Abstract
Transpulmonary pressure (P(L)) calculation requires esophageal pressure (P(ES)) as a surrogate of pleural pressure (Ppl), but its calibration is a cumbersome technique. Central venous pressure (CVP) swings may reflect tidal variations in Ppl and could be used instead of P(ES), but the interpretation of CVP waveforms could be difficult due to superposition of heartbeat-induced pressure changes. Thus, we developed a digital filter able to remove the cardiac noise to obtain a filtered CVP (f-CVP). The aim of the study was to evaluate the accuracy of CVP and filtered CVP swings (ΔCVP and Δf-CVP, respectively) in estimating esophageal respiratory swings (ΔP(ES)) and compare P(L) calculated with CVP, f-CVP and P(ES;) then we tested the diagnostic accuracy of the f-CVP method to identify unsafe high P(L) levels, defined as P(L)>10 cmH(2)O. Twenty patients with acute respiratory failure (defined as PaO(2)/FiO(2) ratio below 200 mmHg) treated with invasive mechanical ventilation and monitored with an esophageal balloon and central venous catheter were enrolled prospectively. For each patient a recording session at baseline was performed, repeated if a modification in ventilatory settings occurred. P(ES), CVP and airway pressure during an end-inspiratory and -expiratory pause were simultaneously recorded; CVP, f-CVP and P(ES) waveforms were analyzed off-line and used to calculate transpulmonary pressure (P(L)CVP, P(L)f-CVP, P(L)P(ES), respectively). Δf-CVP correlated better than ΔCVP with ΔP(ES) (r = 0.8, p = 0.001 vs. r = 0.08, p = 0.73), with a lower bias in Bland Altman analysis in favor of P(L)f-CVP (mean bias - 0.16, Limits of Agreement (LoA) -1.31, 0.98 cmH(2)O vs. mean bias - 0.79, LoA - 3.14, 1.55 cmH(2)O). Both P(L)f-CVP and P(L)CVP correlated well with P(L)P(ES) (r = 0.98, p < 0.001 vs. r = 0.94, p < 0.001), again with a lower bias in Bland Altman analysis in favor of P(L)f-CVP (0.15, LoA - 0.95, 1.26 cmH(2)O vs. 0.80, LoA - 1.51, 3.12, cmH(2)O). P(L)f-CVP discriminated high P(L) value with an area under the receiver operating characteristic curve 0.99 (standard deviation, SD, 0.02) (AUC difference = 0.01 [-0.024; 0.05], p = 0.48). In mechanically ventilated patients with acute respiratory failure, the digital filtered CVP estimated ΔP(ES) and P(L) obtained from digital filtered CVP represented a reliable value of standard P(L) measured with the esophageal method and could identify patients with non-protective ventilation settings.