Use of expiratory change in bladder pressure to assess expiratory muscle activity in patients with large respiratory excursions in central venous pressure

Background: Expiratory muscle activity may cause the endexpiratory central venous pressure (CVP) to greatly overestimate right atrial transmural pressure. Methods: We recorded CVP and expiratory change in intra-abdominal pressure (DIAP) in 39 patients who had a respiratory excursion in CVP from end-expiration to end-inspiration (CVP _ee-CVP _ei) ≥8 mmHg. Uncorrected CVP was measured at end-expiration, and corrected CVP was calculated as uncorrected CVPDIAP. In 13 patients measurements were repeated during relaxed breathing. Results: The CVP _ee-CVP _ei was 15.2 ± 6.3 mmHg (range 8-34 mmHg), and ΔIAP was 7.4 ± 6.0 mmHg (range 0-30 mmHg). Uncorrected CVP was 18.3 ± 6.1 mmHg, and corrected CVP was 10.9 ± 3.9 mmHg. There was a significant positive correlation between CVP _ee-CVP _ei and ΔIAP (r = 0.814). However, some patients with a large CVP _ee-CVP _ei had negligible ΔIAP. In a subset of 13 patients with active expiration who had a relaxed CVP tracing available for comparison, the difference between uncorrected CVP and relaxed CVP was much greater than the difference between corrected CVP and relaxed CVP (7.3 ± 3.0 vs. 1.1 ± 0.7 mmHg, p<0.001). Conclusion: Patients with large respiratory excursions in CVP often have significant expiratory muscle activity that will cause their CVP to overestimate transmural right atrial pressure. The magnitude of expiratory muscle activity can be assessed by measuring ΔIAP. Subtracting ΔIAP from the end-expiratory CVP usually provides a reasonable estimate of the CVP that would be obtained if exhalation were passive.