Comparative performance of two inhaler systems to assess distribution of convective ventilation by 99mTc-labeled aerosol scintigraphy in patients with airway obstruction

Abstract

Aim: Redistribution of convective ventilation, the leading disorder in airway obstruction, is a target of pharmacological and mechanical ventilation treatments for patients with chronic obstructive pulmonary disease (COPD). Convective ventilation is visualized by ventilation scintigraphy using radiolabeled aerosol particles that should ideally deposit in the terminal airspaces, but not in the conducting airways, and have no Brownian motion (which characterizes diffusive ventilation). Currently available commercial systems do not meet these requirements as they do not ensure an optimal size of aerosol droplets delivered at the mouthpiece.

Methods: A new inhaling system (FAI) was developed and designed so as to yield radioaerosol droplets with smaller particle size and to ensure more efficient aerosol delivery to the terminal airways than that obtained with a widely available commercial system (MMI). A cascade impactor was employed to measure the size of the radioactive droplets at the mouthpiece. Preliminary comparative validation was based on ventilation scintigraphy using the two systems (both followed by a standard lung perfusion scan) in control subjects and in patients with airway obstruction. The time required to reach a certain count rate in the lung fields (1 kc/s) was recorded by means of dynamic g camera acquisition during breathing. Subsequent static images allowed assessment of intrapulmonary distribution of ventilation (by both visual and quantitative evaluation) and of the ventilation/perfusion (V/Q) ratios relative to the upper, middle, and lower thirds of the lung fields. RESULTS. FAI yielded 99mTc-labeled droplets with a count median diameter of 1.4 microm and a geometric standard deviation of 2 microm , versus 3 microm and 2, respectively, produced by the commercial inhaler (MMI). The mean time to reach the 1 kc/s count rate was significantly shorter with the FAI than with the MMI both in control subjects (4.7+/-1.7 min versus 8.2+/-2 min, P<0.04) and in airway-obstructed patients (3.4+/-0.8 min versus 8.4+/-2 min, P<0.001). With the MMI, appreciable radioaerosol deposition in the large bronchi prevented reliable quantitative assessment of ventilation, even in the control subjects. With the FAI, radioaerosol deposition in the central large airways was never observed in the controls and was only sporadically or occasionally observed in patients with COPD or asthma, respectively. This feature allowed quantitative ventilation assessment. The FAI-generated radioaerosol particles reached the peripheral respiratory spaces more efficiently than those generated by MMI; on the ventilation scans, the FAI allowed better discrimination than the MMI of the different pathophysiologic conditions.

Conclusion: These findings consistently indicate that the smaller-sized radiolabeled droplets generated by FAI, combined with the better breathing dynamics of the inhaler device, result in better overall performance as compared to the commercial system. This makes scintigraphic images obtained with the new device especially suitable for assessing convective ventilation in COPD patients, a particularly helpful feature for analytically describing the distribution patterns observed in airway-obstructed patients and for evaluating the effects of drugs, mechanical ventilation, and other interventions in such patients.