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

Objective data: P{aO2} / SpO₂ below target ranges.
Clinical signs: tachycardia, tachypnoea, cyanosis, ↑ WOB.
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.