1.1.3 Chrontropy Flashcards
What 3 things does tachycardia cause?
Increases myocardial oxygen consumption
Reduces time for ventricular filling
Reduces time available for coronary blood flow
What does bradycardia cause?
Decreased cardiac output and tissue perfusion
What is heart rate dependent on?
Autonomic nervous system
What does Na, Ca and K do to allow a current?
Na moves into the cell to depolarise it
Ca moves into the cell to depolarise it
K moves out of the cell to repolarise it
Explain what occurs in a pacemaker potential
Slow increase in depolarisation until threshold reached, action potential trigered, channels close and K+ exits cell, repolarise
What is the order of conduction in the heart?
SAN generates pacemaker potential
Depolarisation passed over both atria
reaches AVN = small pause
Cascade down purkinje system to ventricles for contraction
What does the vagus nerve innervate?
SAN
AVN
Small amount of atria
What do adrenergic fibres innervate?
SAN
AVN
Atria
Ventricles
How does sympathetic stimulation increase heart rate?
Sympathetic fibres release noradrenaline which opens more Na channels so depolarisation is faster
How does parasympathetic stimulation slow heart rate?
Parasympathetic fibres release acetylcholine opens fewer Na channels so depolarisation is slower
What receptor do muscle cells use to detect noradrenaline?
B1 adrenergic receptors
What receptors detect acetylcholine in the SAN and AVN?
M2 Muscarinic
Arrhythmias
Abnormalities of cardiac rhythm affecting heart rate, filling of the heart and can therefore compromise cardiac output
Sinus Rhythm
SA node is acting as pacemaker; a QRS complex follows each P wave, PR and QT intervals normal, RR interval regular
Sinus Arrhythmia
Normal QRS complex, PR and QT intervals but RR interval varies in a set pattern
Sinus Tachycardia
Normal response to exercise; seen in fever, hyperthyroidism and as a reflex to low arterial pressure
Sinus Bradycardia
May be abnormal but usually indictive of very fit individuals
What characterises a first degree heart block?
Prolonged PR intervals
Contraction is delayed due to increased time for AV conduction
What characterises a second degree heart block?
More P waves than QRS complexes
AVN fails to transmit all atrial impulses
Atria beat more than once for each ventricular contraction
What characterises a third degree heart block?
P and QRS complexes are completely dissociated
Transmission of impulse from atria to ventricle fails
Atria and ventricles beat independently of each other
What 4 things can cause arrhythmias?
Damage to cardiac tissue
Stretch of the myocardium
Overload of the cell with calcium
Excessive sympathetic stimulation
