The definition of hypertension is: a chronic
increase in blood pressure generally agreed
upon as greater than 140/90 mm Hg. 140 is
the systolic blood pressure. 90 is the diastolic
In very elderly patients these days, we’re
allowing blood pressures as much as 150 systolic
because of stiffening of the arteries with
Another definition of hypertension of course
is if the patient has had high blood pressure
diagnosed in the past and is currently receiving
Hypertension results from increased vascular
resistance. Let’s think about that for a moment.
Remember the formula for blood pressure is
the same as the formula for measuring resistance
or voltage in an electrical current. The voltage
in an electrical current = resistance × flow.
In other words, the amps × resistance.
In the cardiovascular system it’s the same.
Blood pressure = flow × peripheral vascular
resistance. When the peripheral vascular resistance
goes up and the flow is maintained constant,
blood pressure goes up.
Approximately one third of Americans have
hypertension and often there are serious complications.
The reason for this is that patients very
often do not realise that they have high blood
pressure because it’s often without symptoms
until the patient gets to the point that there’s
I’ve just repeated for you the so-called
Ohm’s law. Ohm’s Law is the electrical
law of the circulation that is pressure = current
× resistance. Or in electricity, E is voltage,
I is amperage or flow and R is the resistance.
And in the circulation, remember, it’s the
same formula: blood pressure = cardiac output
(which is the flow in the system) × vascular
resistance. And patients with hypertension
have elevated vascular resistance.
There are two forms of hypertension that are
recognized by physicians: the so-called essential
hypertension – that’s the vast majority
of patients – where there’s no obvious
cause for the hypertension. It’s thought
to be genetics as well as environment. Things
like high salt diet and obesity play into
this. We know families where everybody in
the family has a tendency to hypertension
and other families where there’s no tendency
to hypertension. So there’s clearly genetic
factors involved here.
Secondary hypertension is the result of another
factor, not genetic. It can be the use of
abusive drugs such as cocaine or amphetamine;
it can be due to kidney disease, usually renal
vascular – we’ve already talked about
that in the last lecture and we’re going
to talk some more about that in this one;
pheochromocytoma which is a tumour that releases
adrenaline and noradrenaline; and coarctation
of the aorta – we’ve talked about that
a little bit in the past.
Many of these secondary causes are effectively
treated either by medicine or by surgery.
Let’s talk a little bit about coarctation
of the aorta as a typical example of a secondary
form of hypertension.
Remember what coarctation is? It’s a narrowing
of the aorta, just like the diagram shows
you, just distal to the left subclavian artery.
This results in hypertension of the blood
vessels on the upper part of the body and
low blood pressure in the lower part of the
body because blood doesn’t get down there
because of the stenosis in the aorta just
after the left subclavian artery.
There’s often a pressure difference of 20
mm Hg or more between arm and leg. And remember
that was one of the test we used also for
diagnosing peripheral vascular disease.
Coarctation is usually picked up in young
people before atherosclerosis has developed.
And, again, it requires either surgical repair
or a balloon angioplasty often with a stent
to reopen the narrowed area in the aorta and
restore normal blood flow.
Even after successful therapy, patients who
have had a coarctation are at risk to maintain
high blood pressure and need medical therapy.