I&M I: Lecture 8 - Gas Fluid Physics

Question 1

The Kinetic Theory of Gases

Answer 1

Question 2

The Kinetic Theory of Gases (2)

Answer 2

Microscopic Scale
Mass, velocity, momentum, energy
Small size vs. space between
Constant random motion
Kinetic energy
Frequent collisions

Physical Properties
Mass
Velocity
Momentum
Energy

Variables involved in kinetic energy are mass of molecule and velocity.
Molecules are in constant random motion have frequent collisions.
Equation

Question 3

The Kinetic Theory of Gases (3)

Answer 3

Macroscopic scale
Density
Pressure
Temperature
Increased temp = increased motion

Increased temperature would be increased KE

Question 4

Pressure

Answer 4

Units
Atmospheres
Torr (~mmHg)
cmH2O
Pascals
Pounds per square inch (PSI)

Question 5

Pressure: Unit Conversion

Answer 5

Question 6

Pressure: Unit Conversion

Answer 6

Question 7

Pressure in Anesthesia (not just preceptors)

Answer 7

Gauge pressure
Referenced against ambient pressure

Absolute Pressure
Referenced against a vacuum

Pabs = Pgauge + Pamb

Question 8

Pressure IRL

Answer 8

Convert 100 cmH2O gauge at sea level to cmH2O absolute pressure

Pabs = Pgauge = Pamb
1 bar = 760 torr = 760 mmHg = 1034 CmH2O
Pabs = 100 (gauge) + 1034 (ambient) = 1134 cmH2O

Question 9

TEMPERATURE

Answer 9

FAHRENHEIT
USA & 9 tiny countries
H2O BP 212 ∘F
H2O FP 32 ∘ F
Absolute zero -459 ∘ F

CELSIUS
Science, medicine, and the world
H20 BP 100 ∘ C
H2O FP 0 ∘ C
Absolute zero -273.15 ∘ C

Question 10

Temperature Conversions

Answer 10

∘ C = 5/9 ( ∘F – 32)
∘ F = 9/5 (∘ C + 32)
∘ C = K – 273.15
K = ∘ C + 273.15

Zero K is Absolute Zero

Question 11

Temperature

Answer 11

Critical Temperature (Tc)
Temperature at which a gas can no longer be liquefied no matter how much pressure is applied
N2O = 36.5 ∘C
O2 = -119 ∘ C
CO2 = 31.1 ∘ C

Pseudocritical Temperature
Temperature at which gas mixture may separate into component gases
Entonox = -6 ∘ C
50:50 O2 and N2O

Question 12

Ideal Gases

Answer 12

Pressure
Volume
Number of moles of the gas
R=constant
Temperature

Question 13

Ideal Gases

Answer 13

PV = nRT

Ideal gas law relates
Gas pressure (mmHg or torr)
Gas volume (mL)
Gas temperature (∘C)

Question 14

Boyle’s Law

Answer 14

The pressure and volume of a fixed amount of an ideal gas at a fixed temperature are inversely proportional.

Only works for ideal gases and when temp is not approaching Tc.
All gases obey Boyle’s law at infinitely low temps

As volume increases in the space, pressure decreases
As volume decreases in the space, pressure increases due to more molecule collisions

Question 15

Boyle’s Law IRL

Answer 15

Drawing up syringes
Drawing back on a syringe plunger ⇧ syringe volume and ⇩ syringe pressure
Fluid outside the syringe is drawn into the cylinder until pressures inside and outside cylinder balance

Question 16

Charles’ Law

Answer 16

The volume of a gas varies directly with absolute temperature for a fixed amount of an ideal gas held at a constant pressure

Gases tend to expand when heated

Question 17

Charles’ Law in Action

Answer 17

Convectional heat loss
Warm air rises away from the body as density decreases

ETT/LMA cuff size changes
Cuff warms over time, increasing volume, increasing pressure

Respiratory gas volumes
Measured at ambient temp in machine, actual exchange occurs at core body temp
Physiological volumes > delivered
TV, VC, etc.

Question 18

Gay-Lussac’s Law

Answer 18

The absolute pressure varies directly with the absolute temperature for a fixed mass of ideal gas held at a constant volume

Again shows a linear relationship between pressure and temperature.

Question 19

Gay-Lussac’s Law IRL

Answer 19

Medical gases are stored in cylinders
Fixed volume, high pressures
If cylinders are compromised, explosion risk with increased temps
⇧ temp leads to ⇧ pressure
Molybdenum steel can withstand pressures of up to ~3,000psi

Question 20

Adiabatic Conditions

Answer 20

Adiabatic compression or expansion of gases, where energy is constant (energy is not added or removed)
Rapid compression→↑ temperature
Rapid expansion→↓ temperature

A-D abatic: heat remains constant within the system- it does not leave or enter

This means a system undergoing an adiabatic process is thermally isolated from its surroundings.Essentially, any temperature changes within the system are due to work being done on or by the system, not due to heat exchange.

If a gas is compressed or expanded very quickly, there isn’t enough time for heat to flow in or out, making the process approximately adiabatic

Question 21

Adiabatic Conditions IRL

Answer 21

Gas hammer effect
Gas cylinders connected to machines
Rapid opening  rapid compression in gauges/pipelines  fire or explosion
SLOWLY open cylinders, especially O2

Question 22

Combined Gas Law

Answer 22

Boyle’s + Charles’ + Gay-Lussac’s Laws

K=constant

Question 23

Boyles law- temperature constant
Charles law- pressure constant
Gay-lussac- volume constant

Answer 23

Question 24

Avogadro’s Law

Answer 24

Gases of equal volumes, held at same temp and pressure, contain the same number of molecules

Avogadro’s Number = number of C12 atoms in 0.012Kg
6.023x1023

1 mole C12 = 1 mole O2 = 1 mole H2

At STP, one mole of any ideal gas will occupy a volume of 22.4L
STP = 0∘C and 1 atm (273K and 760 mmHg)

STP- standard temperature and pressure

Question 25

Avogadro’s Law, Applied

Answer 25

Calculate N2O volume in a cylinder N2O is in liquid and gas form within cylinders Cylinder must be weighed to calculate volume Full cylinder holds 2.9Kg N2O Molecular weight = 44g/mol 1 mole @STP occupies 22.4L 1590L full tank (some sources say 1600L) Liquid N2O exists in tank until tank is below ¼ full (397.5L or 400 L) This is where you will see psi decrease As long as there is liquid in the tank, the pressure will read 750 psi, so psi is unreliable to gauge available N2O How long will a 6.1kg E cylinder last at STP, if we have it running at 2L/min?

Question 26

Ideal Gas Law

Answer 26

Combines Boyle’s + Charles’ + Gay-Lussac’s + Avogadro’s Laws

Question 27

Ideal Gas Law IRL

Answer 27

Pressure gauges on cylinders act as content gauges V & T constant: pressure on gauge is proportional to molecule #  gas left Conversion factor = ~0.3 for oxygen E cylinders 625 L/ 2000 psi = 0.3

Question 28

Dalton’s Law of Partial Pressures

Answer 28

The total pressure exerted by a gas mixture is the sum of the individual pressures exerted by each individual gas

Question 29

Henry’s Law

Answer 29

At constant temp, the amount of gas dissolved in a liquid is proportional to the partial pressure of the gas in equilibrium with the liquid ## Footnote The higher the partial pressure, the more molecules dissolved in the liquid.

Question 30

Vapor Pressure

Answer 30

Pressure at a given temperature where the solid, liquid and/or gas states are in equilibrium within a closed container Describes the propensity for a substance to bond with itself Low vapor pressure = poor bonds High vapor pressure = strong bonds Boiling point: temp where vapor pressure = atmospheric pressure ## Footnote Desflurane has highest vapor pressure, lowest boiling point

Question 31

Evaporation

Answer 31

Vaporization at the surface of a liquid, below the boiling temp Liquid changes to gas at the surface of a liquid

Question 32

Van der Waals Forces

Answer 32

Non-Ideal gases have small intramolecular forces at play Dipole-Dipole forces (permanent dipole molecules) Dispersion forces (instantaneous dipoles) Van der Waals forces Distance-dependent interaction between molecules Ideal Gas Law is rewritten to accommodate these forces ## Footnote Distant dependent interactions, the closer the molecules get, there is a light interaction between, where negative and positive charges attract.

Question 33

Diffusion vs. Osmosis

Answer 33

Osmosis is dependent on membrane area & concentration gradient Osmosis is Inversely proportional to membrane thickness Solutes are things like salts or ions. Osmosis- natural transport of water molecules across a selectively permeable material (acting thus as a membrane) that allows the migration of a solvent (usually water) and restricts the passage of solute molecules (salts) or ions. When such a material separates a solution with different solute concentrations, a fluid flows from the region with a low solute concentration (a high fluid chemical potential) to the region with its high concentration (a low fluid chemical potential). The flow increases the fluid pressure in the latter region and decreases it in the former one. These pressure changes lead to a countering hydraulic flow, until the two opposing flows cancel each other and equilibrium is reached.

Question 34

Fick’s Diffusion Laws

Answer 34

Gas diffuses across a semi-permeable membrane at a rate proportional to the surface area of the membrane, the difference in partial pressure of the gas on each side of the membrane, and the diffusion constant. Diffusion rate is inversely proportional to the barrier thickness

Question 35

Fick’s Diffusion Law IRL

Answer 35

https://www.youtube.com/watch?v=tDsK0eyuhAk

Question 36

Graham’s Law

Answer 36

Diffusion or effusion rate is inversely proportional to the square root of molecular size Diffusion = rate at which 2 gases mix Effusion = rate at which gas escapes through a pinhole into a vacuum

Question 37

Solubility Coefficients

Answer 37

Describes how gases split between liquid and gas states after equilibrium is reached High blood:gas coefficient  higher lipophilicity  higher potency  higher solubility High solubility  needs more anesthetic for dissolution  slower onset High lipophilicity  higher potency  lower [] in alveoli and brain for effect MAC decreases as blood:gas partition coefficient increases (generally) Halothane at equilibrium Blood concentration 2.3x higher than alveoli concentration Higher uptake requirement slows induction

Question 38

Diffusion IRL

Answer 38

Nitrous is 35x more soluble than N in blood Diffuses into air-filled space faster than N can reabsorb Hence, N2O contraindications: COPD, pneumothorax, bowel obstruction, intraocular air bubbles (retinal detachment surgery), middle ear procedures Avoid if there is trapped air or potential for trapped air in the body

Question 39

Diffusion IRL

Answer 39

O2 and CO2 obey Fick’s Law O2 Diffusion 1  alveolus 2  alveolar-capillary barrier 3  red blood cell membrane (O2 uptake) Other applications ETT cuff expansion in long cases Diffusion hypoxia

Question 40

Osmosis IRL

Answer 40

Hypotonic Cell gains H2O 0.18-0.3% NaCl + dextrose Occasionally used for acute infection maintenance fluids Isotonic No net H2O movement NS, D5W, LR, D5LR Most common maintenance fluids Hypertonic Cell loses H2O 3% or 5% hypertonic saline Dehydration fluid administration Increases extracellular volume, decreases Na+ and K+ blood serum levels Colloids contain molecules that produce a colloid oncotic pressure Vasculature retains fluids better with colloids than isotonic crystalloids (~4hrs vs 30 mins) Differs in critically ill pts, but in general remember the above Albumin, Hespan, Dextran, etc.

Question 41

Flow

Answer 41

Flow rate = pressure change divided by resistance experienced by the fluid (liquid or gas) Laminar: low velocity, steady flow, parallel to tube Turbulent: unpredictable flow High flows, irregularities in tube, sharp angles

Question 42

Locaitons of Flow Patterns

Answer 42

Question 43

Density & Viscosity

Answer 43

Viscosity A fluid’s resistance to flow Laminar flow is inversely proportional to viscosity Density Measurement of the space between the molecules of a given fluid Turbulent flow is proportional to density

Question 44

Density & Viscosity IRL

Answer 44

Laminar flow is inversely proportional to viscosity Anemia ↓ viscosity and ↑ laminar flow ↑ Hb/Hct ↑ viscosity and ↓ laminar flow Turbulent flow is proportional to density Heliox: ↓ density ↓ turbulence Airway obstruction, epiglottitis, tracheal stenosis, etc.

Question 45

Hagen-Poiseuille Equation

Answer 45

Fluid flow rate through a horizontal, straight tube of uniform internal diameter is proportional to the pressure gradient and r4; it is inversely related to the viscosity of the gas and the length of the tube Only valid for laminar flow

Question 46

HP Equation IRL

Answer 46

Hagan-Poiseuille Equation can be used to determine quicker fluid delivery How can you give blood more rapidly? Large bore IV (↑radius) Increase IV pole height (↑pressure gradient) Use pressure bag on fluid bag (↑pressure gradient) Limit extensions on IV tubing (↓length)

Question 47

Critical Velocity

Answer 47

Direction and speed at which liquid flow converts from laminar to turbulent flow Variables: Gas Air ↑ than O2/N2O mix Gas Temperature Directly proportional ↑ temp, ↑ critical velocity ↓ temp, ↓ critical velocity Increase pt temp with gas humidification & pt warming modalities

Question 48

Reynold’s Number

Answer 48

Defines the flow of liquid through a tube Laminar or turbulent Has no units and is dimensionless

Question 49

Endotracheal Tube Resistance

Answer 49

Question 50

Flow Variances in Practice

Answer 50

Turbulent flow Larger tubes Circuit Large ETTs Trachea Upper bronchioles Higher velocities Higher Densities He vs air vs O2 vs N2O Orifice diam changes Constrictions Mucus plug/thick mucus lining airways Tube kink Circuit connectors Laminar flow Lower bronchioles Increased surface area Decreased velocity Undersized ETT Large drop in flow Wide bore and curved over narrow and sharp angles Respiratory tract obstructions: Heliox reduces density, increases flow Laminar flow during quiet breathing Speaking or coughing becomes turbulent, may lead to dyspnea

Question 51

Bernoulli’s Principle

Answer 51

Fluid speed increases simultaneously with decrease in its potential energy When a gas flowing through a tube reaches constriction, pressure ↓ and velocity ↑

Question 52

Venturi Effect

Answer 52

Decrease in pressure at the point of constriction (Bernoulli’s principle) entrains air from surroundings

Question 53

Laplace’s Law

Answer 53

Within a spherical gas-liquid surface, excess pressure is equal to 2x the coefficient of surface tension divided by the interface radius Pressure is directly proportional to surface tension and inversely proportional to the alveolus radius

Question 54

Laplace’s Law in Numbers

Answer 54

Pulmonary surfactant decreases surface tension 15x Atelectasis without it