CV Control 2: Reg. of Cardiac Output Flashcards
Normal cardiac index range?
2.5-3.5 L / min / m^2
Is oxygen demand the sole determinant of blood flow?
Nope. It's important, but... For kidney, filtration rate. For skin, heat dumping. For GI, absorption. etc. can also determine blood flow.
What is the indicator dilution technique?
What is “diluted”?
To measure CO, tracer is injected into bloodstream, and its “concentration” is measured downstream.
Tracers include oxygen and heat.
What’s the formula for measuring CO with O2 concentrations?
CO = VO2 (total O2 usage) / AVO2D (aterio-venous oxygen difference)
How is thermal diffusion for CO measured?
Temp probe in pulmonary artery, cooled saline injected into RA via catheter.
CO can be derived… with calculus.
Normal mixed venous O2 sat. at rest?
What’s a theoretical minimum necessary mixed venous O2 sat at rest without change in CO?
75%
Theoretically this could go down to 25% without any change in CO (though if you see this, your patient is very very sick).
Review: 3 factors affecting SV?
Preload
Afterload
Inotropic state
With increasing HR, there’s less time for ventricular filling. What fact about ventricular filling in diastole makes helps faster heart rate produce a net increase in CO?
Most of the filling happens in the first 1/3 of diastole, so abbreviating diastole doesn’t decrease SV that much. (unless HR is at extremes, maybe > 200bpm)
What’s the problem with trying to boost CO with drugs that increase HR and inotropic state? (i.e. “whipping” the heart)
This increases the metabolic demands of the heart, so if patient has disorders from CAD, or certain valve disorders, this can actually cause ischemia.
2 ways to modulate preload?
Vasomotor tone.
Renal regulation of intravascular volume.
How to modulate afterload?
Altering systemic vascular resistance.
How to modulate inotropy?
beta adrenergic input
