Humidity Therapy

Front 1

What is humidity?

Definitions

Back 1

HUMIDITY: water molecules in a vaporous or gaseous state within a gas mixture

ABSOLUTE HUMIDITY: actual content or weight of water in a volume of gas

(g/m3 or mg of H2O/ L of gas)

Front 2

What is humidity?

Definitions

Back 2

RELATIVE HUMIDITY:

actual content of water in mixture

--------------------------------------------------

Maximum amount (capacity) that can be held at that temperature

• Expressed as a percentage

• 100% RH: content = capacity -> ‘saturated’

Front 3

What is humidity?

Definitions

Back 3

VAPOUR: substance in the gas phase at a temperature lower than its critical point

• can be condensed to a liquid by increasing its pressure without reducing the temperature

WATER VAPOUR PRESSURE: partial pressure

of water vapour in any gas mixture in equilibrium with solid or liquid water

Front 4

What is humidity?

Definitions

Back 4

SATURATION: the point at which a solution of a substance can dissolve no more of that substance or additional amounts of it will appear as a separate phase

Front 5

What is humidity?

Definitions

Back 5

DEW POINT: the temperature at which the water vapor in a sample of air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates.

Front 6

What is humidity?

Definitions

Back 6

CONDENSATION: the change of the physical state of matter from gas phase into liquid phase

• the reverse of vaporization

Front 7

What is humidity?

Evaporation

Back 7

Process of water moving from a liquid to a gaseous state -
1. ↑ kinetic activity of some molecules allows them to break from the liquid’s surface and become vapour
2. Resultant drop in energy level of the liquid →
temperature drops: ‘evaporative heat loss’

Front 8

What is humidity?

Evaporation

Back 8

3. Ambient heat transfers to liquid → temperature restored → process repeats
4. ↑ temperature of liquid → ↑ molecular activity → ↑ molecules escaping as vapour

Front 9

What is humidity?

Percent Body Humidity

(%BH)

Back 9

• Maximum H2O content at body temperature

BTPS:
• Gas at body temperature
• Pressurized to atmospheric
• 100% saturated with water vapour
• Normal BTPS = 43.9 mgH2O/ L of gas

Front 10

What is humidity?

Percent Body Humidity

(%BH)

Back 10

Temp. Absolute Humidity PB(mmHG)
20°C 17.3mg/L 17.5

25°C 23.0mg/L 23.7

30°C 30.4mg/L 31.7

37°C 43.9mg/L 46.9

40°C 51.1mg/L 55.1

Front 11

What is humidity?

Humidity Deficit

Back 11

• Difference between inspired water vapour (%BH) and actual water vapour in lung

• Amount of humidity not found in inspired gas → must be supplied by the body or artificially

Front 12

What is humidity?

Physiological Control of Heat & Moisture

Back 12

• Function of the upper respiratory tract

• Mucous glands, goblet cells, cell walls

• High vascularization → good heat exchange

• Principle of evaporation

Front 13

What is humidity?

Respiratory Cycle

Back 13

Inspiratory Phase:
• Air warms (convection)
• Picks up water vapour (evaporation)

Expiratory Phase:

• Heat transferred back to mucosa (convection)
• Cools → less able to hold water vapour → rain-out onto mucosa (condensation)

Front 14

What is humidity?

Isothermic Saturation Boundary

(ISB)

Back 14

• Point at 5 cm below carina

Where constant 100% relative humidity @ 37°C → BTPS

Front 15

What is humidity?

Humidity Deficit: Distal Shift of ISB

Back 15

Below 5 cm point in trachea

1. Breathing cold, dry air
2. Upper airway bypassed
3. Other conditions

Front 16

What is humidity?

Purpose of Humidity Therapy

Back 16

Primary Indications:
1. Humidify dry gas being inspired by patient, +/- heat
2. Shift in ISB

Secondary Indications:
Other conditions

Front 17

What is humidity?

Purpose of Humidity Therapy

Primary Indications

Back 17

1. Breathing cold, dry air
• Outdoor winter conditions
• Administration of medical gases @ > 4 LPM:
-Heat & water loss
-Reduced ciliary motility
-Increased irritation of airways
-Increased mucous production
-Thick, sticky mucous

Front 18

What is humidity?

Purpose of Humidity Therapy

Primary Indications

Back 18

2. Bypassed upper airway:
• Placement of artificial airways or tubes
• > 2 h/day → damaged tracheal epithelium as in previous situ
• Minimum humidity: 60% of BTPS
-43.9 mg/ L x 0.60 = 30 mg/ L
-Government regulated: International Institute for Standardization (ISO)

Front 19

What is humidity?

Purpose of Humidity Therapy

Secondary Indications

Back 19

• Managing hypothermia
• Treating bronchospasm from breathing cold air
-cooling & drying of airways → hypertonicity of airway fluid → release of chemical mediators → bronchospasm

Front 20

What is humidity?

Humidity Therapy

Recommended Levels

Back 20

Site %RH AH

Nose/mouth 50% 10mgH20

Hypopharynx 95% 28-34

Trachea 100% 36-40

Front 21

What is humidity?

Symptoms of Inadequate Humidification

Back 21

• Atelactasis (collapse of alveoli)
• Dry, non-productive cough
• Increased airway resistance
• Increased incidence of infection
• Increased work of breathing
• Substernal pain
• Thick, dehydrated secretions

Front 22

Humidity Therapy Equipment

What is a Humidifier?

Back 22

• Device that adds molecular water to a gas

• Simple evaporation from water surface and carried to patient

Front 23

Humidity Therapy Equipment

Physical Principles Affecting Humidifier Efficiency

Back 23

1. Temperature
2. Surface area
3. Time (duration) of contact

Front 24

Humidity Therapy Equipment

Physical Principles Affecting Humidifier Efficiency

Temperature

Back 24

• Most NB factor affecting performance

• Both water and gas

• ↑ temperature of gas = ↑ water vapour

Front 25

Humidity Therapy Equipment

Physical Principles Affecting Humidifier Efficiency

Surface Area

Back 25

• ↑ area of contact = ↑ evaporation

• Goal is to increase contact area between water and gas

• Large or deep reservoirs

Front 26

Humidity Therapy Equipment

Physical Principles Affecting Humidifier Efficiency

Surface Area

Back 26

• Porous material (wicks) submerged in water

• Bubble diffusers – more bubbles

Front 27

Humidity Therapy Equipment

Physical Principles Affecting Humidifier Efficiency

Contact Time

Back 27

• ↑ contact time = ↑ evaporation

• Goal is to increase contact time between water and gas

• Large or deep reservoirs

• Low gas flows

Front 28

Humidity Therapy Equipment

Types of Humidifiers

Back 28

Active
1. Bubblers
2. Passover high-flow
• Aerosol nebulizers
• High-Flow Nasal Cannula

Passive
• Heat & Moisture Exchangers

Front 29

Humidity Therapy Equipment

Types of Humidifiers

Active: Bubblers

Back 29

• Goal: raise water vapour content of the gas to ambient levels
• Low-flow oral & nasal O2 delivery systems
• Bubble gas up through a reservoir of water
• Output approx. 30-40% RH @ 37°C
• Safety pressure relief set @ 2 psi

Front 30

Humidity Therapy Equipment

Types of Humidifiers

Active: Passover High Flows

Back 30

• Provide humidity to all inspired gas

• Minimum of 30 mgH2O/ L @ 31-35°C (ISO)

• Heated to ↑ evaporation and minimize hypothermia

Front 31

Humidity Therapy Equipment

Types of Humidifiers

Active: Passover High Flows

Back 31

• Equipment requiring high gas flows (up to180 LPM)
-High FiO2 systems
-Mechanical ventilators requiring artificial airways and high gas flows

Front 32

Humidity Therapy Equipment

Types of Humidifiers

Active: Passover High Flows

Back 32

• 100% humidity @ BTPS: 43.9 mgH2O/L

• High gas flow enters reservoir inlet port → passes over water surface → picks up water vapour → continues to outlet port

• Heated to ↑ evaporation

Front 33

Humidity Therapy Equipment

Types of Humidifiers

Active: Passover High Flows

Back 33

• ‘Servo-Controlled’ to maintain temperature & RH throughout tubing length - ↓ rain-out
-“rain-out” collects in tubing causing obstruction or restriction to gas flow → increased resistance → backpressure

Front 34

Humidity Therapy Equipment

Types of Humidifiers

Active: Passover High Flows

Back 34

• Tubing to patient heated via threaded wires to reduce rain-out/condensation

• Semi- or automatic water feed systems to maintain water levels in reservoir

Front 35

Humidity Therapy Equipment

Types of Humidifiers

Active: Passover High Flows

Back 35

*May submerge porous paper wick in water reservoir to draw up liquid - ↑ surface area & ↑ contact time (ie. swirling)