Stress Index : is the airway pressure a good surrogate of the transpulmonary pressure?

Rationale Stress Index (b) is a dimensionless number defining the shape of the airway pressure (Paw)-time (t) curve1. When equal to 1, it indicates that respiratory system compliance remains constant during inflation. Lower and higher values respectively suggest recruitment or overstretching. However, since the distending force of the lung is the transpulmonary pressure (PL), we decided to compare lung Stress Index (computed as a function of PL, bPL) with respiratory system Stress Index (computed as a function of Paw, bPaw). Methods Gas flow, Paw and esophageal pressure (Pes) were continuously recorded for 1 minute in 21 acutely lung-injured patients, at different levels of positive end-expiratory pressure. PL was defined as the difference between corresponding Paw and Pes values. Stress Index was computed from the Paw-t curve and the PL-t curve (KeisTEK, Italy). Only curves with adequate fitting (R2<0.95) were included in the analysis. Simultaneous recordings were compared and then divided into 3 groups: 1) bPL<bPaw; 2) bPL=bPaw; 3) bPL>bPaw. Data were studied using linear regression, Bland-Altman analysis and ANOVA on ranks, as appropriate. Statistically significant difference was defined as p<0.05. Results Ninety-six (out of 105) pairs of Stress Index values were analyzed. bPL and bPaw were significantly correlated despite a low R2 (bPaw=0.59+0.42xbPL, R2=0.43, p<0.0001). Bland-Altman analysis showed that the overall mean (range) Stress Index was 1.085 (0.866-1.449). The overall bias, computed as the difference between bPL and bPaw, was 0.053 (CI95% -0.195–0.297). The percentage error, i.e. the ratio between limits of agreement and mean Stress Index, was 22%. In 63 (66%) cases, bPaw reliably reflected bPL (bPL=bPaw). In the remaining cases, bPaw and bPL provided discordant information, so that, for example, lung overdistension could be detected from the analysis of bPL but not bPaw (bPL>bPaw). We found that these discrepancies were primarily related to differences in respiratory mechanics. In fact, the 3 groups had different chest wall (Ew) and lung (EL) elastances relative to respiratory system (Ers) elastance (EW/ERS and EL/ERS, respectively). We did not find any significant difference in absolute elastances. bPL< bPaw (N=8) bPL=bPaw (N=63) bPL> bPaw (N=25) p ERS 30.8±10.9 27.3±7.8 26.4±9.9 NS EW 7.3±3.6 8.1±3.1 9.5±4.0 NS EL 23.5±9.3 19.2±6.7 16.9±7.3 NS EW/ERS 0.24±0.11 0.30±0.10 0.37±0.13* 0.0033 EL/ERS 0.76±0.11 0.70±0.10 0.63±0.13* 0.0033 Data are expressed as mean±SD . *p<0.05 vs bPL< bPaw Conclusions When computed from airway pressure, the Stress Index provides the same information as those obtained from transpulmonary pressure in 66% of the cases. When respiratory mechanics are altered, however, respiratory system Stress Index does not appear to be an adequate surrogate of lung Stress index, possibly providing misleading information. References 1.Grasso et al. AJRCCM 2007;176:761-767.