Is pulmonary resistance constant, within the range of tidal volume ventilation, in patients with ARDS?

Abstract

When managing patients with acute respiratory distress syndrome (ARDS), respiratory system compliance is usually considered first and changes in resistance, although recognized, are neglected. Resistance can change considerably between minimum and maximum lung volume, but is generally assumed to be constant in the tidal volume range (V(T)). We measured resistance during tidal ventilation in 16 patients with ARDS or acute lung injury by the slice method and multiple linear regression analysis. Resistance was constant within V(T) in only six of 16 patients. In the remaining patients, resistance decreased, increased or showed complex changes. We conclude that resistance within V(T) varies considerably from patient to patient and that constant resistance within V(T) is not always likely.

Fig 1

The principle of the slice method involving two steps is shown using an example of a pressure–volume loop. Step 1: the outer loop is based on measured airway pressure (P_aw) and volume. Continuous calculation of the flow-dependent pressure drop across the endotracheal tube yields the inner loop, representing tracheal pressure (P_trach). Step 2: the loop is divided into six slices of equal volume excluding the uppermost and lowermost 5% of the loop. In each slice, one value of resistance and compliance is calculated based on the simple linear RC model. The resulting R_slice and C_slice data drawn over volume give the course of these parameters within tidal volume (compare with Figure 2). Two additional steps may be used to verify the quality of the fit. Step 3: the P_trach–volume loop (thin line) is reconstructed using calculated respiratory parameters and measured flow and volume. The match of the measured and reconstructed P_trach–volume loop give an estimate of the appropriateness of the fit and the model in describing the raw data. Step 4: The absolute difference (mean and sd) between measured and reconstructed P_trach–volume-loop (ΔP) is given for each slice.

Fig 2

R_slice data within V_T (mean and sd, mbar litres⁻¹ s; markers for sd often not visible). Note the different scaling on the y-axis. The numbers (R1–6) refer to slices; volume increases from left to right. R_MLR of the same breaths are plotted as a straight horizontal line, mean values (with sd) are given. The letters mark the patients studied. C_MLR (ml mbar⁻¹) is given above each plot. A symbol showing the course of C_slice over volume as previously described² is also shown.

Fig 3

The mean (with sd) flow rate (ml s⁻¹) is shown for each slice in one breath of each patient. The numbers (F1–6) refer to slices; volume increases from left to right. The letters mark the patients studied. Note the different scaling on the y-axis in patient I. The distribution of flow rate over volume (slices) should not be confused with a plot of flow rate over time, which decreases during pressure-controlled ventilation.