Nursebob's MICU/CCU Survival
Guide
Cardiology in Critical Care
Variables
Affecting
Left Ventricular Function
1. Preload - the pressure in the ventricles at the end of diastole (Left ventricle end diastolic pressure, LVEDP)
A. Frank-Starling Law of the Heart
a. The greater the stretch on the fibers (preload) the greater the ventricular contraction (stroke volume) in the next systole, up to a point.
b. Muscle
fibers can reach a point beyond which contraction is
no
longer enhanced and stroke volume decreased
B. Factors affecting preload:
a. Mitral insufficiency (increases)
b. Mitral stenosis (decreases
c. Aortic insufficiency( increases)
d. Volume of circulating fluid
1) Increased volume increases preload
2) Decreased volume decreases preloade. Drugs
1) Vasoconstrictors increase prelod
2) Vasodilators decrease prelod
2. Afterload - initial resistance that must be
overcome
by the ventricles in order
to open the semilunar valves and propel blood into the systemic &
pulmonary
circulation
Clnically reflected by: arterial systolic
pressure -
SVR
A. Formula for SVR -
SVR = MAP - CVP(RAPI = x 80 converts into dynes/sec/cm-5
C.O. L/minNormil SVR = 900 - 1400 dynes/sec/cm-5
B. Factors affecting afterlod
a. Aortic valvular stenosis (increases afterload)
b. peripheral ateriolar vasoconstriction (increases afterload)
c. Hrpertension (increases afterload)
d. Polycythemia (increases afterlod)
e. Drugs
1) Arteriole vasodilators decrease afterlode
2) Arteriolr vasoconstrictors increase afterload
3. Contractility
A. Factors that increase
a. positive inotropic drugs: digitalis, epinephrine, dobutamine
b. Increased heart rate
c. Sympathetic stimuldon
d. Hypercalcemia
B. Factors that decrease
a. Negative inotropic drugs: quinidine, barbiturates, propranolol
b. Hypoxia
c. Hypercapnia
d. Intrinsic depression: due to cardiac musclt disease or loss of function myocardial tissue due to an M.I.
e. Parasympathetic stimulation
f. Metabolic acidosis
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