Fast Track - Laws Flashcards
Laplace’s Law
P = 2T/r
surfactant allows for same distending pressure regardless of alveolar size; larger alveoli have higher surface tension
Fick’s Law
Vgas= (A x D x dP) / T
rate of gas diffusion is proportional to area, pressure gradient and inversely proportional to thickness
Graham’s Law
applies to gas already in solution and states diffusion through liquid proportional to square root of its density
r1/r2 = √M2/M1
larger mlcl gas diffuses slower than smaller mlcls
Henry’s Law
Mass of gas dissolved into liquid is proportional to the solubility coefficient of the gas and the partial pressure of that gas
C (mol/L) proportional to P (atm) x S (L/atm)
Hooke’s Law
elasticity changes in proportion to force applied; as lung volume increases, recoil force increases linearly
Hysteresis (not a law but important concept)
pressure required to keep lung volume during deflation is < pressure to keep lung volume during inflation; this is bc during inflation energy is expended to recruit alveoli + overcome surface tension
Poiseuille’s Law
P = (8*viscosity of fluid x flow x length of tube) / (pi * R^4)
pressure required for gas flow is proportional to the length of the tube and inversely related to radius
-applies to laminar flow (small airways)
Reynold’s # for turbulent flow
> 2000 = turbulent flow in large airways
Pressure required depends on molecular weight and density (NOT viscosity) of the gas
-heliox has lower density than room air, therefore less pressure required and lower WOB
alveolar air eqn
For patients breathing FIO2 < 0.60: PAO2 = PIO2 - (PaCO2 * 1.2) For patients breathing FIO2 > 0.60: PAO2 = PIO2 - PaCO2 Calculate PIO2: accounts for water vapour of lungs (47 mmHg): PIO2 = FIO2 x (760-47)
mean arterial pressure formula
MAP = [systolic + (diastolic x 2)] / 3
IBW formula
Women = [105 + 5(Height in – 60)]/2.2 Men = [106 + 6(Height in – 60)]/2.2
Bohr eqn
Vd/Vt = (PaCO2 - PECO2 / PaCO2)
SVR eqn
R = P/flow = (MAP - CVP)/Q x 80
PVR eqn
R = P/flow = (PAP-PCWP)/Q x 80
Bohr effect vs. Haldane effect
Bohr effect = CO2 effects O2 binding to Hb; acts at tissues
Haldane effect = O2 affects CO2 binding to Hb; acts at lungs
