CVP does not reflect changes in preload when optimal PEEP is determined

Objective

High intrathoracic pressures during lung recruitment and defining optimal PEEP may cause haemodynamic instability due decreased venous return [1]. The aim of this study was to evaluate volumetric (ITBV) and pressure (CVP) indicators of preload during the manoeuvre.

Materials and methods

Eighteen patients suffering from ARDS were recruited. All patients were ventilated in pressure control mode (FiO₂ = 1.0, respiratory rate = 20, I:E = 1:1). Following basic haemodynamic measurements and blood gas analysis (Tep) alveolar recruitment was done: PEEP was set at 26 cmH₂O, then 40 cmH₂O of pressure amplitude was applied for 40 s (T26). Optimal PEEP was then determined as follows: VT was reduced to 4 ml/kg, then the PEEP was reduced from 26 cmH₂O by 2 cmH₂O every 4 min and the optimal PEEP was defined as 2 cmH₂O above the level of PEEP, where the PaO₂ suddenly dropped by >10%. After setting the PEEP at the optimal level, the '40/40' manoeuvre was applied again and the tidal volume was set as 6 ml/kg, end point (Tep). Haemodynamic parameters were determined by arterial thermodilution (PiCCO) during lung recruitment (T26), then every 8 min until the end point was reached (Tep). Data are presented as the mean ± standard deviation. For statistical analysis a paired t test and Pearson's correlation was performed.

Results

The PaO₂ improved significantly from T0 to Tep: 203 ± 108 vs 322 ± 101 mmHg, P < 0.001. While CI, SVI and ITBVI increased as PEEP was reduced from 26 cmH₂O (T0) to an average of 15 ± 4 cmH₂O (Tep), CVP decreased (Table 1). There was a significant positive correlation between CI and ITBVI (r = 0.699, P < 0.01), and a significant negative correlation between CI and CVP (r = -0.294, P < 0.05).

Table 1

Discussion

As was expected, as PEEP dropped the CI improved due to increased venous return. This increase in preload was reflected by ITBV but not by CVP, which is in accord with findings of a recent animal experiment [2]. These results suggest that CVP cannot be relied on as a measure of preload during lung recruitment or when defining optimal PEEP in ARDS.