The respiratory pressure-abdominal volume curve in a porcine model

Increasing intra-abdominal volume (IAV) can lead to intra-abdominal hypertension (IAH) or abdominal compartment syndrome. Both are associated with raised morbidity and mortality. IAH can increase airway pressures and impair ventilation. The relationship between increasing IAV and airway pressures is not known. We therefore assessed the effect of increasing IAV on airway and intra-abdominal pressures (IAP).

Seven pigs (41.4 +/-8.5 kg) received standardized anesthesia and mechanical ventilation. A latex balloon inserted in the peritoneal cavity was inflated in 1-L increments until IAP exceeded 40 cmH₂O. Peak airway pressure (pP_AW), respiratory compliance, and IAP (bladder pressure) were measured. Abdominal compliance was calculated. Different equations were tested that best described the measured pressure-volume curves.

An exponential equation best described the measured pressure-volume curves. Raising IAV increased pP_AW and IAP in an exponential manner. Increases in IAP were associated with parallel increases in pP_AW with an approximate 40% transmission of IAP to pP_AW. The higher the IAP, the greater IAV effected pP_AW and IAP.

The exponential nature of the effect of IAV on pP_AW and IAP implies that, in the presence of high grades of IAH, small reductions in IAV can lead to significant reductions in airway and abdominal pressures. Conversely, in the presence of normal IAP levels, large increases in IAV may not affect airway and abdominal pressures.