Lecture 5: Electrical Events of the Heart Flashcards
what is the basis of the electrocardiogram?
body = volume conductor
when cardiac muscle cells depolarise, extracellular current between resting and depot causes potential difference that can be measured at the body surface
in what direction does current flow?
from -ve to +ve
what a cell depolarises what is the net charge on the OUTSIDE?
-ve
what are einthovens 5 hypotheis’
- heart is located at the centre of a homogenous volume conductor
- attachment of arms and legs to trunk = equidistant
- limbs behave as linear conductors to trunk
- electrodes placed on limb = located at apices of equilateral triangle
- differences in potential recorded between electrodes represent projection of vector forces
what are the three main ECG leads?
Lead 1: right arm -ve, left arm +ve
Lead 2: right arm -ve, left leg +ve
lead 3: left leg -ve, right arm +ve
which lead is the most accurate vector analysis?
lead 2
what are the limitations to einthovens hypothesis in regards to quadrupeds?
- limb arrangement is much less like an equilateral triangle
- anatomical attachment of forelimbs to body is different
- moving limbs alter amplitude and direction
describe the general features of an ECG wave
P wave = depol of atria
QRS complex = ventricular depot
T wave = ventricular repol
(can’t see atrial repol, hidden behind QRS complex)
What type of contraction is the P-R, and S-T segment?
iso-electric
what is the P-R interval?
the delay between atrial and ventricular depol due to eh delay in the AV node
what is the S-T segment?
plateau of ventricular muscle action potential
Why is the T wave not inverted?
endocardial cells depolarise first, then the epicardial cells epicardial cells (on the outside) REPOLARISE before the endocardial cells ie/ tissue that depol last in the tissue are the first to repol
what can change the mean electrical axis of the heart?
change in position of the heart eg/ mass in the thorax
increase in mass in one of the ventricles
what is an arrhythmia?
an alteration in rate or rhythm
what is bradycardia?
slowing of heart rate
what is tachycardia?
increase in heart rate
what is sinus bradycardia?
slowing of heart rate governed by SA node due to increase in vagal tone eg/ observed in sleeping individuals
what is sinus tachycardia?
inc of HR governed by SA node due to increase in sympathetic tone, normal during exercise, anxiety feat etc.
what is sinus arrhythmia?
variations in HR synchronous with respiration
HR increase towards end of inspiration
HR decrease towards end of expiration
disappears with increase Hr
what is a sinoatrial block?
impulse blocked before it enters atrial muscle = no p wave
what is an atrioventricular block?
transmission through the AV node either slowed or completed impeded
P wave is not always related to QRS
what is premature atrial contractions?
area of atria escapes normal pacemaker domination and initiates a heart beat
may or may not be followed by ventricular contraction
What is a premature ventricular contraction?
not proceeded by the P wave
often followed by missed beat as muscle is in refractory
premature beat has decreased SV
what is a paroxysmal tachycardia
tachycardia arising from ectopic pacemaker
onset and termination normally abrupt, may arise in atria or AV node
may progress to fibrillation
