Biomedical Engineering 403
Cardiac Output, and Interactions between the Heart and Vasculature
Cardiac Output, and Interactions between the Heart and Vasculature
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Today's topic:
Many of these topics are incomplete.
They should be completed when the
extra credit projects are turned in.
These are Dr. Schechtman's very own notes for this lecture.
I hope you find them helpful.
9
Cardiac Output, Interactions between heart and vasculature
Quantity of blood ejected per minute.
Cardiac output related to body size: ~70 ml/kg/min or ~5 l/min in
average-sized human adults.
The output of the two ventricles is usually equal, and quickly equalized
if a discrepency occurs.
Can increase substantially with exercise
Factors affecting cardiac output:
Heart Rate
Myocardial Contractility
Preload
Afterload
Heart Rate
Sympathetic stimulation increases heart rate, parasymp reduces.
HR must remain within a range to acheive adequate perfusion.
Cardiac output falls if heart rate is too slow or too fast.
Slowing heart rate beyond a limit has little effect on stroke volume
because most ventricular filling occurs in early diastole anyway, so the
extra time at the end adds little.
Very rapid heart rate impinges on filling time, decreasing stroke volume,
and hence cardiac output.
So extreme brady or tachy require correction (pharacological or
electrical).
Preload
Starlings law (Frank-Starling mechanism)--The total work output of the
ventricle is related to the presystolic length of the muscle fibers.
Afterload
If output reseistance is high, a standard contraction will force out less
blood than normal. Starling's law remedies this problem. Higher
output resistance overwhelms one beat forcing some blood to remain in the
ventricle; thus, for the next beat the ventricle will be extra full,
so next beat will be more forceful.
Contractility
Starling curve is not fixed. Many things can effect contractility.
Heart rate and contractility are both cardiac factors that can be affected
by autonomic influences on the heart, while preload and afterload are
vascular factors, that are determined in large part by cardiac output.
Atrial Baroreflexes--the force of atrial contraction can be modulated by
changes in arterial BP via baroreceptor reflex:
A drop in arterial pressure senced by baroreceptors triggers
sympathetic stim of atrium and more vigorous atrial contraction.
Ventricular filling increased and stroke volume increased leading to
increase in arterial BP.
The Fick Method of determining cardiac output:
CO=O2 consumption/A-V O2 difference
ml/min ml/L
Plotting Cardiac Function
Cardiac function curve reflects Starling's law
Vascular function curve reflects that increased CO reduces venous pressure
Equilibrium point represents the values of cardiac output and venous
pressure at which the system usually operates (with short term
deviations from these values).
Example: A sudden increase in venous pressure will raise cardiac
output.
The increased cardiac output reduces venous pressure,
which in turn reduces cardiac output.
Figure 1
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Figure 14
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