Aerosol Drugs and Humidity

In depth

The one-page summary above is what the PDF prints. These sections go further.

Humidity physics and troubleshooting

Absolute humidity is the water vapor in a volume of gas (mg/L); relative humidity is that content as a percentage of what the gas could hold at its temperature. Warming a saturated gas lowers its relative humidity, and cooling it causes rainout. At 37 °C, saturated gas holds 44 mg/L, so gas delivered at 30 mg/L leaves a humidity deficit of 14 mg/L that the airway must make up. The isothermic saturation boundary normally sits about 5 cm below the carina; dry gas through a tube moves it deeper, drying mucosa, slowing cilia and thickening secretions until they plug the tube.

AARC 2012 calls for humidification in every invasively ventilated patient: 33–44 mg/L at 34–41 °C at the Y-piece from a heated humidifier, or at least 30 mg/L from an HME, which suits short-term use and transport. Heated-wire circuits keep the inspiratory limb warmer than the chamber so less water condenses; if the circuit runs much hotter than the chamber, relative humidity falls and secretions dry. A probe left inside an incubator or under a radiant warmer reads falsely high and the heater turns down.

Bland aerosol (AARC 2003) has three indications: cool aerosol for upper-airway edema (post-extubation, subglottic), humidity for a bypassed upper airway, and hypertonic or hypotonic saline for sputum induction. Its only listed contraindication is bronchoconstriction. Aerosol is a poorer humidifier for a tracheostomy than a heated humidifier and carries infection risk. Cool mist for croup is traditional but has shown no benefit in trials.

Aerosol generators in detail

Particles of 1–5 µm reach the lower airways; larger ones impact in the mouth and throat, more so with fast breathing, and smaller ones are often exhaled. Jet nebulizers shear liquid with gas at 6–8 L/min from a 4–5 mL fill and leave 0.5–1 mL in the cup; tapping the cup and treating until it sputters extracts more. Breath-enhanced and breath-actuated designs waste less during exhalation.

Vibrating mesh nebulizers push liquid through a plate with thousands of laser-drilled holes. They are quiet, portable, leave little residue and add no gas, which makes them the usual choice on ventilators and for continuous inhaled epoprostenol; because they are efficient, some doses may need adjusting. Ultrasonic nebulizers vibrate a piezoelectric crystal at 1–3 MHz, can warm and degrade drugs, and should not be used for suspensions such as budesonide or for proteins such as dornase. Large-volume ultrasonic units are used for sputum induction.

Continuous albuterol for severe asthma (EPR-3) is 10–15 mg/h in adults and 0.5 mg/kg/h in children, through a large-volume or infusion-fed mesh nebulizer, with cardiac monitoring and potassium checks. Ipratropium mist in the eyes can dilate the pupil and precipitate angle-closure glaucoma, so use a mouthpiece or a well-fitted mask.

Mucoactive drugs and the evidence

Mucolytics break mucus bonds: acetylcysteine splits disulfide bonds and dornase alfa cleaves the DNA released by neutrophils, which is why dornase works in purulent CF sputum. Hypertonic saline is a hyperosmolar expectorant that draws water into the airway surface liquid and raises mucociliary clearance. Acetylcysteine smells of sulfur, irritates the airway and can cause bronchospasm at any dose, so the label advises a nebulized bronchodilator and stopping if bronchospasm progresses; a vial once opened is refrigerated and used within 96 hours.

The CF Foundation (2013) recommends dornase and 7% hypertonic saline for patients 6 years and older. In idiopathic bronchiectasis, dornase given twice daily for 24 weeks increased exacerbations and FEV₁ decline (O'Donnell 1998, 349 patients), so it is not used outside CF. AARC 2015 found no high-level evidence for any aerosol to aid secretion clearance in hospitalized patients without CF and recommends against routine dornase, acetylcysteine and bronchodilators for that purpose.

For a first Pseudomonas aeruginosa culture in CF, the CF Foundation (2014) recommends inhaled tobramycin for eradication, typically 300 mg twice daily for 28 days. Chronic infection is treated with cycles of inhaled tobramycin or aztreonam, given after airway clearance. Inhaled antibiotics need an expiratory filter in hospital to protect staff, and tobramycin is never mixed with dornase in the cup.

Protecting patients and staff

• Pentamidine: 300 mg every 4 weeks by Respirgard II at 5–7 L/min until empty (30–45 minutes). Cough (38%) and bronchospasm (15%) are common, so a bronchodilator is given first if needed. Screen for TB, treat in a negative-pressure booth or room, and use an expiratory filter.
• Ribavirin: SPAG-2 generator only, 20 mg/mL for 12–18 hours a day for 3–7 days. It is contraindicated in anyone who is or may become pregnant, so pregnant staff stay away, a scavenging tent or exhaust is used, and the drug can precipitate in ventilator valves and tubing.
• Sputum induction: a booth or negative-pressure room, an N95 respirator for staff when TB is possible, and a bronchodilator on hand because hypertonic saline can cause bronchospasm.
• Nebulizers are single-patient devices: between treatments clean, rinse with sterile water and air-dry; use sterile fluids and unit-dose vials; at home, clean after every use and disinfect per the manufacturer.
• Valved holding chambers and spacers: wash weekly in warm water with liquid detergent and let them air-dry.

NBRC emphasis and exam pearls

• Thick secretions and an HME: change to a heated humidifier.
• Mist disappears on inspiration from a trach collar: flow is too low for demand.
• Acetylcysteine in an asthmatic: give or pair it with a bronchodilator.
• Dornase: CF only, its own cup, jet or mesh; never ultrasonic.
• Pentamidine: negative pressure, filter, TB screen; ribavirin: pregnancy precautions and scavenging.
• Albuterol with no wheeze and no measurable response: recommend discontinuing it.
• Heart rate jumps with PVCs during albuterol: stop the treatment, stay, monitor, notify.
• Inhaled steroid with thrush: spacer, rinse and spit; do not stop the controller.