Comparative evaluation of pulse contour analysis, thermodilution and partial CO₂ rebreathing techniques for cardiac output assessment in critically ill patients during different levels of positive end expiratory pressure

Objective

To investigate the precision and accuracy of continuous pulse contour cardiac output (PCCO) and the partial carbon dioxide rebreathing technique (NICO) under different levels of positive end expiratory pressure (PEEP).

Design

A prospective, interventional study in the ICU of a university hospital.

Participants

Ten patients undergoing elective CABG surgery.

Methods

After admission to the ICU each patient was ventilated using one of three steps of PEEP for a time period of 30 min, followed by a 15-min equilibration period. All three PEEP settings were applied in randomly assigned order in every patient. A: PEEP 5 cmH₂O; B: PEEP 10 cmH₂O; C: PEEP 15 cmH₂O. The ventilatory settings were adjusted to achieve a tidal volume of 6–8 ml/kg/BW. Hemodynamic measurements were performed in sequence every 3 min during the subsequent 30-min period. The cardiac output (CO) was simultaneously measured using PCCO and NICO. At the end of each 30-min period a bolus thermodilution-derived CO was obtained from thermodilution curves detected in the femoral artery (TPTDCO). Three intermittent consecutive boli consisting of 10 ml ice-cold saline were randomly injected over the ventilatory cycle.

Results

The comparison between the continuous cardiac output measurement methods NICO vs PCCO showed a bias of 0.4 ± 1.02 l/min (bias ± SD), r = 0.58; between NICO vs TPTDCO 0.0 ± 1.22 l/min; r = 0.47; and between TPTDCO vs PCCO 0.24 ± 0.45 l/min; r = 0.93 at a PEEP level of 5 cmH₂O. Mean bias at a PEEP level of 10 cmH₂O was 0.30 ± 1.17 l/min; r = 0.58; 0.1 ± 1.12 l/min; r = 0.59; 0.22 ± 0.61 l/min; r = 0.89, respectively. With increasing PEEP to a level of 15 cmH₂O the mean bias was NICO vs PCCO 0.5 ± 1.37 l/min; r = 0.45; NICO vs TPTDCO 0.7 ± 1.37 l/min; r = 0.4; and TPTDCO vs PCCO -0.29 ± 0.99 l/min; r = 0.92.

Conclusion

The results of this clinical investigation show agreement between TPTDCO and PCCO to satisfy clinical requirements in a setting of postoperative patients after cardiac surgery independent of the level of applied PEEP. In contrast, the NICO monitor show agreement in clinical range until a PEEP level of 10 cmH₂O. With further increasing levels of PEEP the NICO monitor is of limited use in these patients.