Non-Invasive Assessment of Intra-Abdominal Pressure Using Ultrasound Guided Tonometry – a Proof-of-Concept Study

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Abstract

Background:

Intra-abdominal hypertension jeopardizes abdominal organ perfusion and venous return. Contemporary recognition of elevated intra-abdominal pressure (IAP) plays a crucial role in reducing mortality and morbidity. We evaluated ultrasound guided tonometry in this context hypothesizing that the vertical chamber diameter of this device inversely correlates with intra-abdominal pressure.

Methods:

IAP was increased in six 5 mmHg steps to 40 mmHg by instillation of normal saline into the peritoneal cavity of eight anesthetized pigs. Liver and renal blood flows (ultrasound transit time), intra-vesical, intra-peritoneal and end-inspiratory plateau pressures were recorded. For ultrasound-based assessment of intra-abdominal pressure (ultrasound guided tonometry), a pressure transducing, compressible chamber was fixed at the tip of a linear ultrasound probe, and the system was applied on the abdominal wall using different pre-determined levels of external pressure. At each IAP level (reference: intra-vesical pressure), two investigators measured the vertical diameter of this chamber.

Results:

All abdominal flows decreased (by 39% to 58%), and end-inspiratory plateau pressure increased from 15 mbar (14–17 mbar) to 38 mbar (33–42 mbar) (median, range) with increasing IAP (all p < 0.01). Vertical chamber diameter decreased from 14.9 (14.6–15.2) mm to12.8 (12.4–13.4) mm with increasing IAP. Coefficients of variations between and within observers regarding change of the vertical tonometry chamber diameter were small (all < 4%), and the results were independent of the externally applied pressure level on the ultrasound probe. Correlation of IAP and vertical pressure chamber distance was highly significant (r: -1, p: 0.0004). Ultrasound guided tonometry could discriminate between normal (baseline) pressure and 15 mmHg, between 15 and 25mmHg) and between 25 and 40 mmHg IAP (all p≤0.18). Similar results were obtained for end-inspiratory plateau pressures.

Conclusions:

In our model, values obtained by ultrasound guided tonometry correlated significantly with intra-abdominal pressures. The method was able to discriminate between normal, moderately and markedly increased IAP values.

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