Cardiac electrophysiology and ECG Flashcards
why is it important that the coronary arteries are on the surface of the muscle
so they are not compressed upon contraction
what structures drain blood from the upper and lower body into the right atrium
upper - superior vena cava
lower - inferior vena cava
what does it mean that the heart has variable pump output
flow is not always the same
how many pulmonary veins are there
4
2 from each lung that converge before the heart
what allows cardiac action potentials to propagate from cell to cell
gap junctions
what is the duration of a normal cardiac action potential
what mediates this
250 ms
voltage gated Ca2+ channels allowing leading to a plateau phase of the AP
what is the resting membrane potential of cardiac muscle cells
-85 mV
what structures form gap junctions
what happens if there are genetic deformities of these proteins
connexons
mutations are incompatible with life
what are the specialised conductive tissues in the heart (in order)
SA node
AV node
bundle of His
left and right bundle branch
Purkinje fibres
automaticity of heart cells
ability to generate a heart beat without nervous system input
what electrically isolates the atria and ventricles
annulus fibrosus ?
hearts fibrous skeleton - fibrous rings
where does electrical activity normally originate in the heart
what is the pacemaker potential of these cells
SA node
-70mV
why does the SA node depolarise before the AV node
it has a higher pacemaker potential (less negative)
what forms the only site of electrical connection between the atria and ventricles
AV node
what is the rate of SA pacemaker potential
why is heart rate different to this
~90 - 100 bpm
parasympathetic tone slows natural rhythm
chronotropic agents
agents that alter heart rate
examples of positive chronotropic agents and their receptors
adrenaline and noradrenaline
beta-adrenergic receptors
examples of negative chronotropic agent and its receptors
acetylcholine
muscarinic cholinergic receptors
effect of ANS on the membrane potential of the SA node
sympathetic: increases the potential (more rapid depolarisation)
parasympathetic: decreases the potential
what controls the movement of electrolytes through gap junctions
free movement only controlled by concentration gradients
what is an ECG
recording of electrical activity of the heart from the body surface using electrodes to allow a spatial perspective of electrical events
why do an ECG
- suspect disturbances in rhythm and conduction
- localise and assess extent of ischemic damage
- assess size of chambers
- assess effects of changes in electrolyte concentrations
how does the flow of electrical signals provide the recording we see in the ECG
electrical vectors parallel to the axis of the lead result in maximal deflection
vectors perpendicular to axis result in 0 deflection
vectors travelling towards the exploring electrode (+) result in a +ve deflection
vectors travelling away from + electrode result in -ve deflection
p wave
atrial depolarisation
QRS complex
ventricular depolarisation
t wave
ventricular repolarisation
P-R interval
time between atrial and ventricular depolarisation due to AV node conduction block
ST segment
plateau phase of ventricular action
what part of an ECG is important in diagnosis of ventricular ischemia or hypoxia
ST segment
Q-T interval
time for both ventricular depolarisation and repolarisation
roughly estimates the duration of an average ventricular AP
what does ST elevation indicate
myocardial infarction
think “STEMI”
what does ST depression indicate
myocardial ischemia
what does a sinus dysrhythmia ECG show
varied R-R intervals with normal sinus rhythm
what does a atrial fibrillation ECG show
irregularly irregular, often rapid, absent P waves, and erratic, fibrillatory waves between QRS complexes.
what does left ventricular hypertrophy ECG show
increased QRS voltage, particularly in the left precordial leads (V5, V6)
may also display signs like a left axis deviation and ST-T wave abnormalities.
