Ventilators are capable of assessing certain aspects of lung function such as static and dynamic compliance (Cstat and Cdyn) and airway resistance (Raw). We sought to determine the normal limits of these parameters and a new easily determined parameter minute ventilation (VE)X PaCO2, as a reflection of dead space. We attempted to see if simple corrections would increase the separation between normal and abnormal subjects. Increased dead space by the Bohr method is predictive of death in acute respiratory distress syndrome (ARDS), but is difficult to measure.
Mechanically ventilated patients were evaluated. Cstat and Cdyn were measured as well as Raw. PaO2/FiO2 and VE X PaCO2 were calculated. Patients intubated for non-pulmonary reasons with normal radiographs, no history of lung disease, and a PaO2/FiO2 > 400 mm Hg were used as controls. The coefficient of variation (COV) and 95% confidence interval (CI) were determined based on the controls for the parameters Cstat, Cdyn, Raw, and VE X PaCO2. A series of corrections were then applied to see if these reduced the COV and increased the number of patients outside the 95% CI.
There were 17 controls and 227 patients studied. The groups had similar demographics and ventilator settings, but patients weighed more than controls. Dividing Cstat and Cdyn by the patient’s height reduced COV and increased the number of patients classified as abnormal. Indexing Raw for patient size also improved performance. For VE X PaCO2, height resulted in more ARDS patients being classified as abnormal. The mean, COV, range of normal and percentage of patients classified as abnormal are given as follows:. ParameterMeanCOV(%)Normal Range% Abnormal (n=227)Cstat (cc / cmH2O)48.824.6>29.135.7Cstat/Ht (cc / cmH2O / cm)0.28322.4>0.17936.6Cdyn (cc / cmH2O)29.721.4>19.232.2Cdyn/Ht (cc / cmH2O / cm)0.17219.9>0.11633.5Raw (cmH2O / L / sec)13.028.9<19.115.4Raw X Ht (cmH2O X cm / L / sec)2,20524.8<3,10022.9VE X PaCO2 (L X Torr / min)32044.3<55316.8VE X PaCO2 / Ht (L X Torr /min / cm)1.8339.3<3.0018.6
Indexing commonly performed ventilator parameters for patient height reduces variability and increases separation of normal and abnormal subjects. VE X PaCO2/Ht is abnormal in over half of ARDS patients and warrants further study as a predictor of mortality.
Indexing common ventilator parameters may aid in diagnosis and prognosis in the intensive care unit.
R.L. Proffitt, None.