Electrophysiology of CVS Flashcards
What is the anatomy of the heart?
Superior Vena Cava
Inferior Vena Cava
Right Atrium
Tricuspid Valve
Right Ventricle
Pulmonary Valve
Pulmonary Artery
Pulmonary Vein
Left Atrium
Mitral Valve
Left Ventricle
Aortic Valve
Aorta
What is the difference between the heart’s chambers?
The atria are the receiving chambers of the heart and receive blood
The ventricles are the discharging chambers of the heart and discharge blood from the heart
Why is there an anatomical difference between the ventricles of the heart?
The left ventricle is thicker than the right ventricle as it pumps blood to the entire body and so need more force
Left has more cardiac cells
How is blood oxygenated via the heart?
Superior vena cava carries deoxygenated blood from the head, neck and upper limb
Inferior vena cava carries deoxygenated blood from lower parts of the body
Deoxygenated blood from the right atrium enters the right ventricle through the tricuspid valve
Deoxygenated blood form the right ventricle enters the lungs through the pulmonary artery
Oxygenated blood from the lungs is supplied to the left atrium
Oxygenated blood from the left atrium enters the left ventricle through the mitral/bicuspid valve
Arch of aorta arises from left ventricle which pumps oxygenated blood to all parts of the body
What are the three types of events of the cardiac cycle and how can they be measured?
Electrical: ECG (read to find out what is occurring electrically)
Mechanical: Pressure and volume changes
Acoustic: Heart sounds (first and second)
What is the definition of membrane potential?
The difference in charge between the inside and outside of the cell
What is the definition of polar?
Having ‘“poles” both positive (+) and negative (-) poles/sides
What is the definition of depolarisation?
Membrane potential going from more negative to less negative (or positive)
The difference in charge across the membrane, i.e. the poles, getting smaller
What is the definition of repolaristation?
Becoming polarised again
Positive and negative sides become more different
What is the definition of ion channels?
Proteins which act as openings to let specific ions move through
May be “gated”- open in response to certain stimuli
What is a key difference between cardiac muscle and skeletal muscle?
Skeletal muscle is stimulated by the nervous system, but cardiac muscle is stimulated by its own electrical stimulation
How is a cardiac contraction initiated?
By the generation of action potentials at SA (Sino-atrial) nodes
What does depolaristion of the cardiac cells cause?
Electrical activity across the atria and down to ventricles (apex of the heart) which causes cardiac contraction
What is the meaning of automaticity?
The generation of a spontaneous action potential by cardiac cells
Is not identical in all parts of the heart
How does the SA node compare to the other components of the heart?
The SA node has the highest degree of automaticity and so generates the impulses at the highest frequency and so suppresses all the other elements
Can other elements take over if the SA node was suffering difficulties with automaticity?
Yes, but at an expense of a lower heart rate
What is the heart rate that each element generates due to its automaticity?
SA node: 60-100 bpm
AV node: 40-60 bpm
Bundle of His: 40 bpm
Purkinje fibres: 15-20 bpm
When are some moments that the AV node may takeover from the SA node to maintain heart function?
Heart attack as SA node may not function properly
Blockage in artery (AV node receives no oxygen so won’t function)
What is the location of the SA node in the heart?
Autorhythmic cells (main pacemaker) in the right atrium near the entry of superior vena cava
What is the location of the AV node?
Autorhythmic cells near the floor off the right atrium
How is the action potential conducted to the AV node from the SA node?
Internodal pathways connect SA & AV nodes
Electrical activity spreads rapidly to AV node via internodal pathways
Depolarization spreads more slowly across atrial muscle
Conduction slows within AV node (allows atria to completely empty)
What happens if there is an internodal delay?
Herat block, first degree
How is the action potential conducted from the AV node to the Purkinje fibres?
Action potentials pass into Bundle of His
(Wall of the septum between the ventricles)
Bundle divides into left & right bundle branches
Fibres continue downward and divide into many small Purkinje fibres that spread outward among the contractile cells
Will then go to the base of ventricles (apex of heart) and so cause contraction of the heart
What do the different waves on an ECG represent?
P wave: Atrial depolarisation
QRS complex: Ventricular depolarisation and atrial repolarsation
T wave: Ventricular repolarisation
PR interval: Delay in the AV node; atria electrical activity
QT interval: Ventricular electrical activity
How is the cardiovascular generally regulated?
Local: auto-regulation
Neural: automatic nervous system
Humoral: numerous constrictors and dilators
What are the mechanisms of hormone release?
Humoral: in response to changing levels of ions or nutrients in the blood
Neural: stimulation by nerves
Hormonal: stimulation received from other hormones
What occurs if there is a change in stroke volume?
A change in heartbeat
What are the two nervous systems that the heart is associated with, and what are they broadly responsible for?
Parasympathetic nervous system: rest and digest
Sympathetic nervous system: fight or flight
What is the function of the automatic nervous system?
Consists of motor neurones that:
Innervate (supply with nerves) smooth muscle, cardiac muscle, internal organs and skin
Makes adjustments to ensure optimal support for body activities
Operates via subconscious control (sometimes changes)
How is visceral activity controlled?
Most visceral organs are innervated by both sympathetic and parasympathetic fibres, which results in dynamic antagonisms that precisely control visceral activity
How can heart activity be controlled by both sympathetic and parasympathetic fibres?
Sympathetic fibres: Increase heart and respiratory rates, and inhibit digestion and elimination
Parasympathetic fibres: Decrease heart and respiratory rates, allows for digestion and discharging of waste
How do catecholamines affect heart rate?
Sympathetic effect so increases heart rate
Increases slope of pacemaker potential
Increasing rate of depolarisation by increasing Ca^2+ entry
How does acetylcholine affect heart rate?
Parasympathetic affect so decreases heart rate
Reduces slop of pacemaker potential
Hyperpolarises pacemaker cell membrane (increasing K^+ removal) via muscarinic receptors
What is electrophysiology on a cellular level?
Rapid signals can be sent through the body via action potentials
Changes in the voltage across cell membrane (membrane potentials)
Caused by the entry/exit of ions
(Atoms/molecules with extra or fewer electrons)
Ions entering/leaving make the inside of the cell more positive or negative
What are action potentials?
Rapid depolarisations and repolarisations in the cell membrane, via voltage gated ion channels, which are transmitted down the length of the cell
What are cardio myocytes and what are their function?
Cardiac muscle cells
Conduct the signal through the heart using action potentials whilst also contracting
How is the action potential conducted in cardiac muscle?
Signal passes through gap junctions of intercalated discs, where the cytoplasm is continuous between cells
What is Phase 0 of the cardiomyocyte action potential?
Phase 0 - Depolarisation
Action potential from SA node opens fast Na+ channels
Large, but transient (1-2 msec), increase in Na+ permeability - Na+ enters
Accompanied by dramatic reduction in K+ conductance - prevents K+ efflux and repolarization
What is Phase 1 of the cardiomyocyte action potential?
Phase 1 – Early Repolarisation
Fast Na+ channels close
Membrane potential begins to fall - reflects partial repolarization
What is Phase 2 of the cardiomyocyte action potential?
Phase 2 - Plateau
Despite closure of fast Na+ channels, potential remains positive or near 0 for ~300 msec
Largely due to opening of voltage-gated slow Ca2+ channels
Ca2+ entry is involved in excitation-contraction coupling
Why is there a plateau in phase 2?
Na+ influx from slow Na+ channels
Slow leakage of K+ out of cell keeps potential from rising
What is Phase 3 of the cardiomyocyte action potential?
Phase 3 - Repolarisation
Intracellular K+ moves down concentration gradient - leaves cell causing it to repolarize
Repolarization facilitated by closure of Ca2+ and slow Na+ channels
What is Phase 4 of the cardiomyocyte action potential?
Phase 4 – Resting Membrane Potential
Cell repolarized and ready for the next stimulus
What is the importance of troponin?
Check if a heart attack has occurred which will be indicated by high troponin levels
Will remain high for 1-2 weeks post heart attack
When is troponin also present?
Released when heart is damaged/ no perfusion
Need to also check SVT (accessory pathway) between nodes; over long time/over exhorting so troponin may be present
Pericarditis also leads to an increase in troponin levels
How do action potentials spread between the cells?
Gap junctions: direct transfer of ionic current from one cell to the next
What does it mean for cells to be “electrically coupled”?
Cells are connected by a gap junction
Flow of ions from cytoplasm to cytoplasm
Very fast, fail-safe transmission
Almost simultaneous AP generation
How do skeletal myocytes and cardio myocytes differ?
Skeletal:
Dependent of action potential on neural activity
Resting membrane potential= -85mV
Action potential lasts 2.5msec
No extracellular Ca2+ for contractions
Cardiac:
No dependency of action potential on neural activity
Resting membrane potential= -90mV
Action potential lasts ~300msec
Have extracellular Ca2+ for contractions
