blood flow to vital organs,
including the heart. This graph
gives you an idea of how the vapours
affect cardiac function. On the upper
graph to the left, you can see how mean arterial
blood pressure is affected by the vapours,
and basically all of them cause a decrease
in blood pressure, at least mean arterial pressure.
But there is a bit of a difference with Desflurane,
which tends to come down and then stabilize a bit, and
with Sevoflurane that also does that. Comes down,
there's a drop in blood pressure, and then there's a stabilization.
So you don't have to worry about it continuing
to drop quite so much. On the right upper
graph you're looking at cardiac output affected
by vapours. And you can see that Desflurane
has a very slight effect on cardiac output.
Initially a bit of a drop, and then it tends to come back
and stabilize. Sevoflurane, a bit more profound,
but the older drugs, Isoflurane
and particularly Halothane, are continuing drop
in cardiac output as the concentration of
the vapours increased. On the left lower graph we're looking
at Systemic Vascular Resistance. So this
is the resistance against which the heart works
when it contracts. And you can see that Isoflurane
causes quite a profound drop in peripheral resistance,
as does Desflurane. This can be a good
thing in the presence of poor cardiac output,
because it improves the ability of the heart to eject blood
against a lower resistance. However, it may result
in hypotension and decreased blood flow to organs.
In the bottom slide to the right is Heart Rate,
and you can see that Desflurane
initially causes very little change in heart rate and then,
as you increase it, there is fairly marked
increase in heart rate. Isoflurane, on the other hand,
causes quite a nice stable increase
in heart rate, and it stays pretty much at a level
that doesn't change for a period after
that. Heart rate's critical in these patients,
because increases in heart rate increase oxygen
demand by the heart. And so, we don't like
to see tachycardia, particularly in middle aged
people or older people. So how about the induction drugs?
We already mentioned Propofol and Pentothal.
We're now going to talk about Ketamine
and how it affects cardiac function.
Ketamine is often used in patients who appear
to be volume depleted at the time of surgery
or have had trauma, because it increases heart rate
and increases blood pressure moderately.
The problem is that it can lead to increases
in blood pressure that are unacceptable,
and it can lead to tachycardia or increased
heart rate that's unacceptable. On the good
side, it acts as a pain killer. So less opioids needs
to be used. You can give Ketamine without
a whole lot of extra narcotic on board.
The problem with Ketamine, and one
of the reasons why it isn't as widely used as it might otherwise be,
is that people have what are called Emergence
Phenomena when they start to wake up from Ketamine,
which can include hallucinations and quite
strange behaviour. Patients will wake up and be picking
at things in the air, they'll think they see bugs.
They'll be very concerned about noises in the room.
So, at the time Ketamine was used fairly
regularly in the early 80's, the recovery rooms were often
kept dark and very, very quiet. Which meant
you really couldn't see your patient and monitor them adequately.
So that was totally unacceptable and we no longer
accept that as a property within our recovery
rooms. Etomidate, which is a drug
that has been available in the United States for many years,
but in many other countries hasn't been available, because
it wasn't marketed due to cost constraints,
is a very good drug to use
in patients who have had brain trauma.
Because it has very little effect on intracerebral blood pressure,
has very little effect upon the cardiovascular system,
and oxygen, cerebral oxygen delivery tends
to remain stable. But it's not a perfect drug.
It suppresses the release of cortisol
from the adrenal cortex. And this may interfere
with the body's ability to handle the stress
of trauma, surgery, or anesthesia.
And there are some reports that suggest,
that unexpected death following surgery.
Not at the time of surgery, but in the weeks
to maybe a month following surgery, that unexpected
death is higher in patients who have had
Etomidate. So, what does the anesthesiologist
do about these changes?
Low blood pressure can reduce blood flow,
thus oxygen supply to vital organs.
Rapid heart rates can increase the work
of the heart and increase the need
for myocardial oxygen, which is difficult to supply
in the presence of low blood pressure.
A blood pressure that is too high
is also unacceptable, as it can lead to strokes,
an increased cardiac work, which may cause
cardiac ischemia or heart attacks.
So the anesthesiologist can modify these variables
by careful administration of either stimulating
drugs, such as Ephedrine or Phenylephrine,
Epinephrine or Norepinephrine, which increase
blood pressure and heart rate. Of course,
all of these drugs have their pros and cons.
So careful understanding of each is necessary
before one uses them. For instance, Phenylephrine
will increase blood pressure quite nicely,
but only for a very short period of time
and often causes quite a profound drop in heart rate,
whereas the others all increase heart rate
and tend to increase blood pressure.
Constant monitoring of the patient's condition,
during and after anesthesia, has shown to be
the most important aspect of anesthesiology.