Use of bladder pressure to correct for the effect of expiratory muscle activity on central venous pressure

To assess whether subtracting the expiratory change in intra-abdominal (bladder) pressure (Delta IAP) from central venous pressure (CVP) provides a reliable estimate of transmural CVP in spontaneously breathing patients with expiratory muscle activity.

Prospective observational study in a medical ICU.

Twenty-four spontaneously breathing patients with central venous and bladder catheters: 18 with no clinical evidence of active expiration (group 1) and 6 with active expiration (group 2).

Patients in group 1 were coached to change their breathing pattern to one of active expiration for several breaths; those in group 2 were asked to sip water through a straw to briefly interrupt active expiration.

During active expiration end-expiratory CVP (uncorrected CVP) and Delta IAP were measured; Delta IAP was subtracted from uncorrected CVP to obtain corrected CVP. End-expiratory CVP during relaxed breathing (best CVP) was assumed to represent the best estimate of transmural CVP. The absolute difference between corrected CVP and best CVP was much less than the difference between uncorrected CVP and best CVP (2.3+/-2.0 vs. 12.5+/-4.7 mmHg).

In patients with active expiration, subtracting Delta IAP from end-expiratory CVP yields a more reliable (and lower) estimate of transmural CVP than does the uncorrected CVP value.