From a sales brochure kindly provided by Emanuele Isnardi. The entire sales brochure can be downloaded here.
“The S basic unit may be extended to a SM assembly by adding a slide-in unite for the measurement of oesphageal pressure, this making tests of respiratory mechanics possible. As the zero-flow points (i.e. the points of inversion) can be marked automatically, it is possible to determine the elastic axis in the diagrams of mouth volume in relation to oesaphageal pressure even under uncertain conditions (as they often occur in patients suffering from emphysema). This makes evaluation considerably easier.”