The “Air-Bag System”: A Novel Abdominal Compression Device Collaborated with RPM System

Abstract

We developed a novel abdominal compression device which monitors the patient's body volume change as an external respiration signal collaborated with the Real-Time Position Management System (RPM, Varian Medical System, Palo Alto, CA). The “Air-Bag System” consists of a non-elastic air bag connected with an elastic air bag. A non-elastic air bag is placed between the patient's abdomen and the stereotactic fixation frame. Air can be injected into this system to give pressure on the patient's abdomen. A current meter located between two bags measures the air flow back and forth between these bags. This air flow data are processed by personal computer and translated into swinging decoy markers displayed on the computer screen so that an infrared camera of RPM system can capture the marker instead of an infrared reflective marker. The impedance model was adopted for describing the behavior of the air flow in this system. The phase gap between the respiratory motion and the monitoring signal is defined as θ = arctan (X/R). Where R is the resistive part, and X is the reactive part of the impedance. The resistive part represents the viscous forces at the air/wall interface. The reactive part, which is a respiratory frequency dependent parameter, represents the ability of air to store the kinetic energy as potential energy since air is a compressible medium. An elastic air bag also stores potential energy by inflation and deflation. Both values of R and X were obtained experimentally and the theoretical equation was evaluated with the clinical data of 25 patients. 4D-CT data of 93 patients (49 lung cancer patients and 44 liver cancer patients) were acquired by the RPM system with the “Air-Bag System” in order to observe the motion of the diaphragm while free breathing. Both values of R and X were obtained by least squares method with strong correlations (R; r2 = 0.965, X; r2 = 0.973). The theoretical equation was given by θ = arctan(X/R) = arctan((25.178f-1/0.604f)/23.780). Where f is the respiratory frequency. Comparing the 25 clinical data to the theoretical equation, the phase gaps were resulted in good correlations on each respiratory frequency. Diaphragm motion was reduced to 7.1 ± 3.8mm (1.2 - 23.5mm) in the superior to inferior direction with 4D-CT data of 93 patients. The novel abdominal compression device which presses on the patient's abdomen with the air bag reduces the movement of diaphragm without loading additional stress on the patient. This device can also be used for respiratory monitoring which monitors the patient's body volume change as an external respiration signal. The phase gap between the respiratory motion and the monitoring signal was negligibly small under average respiratory frequency.