Deadspace free ventilatory measurements in newborns during mechanical ventilation

Objective: To improve the accuracy of ventilatory measurements in ventilate d newborns by means of a numerical correction when a deadspace free differe ntial measuring method using two pneumotachographs (PNTs) is applied and to investigate the clinical usefulness of this correction procedure. Design: In vitro study and prospective animal study. Setting: Research laboratory of the Clinic of Neonatology and the Animal Re search Laboratory, Charite Hospital Berlin. Subjects: Ten newborn piglets, weighing 610-1340 g (median, 930 g), age <12 hrs. Interventions: The accuracy of both the deadspace free method and the endot racheal flow measurements (conventional method) was investigated using mech anical lung models. A correction procedure for the deadspace free method wa s developed considering signal delay time and tube compliance between both PNTs. This method was applied to the piglets measured during partial liquid ventilation (PLV). Measurements were done before and after lung lavage and during 30 and 120 mins of PLV (30 mL/kg body weight perfluorocarbon). Measurements and Main Results: In vitro measurements showed volume differen ces between both methods of 8%, 12%, 16%, and 17%, respectively, depending on the distance between the PNTs of 10, 60, 120, and 180 cm. After applying the correction algorithm, the differences decreased to 3%, 0%, -2%, and -8 %, respectively. The piglets were measured with 120-cm tube length between the PNTs. The correction algorithm reduced the measured tidal volume before lavage by 7%, after lavage by 14%, 30-min PLV by 12%, and 120-min PLV by 1 0%, corresponding to the changes in respiratory compliance of 1.2, 0.6, 1.0 , and 1.1 mL/cm H2O. Conclusions: The deadspace free method can be advantageously used for conti nuous measurements in newborns despite much higher technical expense. The c orrecting procedure improved the accuracy of the volume measurement remarka bly, especially for lower respiratory compliance.