Effects of PEEP on intrathoracic blood volumes and cardiac function in ARDS patients assessed by a double indicator dilution technique

Introduction

The aim of the study was to evaluate the effects of PEEP on intrathoracic blood volumes and cardiac function measured by the COLD system in ARDS patients.

Methods

We studied eight sedated and paralyzed ARDS patients (age 58 ± 13.8 years, PaO₂/FiO₂ 146 ± 84 mmHg), ventilated in CPPV. All patients had a Swan–Ganz catheter and a 4 F thermistore-tipped, fiberoptic catheter inserted through a femoral artery, both connected to the COLD system. All patients randomly received, for at least 30 min, three levels of PEEP (5, 10 and 15 cmH₂O). We measured: cardiac (CI) and stroke volume (SV_i) indexes; heart rate (HR), central venous pressure (CVP); wedge pressure (WP); intrathoracic (ITBVi), global-end diastolic (GEDVi), right heart end-diastolic (RHEDVi), left heart end-diastolic (LHEDVi), and pulmonary (PBVi) blood volume, all indexed for body surface area. PEEP effects were assessed by one-way analysis of variance for repeated measurements and linear trend analysis. Multiple regression analysis by a stepwise procedure was used to find major determinants of stroke volume index (SVI) and cardiac index (CI) changes induced by PEEP.

Results

Increased PEEP levels resulted in a significant reduction of CI, RHEDVi, PBVi and an increase in CVP. GEDVi, ITBVi, LHEDVi, HR and WP did not change significantly with PEEP. HR changes correlated with pH and PaCO₂ changes. SVi changes were explained by changes of RHEDVi and LHEDVi (Δ SVi = 0.0365 × Δ LHEDVi + 0.122 × Δ RHEDVi, r² = 0.823, P < 0.01). CI changes were positively correlated with GEDVi, ITBVi, RHEDVi, and PaCO₂ changes, and were negatively correlated with pH changes. CI changes were explainable by changes of RHEDVi and PaCO₂ (Δ CI = 0.007 × Δ RHEDVi + 0.113 × Δ PaCO₂, r² = 0.78, P < 0.001).

Conclusions

In ARDS patients, PEEP affected significantly the RHEDVi and PBVi. RHEDVi and LHEDVi changes accounted for 82% of SVi changes induced by PEEP. Changes in PaCO₂ and RHEDVi were the main determinant of CI changes.