can be equipped with additional diagnostic functions it is for example conceivable to compare the determined ratio between alveolar air and dead space volume with a normal value and to generate a ratio as result this ratio can be displayed directly on the device e g with digits by means of an analogue display and or by means of a quasi analogue display another variant is three in each case single stage display elements such as e g light emitting diodes which are either illuminated or dark a display sequence that is very simple and can therefore also be interpreted by the patient himself is a green light emitting diode if the ratio is in the normal range a yellow light emitting diode if the ratio deviates somewhat from the normal value and a red light emitting diode if the ratio deviates from the normal range to a critical extent all lung diagnosis devices operating with an ultrasound measurement zone according to the method of the invention have the advantage that they can dispense with a precise determination of the absolute value of respiratory air density and instead only need to register a ratio of the co 2 densities for which purpose a very high reproducible accuracy can be achieved compared to the.