Week 2 - Gas Laws for Anesthesia Flashcards
What is the equation that describes the relationship between force, pressure, and area?
Force = Pressure x Area (F = PA)
Pressure = Force / Area (P = F/A)
What are the pressure conversions?
1 atm = ? bar = ? mmHg = ? psi = ? kPa
1 atm = 1 bar = 760 mmHg = 14.7 psi = 101.325 kPa
What is Avogadro’s number?
the number of molecules in one mole of a substance
6.022 x 10^23
What is Avogadro’s Hypothesis?
one mole of gas at standard temperature (0*C or 273.15K) and pressure (1 atm) occupies a volume of 22.4 (or 22.7) liters
at equal temp and pressure, equal volumes of gas contain equal numbers of gas particles
How much volume is occupied by 2 moles of Oxygen?
2 moles * 22.4 L/mole = 44.8 L of oxygen
Avogadro’s hypothesis
What are the equations to convert temperatures?
Kelvin = *C + 273.15
C = (F - 32)5/9
What is Charles’ Law?
The volume of a gas is proportional to its absolute temperature as long as pressure and amount of gas are held constant
(Constant = P; V/T = directly related)
V1/T1 = V2/T2 (temp must be kelvin)
increase temp = increase volume
decrease temp = decrease volume
You have 5 balloons with 50mL of helium each filled in Texas (80F). When you get to North Dakota it is 34C. How much Helium do you have to share at the party?
Charles’ Law: V1/T1 = V2/T2
-Convert temp to kelvin
250mL / 299.8K = V2 / 274.15K
228 mL Helium
What is Boyle’s Law?
The volume of a gas is proportional to its pressure as long as the temperature is held constant
(Constant = T; VP = inversely related)
P1V1 = P2V2
Decrease volume = increased pressure
Increased volume = decreased pressure
What gas law describes the function of breathing?
Boyles’ Law
when we contract our diaphragm (increases volume), pressure inside the lungs drips below atm pressure and air flows into the lungs
when diaphragm relaxes (decreases volume), pressure in lungs increases and gas flows out the lungs
What direction does the Boyle’s Law curve shift with change in temperature?
Increased temperature - curve moves right
Decreased temperature - curve moves left
A full e cylinder has a cylinder volume of 4.46 L and a gauge pressure of 2100 psi. If you open the cylinder and slowly release the gas, how much volume (space) will the oxygen from the cylinder occupy?
Boyle’s Law: P1V1 = P2V2
2100 psi x 4.46 L = 14.7 psi x V2
637 L oxygen in the room
What is Gay-Lussac’s Law?
Pressure is proportional to absolute temperature (Kelvin) if volume is constant
(Constant = V; P/T = directly related)
P1/T1 = P2/T2
Increase temp = Increase pressure
Decrease temp = decrease pressure
5L gas at room temp (68F) and pressure (1 atm) gets heated to 37C at a constant volume (closed tank). What happens to the pressure and what is the pressure at 37*C?
Gay Lussac’s Law: P1/T1 = P2/T2
-Convert temp to kelvin
1atm / 293.15K = P2 / 310.15K
1.05 atm = 798 mmHg = 15.4 psi
What is the Universal or Ideal gas law?
Unified the findings of Charles, Boyle, and Gay-Lussac Laws
PV = nRT
P = Pressure (atm) V = Volume (L) T = Temp (K) n = number of moles of gas R = universal gas constant (8.314 Joules/mol K) or (0.082 L-atm/mol-K)
Apply the Universal gas law to opening a cylinder of oxygen to deliver 4 LPM n/c
Oxygen molecules (moles) are released from the cylinder and Pressure inside decreases The "volume" of the cylinder itself remains unchanged V, T, and R are unchanged As "n" decreases "P" decreases (The reading on the pressure gauge decreases as the # of moles of O2 in the cylinder decreaes)
What is Kinetic Molecular Theory?
- Ideal gases consist of large # of tiny particles
- Gas particles are in constant, random motion
- Gas particles are small compared to the distances between them
- The volume of particles is negligible
- Gas particles don’t react chemically with the container or each other
- There are NO forces of attraction or repulsion between gas molecules or container
- Collisions between particles and container are perfectly elastic (no lost energy)
- Pressure is caused by collisions between particles /container walls
- Avg kinetic energy of a collection of gas particles depends on their absolute temp and nothing else
What are the three Ideal gas assumptions?
- Molecules don’t react chemically with each other or container
- Molecules don’t interact with each other or the walls of the container
- Molecular size is small in comparison to the average distance between molecules
At standard temp and pressure (0*C & 1 atm), how many liters of space does 1 mole of an ideal gas occupy?
22.4 liters of space
at room temp 68*F - 1 mole of ideal gas occupies 24 L of space
What is Dalton’s Law?
Dalton’s Law of Partial Pressures:
-the total pressure in a mixture of gases is equal to the sum of the partial pressures of the individual gases in the mixture
- need to know the Fi% of the gas to make it work (needed to calculate the partial pressures of a gas)
What is the equation to find the partial pressure of a gas?
Fi% x Atmospheric Pressure
What is the partial pressure of Oxygen in room air?
RA = Fi% of 21% Atm = 760 mmHg
0.21 x 760 = 159.6mmHg
What is the composition of dry room air?
Nitrogen 78% – 593 mmHg
Oxygen 21% – 159 mmHg
Argon 1% – 8 mmHg
CO2 0.03% – 0.2 mmHg
What is the mole fraction?
The mole fraction of each component is the number of moles compared to the total number of moles of gas present
P1 + P2 = (n1 + n2) RT / V
Mole fraction of Gas 1 = n1/(n1 +n2)
Mole fraction of Gas 2 = n2/(n1 +n2)
The # of moles of gas is directly related to the gases partial pressure (n1/P1 = n2/P2) (n1/n2 = P1/P2)
The partial pressure of a gas is directly related to its mole fraction — n1/(n1+n2) = P1/P1+P2)
Patient is receiving general anesthetic with an ETT:
-Inspired gas mixture 47% N2O, 47% O2, 6% Desflurane
What are the partial pressure of each gas at inspiration?
Dalton’s Law:
N20: 0.47 x 760 = 357.2 mmHg
O2: 0.47 x 760 = 357.2 mmHg
Des: 0.06 x 760 = 45.6 mmHg
357.2+357.2+45.6 = 760 (1atm)
If 100% oxygen is run through the isoflurane vaporizer, What is the % Iso in the oxygen/gas mix coming out of the vaporizer?
Dalton’s Law
SVP of Iso = 238
1 atm = 760
238/760 = 31.3% Iso 100-31.3 = 68.7% O2
What happens if you fill a vaporizer with the wrong agent?
If an agent with a higher saturated vapor pressure is added to the vaporizer of an anesthetic with a lower SVP, the delivered concentration will be higher than the dial setting (over anesthetize)
Lower SVP added to vaporizer of Higher SVP, delivered concentration = lower than dial setting (under anesthetize)
What is Henry’s Law?
The amount of gas that dissolves in a liquid is directly proportional to the partial pressure of gas in the gas phase over/around that liquid
Blood is a liquid and O2, CO2, and anesthetic agents are gases
What is the oxygen carrying capacity equation?
Henry’s Law:
(1.34 x Hgb x SaO2) + 0.003 = CaO2
Each gram of fully saturated Hgb can carry 1.34 mL O2 (constant)
There is 0.003(100) mL O2 dissolved in 100 mL blood (constant)
What is the PaO2 at a FiO2 of 21% and FiO2 of 100%?
FiO2 of 21%: PaO2 = roughly 100 = 0.3 mL/Dl
FiO2 of 100%: PaO2 = about 500 = 1.5 mL/Dl (there is 0.003(500) mL O2 dissolved in 100mL blood)
Alveolar oxygen partial pressure (PAO2) = roughly 6x FiO2
Arterial oxygen partial pressure (PaO2) = roughly 5x FiO2
What is the amount of CO2 dissolved in blood?
What is the amount of CO2 in the blood at a given point in time?
Amount CO2 dissolved in blood = 0.067 mL/Dl
Amount CO2 in the blood at a given point in time = PaCO2 x 0.067
What is Graham’s Law?
The rate of diffusion, for a gas through an orifice, is inversely proportional to the square root of the molecular weight of the gas
r = 1/√MW
- Small molecules diffuse through membranes faster (larger = slower)
- If you know the ratio of the molecular weights, you can figure out how much faster a given gas will diffuse than another gas (Rate1/Rate2 = √MW1/MW2)
How does Graham’s Law help explain the second gas effect with Nitrous Oxide?
Rapid uptake of the smaller molecules leaves a higher concentration of the larger molecules that are left behind
A large amount of an insoluble gas (N2O - Low MW) diffuses rapidly from the lungs (large membrane area) into the blood because of a high pressure gradient
How does Graham’s Law help explain diffusion hypoxia, which can occur with N2O?
Smaller molecules rush out and reduce concentration of the larger molecules
N2O rushes out of the blood into the lungs and reduces the concentration of oxygen in the lungs. The FiO2 can drop below 21% if you don’t increase O2 flow rates to compensate
What is the Meyer-Overton Hypothesis?
The potency of a compound to induce general anesthesia is directly related to the compounds lipid solubility (lipid solubility can be equated to the oil:water coefficient)
Higher coefficient, higher solubility, higher partitioning into compartment (blood, muscle, fat)
Low solubility agents = Higher MAC to produce anesthesia and rapid onset/recovery (more potent)
High solubility agents = Lower MAC to produce anesthesia and slower onset/recovery (less potent)
What are the oil:gas coefficients of Halothane, Isoflurane, Sevoflurane, Desflurane, and Nitrous Oxide?
Hal = 197 Iso = 91 Sevo = 50 Des = 18.7 N2O = 1.3
What is Fick’s Law of Diffusion?
Rate of Diffusion = (P1 - P2) x Area x Solubility / membrane thickness x √MW
- Diffusion is directly proportional to:
- Change in partial pressure
- Area of membrane over which diffusion occurs
- Solubility of gas in the membrane
- Diffusion is inversely related to:
- Membrane thickness
- Square root of the molecular weight
Rate of diffusion decreases over time
Concentration decreases as the square of the distance increases (concentration decreases as you get further down the nerve)
What is the Joule-Thomson Effect?
When a compressed gas (under pressure) is released rapidly into space (a room), cooling occurs
-There is no loss of heat or matter between the cylinder and surrounding so the process is called adiabatic
-Reason why O2 released from O2 tank at high flow is very cold
