Noninvasive evaluation of instantaneous total mechanical activity of the respiratory muscles during pressure support ventilation

OBJECTIVE: The measurement of esophageal pressure (Pes) is the conventional method for the evaluation of the forces applied to the respiratory system by the respiratory muscles. As an alternative to Pes measurement, we propose the calculation of the instantaneous net pressure applied by the respiratory muscles [Pmusc(t)]. DESIGN: Prospective, randomized study. SETTING: A general ICU of a university hospital. PATIENTS: Eight intubated patients submitted to pressure support ventilation for acute respiratory failure. INTERVENTIONS: Four different levels of pressure support were used to unload progressively the respiratory muscles. Pmusc(t) was calculated at all levels of pressure support and compared with Pes corrected for chest wall load as a reference. Pmusc(t) was further used to calculate inspiratory work of breathing, which in turn was compared with data obtained with the conventional method. MEASUREMENTS AND RESULTS: Airway pressure, airflow, and Pes were measured. Both for amplitude and for timing, Pmusc(t) showed good agreement with reference measurements. Work of breathing as calculated from Pmusc(t) agreed well with the measurement obtained with the conventional method (mean difference, 0.057 +/- 0.157 J). CONCLUSIONS: Noninvasive evaluation of Pmusc(t) allows extended monitoring of mechanical ventilation, which is particularly interesting for pressure preset ventilation modes.