Flashcards in Lec 26 Congenital Heart DIsease Flow Dynamics Deck (15)
What determines the driving force for systemic flow? what about the pulmonary circulation?
systemic = pressure difference between aorta and central venous pressure pushing blood through the heart
pulmonary = pressure difference between pulm artery and LA
What is Ohm's law for flow?
delta P = Q * R
pressure drop = mean flow * mean resistance
Why do we measure average rather than instantaneous flow?
- easier to conceptualize and measure
limitation: instantaneous behavior of circulation can be obfuscated by these models
What is the equation for systemic resistance?
Rs = (Pao - Pra) / CI
[R = delta P/flow]
Rs = systemic resistance
Pao = aortic P
Pra = right atrium P
CI = cardiac index
What is cardiac index?
CI = cardiac output indexed to body surface area
What happens to PVR after birth?
starts out really high and declines to adult level
means there is a rapid decline in pulm/systemic resistance ratio
What is normal mature level of PVR [pulm ascular resistance]?
2 woods units
What 3 factors alter resistance to blood flow?
1. blood viscosity
2.. vessel length
3. vessel radius [most important]
What determines blood viscosity?
- high hematocrit [Hct] and protein conc --> high viscosity --> increase resistance
What is poiseulle's equation?
R = 8 * Ln/pi*r^4
What is relationship relative flow through parallel outlets from single reservoir?
relative flow rate proportional to relative resistances
What happens to resistance when resistors are added in parallel?
What do we assume if non-restrictive "large" VSD when we are calculations flows/pressures?
- systolic RV and LV pressures equalize thus aortic and pulmonary artery pressure equal in systole
How do you determine Qp/Qs the pulmonary to systemic flow ratio?
Qp/Qs is inversely proportional to resistance ratio Rp/Rs
--> the ratio of resistances determines the amount of shunting through VSD