Law of the heart: blood pressure equals heart
rate… equals cardiac output times resistance.
Now, let’s talk a little bit about the energy
metabolism of the heart. The heart’s just
like a gasoline motor. It has to have fuel
in order to contract just like the gasoline
motor has to have gasoline in order to function.
And you need to balance, the amount of energy
that’s spent in causing the heart muscle
to contract depending on the amount of work
it has to do versus the amount of fuel that
goes into it. And we usually do this by measuring
something called the oxygen consumption of
the myocardium that tells us how much fuel
the heart is burning because you need oxygen
to burn the fuel in the heart muscle cells
and these two have to be balanced. You can
see why. If the demand is more than the supply,
the heart muscle’s going to run out of energy
and it’s going to be hurting, so that, in
fact, that’s exactly what happens in
ischemic heart disease with atherosclerosis.
When there is narrowing in the blood vessels,
it’s like narrowing in the fuel lines in
your car. You have to go up a hill and suddenly,
you step on the gas and guess what? The car
doesn’t respond because not enough fuel
is getting into the engine. Well, the same
thing happens with the heart. You suddenly
decide to run up a hill. If there is a restriction
in the supply of blood to the heart muscle,
it can’t function adequately. You have an
imbalance between supply and demand- oxygen
consumption and oxygen demand, and that results
in a decrease in heart function just like
the decrease in the motor when you don’t
get enough gasoline into it when you are trying
to go up a hill. We have calculated these
numbers, we can calculate them in the cath
lab. We know what normal values are, we can
measure the amount of oxygen in the artery,
we can measure the amount of oxygen in the
veins. We can calculate all of these kinds
of numbers and they are often done on critically
ill patients or patients in the catheterization
laboratory to give us a sense of… of how
well the heart is functioning, given the amount
of blood flow that it is receiving. We often
do a very simple test in most patients. You
can do this as an outpatient or inpatient.
This is a device that measures the oxygen
saturation just with your fingertip.
You put your fingertip in a little device and it tells
you how well saturated the blood is, that’s
being pumped around the body. Now, we would
like it most times for people to have oxygen
saturations above 90. Of course, if you are
on the top of Mount Everest without an oxygen
mask, your oxygen tension is going to be much
lower than 90. It would probably even get
down into the 70s, which could be dangerous
unless you are really a trained mountaineer
and even in a trained mountaineer, sometimes,
there is a little brain damage because of
the very, very low oxygen at very, very high
altitude. But, most normal people without
heart or lung disease will have values well
in the 90% saturation with oxygen measured
at their fingertip and that’s often a measure
of sort of integrated heart and lung function.
Is the oxygenation adequate? And often, for
example, in my clinic, every time I see a
patient, we measure that to make sure that
the oxygen saturation in the periphery is
adequate. These devices are very easily obtained
and are in pretty much all Cardiology Clinics
and inpatients. One other piece of equipment
I wanted to bring to your attention:
This is the balloon catheter that we use to measure
the pressures on the right side of the heart.
the balloon is out at the tip, and as I said it’s
like a sail on a sailboat. The circulation
pulls the catheter through the right heart
chambers and out into the pulmonary artery.
There is a number of extra little lumen so
we can squirt material, drugs sometimes or just
salt water to increase the blood volume in
patients where we think the blood volume has
become decreased. Finally, the last two slides
are fairly complicated.
They will require you to sort of work your
way through them and think about them.
They show you the integration of the cardiovascular
system. The resistance in the periphery, the
pumping chambers, the two ventricles and how
they are all related. And in a normal person,
these things are exquisitely balanced, so
that the cardiac output, the peripheral resistance
in the lung and the periphery all are balanced
so that there is a normal cardiac output.
Let me give you an example of what happens.
You start to exercise. Suddenly, with the
exercise, the resistance in the peripheral
vessels goes down, afterload goes down, the
heart pumps more blood and you will increase
your cardiac output. Trained athletes can
increase their cardiac output by 4 and 5 fold.
Most of us can usually do 2 and 3 fold without
too much trouble. And a final slide, again
shows the interrelationship of the resistance.
For example, you can see SVR that’s systemic
vascular resistance. You see mean arterial
pressure at the top, you see cardiac output
and heart rate determined by stroke volume
and heart rate. You see the whole circulatory
system, all in relation to the various parameters
that we can measure. In normal people, all
of these things are exquisitely balanced both
by the central nervous system which is constantly
monitoring blood pressure and… and flow
through the circulatory system, and also with
activity from the kidney which puts out hormones
that can increase or decrease the blood pressure.
And we are going to talk a lot more of that
as we get into various ways to diagnose and
manage heart disease.
Thank you very much for going along with me
in this series of discussions relating to
the function of the heart. I know it’s a
lot of material in a very short time.
Those of you who want to dig into it a little deeper,
there is a number of references that will
give you a lot more information than I can
give you in the short time that I have with
you for this topic. Thank you for being here today.