Respiratory Therapy - Key Terms and Definitions

Conditions of Measurement

• STPD (Standard Temperature and Pressure, Dry):
  • Volume of gas at 0^{\circ}C, 760 mm Hg.
  • Without water vapor.
• BTPS (Body Temperature, Barometric Pressure, Saturated):
  • Body temperature: 37^{\circ}C.
  • Barometric pressure: 760 mm Hg (at sea level).
  • Saturated with water vapor.
• ATPD (Ambient Temperature, Pressure, Dry):
  • Ambient conditions, dry.
• ATPS (Ambient Temperature and Pressure, Saturated):
  • Ambient conditions, saturated with water vapor.

Laws of Thermodynamics

• Involves heat, energy, and entropy (the work put into it).

1. First Law:
   • Energy cannot be created or destroyed, only converted.
   • Total energy in a system = heat put into the system - work done.
2. Second Law:
   • Energy changes form, entropy increases in a closed system to achieve the lowest possible energy state.
3. Third Law:
   • Statistical law: impossibility of achieving absolute zero.
   • At absolute zero:
     • All processes cease.
     • Entropy is at a minimum.
   • Thermal equilibrium is only possible at absolute zero.

Heat Transfer

• Conduction: Direct contact between hot and cold molecules.
• Convection: Mixing of fluid molecules at different temperatures.
• Radiation: Radiant heat transfer without physical contact.
• Evaporation: Heat is taken from the air surrounding the liquid, cooling the air.
• Condensation: Heat is given back to air surrounding the liquid, warming the air.

Liquids and Solids

• Melting: Change from solid to liquid state.
  • Melting Point: Temperature at which melting occurs.
  • Latent Heat of Fusion: Extra heat needed to change to a liquid; number of calories required to change 1 g of a solid to a liquid.
• Freezing: Heat energy transferred from liquid to environment (usually by exposure to cold).
  • Kinetic energy decreases, molecules regain stable solid structure.
  • Freezing point = Melting point.
  • Energy required to freeze = energy needed to melt.
• Sublimation: Transition from solid to vapor without becoming a liquid.
  • Example: Dry Ice (Frozen Carbon Dioxide).

Liquid - Vapor

• Vaporization: Change of state from liquid to gas; requires heat energy from surroundings.
  • Eliminates attractive forces between molecules.
  • Latent Heat of Vaporization: Energy required to vaporize a liquid
  • Two types:
    • Boiling Point: Temperature at which vapor pressure exceeds atmospheric pressure.
      • Atmospheric pressure determines the boiling point.
      • Liquified O_2 boils at -183^{\circ}C at 1 atm.
      • Nitrogen’s boiling point is -195.8^{\circ} C.
    • Evaporation: Liquid changes into a gas at temperatures lower than its boiling point.
      • Water heated below boiling point enters atmosphere as evaporation.
      • Heat is taken from the air surrounding the liquid, cooling the air.
      • Some molecules near water's surface escape into the surrounding air as water vapor.
      • Evaporated water molecules:
        • Exert their own partial pressure (water vapor pressure).
        • Occupy and have mass.
        • Obey physical principles of gases.
      • Water vapor, like gas, always occupies space.
      • Dry volume is smaller than a saturated volume at constant pressure and temperature.
      • Invisible molecular water acts like a gas and exerts pressure (water vapor pressure).
      • Water vapor pressure is independent of other gases; depends on temperature and RH.
      • Adding water vapor lowers the partial pressures of other gases.
      • Vaporization continues until air is saturated.
      • At saturation, equilibrium is reached (molecule escaping = molecule returning).
      • Temperature effects evaporation in 2 ways:
        • Warmer air holds more vapor.
        • Warmer air contacting water evaporates faster.
        • Increased kinetic energy helps molecules escape from the surface when water is heated.
      • Gas temperature affects capacity to hold molecular water and water vapor pressure.
      • In a closed system, air maintains more saturation, contains more vapor pressure, and exerts higher vapor pressure.

Humidity

• Water in the gaseous state.
• Amount of water vapor in the atmosphere, involving kinetic activity of water molecules in air.
• Absolute Humidity (AH): Actual amount/weight of water vapor in air (actual water vapor content).
  • Measured by weighing water vapor extracted from air.
  • Saturated air at 37^{\circ}C and 760 mm Hg: 43.80 mg/L.
  • AH = \% \text{ saturated } \times \text{ water vapor content}
• Relative Humidity (RH): Ratio of actual water vapor content to saturated capacity at a given temperature.
  • Expressed in relative terms when a gas is not fully saturated.
  • 100% RH: gas is fully saturated.
  • Water vapor content = capacity.
  • 20^{\circ} C has a capacity to hold 17.30 mg/L.
  • Slight cooling causes condensation.
  • RH= \frac{\text{actual water vapor content}}{\text{capacity}} \times 100
• Body Humidity (BH): Ratio of actual water vapor content to water vapor capacity in saturated gas at body temperature (37^{\circ}C).
  • Capacity at 37^{\circ}C: 43.8 mg/L.
  • BH\% = \frac{\text{Actual water vapor content}}{\text{capacity (fixed 43.8)}} \times 100
• Humidity Deficit (HD): Represents water vapor the body must add to inspired gas to achieve saturation at body temperature (37^{\circ}C) when BH is less than 100%.
  • Capacity at 37^{\circ}C: 43.8 mg/L.
  • HD = \text{capacity (fixed 43.8)} - \text{Actual water vapor content}
• Pressure will be largest in correlation with volume at 100% saturated vapor.

Condensation and Dew Point

• Condensation: Heat is given back to air surrounding the liquid, warming the air.
• Slight cooling of saturated gas (100% RH) causes water vapor to turn back into a liquid state.
• Dew Point: Temperature at which condensation begins.
• Cooling a saturated gas below its dew point causes more water vapor to condense into liquid water droplets.
• Condensed moisture deposits on surfaces, such as walls of a container, tubing, or particles suspended in the gas.

Pascal’s Principle

• Pressure of a liquid acts equally in all directions.

Buoyancy (Archimedes Principle)

• Liquids exert a buoyant force.
• Pressure below a submerged object exceeds pressure above it.
• If an object’s weight density exceeds the weight of water, the object will sink.
• Gases also exert a buoyant force (less than liquids).
• Buoyancy helps keep solid particles suspended in gases.

Law of Laplace / Surface Tension

1. Surface tension: Force exerted by like molecules at a liquid’s surface.
2. Surface molecules contract into the smallest possible surface area (retaining a spherical shape).
3. Surface tension quantified by the force needed to produce a “tear” in a fluid surface area.
4. Surface tension increases pressure inside a liquid drop or bubble.
5. Cohesive forces affect molecules inside the drop equally from all directions.

Laplace’s Law

• Spherical structures demonstrate the interaction between distending pressure and surface tension forces as the sphere’s radius varies.
• Pressure varies directly with surface tension and inversely with radius.

Pulmonary Surfactant

• Substance in lungs that helps maintain surface tension.
• Prevents over-distension on inhalation.
• Helps prevent alveoli collapse on exhalation.
• Helps stabilize alveoli across the lungs.

Patterns of Flow, Resistance and Compliance

• Both liquids and gases can flow.
• Energy loss occurs due to opposition to flow (flow resistance).
• Frictional resistance exists in the liquid/gas itself or between the liquid/gas and the tube wall.

Airway Resistance (RAW)

• Resistance to ventilation by movement of gas through the airways.
• Accounts for approx. 80% of frictional resistance
• 80% of RAW occurs in nose, mouth, trachea, upper airways (areas of turbulent flow).
• In terminal bronchioles, there is laminar flow, velocity decreases, and resistance is relative to volume.
• Resistance is highly dependent on lung volume; if lung volumes decrease in the terminal bronchioles i.e., smaller airways 20%) airway diameters also decrease and in turn airway resistance to flow increases in upper airways (i.e., Wheezing on exhalation).
• With increased airway resistance and decreased lung volume, there is an increase in driving pressure (Boyle’s law).

Lung Compliance

• Lung compliance is the result of tissue elastic forces (ability to stretch) and surface tension.
• As lung volume is lost, lungs become stiffer, and ventilation becomes more difficult.
• Static compliance measurement uses Volume, plateau pressure (pressure at end of inhalation prior to exhalation) and PEEP (positive expiratory pressure).

Laminar Flow

• Fluid moves in discrete cylindrical layers or streamlines.
• Viscosity is the force behind laminar flow.
• Changes in tube diameter greatly affect viscosity of flow.
• Driving pressure and flow is linear.
• Confined to small peripheral airways.
• Laminar flow is proportional to driving pressure and inversely proportional to resistance.
• When flow is doubled under laminar conditions, the pressure is doubled.

Poiseuille’s Law

• The difference in pressure required to produce a given flow, under conditions of laminar flow through a smooth tube of fixed size.
• Involves laminar flow.
• Determining factors involve: change in pressure, viscosity, length, radius, and flow.
• Driving pressure increases whenever viscosity, tube length, or flow increases.
• Greater pressure required to maintain a given flow if tube radius decreases.
• Poiseuille’s equation can also be used to express flow resistance, as mentioned above with the decrease in volume changes in flow, pressure, decrease in radius and increasing resistance.

Turbulent Flow

• Prevents significant changes in flow through a tube.
• Irregular eddy currents are formed in disorganized chaotic patterns which causes a higher amount of resistance.
• Turbulent flow favors increased velocity, density, tube diameter and viscosity.
• Driving pressure is proportional to gas density.
• Factors involving changeover from laminar to turbulent flow is Reynold’s number.
• Reynolds number for laminar flow is less than 2000.
• Turbulent flow is in the large airways such as the nose, pharynx, larynx, trachea.

Transitional Flow

• A mixture of both laminar and turbulent flow.
• Main flow in respiratory track.
• Also known as tracheobronchial flow.
• Occurs when a tube narrows, branches or irregularities occur in the tube surface.