Pertechnegas, at the point of production from the machine, is a micro-aerosol mixture of Technetium oxides. Immediately on contact with the water vapour of the lung, these oxides hydrolise and revert to the pertechnetate ion. The ions cross the alveolar-capillary boundary into the pulmonary venous blood stream exactly as though they had been inhaled directly as sodium pertechnetate in saline, reflecting a subsequent identical bio-distribution (Monaghan P, Murray IPC, Mackey DWJ et al. An improved radionuclide Technique for the Detection of Altered Pulmonary Permeability. J Nucl Med 32:1945-1949, 1991).

The gas phase chemistry of the mechanism for this process is not known or indeed knowable at the level of nanograms of active material, but given that trace amounts of carbon dioxide (180ppm) and carbon monoxide (60 ppm) are detected in Pertechnegas, it is assumed that reduction of the Technetium to the metal ( 0 ) state does not occur, or that if it does, the carbon chains (C=C=C) which form the graphite sheath in Technegas are oxidised first, leaving the metal crystal bare to oxidise almost instantly which it would do in such a finely divided state.

The half-time clearance of Pertechnegas in the normal lung is about 7-10 minutes, and in the immune-compromised lung it is as low as 2 minutes. Smokers free of clinical disease also show a reduced clearance half-time (to as low as 3.5min.), and so this figure is usually set as the boundary for a diagnosis of membrane disease.

The physiological behaviour of Pertechnegas once inhaled is identical to Pertechnetate, in that it is a measure of the 'leakiness' of the alveolar-capillary membrane. There is every reason to believe that apart from the clearance half-times, pathology currently being demonstrated with Tc-labelled DTPA, (see for example: Susskind H. Technetium-99m-labelled-DTPA aerosol to measure alveolar-capillary membrane permeability. J Nucl Med1994;35:(2), 207-209 [editorial] ) will be equally demonstrated with Pertechnegas. To date, Fibrosing Alveolitis and radiation pneumonitis (bibliography references #'s 56,57) have been demonstrated very effectively with Pertechnegas.

Despite the short clearance half-times, the fact that the patient can obtain a clinically useful activity in the lung in one or two breaths, means that (a) the start point for the clearance curve is sharper and (b) patients with severely compromised lungs can perform the manoeuvre - precisely those in whom the accurate diagnosis is more critical.