Non-invasive indirect monitoring of intra-abdominal pressure using microwave reflectometry: system design and proof-of-concept clinical trial.

Monitoring intra-abdominal pressure (IAP) has become a standard in intensive care units. Correlation between the abdominal wall's thickness (AWTh) and IAP has been reported previously. The abdominal wall can be modeled as a compound of parallel dielectric slabs; changes in their width have a direct effect on the reflection coefficient of the abdominal wall at microwave frequencies. This work describes the design of a reflectometry system and its proof-of-concept trial on five patients during laparoscopic surgery. The system complies with IEEE Std. C95.1-2005 concerning exposure of humans to microwave electromagnetic fields in controlled environments. The results putatively show an inverse correlation between IAP and the reflection coefficient, and a strong dependence on the body mass index. A better understanding of the dynamics in the changes of the AWTh (during intra-abdominal hypertension) will allow further development of a microwave-based technique for the continuous non-invasive indirect monitoring of IAP in critical patients.

Optimized electrical bioimpedance measurements of abdominal wall on a porcine model for the continuous non-invasive assessment of intra-abdominal pressure.

This work describes the optimization of electrical bioimpedance measurements for indirect intra-abdominal pressure (IAP) assessment. The experimental run was performed on a female Sus scrofa domesticus (domestic pig). Different values of IAP were induced by inflation of the abdominal cavity, using a trocar placed near the umbilicus over the linea alba. The whole experiment was run within 1 h of the subject being sacrificed. The abdominal wall thickness was measured at an IAP of 5 mmHg. An exponential trend linking between the bioimpedance values at 99.8 kHz and the IAP was found. Non-optimized electrode placement presented a strongly reduced sensitivity to IAP changes above 7 mmHg. Upon optimization and placing the electrodes with a separation of about 3.6 times the measured abdominal wall thickness, the sensitivity for high IAP drastically increased, allowing continuous non-invasive monitoring of IAP, confirming the optimization method proposed in this work.