Cardiac Output (Part 1): Electrical events and heart rate Flashcards
Cardiac Output
The heart is the pump that moves the blood (creates a pressure difference) –
its activity is described as the cardiac output
Cardiac output = Heart rate (HR) x Stroke volume (SV)
Changes in cardiac output
Sleep 10% ⇓
Excitement, stress 30% ⇑
Pregnancy 40% ⇑
Exercise 600% ⇑
Heart rate is driven by
waves of electrical activity that induce the cardiac muscles to contract
Delay at AV node allows
atria to contract before ventricles
Spread of action potentials via the purkinje fibres results in
coordinated contraction of ventricles
What causes the pacemaker cells of the S-A node to trigger an action potential?
Low resting membrane potential (-60 to -70 mV)
Na+ leakage that leads to depolarization
Mechanism underlying the ‘spontaneous’ action potentials in the pacemaker cells (sino-atrial nodes)
Sodium ions “leaking” in through the F-type [funny] channels and calcium ions moving in through the T-type (transient) channels cause a threshold graded depolarization.
The rapid opening of voltage-gated
calcium L-types channels is responsible
for the rapid depolarization phase.
Reopening of potassium channels and
closing of calcium channels are responsible for the repolarization phase.
The bigger the heart
the slower the heart beat – elephants 25 beats/min and shrew 600 beat/min
Many heart cells demonstrate intrinsic rhythm but the overall driver of heart rate is the
sino-atrial (S-A) node
Mechanism of contraction of the ventricular cardiomyocytes
The rapid opening of voltage-gated sodium channels is responsible for the rapid depolarization phase.
The prolonged “plateau” of depolarization (contraction) is due to the slow but prolonged opening of voltage-gated calcium channels and closure of potassium channels.
Opening of potassium channels results in the repolarization phase.
How does calcium produce contraction of the cardiac muscles?
Excitation-Contraction Coupling in Cardiac Muscle
Calcium ions regulate the contraction of cardiac muscle:
The entry of extracellular calcium ions causes the release of calcium from the sarcoplasmic reticulum, the source of about 95% of the calcium in the cytosol.
Refractory period
time required before it is possible to re-stimulate muscle contraction
Cardiac muscle (250ms)
Skeletal muscle (1-2ms)
Prevents tetanus
Cardiac muscle has a prolonged refractory period which allows for
ventricles to fill with blood prior to pumping
Sino-atrial is regulated by the
Autonomic Nervous System
Parasympathetic innervates just the
SA and AV nodes
Sympathetic system innervates the WHOLE heart
Both systems are tonically active but parasympathetic dominants a rest
Vagus nerve
it is apart of the parasympathetic system and reduces heartbeat
sympathetic ganglia
part of the sympathetic system increases the heart beat ( also regulates force/stroke volume)
Parasympathetic and sympathetic nervous system regulate the
rate of depolarisation in the sino-atrial node
what does the parasympathetic neurons do i heart rate
1.Parasympathetic neurons -
release acetylcholine
2.m2 muscarinic receptors
of SA node
3.↑ K+ efflux, ↓ Ca2+ influx
4.Hyperpolarizes cell and
↓ rate of depolarization
5.↓HR
bradycardia
Both the parasympathetic and sympathetic
nerves are tonically active
At rest – parasympathetic dominates and heart rate reduced from 100 to 70
what does the sympathetic neurons do in heart rate
1.Sympathetic neurons -
release noradrenaline
2.b1 adrenergic receptors
of SA node
3.↑ Na+ and Ca2+ influx
4.↑ Rate of depolarization
5.↑ HR
tachycardia
Adrenaline = epinephrine
Electrocardiogram was discovered by
Willem Einthoven
Measured the ELECTRICAL activity of the heart
Assigned the letters P, Q, R, S and T to the various deflections
Described the electrocardiographic features of a number of cardiovascular disorders
Awarded the Nobel Prize in Medicine for his discovery in 1924
Einthovenb’s electrocardiogram recorded using a string galvanometer
This ECG opened the modern era of cardiac physiology
ECG is a summation of the spread of
action potentials through the various sections of the heart
P
Atrial depolarization
QRS
Ventricular depolarization
T
Ventricular repolarization
PQ segment
Atrial contraction
QT segment
Ventricular contraction
Electrical events of the Cardiac Cycle
- p wave :atrial depolarization
- PQ or PG segment : conduction through AV node and A-V bundles
- Q wave
- R wave
- S wave
- ST segment ventricals contract
- T wave: ventricular repolarization
ECG is a powerful tool for
diagnosing heart disease
Provides valuable information concerning the electrical but NOT mechanical activity of the heart
Heart rate
Heart rhythm
Disturbances of rhythm and conduction
(arrhythmia, pacemaker)
Conduction velocity
Anatomical orientation of the heart
Relative size of chambers
Condition of tissue within the heart
Damage to the myocardium
Influence of certain drugs
sinus arrhythmia
normal
inspiration (faster – vagal parasympathetic activity ⇓ > heart rate ⇑)
expiration (slows – vagal parasympathetics activity ⇑ > heart rate ⇓)
Ventricular ectopic beat
heart missed a beat’
Aberrant firing of myocytes in ventricles – ill synchronised and fails to eject blood
Common after heart attacks
Third Degree
Completely failure of conduction from atria to ventricles
Atria and ventricles beat independently
HEART PACEMAKER
Ventricular fibrillation
Result:
Random Firing of hearts
Fibrillating ventricles cannot pump blood
Fatal after a few minutes
450,000 deaths/year in the USA
Caused by:
Myocardial Infarction (heart attack)
Electrical shock
Drug intoxication
Impaired cardiac metabolism
Treatment: CPR followed by electronic defibrillator
Cardiac output (I)
Electrical events, heart rate and its measurement using the electrocardiogram (ECG)
Cardiac Output = Heart rate x Stroke volume
Spontaneous depolarisation of the sino-atrial (S-A) node is the driver of heart rate (100 beats/min) and this is tonically reduced by the parasympathetic nervous system (70 beats/min)
- Sino-atrial node is regulated by the parasympathetic (reduce) and sympathetic (increase) nerves
- Electrical activity flows from sino-atrial node – atrioventricular node – purkinje fibres - cardiomyocytes
Electrical activity in the heart can be measured using the electrocardiogram (ECG) and is a powerful tool for diagnosis of disease
