Cardiac Output Flashcards
name the three contributors to altering stroke volume
preload
afterload
contractility
how must contractility, preload, and afterload change to increase the SV?
contractility and preload increase
afterload decreases
what two things is preload proportional to?
LVEDV and LVEDP
where does preload lie on the PV loop?
right at end of diastolic filling and just prior to isovolumetric contraction
what really determines the preload of the heart?
the tension in the ventricular walls…laplaces law
what physical exam technique can we use to measure the preload and why does this work?
measuring the diastolic jugular venous pressure gives a good estimate of the preload of the RV because the jugular is almost directly connected to the RA and in diastole the RA is about equal in pressure to the RV
explain the Frank Starling Mechanism
as you increase the force or load on the muscle…it leads to a better overlap of actin and mysoin..yielding an optimal length that will cause the greatest force in contraction
what effect does nitroglycerin have on the vessels?
it is a potent venous dilator
when you decrease the venous tone, or dilate the venous system…what effect does this have on preload?
due to the venous system needing constriction to increase flow back to heart, when you dilate the venous system you get a lesser preload of the heart and less oxygen demand
name three ways to regulate preload
blood volume
venous tone
body position…gravity plays big role in venous system
what is the easiest thing to really compare afterload to?
systemic vascular resistance
describe a common way of increasing the afterload of the heart that has to do with valves
aortic valve stenosis…less compliance here leads to higher pressure to overcome for the heart
if there is an increase in afterload…what must happen to the isovolumetric line in the PV loop?
it will reach a higher peak since the heart must overcome a higher pressure to begin ejection
define afterload in terms of the heart muscle
afterload is the wall tension during contraction of the heart
with afterload causing an increase in necessary pressure for ejection, explain how the SV is lessened
the myofibrils and myosin and actin shorten at a decreased velocity and with the finite time for ejection the stroke volume decreases
how do we mathematically define afterload?
the SYSTOLIC WALL STRESS
what is wall stress equivalent to?
pressure times radius divided by wall thickness times 2
if you increase afterload what happens to O2 demand of the heart?
increased
which two anti anginal agents decrease the afterload of the heart?
calcium channel blockers and nitrates
what class of sympathetic agents increase heart contractility and what is the mechanism?
beta agonists…like dobutamine
increase intracellular calcium
does the contractility of the heart depend on the load or stress on the heart?
HELL NO
what is contractility truly proportional to?
how many actin and mysoin crossbridges are made…so really intracellular calcium
name four determinants of contractility
drugs that increase intracellular Ca (sympathetic tone)
Myosin ATPase activity (catalyzes ATP at higher rate)
ATP levels
number of sarcomeres
the line depicting contractility on a PV loop takes what form?
a simple line with y=mx+b slope
a tachycardic rate causes what with preload and contractility?
decrease in preload and increase in contractility
less filling time to refill leads to decreased preload
shorter time between beats means more intracellular calcium so increased contractility
when exercising, what is the main contributor to increased CO?
HR increase
with exercise, what happens to SVR and how does this affect the heart?
decrease in SVR due to arteriodilation of muscular arteries
leads to a decrease in afterload of the heart so increased SV
what happens to venous return during exercise? effect this has on heart?
increased venous return,..increased preload…increased SV
