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Lecture 12

PHGY 210 Lecture Notes - Lecture 12: Premature Atrial Contraction, Spiral Wave, Wavefront

Course Code
PHGY 210
Michael Guevara

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Lecture 12 - Cardiovascular Physiology (Part 6 of 9)
(1) Mapping of Cardiac Electrical Activity
*The QRS complexes in the previous figure is not generated by the SA node. Weʼll see
what itʼs generated by in a minute.
To understand whatʼs happening in V. Tach you have to map the spread of action
potentials in the ventricular muscle. The first way that this was done was using this kind
of set up. You see the meshwork which is called the sock electrode and the little circles
are electrodes of which there are probably around 64. This is placed over the heart on
the epicardial surface of the heart. Everytime an action potential comes along (the
upstroke of the action potential wavefront sweeps under the electrode), this electrode is
taken off through a cable into a voltmeter and when you get an action potential you get a
little blip. These go off to the computer which decides which of the 64 guys fired first,
which of them fired second etc. And it makes an activation map of the spread of the
action potentials on the ventricle (will see this on the next slide). You can also record
from the endocardial muscle which looks like a little balloon and the little black spots are
electrodes. You blow this thing up so that the surface of the balloon is pressed to the
surface of the heart and you can record action potentials from endocardial muscle.
(2) Reentrant Ventricular Tachycardia
This is an activation map. If you look from the bottom of the heart and you look up, youʼll
see the apex in the middle and the base is like a circle. This is a recording in 3D so to
show it on a sheet of paper we have to flatten it out so itʼs a bit like if you look at the
earth in an atlas. Looking at the heart from underneath. Each of these lines shows you
the position of the wavefront as computed by the computer at different points in time. In
200 ms youʼre back to where you started. In the time that the action potential has
actually circled around to here, the actions here have repolarized and they are out of
their refractory period so theyʼre now going to be depolarized again because the
wavefront is going to continue to move in this region so itʼll just zip around and around
and around. But what is it zipping around? This was a patient who had maintained
ventricular tachycardia and this was a couple of months after he had his heart attack,
The three forked area at the center is the area of his heart muscle that died - itʼs scar
tissue and itʼs never growing back. Itʼs fibroblasts that have grown into the area that the
cells have died and they make a scar and if you look at the heart you see it sometimes.
This forms an anatomical obstacle because the action potentials cannot go through this
guy. Theyʼre not excitable cells. So they action potential goes as far as there and stops.
To get this kind of reentrance montion (reentry is one of the mechanisms of producing
arrhythmias) you need that scar tissue. In the real circus, you have a bunch of
spectators seated around the ring and in the ring there are animals! This movement was
named when there were real circuses so itʼs called circus movement reentry. Weʼll see
later a different form of reentry thatʼs not called circus and does not require an
anatomical obstacle in the middle. This is 200 ms right? It takes about 200 ms to make
the full circle. If something is going around in a circle be it with circus or spiral
movement, every 200 ms the muscle is being activated. Is ventricular muscle happy to
be contracting five times per second if it evolved to contract maybe one or 2 times per
second? No! So it gets tired. So it goes zip zip zip like 6 times and on the 6th time, the
wavefront is there and this is not a perfectly homogenous chunk of muscle, there are
inhomogeneities in the muscle and it might be that the area in red is the most tired cells
of all and on the 6th time they decide that theyʼve had enough and theyʼll block and wonʼt
conduct the action potential. So the AP wonʼt travel into that area but the other areas can
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keep going so he green part will continue in itʼs direction and the the purple part will
continue in itʼs direction and now you have two circulating wavefronts! And maybe they
will break into different wavefronts and before you know it you have many wavefronts in
the ventricle of the heart and thatʼs the breakup of the wavefront. So with the guy on the
train, the single wave front that zips around a few times is the V. Tach so five, six seven,
eight beats that breaks up into multiple wavefronts and then you have V. Fib.
(3) George Ralph Mines (1886-1914)
He didnʼt live to a ripe old age (he was 27). He was a McGill physiology prof. He was
here for a couple of months and then he was found on the floor of the Strathcona
building, unconscious. He was taken to the Royal Vic where he died. When he was
found though, he was hooked up to the recording equipment. The speculation was that
he was doing an experiment on himself and it went wrong. He was the guy who first saw
circus movement reentry.
(4) Spiral Wave (movie)
Is this circus movement reentry? No! Thereʼs no obstacle! Thereʼs a second form of
reentry called spiral wave reentry.
(5) Computer Simulation of Reentry in Homogeneous Muscle
This was first seen the 1950s in chemical reactions. They saw this oscillating reaction,
Art Winfree found these paper about this chemical reaction in a petree dish and he
thought about reentry. He found this spiral wave in the heart. The video was a computer
simulation. The mathematical model tells him that the voltage will be such and such and
he does a numerical equation of the differentials. He put in little resistors to represent the
gap junctions between each two cells. This movie was started up by a premature
stimulus. The blue means that the voltage is very negative and the red means that itʼs
very positive. So the blue is resting and the red is the activation wave front. This is a
snapshot at a point in time. This is called spiral wave reentry.
(6) Pulmonary Vein Ablation for Treatment of Atrial Fibrillation
The same thing can happen in the atrium, you can have atrial fibrillation. In that case,
thereʼs a whole bunch of activation wavefronts that are circulating in the atria. Not just a
single wavefront as during the normal sinus rhythm. In the baseline of the ECG instead
of seeing a nice P-wave you see this fibulatory activity because at any point in time the
atrium is being activated and this is called atrial fibrillation. If you feel the pulse of a
patient with A. Fib, you would feel an irregular pulse. This is the first clue. A. Fib like V.
Fib is often induced by a premature beat. The beat however, comes from the atrial
muscle. After the atrial premature contraction, we get all this irregular stuff (A. Fib). The
mechanism is a bit more sophisticated than sock electrode. Weʼre looking at the back of
one chamber of the heart (in the left atrium). This is a map of where the eptopic focus
was. The premature atrial contraction is coming from an abnormally located site (an
ectopic site). This is the site of the ectopic pacemaker which has been mapped. In most
patients the ectopic atrial site is not in the left or right atrium itʼs actually in the pulmonary
veins. So thereʼs an operation that they do now where the go into the heart with a probe
whose tip generates either heat or cold and you take the probe and apply it to one spot
on the surface of the atrium and you zap it and if youʼre hot or cold enough the area
under that probe will eventually die. You make a ring of zaps around the orfice of the
pulmonary veins and you do this for the other veins. So that ectopic focus is still there
and it fires off but now the action potentials travel down into the atrial muscle but when
they encounter the ring structure, theyʼll stop traveling because after the muscle cells
have died and have been replaced with scar tissue it contains the ectopic activity to the
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