Respiratory Care: Humidification, Aerosol, and Oxygen Therapy

Humidification Fundamentals

• Primary purpose: condition inspired gas to approximate upper-airway conditions so that the lower airway receives gas at ext{BTPS}=37\,^\circ\text{C},\;100\%\text{RH},\;43.8\;\text{mg H}_2\text{O·L}^{-1} and 47\;\text{mmHg} water-vapor pressure.
• Isothermic saturation boundary (ISB)
  • Normally 5 cm below carina; moves distally when gas is cold/dry, or when there is a bronchopulmonary fistula (air leak → drier gas).
• Heated humidification mandatory whenever natural upper-airway function is bypassed or overwhelmed (ETT, trach, NIV such as BiPAP, high-flow nasal cannula, mechanical ventilation).
• Pre-term neonates
  • Need warm, fully humidified environment to mimic in-utero conditions.
  • Incubators + heated humidifiers maintain thermoneutrality and hydration.

Recognising Inadequate Humidity

• Clinical signs to prompt ↑ humidity/temperature:
  • Atelectasis on CXR or auscultation
  • Dry, non-productive or weak cough
  • ↑ Raw → ↑ WOB
  • Infection / hospital-acquired pneumonia
  • Substernal chest pain/burning sensation
  • Thick, dehydrated secretions or new “noisy” breath sounds

Hazards & Troubleshooting of Heated Humidification

• Burns
  • To practitioner when handling hot plates (Fisher & Paykel heater).
  • To patient if delivery gas >37!\,^{\circ}\text{C} (e.g. heater set at 40!\,^{\circ}\text{C}).
• Incorrect set-point questions on exams
  • “Best” answer is usually 35!\,^{\circ}\text{C}\text{ and }43.8\;\text{mg·L}^{-1}—slightly below core temperature but adequate.
• Condensation (“rain-out”)
  • Obstructs circuit → ↓ flow/auto-triggering → patient–ventilator asynchrony.
  • Solutions: raise heater temp, use circuit heaters, water traps, adjust ventilator alarms.
• Temperature probe placement (infants vs adults)
  • Adults: probe near Y-connector/ETT.
  • Pre-term infants in warm incubator: place probe outside incubator to avoid falsely high reading.
• Condensate disposal = biohazard; drain away from patient using PPE.

Heat Moisture Exchangers (Passive Humidity)

• HME = “artificial nose”; captures patient’s exhaled heat/moisture, returns during next breath.
• Hazards/contraindications
  • Added dead space → hypercapnia in small tidal volumes or high RR.
  • Clogging with secretions/condensate → ↑ Raw.
  • Minute ventilation >10\;\text{L·min}^{-1}, thick secretions, or hypothermia ⇒ avoid.
• Replace PRN (when flow resistance ↑, secretions visible, WOB ↑).

Humidifier vs Nebulizer

• Humidifier
  • Produces molecular water (vapour). Invisible.
• Nebulizer
  • Produces liquid aerosol (cloud). Visible. “Nebula” = cloud.

Variables Affecting Water Content Pick-Up

• Contact time (inverse to flow). 2 L·min⁻¹ cannula bubbles longer → ↑ humidity vs 4 L·min⁻¹.
• Surface area of water (diffuser, wick, depth).
• Water depth (longer bubble path → ↑ humidity).
• Outlet size of device does NOT change humidity generation; only affects rain-out post-chamber.

Typical Water Outputs

• Bubble humidifier (unheated): 15\text{–}20\;\text{mg·L}^{-1}.
• Large-volume nebulizer (unheated): ≈35\;\text{mg·L}^{-1}.
• Heated LVN: 37\text{–}55\;\text{mg·L}^{-1} (> body capacity → excess rain-out).

Aerosol (Bland) Therapy

• Indications (no medication—just sterile water/NS):
  • Mobilise thick, non-productive secretions.
  • Provide humidity for bypassed airway (trach collar, T-piece, aerosol mask during weaning).
• Device selection
  • Trach collar or T-piece for artificial airways.
  • Aerosol mask (same shell as Venturi mask) for non-intubated.
  • Blue 22 mm tubing used because high aerosol density would clog small cannula tubing.
• Contraindications / Caution
  • CHF / pulmonary oedema (lungs already “wet”).
  • Fluid-restricted patients; infants (easy fluid overload).
  • Electrolyte imbalance risk (dilutional effect of absorbed water).
• Ethical principle: “Do no harm” → therapist must understand indications/risks before setup.

Active vs Passive Systems

• Active = anything with external heat/humidity source (heated wick, Fisher-Paykel chamber, LVN, high-flow nasal cannula with heater).
• Passive = HME (heat-moisture exchanger).
• Manual vs Automatic refill
  • Manual: staff pours water periodically.
  • Automatic: bag and feed-line maintain level, saving labour.

Device Change-Out & Maintenance

• Replace HME PRN; replace circuits per policy.
• Treat condensate as infectious; drain to trap, not toward patient.
• Never “hover” over nebuliser mist—maintain distance + PPE.

Typical Selection Algorithm Highlights (text p. 828)

• Cannula ≤4\;\text{L·min}^{-1} → no external humidification required.
• Cannula >4\;\text{L·min}^{-1} → add bubble humidifier.
• Venturi mask often used without humidifier due to large air entrainment ports; non-rebreather seldom humidified (high flow only during inspiration, short-term use).
• Ventilator/Trach/NIV → heated humidifier or HME if not contraindicated.
• Cool aerosol T-piece used during ventilator weaning trials (1 h) – heat usually unnecessary.

Oxygen Therapy Essentials

Goals of O₂ Therapy

• ↓ Cardiopulmonary work (HR, WOB).
• Treat or prevent hypoxemia → avert tissue hypoxia.
• Decrease pulmonary vasoconstriction & hypertension → unload right ventricle (prevent cor pulmonale).
• Part of standard “bundle” in certain conditions: STEMI/MI, major trauma, sepsis, CO poisoning.

Assessment of Need

1. Objective data: P{aO2} / SpO₂ below target ranges.
2. Clinical signs: tachycardia, tachypnoea, cyanosis, ↑ WOB.
3. Specific diagnosis/protocol.

Oxygen Classified as a Drug

• Requires physician order but RTs titrate per protocol; aim for lowest FiO₂ achieving target saturation.

Oxygen Toxicity

• Pathophysiology
  • Prolonged P{aO2}>150\;\text{mmHg} or FiO₂ ≥0.60 for >24–48 h → reactive O₂ species → diffuse alveolar damage → Acute Lung Injury (ALI)/ARDS.
• Vicious cycle (pg 903): ↓ gas exchange ⇒ ↑ FiO₂ ⇒ ↑ toxicity.
• Mortality for ARDS ≈60 %.

Other O₂ Hazards

• Retinopathy of prematurity (ROP) in neonates—keep SpO₂ within NICU targets.
• Absorption atelectasis
  • High FiO₂ washes out alveolar N₂; gas in poorly ventilated units absorbs → collapse.
• Fire risk—strict No-Smoking + spark precautions.

Hyperbaric Oxygen Therapy (HBO)

• Indications: CO poisoning, air/gas embolism, decompression sickness, refractory gangrene/diabetic ulcers.
• Works by ↑ dissolved O₂ (Henry’s Law) + displacing N₂ from bubbles.

Oxygen Delivery Systems & Bedside Examples

• Non-rebreather mask (NRB) delivers highest immediate FiO₂ (~0.8–0.95) — first-line “STAT” device in ED.
• High-flow nasal cannula (HFNC) can exceed NRB performance but requires setup time.
• Partial rebreather = NRB without one-way flaps → more air entrainment.
• Simple mask 6–10 L·min⁻¹ → FiO₂ ≈0.35–0.55.
• Nasal cannula rule of thumb: every 1\;\text{L·min}^{-1} ↑ ≈+4\% FiO₂ starting at 24\%; thus 6 L ≈44\%.

Bedside Calculation Example

• Physician orders 40 % at 60 L·min⁻¹ without blender.
  • 40 % ⇒ air:O₂ ratio 3:1 (air=45 L, O₂=15 L) because \frac{3+1}{1}=4\;\text{parts},\;60/4=15.

“Mask-to-Cannula for Lunch” Scenario

• Patient on simple mask 6 L·min⁻¹ (~0.45 FiO₂) wants to eat.
  • Switch to 6 L·min⁻¹ nasal cannula (≈0.44 FiO₂) → similar oxygenation while allowing oral intake.
  • If patient remains stable, may continue cannula and titrate downward per SpO₂.

Arterial Blood Gases (ABGs)

• Instructor emphasis: ABG interpretation is “gimme points” on every exam; practice until routine.
• Always correlate FiO₂ changes with ABG trends; document rationale when adjusting O₂.

Ethical & Practical Take-Home Points

• “Do no harm” → understand device function, indications, and contraindications before application.
• O₂ and humidity adjustments often protocol-driven; do not hesitate to wean when safe.
• Use personal protective equipment (PPE) and infection-control practices when handling humidifier condensate or aerosol clouds.
• All respiratory connectors are designed to fit; if a part does not connect naturally, it is likely wrong device/port.
• Keep vigilant for exam questions mixing temperature, absolute humidity, and pressure data—select the best (not necessarily perfect) option.