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Humidity

Absolute Humidity

  • Definition: The actual weight of water present in a given volume of gas.
  • Units of Measurement: Commonly expressed in milligrams per liter (mg/L) or as water vapor pressure (PH₂O).
  • Alveolar Gas Content: At a temperature of 37°C, the water vapor content is 43.8 mg/L.
  • Influence of Temperature and Pressure: The amount of water vapor a gas can hold is influenced by temperature and pressure considerations.

Potential Humidity

  • Definition: The maximum amount of water that a volume of gas can hold at a specific temperature.
  • Measurement: Refers to the weight of water vapor or the PH₂O in a saturated gas.
  • Temperature Dependence: Maximum absolute humidity increases with temperature.
    • Example: At 37°C, maximum absolute humidity is 43.8 mg/L with a partial pressure of 47 mm Hg.
    • Implication: If the temperature changes, so will the potential humidity.
    • Query: What vapor generates a pressure of 47 mm Hg?

Relative Humidity (RH)

  • Definition: A comparison between the actual amount of water vapor present (absolute humidity) and the maximum amount that can be held (potential humidity).
  • Calculation Formula: %RH = (absolute humidity / potential humidity) x 100.
  • Saturation: When actual water vapor content equals potential humidity, the gas is considered saturated (RH = 100%).
  • Example Calculation: At 37°C if actual water vapor pressure = 33 mm Hg, then RH is:
    • X = rac{33}{47} = 70 ext{ %}

Continued Discussion of Relative Humidity

  • Additional Example: For air at 21°C with an absolute humidity of 10 mg/L, if water content remains constant, increasing temperature decreases relative humidity.
    • Increase in temperature increases air's capacity to hold water, hence lower relative humidity.
    • Chart Reference Needed: For additional calculations between relative humidity and temperature.

Body Humidity

  • Definition: The amount of water vapor required to saturate alveolar air at body temperature and pressure (BTPS).
  • Normal Physiological Conditions: At normal conditions, body humidity is 43.8 mg/L or 47 mm Hg.

Percentage of Body Humidity (%BH)

  • Definition: Comparison of water vapor amount in a volume of gas to that in saturated gas at body temperature.
  • Calculation Example:
    • Saturated air at room temperature (21°C) can hold 18.0 mg/L of water. Inspired air, heated to body temp (37°C), holds 43.8 mg/L.
    • Relative humidity at body temperature for inspired air is:
    • ext{%BH} = ( rac{18 ext{ mg/L}}{43.8 ext{ mg/L}}) imes 100 = 41 ext{ \%}

Humidity Deficit

  • Definition: The difference between the inspired absolute humidity and the water vapor content in alveolar air.
  • Clinical Relevance: The respiratory mucosa must satisfy the humidity deficit; otherwise, mucosal crusting may occur.
  • Example Calculation:
    • If a patient breathes air at 21°C with RH of 50%, find humidity deficit:
      • Absolute humidity = RH x potential humidity = 0.50imes18.0extmg/L=9.0extmg/L0.50 imes 18.0 ext{ mg/L} = 9.0 ext{ mg/L}.
      • Humidity deficit = body humidity – absolute humidity = 43.8extmg/L−9.0extmg/L=34.8extmg/L43.8 ext{ mg/L} - 9.0 ext{ mg/L} = 34.8 ext{ mg/L}.
    • Conclusion: 34.8 mg/L must be added by respiratory mucosa.

Clinical Implications for Humidity Therapy

  • Objective of Humidifying Therapeutic Gases: To increase RH of gas supply (which starts at 0% RH) to approximately 50% RH (like room air).
  • Ether Content: Humidification does not add additional water vapor to the patient, but prevents a humidity deficit from inhaling dry gases.

Matching Body Conditions (BTPS)

  • Normal Conditions: Inspired air should reach a temperature of 37°C and be saturated with water vapor by the time it reaches the carina.
  • Importance of Respiratory Mucosa: The upper airway adds heat and moisture; bypassing it (through artificial airways) leads to inadequate warming and humidifying of inspired air.

Consequences of Inadequate Humidification

  • Results may include:
    • Decreased ciliary activity
    • Drying and thickening of secretions
    • Retention of thick secretions leading to complications like atelectasis and pneumonia.
  • Preventive Measures: Additional humidity should be supplied to the patient’s airway. Gas should be between 32°C to 37°C with %BH between 80% to 100%.

Humidifiers

  • Factors Affecting Efficiency:
    • Temperature of the water and gas.
    • Length of contact time between water and gas.
    • Surface area for gas-water contact.
Types of Humidifiers

  1. Pass-over Humidifiers:

    • Gas passes over liquid surface, allowing evaporation to occur.
    • Efficiency is low due to limited contact time and surface area.
    • Heating either the gas or water can improve efficiency.
    • Cannot achieve 100% RH unless gas/water temperature is sufficiently high.

  2. Bubble Diffusion Humidifiers:

    • Gas is forced below the liquid surface, creating tiny bubbles that increase surface area for humidification.
    • Typically only achieves 20-30% RH, mainly used for humidifying therapeutic gases.
    • Can be heated for 100% body humidity, suitable for low flow oxygen delivery (e.g., nasal cannula).

  3. Wick Humidifiers:

    • Water absorbed by wick (blotting paper type) creates high surface area for evaporation.
    • Can achieve high RH levels (>90%) when paired with heating elements.
    • Must keep wick saturated.

  4. Cascade Type Humidifiers:

    • More advanced, forcing gas down while water is heated.
    • Efficient, but requires correct tubing to avoid condensation and ensure gas reaches the patient at 37°C.

  5. Spinning Disk Humidifiers:

    • Used for room or tent humidification, utilizes centrifugal action to aerosolize water.
    • Low efficiency and can be contaminated, functions as a nebulizer.

  6. Hygroscopic Condenser Humidifiers (Artificial nose):

    • Recycles exhaled moisture and heat to humidify inhaled air.
    • Disadvantages include increased dead space, morphological mucous trapping, and potential flow resistance.

Aerosol Therapy

AEROSOLS
  • Definition: Fine liquid or solid particles suspended in air.
  • Characteristics of Aerosol Therapy:
    • Key factor is particle size; aerosols emerge at velocities of 100 km/h.
    • 80% of drugs are deposited in the oropharynx, 10% in inhaler walls, and 10% in the lungs.
    • Optimal Particle Size: Practice suggests particles sized 2-5µ are ideal for lung penetration and effect.

Indications for Aerosol Therapy

  • Use cases include bronchoactive aerosol deposition, secretion clearance, sputum induction, humidification of gases, preventing dehydration, and relieving bronchospasm.

Hazards of Aerosol Therapy

  • Risks may include:
    • Bronchospasm
    • Overhydration
    • Overheating of inspired gases
    • Contaminated aerosol delivery
    • Tubing condensation draining into the airway.

Devices for Inhalation Therapy

  1. MDI: Metered Dose Inhalers.
  2. DPI: Dry Powder Inhalers.
  3. Nebulizers (uses inertial impaction for particle depostion).
Metered Dose Inhalers (MDI)
  • Characteristics: Widely used, often propelled by CFCs. Aerosol flow is approximately 30 m/s or 100 km/h.
Components of MDI
  • Canister, actuator, metering valve, drug/propellant mixture.
  • Function: Metering valve delivers a precise dose; requires shaking before use.
Advantages of MDI
  • Cost-effective, portable, rapid drug delivery, consistent dosing, widely available.
Disadvantages of MDI
  • Coordination difficulties for use, requires patient education time, potential cold freon effect, CFC contents.
Spacers
  • Holding chambers or attachments for MDIs facilitate drug delivery without requiring coordinated breathing.
    • Two types: Small volume (tube) and large volume (conical).
  • Advantages: Increased lung deposition (~130%), reduced oral deposition, beneficial for children, eliminates cold freon side effects.

Comparison and Efficiencies Between MDIs and DPIs

  • MDIs: Contains CFC, require hand-breath coordination, timing for teaching.
  • DPIs: No CFC, do not require coordination, rely on high inspiration flow (>28 L/min).

Nebulizers

  • Converts aqueous drug solutions into fine mists for inhalation.
  • More effective for deep lung delivery with two types: jet and ultrasonic.
Jet Nebulizers
  • Utilize Bernoulli's principle to aerosolize medication; effectiveness contingent upon airflow rates.
Artificially Indicated Usage
  • Effective in children, the handicapped, acute asthma cases, and for high-dose medication delivery.

Disadvantages of Nebulizers

  • Can be expensive, require maintenance, and potentially transmit airborne infections.

Choice of Therapy by Age Group

  • Infants: Nebulizers
  • Children under 4: Nebulizers
  • Age 4+: DPI/MDI/Spacer
  • Age 7+: DPI/MDI
  • Adults: MDI/DPI
  • Acute episodes: Nebulizers

Available Drugs for MDI

  • Beta Agonists: Salbutamol, terbutaline, fenoterol, etc.
  • Steroids: Beclomethasone, budesonide, fluticasone.
  • Antimuscarinics: Ipratropium.
  • Mast Cell Stabilizers: Sodium cromoglycate, nedocromil sodium.

Available Drugs for Nebulization

  • Similar to MDI lists with options including beta agonists, antimuscarinics, corticosteroids, and antibiotics.

Beta 2 Adrenergic Agonists

  • Mechanism of Action: Stimulation leads to increased cyclic AMP (cAMP) in bronchial cells which causes relaxation, also influences mast cells to reduce mediator release.
Benefits of Inhaled Steroids
  • Reduce hospital admissions, potentially decrease asthma mortality, help prevent long-term lung damage.
Side Effects of Inhaled Steroids
  • Adrenocortical suppression, bone metabolism effects, growth impacts in children, skin thinning, cataracts.

Inhaled Steroid Advancements

  • Commonly used include beclomethasone & budesonide, with limited adverse effects at specific dosages. Fluticasone noted for being safer in children due to absorption properties.

Particle Deposition and Size Challenges

  • Particle Size Impacts: >5μm generally ineffective, while <2μm results in systemic absorption rather than clinical effects.

Visual Aid Reference: Figure comparing particle size and airway deposition (not provided in this text).