Physical Principles of Respiratory Care

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Physical Principles of Respiratory Care
Chapter 6

Physics helps our understanding of how respiratory care equipment works

States of Matter Physics is the branch of science that deals with the interactions of matter and energy.

According to the law of conservation of energy, energy cannot be created or destroyed: energy can only be transferred.

Kinetic Theory States that the atoms & molecules that make up matter are in constant motion.

The 3 states of matter

States of Matter Solids – have a high degree of internal order; their atoms have a strong mutual attractive force Liquids – atoms exhibit less degree of mutual attraction compared with solids, they take the shape of their container, are difficult to compress, exhibit the phenomenon of flow Gases – weak molecular attractive forces; gas molecules exhibit rapid, random motion with frequent collisions, gases are easily compressible, expand to fill their container, exhibit the phenomenon of flow

The energy of position, and the energy of motion.
States of Matter All matter possesses energy. There are 2 types of internal energy: The energy of position, and the energy of motion. Internal energy of matter Potential energy (Position) The strong attractive forces between molecules that cause rigidity in solids Kinetic energy (Motion) Gases have weak attractive forces that allow the molecules to move about more freely, interacting with other objects that they come in contact with Internal energy and temperature The two are closely related: internal energy can be increased by heating or by performing work on it. Absolute zero = no kinetic energy

Potential energy is stored energy.
Kinetic energy is the energy that an object possesses when it is in motion.

Physical properties of a solid:
Possess the least amount of KE Mostly Potential Energy in intermolecular forces holding particles together Can maintain their volume & shape

Physical properties of a liquid:
Intermolecular, cohesive forces are not as strong They exhibit fluidity (particles sliding They exhibit a buoyant force Essentially incompressible Assume the shape of their container

Physical properties of a gas:
Extremely weak – if any – cohesive forces Possess the greatest amount of KE & the least amount of Potential Energy Motion of atoms & molecules is random Do not maintain their shapes & volumes but expand to fill the available space Exhibit the phenomenon of flow Exhibits the least thermal conductivity Uses: Gas therapy (Oxygen, Heliox, Nitrous oxide…HHN/SVN…)

Change of State Liquid-solid phase changes (melting and freezing)
Melting = changeover from the solid to the liquid state Melting point = the temperature at which melting occur. Freezing = the opposite of melting Freezing point = the temperature at which the substance freezes; same as its melting point

Fahrenheit Scale the freezing point of water at 32 degrees and the boiling point at 212 degrees. These two points formed the anchors for his scale. Celcius Scale the freezing temperature for water to be 0 degree and the boiling temperature 100 degrees. The Celsius scale is known as a Universal System Unit. It is used throughout science and in most countries. Kelvin Scale There is a limit to how cold something can be. The Kelvin scale is designed to go to zero at this minimum temperature. At a temperature of Absolute Zero there is no motion and no heat. Absolute zero is where all atomic and molecular motion stops and is the lowest temperature possible. Absolute Zero occurs at 0 degrees Kelvin or degrees Celsius or at -460 degrees Farenheit.

Change of State (cont.) Properties of liquids
Pressure – depends on the height and weight density. Buoyancy – occurs because the pressure below a submerged object always exceeds the pressure above it Viscosity – the force opposing a fluid’s flow. The greater the viscosity of a fluid, the greater the resistance to flow. Blood has a viscosity five times greater than that of water

Pressure Pressure is measured in cmH2O, mmHg or PSI
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth (or that of another planet). In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the mass of air above the measurement point.

Pressure Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges. A manometer could also refer to a pressure measuring instrument, usually limited to measuring pressures near to atmospheric. The term manometer is often used to refer specifically to liquid column hydrostatic instruments.

Pressure Static pressure is uniform in all directions, so pressure measurements are independent of direction in an immovable (static) fluid. Flow, however, applies additional pressure on surfaces perpendicular to the flow direction, while having little impact on surfaces parallel to the flow direction. This directional component of pressure in a moving (dynamic) fluid is called dynamic pressure.

Change of State (cont.) Heat transfer
Conduction – transfers heat in solids Convection – transfers heat in liquids and gases (Example: heating homes or infant incubators) Radiation – occurs without direct contact between two substances - example: microwave oven Evaporation/Condensation: requires heat energy to occur Sublimation - change from a solid to a gas without an intermediate change to a liquid - example dry ice turning into CO2

Convection &feature=relmfu Radiation &feature=relmfu Condensation/evaporation feature=related

Change of State (cont.) Pascal’s Principle. Liquid pressure depends only on the height and weight density of the liquid and not the shape of the vessel or total volume of a liquid.

Pascal's law Pascal's law states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio (initial difference) remains the same. Pascal’s Law states that when you apply pressure to confined fluids (contained in a flexible yet leak-proof enclosure so that it can’t flow out), the fluids will then transmit that same pressure in all directions within the container, at the same rate. The simplest instance of this is stepping on a balloon; the balloon bulges out on all sides under the foot and not just on one side. This is precisely what Pascal’s Law is all about – the air which is the fluid in this case, was confined by the balloon, and you applied pressure with your foot causing it to get displaced uniformly.

Change of State (cont.) Cohesion and adhesion
The attractive force between like molecules is cohesion. The attractive force between unlike molecules is adhesion. The shape of the meniscus depends on the relative strengths of adhesion and cohesion. H20: Adhesion > Cohesion Mercury: Cohesion > Adhesion Mercury H20

Cohesion and Adhesion Cohesion: Water is attracted to water Adhesion: Water is attracted to other substances Adhesion and cohesion are water properties that affect every water molecule on earth and also the interaction of water molecules with molecules of other substances. Essentially, cohesion and adhesion are the "stickiness" that water molecules have for each other and for other substances. The water drop is composed of water molecules that like to stick together, an example of the property of cohesion. The water drop is stuck to the end of the pine needles, which is an example of the property of adhesion. Notice I also threw in the all-important property of gravity, which is causing the water drops to roll along the pine needle, attempting to fall downwards. It is lucky for the drops that adhesion is holding them, at least for now, to the pine needle.

Change of State (cont.) Liquid to vapor phase changes
Boiling – heating a liquid to a temperature at which its vapor pressure equals atmospheric pressure. Saturation – equilibrium condition in which a gas holds all the water vapor molecules that it can. Dew point – temperature at which the water vapor in a gas begins to condense back into a liquid. Evaporation – when water enters its gaseous state at a temperature below its boiling point.

Evaporation: Heat is taken from the surrounding air by the liquid via convection thereby cooling the air This heat transfer increases the KE in the liquid thus more molecules will have sufficient energy to escape from a liquid to a gaseous state, and vaporize. If air temperature increases, KE increases and more evaporation occurs.

Condensation (conversion from a gas to a liquid) is the opposite of evaporation
The hygroscopic humidifier (artificial nose) traps the condensation from the patient’s exhaled gas and re-humidifies the dry incoming air on inhalation

Vapor pressure Vaporization: the change of matter from a liquid to a gaseous form Water vapor pressure – the direct measure of the kinetic activity of water vapor molecules Reducing the pressure above a liquid lowers its boiling point. Ex. water boiling in mountains

Water Vapor Pressure When a gas is in contact with a liquid, and is in equilibrium (saturated) with the liquid, the partial pressure of the gas is a function of temperature. The one gas to which this applies in a normal respiration is water. The lungs and airways are always moist, and inspired gas is rapidly saturated with water vapor in the upper segments of the respiratory system. The temperature in the airways and lungs is almost identical with deep body temperature (approximately 37°C); at this temperature water vapor has a partial pressure of 47 mmHg. (Note that the gaseous form of a liquid frequently is termed a "vapor"). Using the value of 47 mmHg, we can calculate partial pressure of oxygen and nitrogen in inspired air, after the gas mixture becomes saturated with water vapor in the upper airway (so-called tracheal air): Ptotal = 760 mmHg PH20   = 47 mmHg mmHg for remaining inspired gases (21% O2 and 79% N2) PO2 = 0.21 · 713 = 150 mmHg PN2 = 0.79 · 713 = 563 mmHg

Water Vapor Pressure That is, since water vapor partial pressure must be 47 mmHg in a saturated gas mixture at 37°C, the total pressure remaining for the inspired gases is only or 713 mmHg. The composition of this remaining gas is 21% O2 and 79% N2, giving the partial pressures indicated above which is then substrated by the partial pressure of PaCO2 (PACO2, is a product of the amount of CO2 diffused into the lung) PAO2 = FIO2 (Pb-PH2O) – (PaCO2/0.8)

Humidification Absolute humidity: the actual content or water vapor present in a given volume of air Relative humidity: the actual water vapor present in a gas compared with the capacity of that gas to hold the vapor at a given temperature If the water vapor content of a volume of gas equals its capacity, the relative humidity of the gas equals 100% Both are essential in effective ventilation. Prevents drying of airway mucosa and irritation. Various respiratory care devices are used to ensure adequate humidification of inspired gases. wKUXc

Humidity The NOSE is the bodies natural humidifier and filter, when bypassed we must use a artificial humidifier

Humidity Terms Vapor pressure – Pressure water as a vapor or gas exerts and is part of the total atmospheric pressure. Water vapor pressure in the lungs exert 47 mmHg Absolute Humidity – the actual amount (in mg./l) of water vapor in the atmosphere Relative Humidity – the percent of water vapor in the air as compared to the amount necessary to cause saturation at the same temperature. % Body Humidity – the relative humidity at 37 degrees Celsius Humidity Deficit – the amount of water vapor needed to achieve full saturation at body temperature (44 mg/l - A.H) Isothermic Saturation Boundary – At or just below carina (end of trachea) The point at which inspired gases are fully 100% saturated and warmed to body temperature (44 mg/L at 37oC)

Humidity Uses of Humidity therapy Humidification of inspired gases
Thinning of bronchial secretions Sputum induction Solutions Used Sterile water used in humidifiers and continuous nebulizers (Hypotonic) (Normal) Isotonic saline (.9% Na) with (Aerosol / Medicine) Treatments Hypertonic saline (10%) (for sputum induction)

Example A gas is flowing thru a ventilator circuit at 50 C with a relative humidity of 100%. As it flows thru the tubing it is cooled to 37 C by the surrounding ambient temperature of the room. What effects will occur within the tubing? What will occur to the ambient temperature of the air surrounding the tubing? Condensation will occur on the inside surface of the tubing as the water vapor reaches its dew point There will be visible droplet formation when dew point is reached There will be warming of the adjacent air due to convection

Critical temperature:
The temperature reached in which gaseous molecules cannot be converted back to a liquid, no matter what pressure is exerted on them. The highest temperature at which a substance can exist in a liquid state. Critical pressure: The critical pressure of a substance is the pressure required to liquefy a gas at its critical temperature.

Critical Temperature Gases can be converted to liquids by compressing the gas at a suitable temperature. Gases become more difficult to liquefy as the temperature increases because the kinetic energies of the particles that make up the gas also increas Gas Liquid

critical temperature (oC)
The critical temperature of a substance is the temperature at and above which vapor of the substance cannot be liquefied, no matter how much pressure is applied. Every substance has a critical temperature. substance critical temperature (oC) NH3 132 O2 -119 CO2 31.2 H2O 374

Tubes containing water at several temperatures
Tubes containing water at several temperatures. Note that at or above 374oC (the critical temperature for water), only water vapor exists in the tube.

critical pressure (atm)
The critical pressure of a substance is the pressure required to liquefy a gas at its critical temperature. Some examples are shown below. substance critical pressure (atm) NH3 111.5 O2 49.7 CO2 73.0 H2O 217.7

Critical temperature & critical pressure examples
Water boils at 100 C and has a critical temperature of 374 C. Oxygen has a boiling point of -183 C and a critical temperature of about -119 C Below -183 C, oxygen can exist as a liquid. Above – 183 C, liquid oxygen becomes a gas. Above 217 atm, and a temperature of 374 C gaseous water cannot be converted back to a liquid no matter how much pressure is added

Bulk Oxygen System Here, the liquid O2 is allowed to exceed its critical temp & convert to gas.

Temperature Adding heat to a thermometer
changes its physical properties. A mercury (nonelectrical thermometer) expands or contracts as temp. changes. A thermistor (electrical thermometer) operates by the electrical resistance of metal changing with changes in temp. As the temp. increases, resistance to current flow decreases and is shown as an increased temp. reading

Viscosity The internal force that opposes the flow of fluids (equivalent to the frictional forces between solid substances) The greater the viscosity, the greater the opposition to flow The stronger the cohesive forces, the greater the viscosity

Surface Tension A force exerted by like molecules at a liquids surface
For a given liquid, surface tension varies inversely with temperature Surface tension, like a fist compressing a ball, increases the pressure inside a liquid drop or bubble The smaller the bubble, the greater the inflation pressure Inflation pressure can be lowered if surface tension is lowered The smaller the bubble, the greater the surface tension When connected, small bubbles tend to empty into larger bubbles ETOH has a low surface tension and is used to treat pulmonary edema

The pressure difference between the inside and outside of a bubble depends upon the surface tension and the radius of the bubble. The relationship can be obtained by visualizing the bubble as two hemispheres and noting that the internal pressure which tends to push the hemispheres apart is counteracted by the surface tension acting around the circumference of the circle.

Surface tension The amount of net pressure required for inflation is dictated by the surface tension and radii of the tiny balloon-like alveoli.

Capillary Action A phenomenon in which a liquid in a small tube moves upward, against gravity Involves both adhesive and surface tension forces Small capillary tubes create a more concave meniscus and thus create a greater area of contact with the liquid along its glass surface. The strong adhesive force of the liquid to the glass coupled with the surface tension properties of the liquid combine to cause the liquid be pulled upward. Liquid will rise higher in tubes with smaller cross-sectional areas

Temperature Temperature Scale Calculations:
Conversion of Celsius to Kelvin: °K = °C +273 Conversion of Fahrenheit to Kelvin: °K = °F + 460 Conversion of Celsius to Fahrenheit: °F = (9/5 x °C) + 32 (°C x 1.8 ) + 32 Conversion of Fahrenheit to Celsius: °C = 5/9(°F – 32) (°F – 32) divided by 1.8

Temperature Temperature Scale Calculations:
Conversion of Celsius to Kelvin: °K = °C +273 Conversion of Fahrenheit to Kelvin: °K = °F + 460 Conversion of Celsius to Fahrenheit: °F = (9/5 x °C) + 32 (°C x 1.8 ) + 32 Conversion of Fahrenheit to Celsius: °C = 5/9(°F – 32) (°F – 32) divided by 1.8

First-year students at Med School were receiving their first Anatomy class with a real dead human body. They all gathered around the surgery table with the body covered with a white sheet. The professor started the class by telling them: "In medicine, it is necessary to have 2 important qualities as a doctor. The first is that you not be disgusted by anything involving the human body." For an example, the professor pulled back the sheet, stuck his finger in the EYE of the corpse, withdrew it and stuck his finger in his mouth." “Go ahead and do the same thing," he told his students. The students freaked out, hesitated for several minutes, but eventually took turns sticking a finger in the EYE of the dead body and sucking on it. When everyone had finished, the Professor looked at them and told them, "The second most important quality is observation. I stuck in my Middle finger and sucked on my index finger. Now learn to pay attention to your patients, their life may depend upon it."

Gas Laws

Gas Laws (why?) During mechanical ventilation, volumes, pressures, flows & the temperature of delivered gas are routinely manipulated to better match the patient’s condition.

Pressure = Force Area Caused by collision of gas molecules with solid or liquid surfaces. Measurements reported in psi, mmHg, torr, cmH2O, and kPa.

Properties of Gases Gaseous diffusion – the movement of molecules from areas of high concentration to areas of lower concentration Gas pressure All gases exert a pressure. Gas pressure in a liquid is known as gas “tension.” Atmospheric pressure is measured with a barometer.

Properties of Gases (cont.)
Components of a mercury barometer

Properties of Gases (cont.)
Gas pressure (cont.) Partial pressure = the pressure exerted by a single gas in a gas mixture Dalton’s law – the partial pressure of a gas in a mixture, is proportional to its percentage in the mixture. So the greater percentage that a gas occupies in a mixture, the greater its partial pressure Solubility of gases in liquids (Henry’s law) The volume of a gas dissolved in a liquid is a function of its solubility coefficient and its partial pressure. Solubility coefficient: The volume of gas dissolved per unit volume of liquid at standard atmospheric pressure and at a specified temperature

Boyle’s Law: The volume that a gas occupies when it is maintained at a constant temp. is inversely proportional to the absolute pressure exerted on it. Body box plethesmography uses Boyle’s law to determine the volume of air remaining in the lungs after a full expiration. This is used to determine Residual Voume, Total Lung Capacity and Functional Residual Capacity

A factor in breathing: For inhalation to occur, 1. Diaphragm muscle flattens/moves down 2. Increases the volume of the chest cavity 3. Pressure within the chest decreases 4. Air pressure within the lung is now less than atmospheric air pressure 5. Air flows into your lungs.

Boyle’s Law – Application

Charles Law: If pressure remains constant, the volume of a gas varies directly with the temperature, expressed in Kelvin. As the temperature increases, the volume of the gas will increase. As the temp. decreases, the volume will also decrease.

In both parts of this diagram the gas is at the same pressure, as the temperature increases, the volume of the gas also increases If the gas expands exponentially, the kinetic energy will also increase to the same degree

APPLIED IMPORTANCE: Temperature also plays a role in the solubility of a gas in a liquid. As the temperature is increased the solubility of a dissolved gas is actually decreased. Clinical example: When an ABG is iced, the temperature of the plasma decreases. This decreases the amount of oxygen that can be displaced off the RBC and dissolved into the solution

Gay-Lussac’s Law: “With volume remaining constant, pressure and temperature are directly related”

Gay-Lussac’s Law Example: Drive to Las Vegas what happens to tire pressure? What is constant? What varies?

COMBINED GAS LAW = P1 x V1 T1 P2 x V2 T2
States: “The state of an amount of gas is determined by its pressure, volume, and temperature” The absolute pressure of a gas is inversely related to the volume it occupies & directly related to its absolute temp. =

COMBINED GAS LAW It describes the macroscopic behavior of gases when any or all of the variables change simultaneously. It is useful in determining pressure, volume or temperature corrections in arterial blood-gas measurements and during PFTs. mM&feature=related

DALTON’S LAW OF PARTIAL PRESSURE
States: “The sum of the partial pressures of a gas mixture equals the total pressure of the system and that the partial pressure of any gas within a gas mixture is proportional to its % of the mixture”. Example: OXYGEN = 21% NITROGEN = 78% TRACE GASES = 1% 100% Atmospheric At 100% atmospheric, these gases exert a pressure of 760mmHg at sea level

DALTON’S LAW OF PARTIAL PRESSURE
Denver, CO 640 mm Hg x 21% = 134 mm Hg Seattle, WA 760 mm Hg x 21 % = 152 mm Hg

PO2 = (PB – PH2O) (FIO2) PO2 = (760 mm Hg – 47 mm Hg) (0.21) PO2 = 150 mm Hg

Graham’s Law Molecular weight of Oxygen = 31.99
States “the rate of diffusion of a gas through a liquid is directly proportional to its solubility coefficient and inversely proportional to the square root of its density”. Describes the diffusion rate of one gas into another gas. Gram molecular weight equals the number of particles in a given amount of matter. Molecular weight of CO2 = 44.01 Molecular weight of Oxygen = 31.99 CO2 diffuses 20x faster than O2

Henry’s Law Describes the diffusion rate, or dissolving of a gas molecules into liquid. It states “For a given temperature, the rate of a gas’s diffusion into a liquid is proportional to the partial pressure of that gas and its solubility coefficient”.

Henry’s Law in Respiratory Care?
Relates to the solubility of gases, such as oxygen into and carbon dioxide out of the blood. It is known that mL of oxygen dissolve in every milliliter of blood at a temp. of 37 C and 1 atm of pressure.

Henry’s Law Oxygen and Carbon Dioxide transport can change significantly with changes in body temperature. The normal Pa02 at 37°C is approx torr. As a patient’s temperature rises from 37°C to 39°C, Pa02 increases to 110 torr due to the increased solubility. Likewise PCO2 increases 10% from 40 to 44 torr.

Fick’s Law States: The flow of a gas across a semi-permeable membrane into a membrane fluid phase is directly proportional to: The surface area available for diffusion, the partial pressure gradient between the two compartments & The solubility of the gas.

Fick’s Law in Respiratory Care?
Alveolus A/C membrane CO2 O2 O2 O2 CO2 Pulmonary capillary O2

Fluid Dynamics

The Bernoulli Principle
When a fluid flows through a tube of uniform diameter, pressure decreases progressively over the tube length. As fluid passes thru a constriction, the pressure drop is much greater

The Bernoulli Principle
Jet Entrainment Source Gas Area of negative pressure

The Venturi Principle States: “The pressure drop that occurs as the fluid flows thru a constriction in the tube can be restored to the preconstriction pressure if there is a gradual dilation of the tube”. lated

The Venturi Principle Va = Flow before restriction.
Vc = Flow from entrainment plus driving flow. Pa = Original lateral pressure Pb = Falling lateral pressure at the restriction. Pc = Restored lateral pressure passed the restriction.

Venturi Mask The reduced pressure within the restriction may be used to introduce gases (usually air) into a low-pressure region of gas flow.

POISEUILLE’S LAW Fluid viscosity, tube length and radius determine resistance to flow. As the radius of a tube decreases by ½, resistance increases 16 times. Increased resistance to flow can be caused by a decreased airway size secondary to an increase in airway secretions, bronchospasm, intubation, etc.

Poiseuille’s Law: Poiseuille’s Law: the law that the velocity of a liquid flowing through a capillary is directly proportional to the presence of the liquid and the fourth power of the radius of the capillary and is inversely proportional to the viscosity of the liquid and the length of the capillary.                                                                               The variables in Poiseuille’s Law are: The driving pressure gradient (the heart pumping) Viscosity of the fluid (a person who is anemic can affect the viscosity of his/her blood) Tube length (the veins and arteries) Fluid flow (depending on the pressure and viscosity) Tube radius (can be affected by clogged arteries) And the constants (8 and 3.14)

POISEUILLE’S LAW Mucous plug removed from a patient’s airway.

Reynolds Number The changeover from laminar to turbulent flow depends on several factors including: Fluid density Viscosity Linear velocity Tubing length In a smooth bore tube, laminar flow becomes turbulent when the Reynolds Number > 2000 Factors that favor turbulent flow include: High gas velocity High gas density Low gas viscosity Large tube diameter

Fluid Dynamics (cont.) Fluidics and the Coanda effect
Fluidics is a branch of engineering that applies hydrodynamics principles in flow circuits. The Coanda effect (wall attachment) is observed when fluid flows through a small orifice with properly contoured downstream surfaces.

Coanda Wall Effect

Coanda Wall Effect Basis for fluidic devices used in several mechanical ventilators. Main advantage: fewer valves and moving parts that can break.