Estimation of the transpulmonary pressure from the central venous pressure in mechanically ventilated patients

Purpose: 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. Methods: : Twenty patients with acute respiratory failure (defined as PO 2 /FiO 2 ratio below 200 mmHg) treated with invasive mechanical ventilation and monitored with 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). Results: : 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 (mean bias -0.16, 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 (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). Conclusions: : In mechanically ventilated patients with acute respiratory failure, Δf-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 .