase 3 into early phase 4
(5) extends the relative refractory period
V. Conduction system
A. Inherent firing rate is the rate at which the SA node or another pacemaker site normally generates electrical impulses
B. SA Node - Sinoatrial node
1. Dominant or primary pacemaker of the heart
2. Inherent rate 60 – 100 beats per minute
3. Located in the wall of the right atrium, near the inlet of the superior vena cava
4. Once an impulse is initiated, it usually follows a specific path through the heart, and usually does not flow backward
C. Intra-atrial tracts - Bachmann's bundle
As the electrical impulse leaves the SA node, it is conducted through the left atria by way of the Bachmann's bundles, through the right atria, via the atrial tracts
D. AVJunction - Made up of the AV node and the bundle of His
1. AV node
a) Is responsible for delaying the impulses that reach it
b) Located in the lower right atrium near the interatrial septum
c) Waits for the completion of atrial emptying and ventricular filling, to allow the cardiac muscle to stretch to it's fullest for peak cardiac output
d) The nodal tissue itself has no pacemaker cells, the tissue surrounding it (called the junctional tissue) contains pacemaker cells that can fire at an inherent rate of 40 – 60 beats per minute
2. Bundle of His
a) Resumes rapid conduction of the impulses through the ventricles
b) Makes up the distal part of the AV junction then extends into the ventricles next to the interventricular septum
c) Divides into the Right and Left bundle branches
3. Purkinje Fibers
a) Conduct impulses rapidly through the muscle to assist in depolarization and contraction
b) Can also serve as a pacemaker, discharges at an inherent rate of 20 – 40 beats per minute or even more slowly
a) Are not usually activated as a pacemaker unless conduction through the bundle of His becomes blocked or a higher pacemaker such as the SA node or AV junction do not generate an impulse
b) Extends form the bundle branches into the endocardium and deep into the myocardial tissue
VI. Ectopic beats & arrhythmias
A. Any cardiac impulse originating outside the SA node is considered abnormal and is referred to as an ectopic beat
B. Ectopic beats can originate in the atria, the AV junction, or the ventricles, and are named according to their point of origin
C. Rate suppression can occur following an ectopic beat, but after several cycles return to basic rate
D. A series of 3 or more consecutive ectopic beats is considered a rhythm
E. The two causes for ectopic beats include:
1. Failure or excessive slowing of the SA node
a) ectopic beats resulting from sinus node failure serve as a protective function by initiating a cardiac impulse before prolonged cardiac standstill can occur; these beats are called escape beats
b) if the sinus node fails to resume normal function, the ectopic site will assume the role of pacemaker and sustain a cardiac rhythm; this is referred to as an escape rhythm
c) after the sinus node resumes normal function, the escape focus is suppressed
2. Premature activation of another cardiac site
a) impulses occur prematurely before the sinus node recovers enough to initiate another beat; these beats are called premature beats
b) premature beats are produced by either increased automaticity, or by reentry
3. Abnormal conduction system
VII. Automaticity
A. Special characteristic of cardiac cells to generate impulses automatically
B. If the cell automaticity is increased or decreased an arrhythmia can occur
1. Reentry events – reexcitation of a region of cardiac tissue by a single impulse, continuing for one or more cycles and sometimes resulting in ectopic beats or tachyarrhythmias
2. Retrograde conduction
a) When an impulse begins below the AV node
b) Can be transmitted backward toward the AV node
c) Conduction usually takes longer than normal and can cause the atria and ventricles to be ``out of synch"
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