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Physical Principles of Respiratory Care

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  1. Physical Principles of Respiratory Care Chapter 6

  2. Physics helps our understanding of how respiratory care equipment works

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

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

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

  6. The 3 states of matter

  7. 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

  8. 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

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

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

  11. Physical properties ofa 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

  12. Physical properties ofa 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…)

  13. 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

  14. Fahrenheit Scalethe freezing point of water at 32 degrees and the boiling point at 212 degrees. These two points formed the anchors for his scale. Celcius Scalethe 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 ScaleThere 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 -273.15 degrees Celsius or at -460 degrees Farenheit.

  15. 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

  16. 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.

  17. 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.

  18. 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.

  19. 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

  20. Heat Transfer • Conduction: • Convection • • Radiation • • Condensation/evaporation •

  21. 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.

  22. Pascal's law • Pascal's lawstates 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.

  23. Change of State (cont.) Mercury H20 • 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

  24. Cohesion and Adhesion • Cohesion: Water is attracted to waterAdhesion: 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. •

  25. 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.

  26. 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.

  27. 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

  28. 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

  29. 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 mmHgPH20   = 47 mmHg---713 mmHg for remaining inspired gases (21% O2 and 79% N2) • PO2 = 0.21 · 713 = 150 mmHgPN2 = 0.79 · 713 = 563 mmHg

  30. 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 760-47 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)

  31. 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. •

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

  33. 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)

  34. 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)

  35. Example 0 • 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 0

  36. 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.

  37. 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

  38. critical temperature • 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.

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

  40. Critical Pressure • The critical pressure of a substance is the pressure required to liquefy a gas at its critical temperature. Some examples are shown below.

  41. 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

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

  43. 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

  44. 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

  45. 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

  46. Surface Tension • • 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.

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