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Gas Physics and Gas Laws Flashcards

1
Q

Kinetic Gas Theory

A

Kinetic theory is about how ideal gases behave

1) The space between the molecule is large compared to the actual diameter of the molecule
2) Particles are in constant random motion
3) Collisions are completely elastic in nature
4) Increased heat (Ke)= Increased pressure
5) Ideal gases neither repel or attract one another

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2
Q

Dalton’s Law of Partial Pressure

A

The total pressure in a container is the sum of all the partial pressure of the constant gases

Ptotal= P1 + P2 + P3 + etc.

The partial pressure of any gas in a gas mixture is proportional to its percentage within the mixture (fractional concentration)

The partial pressure of each gas is the pressure it would exert if it was alone in the container

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3
Q

Daltons Law and Water Vapour

A
  • Water vapour does not follow Dalton’s law as the pressure primarily depend upon temperature.
  • Because water vapor displaces the partial pressure of other gases the partial pressure of water vapour (PH2O) must be subtracted from the total pressure of the gas mixture when the partial pressure of the gas is saturated with water vapour.
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4
Q

The Gas Laws Units of Measure

A
  • Temperature must be in Kelvin
  • Pressure can in Pascals, kPa, psi etc.
  • Volume can be in litres, M3, etc.
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5
Q

Boyles Law

A

P1 x V1 = P2 x V2

Boyle like girls which is why it is P and V

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6
Q

Charles Law

A

V1 / T1 = V2 / T2

Charles like hot girls (VT)

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7
Q

Gay Lussac

A

P1/T1 = P2 / T2

Like hot guys

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8
Q

Combined Gas Law

A
  • This is the Pivernt equation where the gas content and the number of moles are not affected
  • When do we use Pivernt and when do we use the combined gas law

[(P1 x V1) / T1)] = [(P2 x V2) / T2)]

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9
Q

Ideal Gas Law

A

P= Pressure (Pascals or atm)

V= Volume (m3 or L)

= Number of Moles

T= Temperature (Kelvin)

R=Boltzmann Universal Gas Constant and will be unit specific

R=For atmosphere/ Liters ® R= 0.08205 or 8.2 x 10-2

Atm * L / mol * K

R=For Pascals/ meters3 ® R= 8.31

Pa * m3 / mol * O K

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10
Q

What is a Fluid

A

Anything that is cpaable of flow

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11
Q

Flow

A
  • A pressure gradient is necessary in order to have flow
  • Flow can be defined as movement of an amount of liquid or gas passing a given point per unit of time.
    • Flow ( ) = Volume/Time
    • Flow is most often expressed in liters per minute (Lpm)
    • Flow in an open system is difficult to measure, but can be estimated
    • Flow in a closed system is our area of interest.
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12
Q

Hydrostatic Pressure

A

Hydrostatic Pressure:P=d*h*g

If there is no flow then all liquid levels will be equal and pressure will be the same at any given horizontal point

In a flowing lfuid the pressure will drop progressivly as the fluid flows

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13
Q

Laminar Flow

A

Laminar flow is in cylindrical layer (streamline)

Seen in tubes with smooth surfaces and fixed radii

Pressure required to produce flow is direct related to viscosity of fluid and length of tube and inversly related to radius of tube

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14
Q

Turbulent flow

A

Turbulent flow is chaotic and Poiseuille’s Law is not applicable

Turbulent flow is influence more by density and less by viscosity

The driving pressure required to achieve a given flow is proportional to the square of the flow.

  • Turbulent flow occurs when the velocity at which fluid is moving increasing sharply, when the tube radius varies, and when the tubes have rough uneven surfaces.
  • The likelihood of turbulent flow will depend upon Reynolds Number
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15
Q

Transitional Flow

A
  • Transitional Flow is a combination of turbulent and laminar flow.
    • When turbulent flow dominates the driving pressure varies with the square of flow. Gas flow in larger airways is turbulent and laminar flow predominates in the smaller airways and transitional flow occurs at the point where the airway divide (ex. Main stem bronchi divide into the lobar bronchi)
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16
Q

Resistance

A

Delta P/Flow

  • Resistance is that which opposes flow in a SYSTEM
  • Resistance is the difference between the pressure at 2 points divided by the flow.
  • Resistance is the total of factors that will increase pressure needed to maintain a specific flow
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17
Q

Viscosity

A
  • The INTERNAL force opposing a fluids flow
  • Viscosity of fluids is influenced by cohesive forces
    • The greater the cohesive forces, the greater the fluids viscosity (direct relationship)
    • If the viscosity is greater, it is reasonable to say the there is more likely to be opposition to flow
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18
Q

Poiseuille’s Law Equation

A
  • Hagen Poiseuille’s Law describes what happens with flow under laminar conditions; usually done mathematically

Delta P* Pie * R4

8 * L* n

Where

= Driving Pressure

R4= Radius to the 4th power

L= Length

N- Viscosity factor

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19
Q

Poiseuille’s Law

A

The most important fact to note is the relationship between flow and the radius of the tube that the fluid is moving through.

The relationship is a direct one.

As we increase the radius of a tube, we increase the flow.

The important thing to note is how much changes in radius change the flow rate. Flow is directly related to the radius to the fourth power. If the radius is halved, the flow rate decreases to 1/16th of its previous value. If the radius is reduced to 1/3 the flow rate decreases to 1/81st of its initial value.

Small changes in the radius lead to large changes in the flow rate!! Clinically, it is important to realize that if we decrease the radius of a tube that gas or blood is flowing through, that we can greatly affect the flow through that tube.

The flow of a liquid or gas through a tube has to take into consideration-the driving pressure forcing the fluid through the tube (pressure gradient) and the resistance the fluid must overcome as it flows through the tube.

Based on this equation the following can be stated

Poiseuille’s Law assumes that flow is laminar

The more viscous the fluid the greater the pressure gradient required to move it through a given tube

The resistance offered by a tube is directly proportional to its length; the pressure requires to achieve a given flow through a tube must increase in direct proportion to the length of the tube

Because resistance to flow is inversely proportional to the fourth power of the radius, small changes in the radius of the tube cause profound decreases in the flow of fluid through a tube (ex. Deceasing the radius by one half increases the resistance by 16 fold)

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20
Q

Poiseuille’s Law and Viscosity

A
  • Air and water have viscosity that must be dealt with when considering them in medical applications
  • Blood is 4-5 times as viscous as water, which has ramifications on the heart
    • Increase hematocrit leads to increased workload on the heart
  • Mucous produced by the body must be in a very narrow range for viscosity in order to properly function
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21
Q

Poiseuille’s Law and Resistance

A
  • Assuming that the length & radius of the airway & viscosity of the gas does not change significantly Poiseuille’s Law can be refined to:

Raw=Delta P/Flow

  • Resistance in the airway (Raw) is equal to the difference in pressure in the airway between when there is flow and when there isn’t flow, divided by the flow itself.
    • Units for Raw are cmH2O/L/sec
    • In the airway, we are concerned with the Peak pressures and the Plateau pressures
22
Q

Reynolds Number

A
  • Airway conditions will vary between highly turbulent and high velocity trachea and major bronchi to the quiet, laminar, and very slow flow small airways
    • Remember laminar flow is characterized by the formation of streamlines or layers of flow based on velocity.
  • Fluid flow will become turbulent when the velocity that the fluid is flowing increases sharply, there is also several other factors that can create turbulent flow such as a change in deity and viscosity of the gas or a change in the radius of the tube and all these factors are combined together to create Reynolds Number
  • Reynold discovered that velocity (v), fluid density (d), tube radius (r) and viscosity (n) all played a part in flow.
  • Turbulent flow is created through and increase in velocity, density, or radius or a reduction in the viscosity of the gas
    • In other words turbulent flow is directly related with velocity, density, and radius and indirectly related with viscosity
  • In a smooth bore tube, turbulence will be assured when the Nr is 2000 or above. (unitless)
    • Turbulent flow can occur at a lower number if the tube has a surface that is rough and irregular
  • If the flow becomes truly turbulent, Poiseuille’s Law no longer applies and greater driving pressures are needed to maintain flow.
  • To double the flow under very turbulent conditions we would need to increase pressure four fold.
23
Q

Reynolds Number Equation

A

= V* D* 2r

n

V=Velocity (cm/sec)

D=Density of the Gas (g/cm3)

R=Radius of Tube (cm)

N=Viscoity of the Gas (g*cm*sec / sec2*cm2 )

24
Q

Ohms Law

A

Ohm’s Law R=Volts/Amps

  • There is a direct analogy between Ohm’s Law and the flow of fluids in a tube.
  • Electricity is produced by the flow of electrons through a conductive path or circuit. Electric current is influence by the force pushing the electrons through a conductive path (voltage) and the resistance the electrons must overcome as they flow through the conductive path
  • Electric current can be measures with an ammeter with the standard unit of measurement being an ampere
  • Voltage is defined as the electrical potential required for 1 amp of electrify to move through 1 ohm
25
Q

Voltage

A
  • Voltage can be compared to the difference in pressure at the beginning of the tube and the pressure at the end.
  • Electromotive force
    • The relative difference in electron concentration between the negative (cathode) and positive poles (anode) of an electrical source.
    • The potential difference between 2 points
  • When voltage if held constant then resistance and current are inversely related
26
Q

Current

A
  • The actual flow of electrons between the two poles
    • Measured in amperes (or amps)
    • Amps are coulombs/second
      • Coulomb is the terms for 6.25 X 1018 electrons passing a given point per second.
      • 1 amps=1 coulmob per second
27
Q

Parallel vs. Series

A
  • The resistance described earlier can be further divided into these two main types.
  • If we have several spots where electron flow (or fluid flow) is restricted, we can arrange them in either a continuous pathway or a branched version where several pathways are available.
  • This will change the total resistance dramatically.
  • Blood is unable to flow through one capillary then another and another in a series manner without a pump the size of Manhattan
  • Resistance can be drastically reduced through parallel series which is how the normal branching of the arterial system operates
  • In the airways
    • Compliance calculations
  • 1/comp total = 1/comp(lung) + 1/comp(chest wall)
28
Q

Bernoulli’s Principle

A
  • As the forward velocity of a fluid (or gas) flowing in a tube increases—its lateral wall pressure decreases.
  • A restriction in the tube will cause an increase in the forward velocity and an increase in the forward pressure. This will also cause a decrease in the pressure experienced by the walls of the tube due to the concept of the conservation of energy.
29
Q

Conservation of Energy

A
  • The concept of the conservation of energy, simply stated is that energy can neither by created or destroyed.
    • Energy can only be changed or transformed.
  • Kinetic Energy = the energy of the object in motion (in this case fluid or gas flowing).
    • KE is directly related to mass and velocity

KE = ½MV2

  • Potential Energy = The energy of the objects position (i.e. The water at the top of the water fall; it has the potential to fall)
  • Pressure Energy = The radial or outward force exerted by the fluid (or gas) that is under pressure. This is measured as the lateral pressure.
30
Q

Entrainment

A

As a fluid flows through a restriction velocity will increase and the lateral pressure will decrease, which is the basis for entrainment

When velocity increases enough, it can decrease lateral pressure below atmospheric and create a negative pressure which can pull a fluid or gas into the stream of flow (entrainment)

31
Q

Venturi Principle

A
  • This principle states that the pressure drop across a restriction can be restored to pre-restriction pressure if there is a gradual dilation of the tube distal to the restriction. Provided that the dilation is ≤ 15°.
  • This principal is used in things such as air entrainment masks, aerosol generators, etc. With these devices a lateral port distal to the constrictions used to entrain a second gas or liquid into the main gas flow. The increased gas flow is accommodated by a gradual dilation of the tube downstream of the constriction
  • A drawback of the venturi system is that if there is a build-up of pressure downstream from the entrainment port, there will be a decrease in the amount of fluid or gas entrained.
    • Would this have an effect on the amount of Oxygen delivered to your patient?
      • Absolutely, the amount delivered would increase!
32
Q

Pitot Principle

A
  • Pitot tubes don’t restore the pressure to pre-restriction pressures. The design allows the pressures to remain low, however, the velocity of the liquid or gas is maintained, which allows for greater entrainment.
  • This can also serve to decrease the effect that pressure build-up downstream has on entrainment.
  • Eg. Most aerosol devices–nebulizers.
33
Q

The Coanda Effect

A

Coanda discovered that if he could carefully place these post-restriction extensions, he could deflect the flow a full 180° by extending the contoured wall

When the lateral wall pressure of a fluid decreases such as what happen when a fluid flows through a constriction, there will be an increase in the velocity of fluid flow, and if the wall does not have a side port to entrain other fluids then the low pressure adjacent to the wall will draw some of the stream of fluid against the wall

34
Q

Fluidics

A
  • Fluidics will change the flow of gas through different mechanisms
  • Or/Nor: will direct gas flow out of one output, unless acted upon by an outside force. When input is at A1, A2 or A3, flow will move to Y until the input flow is removed and then it will move back to X.
  • And/NAND: flow will preferentially move out of X, unless both A and B have inputs, in that case flow will move out of Y.. This will only work if there is input from A and B at the same time
  • Flip Flop: This is still commonly used especially in neonatal, when we want to maintain a constant pressure.flow with pick a side, any side and will flip to the other side when an input comes from a or b . There is not a straight line of flow so it can go to X or Y. By directly the flow either toward or away from patient we can keep a constant floweven when they change pressure through breathing in and out
35
Q

Andvantage and Disadvantage of Fluidics

A

Advantage: Gas flow is regulated by gas jets, not mechanical valves, therefore, fewer valves and moving parts therefore less things can get broken

Disadvantage: They consume more gas than more conventional ventilators, because gas flow is used to power the fluidic gates

36
Q

Colloids

A
  • Molecules that are small enough to maintain a homogenous appearance, but large enough that they don’t dissolve, and won’t diffuse through a membrane
    • Range in Size: 1-100nm
  • They stay suspended in the mixture
  • Tend to attract and hold water
  • Sometimes referred to as dispersions or gels
  • Molecules are well distributed in the dispersion; and tend not to settle out in circumstances of low/no flow;
  • Examples: Protoplasm inside a cell; Milk; Jell-O
  • Every colloid consists of two parts: colloidal particles and the dispersing medium. The dispersing medium is the substance in which the colloidal particles are distributed. In muddy water, for example, the colloidal particles are tiny grains of sand, silt, and clay. The dispersing medium is the water in which these particles are suspended.
  • Higher osomositc pressure than suspensions
37
Q

Suspensions

A
  • Large molecules
  • Will settle out, due to gravity, if left to stand for any period of time
  • Dispersion throughout the liquid is done by shaking
  • Do not behave like true solutes and solvents
  • Examples:
    • Many liquid antibiotics (Amoxicillin)
    • Red Blood Cells in Plasma
38
Q

Solutions

A
  • Mixture of one or more substances in a single phase
  • Stable and evenly dispersed
  • Solutions can be:
    • Unsaturated
      • I could get more in
    • Saturated
      • Pretty much full
    • Super-saturated
      • Full and moving fast enough to hold a little more; will settle out when still
      • We got more in but we had to do something to get more in
  • Will affect how much we can get inside that liquid-temp etc
  • This is important in oxygen therapy
  • Solute
    • Substance that dissolves in another substance.
    • Eg. Sugar, salt, electrolytes
  • Solvent
    • Medium in which the solute dissolves.
    • Eg. Water, alcohol, oils (fats)
39
Q

Solubility

A
  • The ability of a substance (solute) to dissolve in another substance (solvent)
    • Term used to describe the nature of the substance itself
  • Solutes can exist in one of two states:
    • Undissociated substances
      • Will not break down
      • Non-ionic gases in the lung or in blood, sugars, etc.
        • CO2 does not dissociate into carbon and oxygen
    • Dissociated substance
      • Will break down
      • Ionic, reactive, all the electrolytes, blood pH buffers, salt in water, etc.
        • NaCl dissociates (breaks the ionic bonds) in water, into ions Na+ and Cl-
          • The water will surround each particle breaking ti down into soidum and choride ions
          • If you want to change it back to NaCl then you need to evapaorate the water out as that is what is holding the Na and Cl apart
40
Q
  • Solubility is dependent on:
A
  • Concentration
    • Unsaturated, saturated and super-saturated
  • Partial Pressure
    • Solubility of gases in a liquid is directly proportional to partial pressure of the gas, this is the basis of oxygen therapy, Henry’s Law.
  • The nature of the substances
    • Depends on the forces of the solute-solute molecules and the solvent-solvent molecules
    • Solubility co-efficent
  • Temperature
    • Solubility of gases is inversely proportional to temperature: More CO2 dissolves in cold Diet Coke.
    • Most of the stuff we will look at will be @ 37 C (Body Temp)
  • We need to know the above about the patient’s blood in order to get O2 into the patient
  • The ease in which a solute dissolves in a solvent is dependent on other factors:
    • The nature of the solutes
      • Varies widely with each substance (sugar vs. gravel)
    • The nature of the solvent
      • Some solvents dissolved some substances well and others poorly. Eg; lipid soluble or water soluble proteins. ETOH dissolves extremely well in water
41
Q

Osmosis

A

Movement of water across a semi-permeable membrane

Osmotic Pressure: The force driving solvent particles from one side of a semi-permeable membrane to the other side

42
Q

Hypertonic Solutions

A
  • Action:
    • Higher osmolar concentration than plasma
    • They will increase the osmotic pressure of the extracellular fluid and draw the fluid from the cells into the circulation
    • The therapeutic benefit of hypertonic solution is that it will draw fluid from the blood vessels in order to increase circulating blood volume which will increase pressure in the blood vessels
  • Indications
    • Used to treat water intoxication (if a patient ingested too much water)
    • If a patient received too much D5W for fluid or electrolyte loss
  • Considerations
    • Continuous infusion can cause cellular dehydration
  • Examples
    • 3% Sodium chloride
    • D10NS (D10W), D5NS (D5W)
43
Q

Hypertonic Solutions

A
  • Action:
    • Lower osmolar concentration than in the plasma
    • Causes fluid to shift from the extracellular space into the cell
  • Indications
    • Hydrate and prevent dehydration
    • Increase diuresis in dehydrated patients
  • Considerations
    • Cause cells to swell and cause water intoxication
  • Examples
    • DSW
    • 0.45% NS (half normal saline)
44
Q

Active Transport

A
  • Many substances must travel across boundaries or cell membranes; sometimes this has to be done against concentration gradients.
    • Requires a carrier protein that will combine specifically and reversibly with the substance needing to travel
  • These require energy in the form of ATP.
  • The most famous is probably the sodium/potassium pump
  • Essential for healthy cell survival
  • Moving against the concentration gradient which requires energy
45
Q

Henry’s Law

A
  • Henry discovered that the weight (or amount) of gas that can be dissolved in a liquid is directly proportional to the partial pressure (driving pressure) of that gas.
  • Each gas has a solubility coefficient
    • The measurement of the amount of gas that will dissolve in a liquid per unit of pressure (mmHg.) The nature of the substance, temperature and pressure will affect the constant (Cs). (See Module 7A)
  • The greater the pressure on top of the liquid the more stuff you can put into the liquid
  • Pressure of oxyegn when it gets to the carina=PiO2
  • PAO2 is the pressure of O2 @alveoli and the big difference between this and PiO2 is the amount of CO2 so in the avleoli you also have to take into account the pressure of CO2
  • When a gas is confined in a spaceadjacent to a liquid a certain number of gas molecules dissolve in that liquid phase
  • For a given temperature the mass of the gas dissolved in a specific volume of liquid is directly porportional to the product to the product of the artial pressure of the gas and its solubility coefficient
46
Q

Solubility of Oxygen

A
  • Henry described the concept that a substance (eg. a gas) will exhibit a solubility rate according to its physical nature and that will render a solubility factor (or CoEf)
  • Solubility CoEff for O2 = 0.023 ml O2/ml plasma/760 mmHg at 37°C
    • You can see this CoEff is at a full atmosphere of pressure
  • A more realistic measure is vol%. (volumes percent)
  • Vol% = Volume of gas (in ml) dissolved per 100ml of plasma
  • The solubility of a gas in a liquid is equal to the volume of gas (L) that will saturate 1 L of liquid at STPD

Where 0.023 is the Solubility Coefficient (CoEff) 760 is one standard atmosphere of pressure and the100 represents 100 mL of plasma.

This 0.003 becomes a useful measure – 0.003 vol %

If the pressure of O2 in a given sample of blood was 100mmHg:

Then, 0.003 * 100mmHg = 0.3 vol%

So, 3/10 of a ml of O2 is dissolved in 100ml of plasma. That’s not very much!!

Solubility Coefficient for CO2 = 0.51ml/ml plasma/760 mmHg at 37°C

0.51 = 0.00067 x 100 = 0.067

760 mmHg

= 0.067 ml/100ml plasma/mmHg of CO2

If PaCO2 is 40 mmHg then,

0.067 x 40 mmHg =

2.7 ml of CO2 is dissolved in 100ml plasma. More than O2

  • So, to sum up:
    • 0.023mL of O2 will dissolve in 1 mL of plasma @ 760 mmHg
      • 0.003ml/100ml plasma/mmHg
    • 0.51 mL of CO2 will dissolve in 1 mL of plasma @ 760 mmHg
      • 0.067ml/100ml plasma/mmHg
  • Henry Law also talks abouts about the nature of O2 and CO2 through the use of solubility coefficient
47
Q

Graham’s Law

A

Kinetic energy is the driving force behind the diffusion. Due to the fact that gases have high KE, gases will diffuse more rapidly than liquids.

  • As we increase the temperature of a gas, we see increased KE, due to increase frequency of collisions
  • Graham discovered that the rate of diffusion of a gas is inversely proportional (not equal to) to the square root of its GMW
    • Difgas = 1/ √gmw
  • According to Grahams’ Law, lighter gases will diffuse more rapidly than heavier ones.
    • Gases of a lighter GMW will diffuse in a liquid or gaseous mixture faster than gases with a higher GMW
  • The bigger the molecule the harder it will be to diffuse it
  • CO2 is heavier but will diffuse faster, so it breaks this law
  • So pressure, temp, GMW, and solubility coefficent will all play a role in diffusion
48
Q

Graham’s Law and Henry’s Law

A
  • In a gaseous mixture O2 diffuses 1.17 times faster than CO2 due to its lower GMW (this is when it is mixing a gas into a gas, which tends to not be what we do in the body). Now let’s add liquid co-effs (constants).
  • We can combine Graham’s and Henry’s laws to describe how gases diffuse in the alveoli and across the A/C membrane
    • Lighter gases will diffuse faster than heavier ones
    • Gases will diffuse in a liquid solution at a known CoEff per unit of pressure
  • Cs = Constant for solubility of a given gas.
  • So, the rate of diffusion of a gas is directly related to the Cs of that gas and inversely proportional to the square root of the GMW of that gas
  • The higher the Cs the more soluble it is
  • Even though it is a bigger molecule CO2 will diffuse better because there is a much higher solubility coefiicent, meaning the solubility coefficient will have a bigger influence when we are diffusing into a liquid but if we are diffusing a gas into a gas then the size of the molecule will have a bigger influence
  • Because CO2 diffuses so much better than O2 through the A/C membrane, any defect that limits the diffusion of O2 into the blood, will not necessarily have the same effect on the CO2
    • Meaning, just because O2 can’t get in, does not mean CO2 can’t get out.
  • Carbon Monoxide (CO) is a colorless, odorless, toxic gas given off as a by-product of combustion
  • Human hemoglobin has a very high affinity for CO
    • About 210-250 times that of Oxygen
  • Cs of CO = 0.0175ml/ml plasma/760mmHg
    • GMW of CO = 28
  • CO has a similar Cs to O2
  • O2 diffuses better than CO
  • O2 can be both diffusion and perfusion limited
  • CO is only diffusion limited
    • Because of its high affinity for hemoglobin, it will not build up a partial plasma, it will bind with hemoglobin faster than it can diffusion across the A/C membrane
  • This means that CO becomes a very useful gas in determining whether or not a patient has a A/C membrane diffusion defect or not.
    • Useful in PFT with DLCO and is used to determine the degree of lung dysfunction
49
Q

Fick’s Law of Diffusion

A

The rate of diffusion across tissue (A/C membrane) is directly proportional to the surface area (A), the diffusion constant (Cs) and the difference in partial pressure between each side of the tissue (∆P); and is inversely proportional to the thickness (T) of the tissue.

Diff (gas) = [(Ax Cs)/ T] * Delta P

  • The flow of a gas across a semipermeable membrane per unit of time into a membrane fluid phase is directly proportional to the surface area (A), gas available for diffusion, the partial pressure gradient between the two compartments (delta P), and the solubility of the gas (S)
  • Flow is inversely related to molecular weight of the gas and T
  • In a perfect world, the concentration gradient for diffusion of O2 from the alveoli to the blood is ~60mmHg
    • Ideally, alveolar PO2 = 100mmHg and PvO2 = 40mmHg, giving a ∆P of 60mmHg
  • In a perfect world, the concentration gradient for diffusion of CO2 from the blood to the alveoli is ~6mmHg
    • Ideally, alveolar PCO2 is 40mmHg and PvCO2 = 46mmHg, giving a ∆P of 6mmHg
  • So even though the concentration gradient for CO2 is a 10th that of O2, we know that CO2 diffuses ~19 times faster than O2, and has little difficulty crossing the A/C membrane
  • Concentration gradients play a very important role in diffusion of O2 and CO2
  • How can we apply Oxygen Therapy to this?
    • As we increase the FiO2 of the inspired Oxygen, we increase the ∆P, causing oxygen movement across the A/C membrane to increase
  • Many diseases lead to a thickening of the A/C membrane and could be labeled diffusion defects.
    • Increasing the thickness (T) is part of Fick’s equation
  • Some examples of diseases that could be classified as diffusion defects:
    • Pulmonary Fibrosis
      • Scar tissue formation causing the lining of the alveoli to thicken
    • Pneumonia
      • Increased gunk in the alveoli for the oxygen to pass through
    • Pulmonary Edema
      • Increased fluid in the alveoli
      • Increase fluid in the space between the alveoli and capillary which will increase the thickness
    • ARDS (Acute Respiratory Distress Syndrome)
      • Formation of the hyaline membrane due to inflammatory cells causing an increase in the A/C membrane
    • Damage from this disease will be permeant
    • Many diseases can lead to problems with the area portion (A) of Fick’s equation. Some examples include:
      • Emphysema
      • Pneumonia
      • Non-ventilated alveoli or any alveoli not available to participate in gas exchange
50
Q

Alveolar Gas Equation

A

PAO2 = (PB – PH2O) * FiO2 – (PaCO2 * 1.25) [or /0.8])

Pb = Barometric Pressure

PH2O = Water Vapour Pressure

FiO2 = Fraction of Inspired Oxygen

PaCO2 = Partial pressure of arterial CO2

*Because alveolar CO2 is almost identical to arterial CO2, you can substitute arterial CO2

  • Partial pressures play a huge role in the diffusion of oxygen from the alveoli into the blood
  • We can use Dalton’s Law of Partial Pressure to calculate the PiO2 (at the carina)
  • Once we breathe in oxygen all the way to the alveoli we have to account for the water vapour and CO2
51
Q

Respiratory Exchange Ratio

A

Respiratory Exchange Ratio (RER) is the ratio of removal of CO2 to the uptake of O2

=CO2 added to alveoli/ O2 removed from alveoli

Normal is 0.8

  • The partial pressure in the alveolar is most useful when we are providing mechanical ventilation, as this is one of the few times we can be very precise about our FiO2. To make this information the most useful we will use it with other information
  • A-a gradient — (PAO2 - PaO2) — A-a dO2 — P(A-a)O2
    • This number will widen with V/Q mismatch, diffusion defects or perfusion issues
    • Should be a small number, when on room air (5-10 mmHg)∗
    • Should not be greater than ~65 mmHg when on 100% ∗
      • Some sources suggest less than 4 mmHg per every 10 years of age
      • i.e. If pt. is 60 yrs old, should be less than 24 mmHg
      • ∗ May vary with the resource used

CBRC (20 decks)

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