Abstract
Background:
The radioisotope Technetium (99mTc), bound to pertechnetate dissolved in 0.9% saline, is an aqueous solution with physicochemical properties similar to inhalation solutions of albuterol, ipratropium, tobramycin, and epoprostenol, among others. 99mTc, a readily detectable gamma-emitter that physically behaves in a nebulizer comparably to aqueous drugs, is used as a surrogate because its radioactivity is easily and rapidly measured via gamma camera which also provides an image of radioaerosol deposition. This report is a teaching example of the utility of nebulizing radiolabeled saline for in vitro assessment of nebulizer performance. Distribution of nebulizer output aerosol is shown as it transits and deposits within the components of a ventilator system during treatment.
Methods:
A breath-enhanced nebulizer (i-AIRE, InspiRx, Inc., Somerset NJ) was charged with 1567 µCi of 99mTc-saline (6 mL) and operated on a ventilator/heated humidifier system and test lung. HEPA filters at the distal tip of the ETT and the distal end of the expiratory limb captured Inhaled Mass (IM) and exhaled aerosol. Post-treatment, filters plus ventilator system tubing and components were scanned by the gamma camera for measurement and imaging of radioactive aerosol deposition and distribution. All gamma counts were expressed as % of nebulizer charge to represent the fate of the drug mass initially placed in the nebulizer.
Results:
Measurement of all system components (known as a full Mass Balance) demonstrated: nebulizer residual post-treatment 34.3%, flex tube from nebulizer to humidifier 6.2%, humidifier 4.4%, inspiratory limb and patient-Y 10.7%, closed system suction 0.2%, ETT 0.7%, IM filter 23.5%, expiratory limb 0.4%, and expiratory filter 18.4%. Different patterns of deposition in the various system components are shown in the Figure. “Hot spots” indicate areas of greater aerosol deposition as a results of flow patterns, entrance/exit effects, impaction/sedimentation due to particle size and other factors.
Conclusions:
The utility of this technique is illustrated by this demonstration. Aerosol delivery (IM) to a patient in a system with different variables can be simulated and predicted on the test bench. Deposition imaging of system components can help explain system losses, the influence of ventilator settings, humidification, and overall behavior of a nebulizer/ventilator system.
Gamma camera radioaerosol deposition images of ventilator system components.
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