Noninvasive Assessment of Intra-Abdominal Pressure Using Ultrasound-Guided Tonometry: A Proof-of-Concept Study.

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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 IAP. 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), intravesical, intraperitoneal, and end-inspiratory plateau pressures were recorded. For ultrasound-based assessment of IAP (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 predetermined levels of external pressure. At each IAP level (reference: intravesical pressure), two investigators measured the vertical diameter of this chamber. All abdominal flows decreased (by 39%-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 25 mmHg) and between 25 and 40 mmHg IAP (all P ≤ 0.18). Similar results were obtained for end-inspiratory plateau pressures. In our model, values obtained by ultrasound-guided tonometry correlated significantly with IAPs. The method was able to discriminate between normal, moderately, and markedly increased IAP values.