1998 OPEN FORUM Abstracts
Symposium: MDI Aerosol Delivery . . . In the Critical Care Setting
Joseph L. Rau, Ph.D., RRT
It would seem that intubated patients offer an ideal route for direct aerosol treatment of the lung. Early research contradicted this expectation. Fraser et al (1981) measured 1.6% and MacIntyre et al (1985) found 2.9% of a nebulizer dose reaching the lung. The usually quoted 10% metered dose inhaler (MDI) dose (Zainudin, 1991) through an endotracheal tube (ETT) was demonstrated by Bishop et al (1989) to be reduced, to between 3.0 and 6.5% of MDI dose delivered. Since the time of these studies, a number of factors have been identified with the ventilator, and with the aerosol generator system, that can affect drug delivery to intubated patients. This review focuses on primarily bronchodilator (BD) delivery to mechanically ventilated, intubated patients.
Studies of drug delivery have shown MDIs to be equally effective to SVNs, for in vitro and in vivo lung deposition (Diot, 1995; Fuller, 1990) as well as for clinical response (Gay, 1991; Fernandez, 1990). Dose delivery varies in studies, with as much as 25-30% delivery through an ETT in vitro, and as little as 5% in vivo.
A number of factors affect the efficiency of MDI dose delivery. Humidified gas has been shown to consistently lower drug delivery compared to dry gas, by as much as 40% (Diot, 1995; Fink, 1996). MDI actuation synchronized with the beginning of a ventilatory breath improves delivery, along with a decelerating inspiratory flow wave during mechanical ventilation (CMV). The type of MDI adaptor, and in general, the configuration of the generator/delivery system also affects dose delivery. Elbow adaptors directly on an ETT are the least efficient, and reservoir devices the most efficient in maximizing drug delivery, with nonchamber inline ("tee") adaptors in between. Rau et al (1998) found consistently lower delivery of aerosolized corticosteroids compared to beta agonists, with nonchamber inline and reservoir devices.
Clinical response (peak pressures, expiratory flow, inspiratory resistances, static compliance) has been shown with MDI delivery of bronchodilators (Fernandez, 1990; Gay, 1991). Both Dhand (1996) and Manthous (1995) demonstrated significant BD response with 4-5 MDI actuations, when optimal technique (including use of a reservoir) was employed, with little increase above 8-10 actuations. An interesting study by Wollam et al (1994) found that potential indications for BD treatment (history of reactive airway disease, presence of auto-PEEP, or wheezing) were a probable but not perfect guide to BD response on the ventilator. Their study recommended trial of a BD if a potential indication exists, but that BD use may not be clinically useful in its absence.
REFERENCE LIST AVAILABLE AT LECTURE.
The 44th International Respiratory Congress Abstracts-On-Disk®, November 7 - 10, 1998, Atlanta, Georgia.