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# 10 equations

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by

## Russell Buhr

on 22 July 2013

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#### Transcript of 10 equations

Let's simplify this:
PAO2 = (760-47)FiO2 - PaCO2 (1.2)
therefore, at sea level on room air:
PAO2 = 150 - PaCO2(1.2)

Conceptually Simplified:
Delivery O2 = Cardiac output (Hgb x SaO2)
Mean arterial pressure = cardiac output x systemic vascular resistance
Metabolic need = Cardiac output x (SaO2 - SvO2)
Flow = K[P(in)-P(out)] - [Onc(in)-Onc(out)]
Wall tension = [Pressure x radius^4]/wall thickness
The 10 Equations
AKA: everything you really need to know about internal medicine
Oxygenation
Alveolar gas equation: air to the alveoli
Simplified:
Diffusion = [pressure gradient x area] / wall thickness
Fick's Law of Diffusion: alveoli to blood
Oxygen Delivery:
blood to tissues
Supply andDemand
Ohm's Law:
Hemodynamics
Stroke Volume
How much is enough?
Spigots & Drains
Excessive CO2
Starling's Law:
Fluid across a membrane
How does the body handle
fluid in its chambers?
Assuming a normal PaCO2, what would the PAO2 be?
PAO2 = 150 - 40(1.2)
PAO2 = 150 - 48
PAO2 = 102
ABG (room air) @ sea level: pH 7.40, PaCO2 40, PaO2 90
Adapted from: Wiese J. "Teaching in the Hospital"
Adapted from: Wiese J (ed.). "Teaching in the Hospital"
Adapted from: Wiese J (ed.) "Teaching in the Hospital" ACP Press, American College of Physicians, Philadelphia 2010
PAO2 = [(760-47)(0.21)] - [(40)(1.2)] = 102
PaO2 on ABG = 90
A-a gradient = PAO2 - PaO2
Normal ~ (1/3 pt's age, minimum of 10)
CaO2 = (SaO2 x Hb (g) x 1.34 (mL/g)) + .003(PaO2)
Cardiac
Output
Increase
heart
rate
Increase
Stroke
Volume
Cardiac output =
stroke volume x heart rate
Increased
Sympathetic Tone
Increase
Venous
Return
(LVEDV)
Why do I get more short of breath running in humid weather?
PAO2 = (760 - Vapor Pressure H2O)(0.21) - (PaCO2)(1.2)
Why do I get more short of breath running at altitude?
PAO2 = (Barometric pressure - Vapor Pressure H2O)(FiO2) - (PaCO2)(1.2)
As humidity increases, vapor pressure increases
As altitude increases, barometric pressure decreases
FiO2 does NOT change
Stroke volume = preload x contractility
% metabolic need being met
Delivery of O2 = CO x Hb x SaO2
Insult 1 -
compensation
Insult 2 -
compensation
Insult 3 -
decompensation
death
ICU
Floor
Outpatient
Metabolism = the facuet
Minute ventilation = the drain
Ventilation and perfusion are not equal across the lung
Illness or exertion drive up metabolism
PaCO2 = CO2 production - minute ventilation
Minute ventilation = (respiratory rate)(tidal volume)(1-deadspace %)
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