Exam 3: Gas Laws & Cylinders Flashcards
Dalton’s Law
the total pressure of a gas is equal to the sum of the pressures of the individual gases in a mixture
P1+P2+P3=total pressure
What are the implications of Dalton’s Law to anesthesia?
We have to make sure the patient is breathing in a minimum of 21% fiO2 with our other gases
-vapor pressure as a percent of atmosphere
Calculating the PP of unmeasured gas in a mixture:
Ptotal is 200 mmHg, gas 1 is 50 mmHg, gas 2 is 40 mmHg; what is gas 3?
Gas 3 is 110 mmHg
Calculate the total pressure if pressure in mmHg is 40 for gas 1, 30 for gas 2, and 100 for gas 3.
170 mmHg
At sea level agent monitor says ETiso= 8 mmHg. What is volume % for isoflurane?
1%
At sea level agent monitor says ETsev=2%. What is PP for sevoflurane?
15.2 mmHg
Describe atmospheric pressure, barometric pressure, and ambient pressure
Atmospheric pressure is the weight of atmosphere pushing against an object; at sea level it is 760 mmHg
atmospheric pressure can be used interchangeably with barometric pressure
Ambient pressure is the pressure of the surrounding medium on you or an object. If you are underwater, the ambient pressure= atm + water (hydrostatic) pressure
If an object is sitting out in room air, then
ambient pressure=atmospheric pressure
What is the alveolar gas (Air) equation?
PAO2= PiO2 - (PaCO2/RQ) + F
F is a correction factor that is usually ignored
PIO2 is the partial pressure of O2 in inspired air and equals PIO2= FiO2 x PB-PH20 (alveolar)
RQ is the respiratory quotient and is typically 0.8
Why is the alveolar gas equation useful?
it can tell us if there is a diffusion problem since we have the calculation PAO2 and can measure PaO2
RQ- Respiratory Quotient is reflective of
8 CO2 molecules are produced for every 10 O2 molecules burned so our ratio is 0.8
What is the alveolar-arterial oxygen gradient?
(PAO2-PaO2)
This is not as good when dealing with high FiO2
Normal range is 5-15 mmHg
What alveolar-arterial calculation should be utilized when dealing with high fiO2?
PaO2/PAO2
ratio should be 0.75 or greater if there is no diffusion problem
What are normal reasons for alveolar arterial oxygen diffusion differences?
regional VA/Q mismatch (largest influence)
Normal anatomic shunt of the blood (Thebesian circulation of the bronchial and circulatory system)
What are reasons for pathological A-a O2 differences?
- Significant VA/Q mismatch
- Intrapulmonary shunts
- A-v-cap membrane diffusion block
What is an additional way to calculate arterial-alveolar ratio?
PaO2/FiO2 ratio
normal >200
<200 observed in ARDS
Henry’s Law
the amount of gas dissolved in a liquid is directly proportional to the pressure applied to the gas as it overlies the liquid
Henry’s Law formula
Cgas= Pgas/Kh
where C= dissolved gas concentration
Kh= solubility constant of gases leaving the solution
Pgas= partial pressure of the gas above the liquid and represents gas molecules entering the liquid
What is the solubility of CO2 compared to O2?
CO2 is 22x more soluble than O2
How many times more diffusable is CO2 compared to O2?
20x more diffusable
What is the amount of oxygen delivered to the tissues?
= O2 content x cardiac output
What is the oxygen content equation?
Arterial O2 content= (1.34 mL O2/gm hgb x 15 gm hgbb/ 100 ml blood x % saturation) + (0.003 x PaO2)
How would our venous O2 content calculation differ from our arterial O2 content?
the % saturation would be about 75%
How much oxygen should we expect to carry in 1 deciliter of blood?
20.4 mL
