Quantitative relationship between increased intra-abdominal pressure and intrathoracic pressures and volumetric filling parameters in the pig

Background

The central venous pressure (CVP) and the pulmonary artery occlusion pressure (PAOP) are dependent on intrathoracic pressure (ITP). A few studies reported 9–73% transmission of intra-abdominal pressure (IAP) to the thoracic compartment. In a porcine model of intra-abdominal hypertension (IAH), we studied this transmission and the impact on pressure and volume parameters of the cardiac preload.

Methods

In nine anesthetized pigs (46 ± 3 kg), ventilated with a constant tidal volume and 5 cmH₂O PEEP, the IAP was increased stepwise from baseline to 30 mmHg. The ITP was measured from a balloon catheter (air) in the distal oesophagus and the abdomino-thoracic pressure transmission (APT = [ITP – PEEP × 0.735] / IAP) was calculated. A pulmonary artery catheter was used to assess cardiac filling pressures and the transmural CVP (CVPt = CVP – ITP) and PAOPt were calculated. All pressures were measured end-expiratory. The global end diastolic volume index (GEDVI) and extravascular lung water index (EVLWI) were measured by transpulmonary thermodilution (PICCO; Pulsion).

Results

With increasing IAP, the ITP increased significantly (Table 1). The APT decreased significantly from 48% to 27% (mean 35%) with unchanged EVLWI. While the CVP and PAOP parallelled changes in IAP, the CVPt, PAOPt and GEDVI remained constant.

Table 1

Conclusion

About 35% of the IAP is transmitted to the thoracic compartment, but the APT decreases with increasing IAP, due to decreasing thoracic compliance. In the setting of IAH, the CVPt and PAOPt or volumetric parameters are superior in the assessment of cardiac preload.