Cardiac action potential and ECG Flashcards
What is autorhythmicity?
When the heart contracts rythmically as a result of APs it generates itself
What are the two specialised types of cardiac cell?
1) Contractile cells (99%; normally dont initiate APs)
2) Autorhythmitic cells (do not contract; initiate or conduct APs)
Compared to nerve or skeletal muscle cells, cardiac autorythmitic cells do not have a resting membrane potential. What do they display instead?
Pacemaker activity
What is the pacemaker potential?
An autorythmitic cell membranes slow drift to threshold
Autorythmitic cells cyclically initiate APs which then spread through the heart to trigger contraction without what?
Any nerve stimulation
Specialised non-contractile cells that demonstarte autorythmicity are located at four specific sites. What are these sites?
1) The sinoatrial node (SV node)
2) The atrioventricular node (AV node)
3) The bundle of His (atrioventricular bundle)
4) Purkinje fibres
What is the normal pacemaker of the heart?
The sinoatrial node
What does it mean by the cells of the heart are linked electrically?
The rate of the fastest will be the rate of all
The AV node forms the only conducting pathway between what two components?
The atrial muscle and Bundle of His and hence the ventricles
When the AV node introduces a considerable delay to spread of excitation, what does this allow for?
Allows time for blood to move from the atria to the ventricles
What do AV nodes have if impulses from the SA node fail to reach them?
Well developed latent powers of rythmciity and can take over pacemaking impulses
The cycle of voltage change across cardiac myocytes occurs in 5 distinct phases. What are these phases?
1) Depolarisation
2) Early ropolarisition
3) Plateau phase
4) Late depolarisation
5) Resting potential
SA nodes fire APs at a rate of what?
70-80 APs per min (at rest)
Neighbouring cardiac muscle cells are linked by the presence of gap junctions which allow what?
Allow the rapid spread of APs from cell
e.g an AP originating in the SA node first spreads throughout both atria, stimulating the simultaneous contraction of R & L atrial muscle
What is the ECG a record of?
The overall spread of electrical activity through the heart during polarisation and repolarisition.
It is not a recording of a single AP in a single cell at a single point in time
Electrical currents generated by cardiac muscle are conducted through what?
Body fluids
What do the orientation of the ECG electrodes determine?
Whether an upward or downward deflection is determined by the way the electrodes are orientated with respect to the current flow in the heart
How many conventional electrode systems, or leads do ECG records routinely consist of? And what id the specific arrangement of each pair of connections called?
12
Lead
The 12 different leads each record electrical activity in the heart from different locations. What are these locations?
6 different arrangements from the limbs
6 different chest leads at various sites around the heart
What do the 6 limb leads include?
I, II, III, aVR, aVL and aVF
Why are leads I, II and III called bipolar?
Two recording electrodes are used
Where is the electrode that serves as a ground and not as a recording electrode positioned?
On the right leg
Why are leads aVR, aVL and aVF called unipolar leads?
Even though two electrodes are used, only the actual potential under one electrode ], the exploring electrode is recorded. The other electrode is set at a zero potential and serves as a neutral refernece point
What are the 6 chest leads called and are they unipolar or bipolar?
V1 to V6
Unipolar
Where is lead I, II and III placed?
Lead I - Right arm and Left arm
Lead II- Right arm and Left leg
Lead III- Left arm and left leg
A normal ECG has 3 distinct waveforms; The P wave, the QRS complex and the T wave. What does each wave represent?
P wave- Atrial depolarisation
QRS- Ventricular depolarisation
T wave- Ventricular repolarisiton
True or False; No wave is recorded for SA nodal depolarisation?
True
Why is the P wave smaller than the QRS complex?
The atria have a much smaller muscle mass than the ventricles
What 3 points in time when no current flow is taking place in the heart musculature, the ECG remains at baseline?
1) During AV nodal delay
2) When the ventricles are completely depolarised and the cardiac contractile cells are undergoing the plateua phase of their AP
3) When the heart muscle is completely depolarised and at rest and ventricular filling is taking place
When are waves recorded?
Only when the potential is changing across cell membranes
The size of the potential depends omn what?
Depends on the size of the extracellular current
