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Towards a continuous non-invasive assessment of intra-abdominal pressure based on bioimpedance and microwave reflectometry: A pilot run on a porcine model

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Abstract

In this work we describe two novel indirect intra-abdominal pressure (IAP) assessment techniques, based on electrical bioimpedance and microwave reflectometry measurements. A pilot run was performed on a female Sus scrofa domesticus (domestic pig). IAP was induced by inflation of the abdominal cavity using a trocar placed near the umbilicus over the linea alba at a depth of 1 cm. The abdominal cavity was inflated to different IAP values. The whole run was done within one hour since the subject was sacrificed. An exponential trend linking between the bioimpedance values at 99.8 kHz and the IAP was found, with drastically reduced sensitivity for IAP above 7 mmHg. Also, a linear relationship between IAP and the microwave reflectometry measurements at 4.25 GHz was established. Although further research is needed to optimize the sensitivity of the bioimpedance system, we draw the conclusion that the use of electromagnetic spectrometry methods might be developed into novel techniques for performing a continuous non-invasive assessment of changes in IAP.

Introduction

More than half of the patients in intensive-care units suffer from intra-abdominal hypertension (IAH) during the hospitalization [1]. IAH is directly associated with increased morbidity and mortality [1], [2], [3], [4]. Different degrees of Intra-Abdominal Pressure (IAP) have been proposed [2], [3], [4]: IAP of 10 mmHg already affects blood flow leading to low organ perfusion. In some cases, abdominal decompression, either surgical or non-surgical [5], [6], [7], [8], is indicated. Therefore, continuous monitoring of IAP in every critical patient has become a standard [3], [4].

Invasive indirect measurements of IAP in ICUs are mostly performed by means of transduction using balloon catheters [4], [5], [6], [9]. The most widely used is the Kron's intravesical catheter (1983) published by Iberti [10] and modified by Cheatham in 1998 [11]. In 2013, the World Society of the Abdominal Compartment Syndrome published a list of risk factors for which IAP monitoring is indicated, and recommended the use of Kron's intravesical catheter for trans-bladder measurement of IAP, as the standard [12].

As for non-invasive monitoring of IAP, Intra-Abdominal Volume (IAV) and Abdominal Wall Compliance (CAB) have been proposed as a correlative parameter: Relative changes in the abdominal perimeter, correlate well with changes in IAP in patients with healthy BMI [13]. In 2008, van Ramshorst et al. [9] showed that the abdominal wall tension also correlates with IAP. Later in 2011, van Ramshorst et al. [14] published a prototype which allowed intermittent non-invasive assessment of the abdominal wall tension.

In light of [9], [13], [14], we have proposed [15], that changes in the IAP ultimately alters the abdominal wall thickness (AWTh) and, therefore, can be continuously and non-invasively monitored by measuring either the overall surface electrical bioimpedance or the microwave reflection coefficient at the abdominal surface. In this work, we present preliminary results of a pilot run on a porcine model.

Section snippets

Sus scrofa domesticus

The experiment was performed on a cadaver of a female Sus scrofa domesticus (domestic pig) weighing 49.9 kg. Before the experiment the porcine received 3.5 l of NaCl solution 0.18 M, and was put in supine position as shown in Fig. 1. Caudal and next to the umbilicus, over the linea alba, a trocar was placed, 1 cm deep, reaching the intraperitoneal abdominal cavity. Through the trocar the abdominal cavity was inflated to different pressures with a precision of 0.1 mmHg. Fig. 2 shows the location

Bioimpedance

The absolute values of the complex impedance measured at 99.8 kHz (the highest frequency measurable with the system available to us – lower frequencies presented lower sensitivity [15]), for different intra-abdominal pressures are presented in Fig. 4. Table 1 presents the complex impedance values of each measurement.

Together with the measurements and their accuracy range, an exponential fitting function is shown in Fig. 4. The exponential behavior of the changes in the bioimpedance is expected

Conclusions

A pilot study on a porcine model of the feasibility of a continuous non-invasive assessment of the intra-abdominal pressure based on bioimpedance and microwave reflectometry was presented. The study's results are in accordance to theoretical considerations and show a solid correlation between IAP and bioimpedance measurements, for pressures up to about 7 mmHg, and also between IAP and S11 in the whole range of measured IAP.

Although further research is needed to optimize the sensitivity of the

Ethical statement

The experiment and the preparation were done according to the ethical considerations of The Center of Innovative Surgery of Hadassah Medical Center in Jerusalem, Israel, and local regulations (the strictest among them).

Conflict of interests

The authors declare no conflict of interests.

Acknowledgment

The authors warmly thank David Kushnir, Chief Operation Officer at The Center of Innovative Surgery of Hadassah Medical Center in Jerusalem, Israel.

Support for this research was granted by the Lev Academic Center: Internal Grant 5777.

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