Work of breathing-tidal volume relationship: Analysis on an in vitro modeland clinical implications

Objective. Work of breathing (WoB) is currently employed to assess the afte rload on the respiratory muscles and to estimate the energy expenditure for breathing. Since WoB depends on the ventilated tidal volume (TV), WoB*L-1, the indicized form of WoB has been employed as a measure of WoB which is i ndependent of TV. Actually, the independence of WoB*L-1 from the ventilated TV has never been demonstrated. The aim of this study was to verify the pr edicted TV-independence of WoB*L-1 on an in vitro model. Methods. Our exper imental model was constituted as follows: two endotracheal tubes, with inte rnal diameter measuring respectively 6.5 and 8.5 mm, were alternatively con nected with two rubber balloons whose compliance was respectively 0.02 and 0.06 L/hPa; the system was mechanically ventilated at ten different tidal v olumes, ranging from 0.3-1 l. Flow rate was kept constant (35 l/m) during t he whole experiment. Results. Both elastic components of the model showed a static volume-pressure relationship which was linear in the experimental r ange of TV. In all combinations of resistance and compliance WoB increased quadratically whereas WoB*L-1 increased linearly with the growing TV (p < 0 .001). Conclusions. These results demonstrate the TV-dependence of WoB*L(-) 1 and suggest that WoB*L-1, if TV changes, cannot be considered as an index of respiratoy muscle afterload and should not be used as a guide for weani ng patients from the mechanical ventilation. Finally, we introduced a new p arameter (WoB(1L)) which seems to be a more TV-independent measure of respi ratory work.