RESPIRATORY MECHANICS BY LEAST-SQUARES FITTING IN MECHANICALLY VENTILATED PATIENTS - APPLICATIONS DURING PARALYSIS AND DURING PRESSURE SUPPORT VENTILATION

Objective: To evaluate a least squares fitting technique for the purpo se of measuring total respiratory compliance (C-rs) and resistance (R( rs)) in patients submitted to partial ventilatory support, without the need for esophageal pressure measurement. Design: Prospective, random ized study. Setting: A general ICU of a University Hospital. Patients. 11 patients in acute respiratory failure, intubated and assisted by p ressure support ventilation (PSV). Interventions: Patients were ventil ated at 4 different levels of pressure support. At the end of the stud y, they were paralyzed for diagnostic reasons and submitted to volume controlled ventilation (CMV). Measurements and results: A least square s fitting (LSF) method was applied to measure C-rs and R(rs) at differ ent levels of pressure support as well as in CMV. C-rs and R(rs) calcu lated by the LSF method were compared to reference values which were o btained in PSV by measurement of esophageal pressure, and in CMV by th e application of the constant flow end-inspiratory occlusion method. I nspiratory activity was measured by P-0.1. In CMV, C-rs and R(rs) meas ured by the LSF method are close to quasistatic compliance (-1.5 +/- 1 .5 ml/cmH(2)O) and to the mean value of minimum and maximum end-inspir atory resistance (+0.9 +/- 2.5 cmH(2)O/(l/s)). Applied during PSV, the LSF method leads to gross underestimation of R(rs) (-10.4 +/- 2.3 cmH (2)O/(l/s)) and overestimation of C-rs (+35.2 +/- 33 ml/cmH(2)O) whene ver the set pressure support level is low and the activity of the resp iratory muscles is high (P-0.1 was 4.6 +/- 3.1 cmH(2)O). However, sati sfactory estimations of C-rs and R(rs) by the LSF method were obtained at increased pressure support levels, resulting in a mean error of - 0.4 +/- 6 ml/cmH(2)O and -2.8 +/- 1.5 cmH(2)O/(l/s), respectively. Thi s condition was coincident with a P-0.1 of 1.6 +/- 0.7 cmH(2)O. Conclu sion: The LSF method allows non-invasive evaluation of respiratory mec hanics during PSV, provided that a near-relaxation condition is obtain ed by means of an adequately increased pressure support level. The mea surement of P-0.1 may be helpful for titrating the pressure support in order to obtain the condition of near-relaxation.