Heartbeat and ECG Flashcards
(63 cards)
Describe how a cardiac pacemaker cell works
At rest there is constant sodium influx into the cell coupled with simultaneous potassium efflux (prevents depolarisation) –> potassium efflux decays with time –> membrane potential slowly depolarises –> action potential generated when outward potassium reaches critically low level and there’s transient increase in sodium influx –> potassium efflux then ‘reset’ to a high level again –> cycle repeats
What is happening biochemically in a cardiac pacemaker cell at rest?
There is a constant sodium influx (leak) into the cell at rest, and this would normally depolarise the cell but a simultaneous outward potassium current is preventing this.
Why are cardiac pacemaker cells myogenic?
Due to the constant inward sodium current, the pacemaker cells in the SAN are spontaneously active and therefore the rate of stimulation simply depends on the rate of decay of outward potassium current.
What is the voltage of the cardiac pacemaker cells at rest?
~ -70mV
What is the voltage of the cardiac pacemaker cells before generating an action potential?
~ -40mV
Where is the SAN found?
On the wall of the right atrium near the entrance of the superior vena cava
Describe the SAN
A group of modified cardiac muscle cells that generate impulses (pacemaker) that is sent to adjacent atrial cells by gap junctions
What is the blood supply to the SAN?
Right coronary artery
Where is the AVN located?
Located on the inter-atrial septum close to the tricuspid valve (right side of heart)
Describe the function of the AVN
Transmits the impulse from the SAN down to the ventricles down the bundles of His and then up the Purkinje fibres, to cause upwards contraction from the apex of the heart
What nervous stimulation is there to the SAN?
Sympathetic and parasympathetic
Describe the parasympathetic nervous innervation of the SAN
Parasympathetic nerves from the vagus act via interneurons within the node to inhibit the closure of the potassium channels via muscarinic receptors, making the pacemaker cells slow down.
Describe the sympathetic nervous innervation of the SAN
Sympathetic nerves at the SAN increase closure of the potassium channels by beta adrenoreceptor actions, which makes pacemaker cells ‘speed up’
How does adrenaline act to increase the force of contraction of the myocardium
Adrenaline in the blood acts on beta receptors throughout the myocardium as well as in the SAN, in order to produce ionotropic actions (increased force of contraction)
What nervous stimulation is there to the AVN?
Both parasympathetic and sympathetic nervous system (weaker inputs than into the SAN)
Describe the spread of the action potential from the SAN in the heart
SAN –> both atria within 60ms –> 60ms delay to AVN to allow time for atrial to contract and push blood into ventricles before ventricular contraction –> fast conduction muscle fibres leave AVN –> travel down IV septum –> down the left and right bundles of His to the apex of the heart –> activates the Purkinje fibres which then move the impulse up the walls of the ventricles to cause upwards contraction –> blood moves into the great vessels
What is a first degree atrioventricular block?
Where the PR interval is lengthened beyond 200ms
How may conduction through the bundles of His or Purkinje fibres be impaired?
Conduction can be damaged by ischaemia
What type of calcium ion channel is present in cardiac muscle?
slow, L type
Describe the action potential graph in ventricular muscle
Fast rise in voltage (depolarisation) due to opening of the sodium channel, this is then followed by a prolonged depolarisation phase known as the plateau (due to late and prolonged entry of calcium into the cell –> muscles can contract for much longer than skeletal muscle) – followed by a refractory period whereby potassium efflux occurs
What is the function of L-type calcium channel antagonists?
Drugs which block these slow calcium channels are used to decrease the force of ventricular contraction and thus the work (and oxygen demand) of the heart
How many leads are there in an ECG?
12
What produces an ECG trace?
Although an ECG is caused by the cardiac action potentials, it is not the same shape as the cardiac action potential graphs; generally the ECG is mainly generated at the start and the end of cardiac action potentials
What does the R wave signify in an ECG?
The start of ventricular depolarisation
