To the Editor:
We read the report by Dr Walsh et al1 in the May issue of Respiratory Care with great interest. In this brilliantly conceived and carefully executed study, the researchers repeatedly refer to the breathing circuit filter that they used to collect ribavirin aerosol (Respirgard II 303, Vital Signs Division of CareFusion, San Diego, California) as an “absolute filter.” One of us (RRD) had occasion to challenge that same filter with aerosol generated by the small-particle aerosol generator (SPAG-2) and reported our findings both in this journal2 and in another clinical journal.3 In both instances, the Respirgard filter was found to be decidedly not an absolute filter for ribavirin. Admittedly, though, we challenged that device with an aerosol that was elaborated by the SPAG-2 in its initial incarnation (during which time it was distributed by ICN Pharmaceuticals, the original manufacturer of ribavirin), so we recognize that the specifications of the current SPAG-2, distributed by Valeant Pharmaceuticals, might be substantially different. The ICN's SPAG-2 generated a monodisperse aerosol of ribavirin with particles having a mass median aerodynamic diameter (MMAD) of 2.2 μm, whereas the MMAD of the monodisperse particles elaborated by the Solo (Aerogen, Galway, Ireland), also used by Walsh et al1, are reported to exhibit an MMAD of 3.4 μm. In addition, we readily concede that it is distinctly possible that the specifications of the Respirgard II 303 could have been appreciably enhanced since we tested it. The actual performance of this filter is of more than casual interest to us, in view of the fact that it is an integral component of the Respirgard II nebulizer that is commonly used to administer the NebuPent brand of pentamidine isethionate to immunocompromised patients as a means of prophylaxis against Pneumocystis carinii pneumonia. That drug is known to exert teratogenic effects, and if the performance of that filter is anything less than “absolute,” caregivers of child-bearing age could be unwittingly exposing themselves to pentamidine aerosol while administering such treatments, triggering potentially harmful effects in their offspring.
With these issues in mind, we decided to perform a semiquantitative test of the Respirgard II 303 by interfacing it to the outlet of a Solo vibrating mesh nebulizer (VMN). We instilled about 6 mL of methylene blue into the VMN and actuated it in the continuous mode. Before the initiation of the run time, a Respirgard II 303 was mounted downstream of the VMN, and a filter that is known to be an absolute breathing circuit filter (BB-50T; Pall Biomedical Products, Port Washington, New York) was mounted immediately downstream of the Respirgard II 303. During the subsequent run, oxygen from an E cylinder, adjusted to a flow of 15 L/min (or 250 mL/s), mobilized the VMN's aerosol plume to the filters arranged in series. After a run time of 6 min duration, the filters were inspected and photographed. The proximal (upstream) face of the Respirgard II 303 was observed to display dense bluish discoloration, as expected, and that filter's downstream face was also observed to be discolored. More importantly, the proximal face of the BB-50T exhibited obvious bluish discoloration (Figure 1), confirming that the Respirgard II 303 had failed to block some of the VMN-generated aerosol.
Respirgard II 303 observed displaying dense bluish discoloration with visible discoloration of the BB-50T downstream of the Respirgard II 303.
Do our findings suggest that the study of Walsh et al1 is fatally flawed? Not at all! In point of fact, if they had captured ribavirin aerosol with a device that actually was an absolute filter, they would have verified an even larger delivered dose of ribavirin than that which was reported in their monograph. Consequently, their clinical study is just as convincing, and fully as important, as it initially appeared to be, and they are to be commended for their efforts.
On the other hand, readers need to be aware that the Respirgard II 303 is decidedly not an absolute filter. When this term is invoked in the usual context, an absolute filter is capable of blocking 100% of whatever particles and/or microorganisms by which it is bombarded. Hence, caregivers need to mount appropriate precautions whenever they might be exposed to harmful pharmaceutical agents, bacterial/viral microaerosols, or airborne endotoxins while practicing in a hazardous environment.
Footnotes
The authors have disclosed no conflicts of interest.
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