CHAPTER 5 â€¢ VASCULAR FUNCTION
123
In this problem, CO is increased by 30% and
MAP is decreased by 10%:
SVR =
0.9 MAP
1.3 CO
0.69
Therefore, SVR is decreased by 31% (0.69
SVR is the equivalent of a 31% decrease), and
the drug is a vasodilator. Note: In solving this
problem, MAP and CO cannot be multiplied
by their percentage change.
CASE 5-1
The total coronary resistance (RT) equals
the sum of the series resistance elements.
Therefore, the left main coronary artery
resistance (Rl) would be in series with the
remainder of the resistance elements (RX),
so that RT = Rl + RX. Normally, RL
= 0.01(RT)
and RX
= 0.99(RT
) because RL
is 1% of R^ and
therefore RT
= 0.01(RT) + 0.99(RT) = 1(rT).
Decreasing the vessel diameter by 50%
increases Rl by a factor of 16 because R ^
1/r4. Therefore, the resistance of the stenotic
vessel will be 16 times its normal resistance,
so that Rl = 16(0.01)RT, or RL
= 0.16(RT). We
can now say that RT = 0.16(RT) + 0.99(RT).
Therefore, RT = 1.15(RT), which means that
total coronary resistance increases by only
15% (1.15 - 1.00) X 100] when the resistance
of the left main coronary artery increases
1500% (16-fold increase).
SUGGESTED RESOURCES
Belloni FL. Teaching the principles of hemodynamics.
Am J Physiol 1999;277 (Adv Physiol Educ
1999;22:S187-S202.
Berne RM, Levy MN. Cardiovascular Physiology.