Automated Insufflation System

[AshLu]

INTRODUCTION

Laparoscopic surgery is preferred over open surgery because only a few 3-5 mm incisions are necessary to

complete the operation compared to the traditional 20-30 mm incision. In order to start the laparoscopic

procedure, an insufflator is used to lift the abdominal wall off the internal viscera with an inert gas, such as CO2

,

thus creating pneumoperitoneum. Once the abdominal cavity is expanded, laparoscopic tools are inserted through

ports, allowing the surgeon to work within the cavity. If the abdominal pressure created by pneumoperitoneum

exceeds 20mmHg, the patient is at risk of suffocation and venous embolism [1]. Suffocation occurs because the

increased pressure prevents the expansion of the diaphragm and lungs, thus preventing the patient from inhaling.

Venous embolism occurs when CO

2 diffuses into unwanted cavities and vessels, creating air bubbles. On the other

hand, if the abdominal pressure is too low, the minimal work space can cause the surgeon to accidently operate on

unintended viscera.

Our solution is to create an automated system so that nurses and surgeons do not have worry about constantly

monitoring abdominal pressure. This allows them to focus on the task at hand and thus improves patient safety. In

addition, our solution can potentially eliminate unneeded staff from the operating room, thus saving the hospital

over head costs associated with each surgery.

EXPERIMENTAL SETUP

A schematic of our system is shown in Figure 1 and consists of the following components:

• CO2 tank: The CO2 tank used to inflate the test lung was an E size cylinder, containing 6.5 cubic feet ofUSP Grade 99.8% compressed CO2 with a maximum output pressure of 650 psi.
• Insufflator: The insufflator regulated the flow of high pressure CO2 from the tank, stepping it down to an appropriate range for our simulations (EDER E1-03500-A2).
• SCR: The SCR was a 110 V solid state control relay (Guardian MSSR-2B-10).
• Inflate Solenoid/Deflate Solenoid: A V11 series three-way solenoid air valve configured to be in the closed state until energized with a 110 volt electrical potential was used. When the inflate solenoidopened, it allowed gas to fill the test lung. The deflate solenoid opened when the pressure in the test lung was too high and caused escape of CO2 (Johnson Controls Air Valve V11HAA-100).
• SR560 Preamplifier: A Stanford Research systems SR560 low-noise preamplifier was used to conditionthe signal to minimize the noise that was seen on the LabVIEW output panel from the Transpac pressuresensor.
• Test lung: The test lung was used to simulate the abdominal cavity.
• Transpac Pressure Sensor: A Transpac II pressure sensor (part of the IV monitoring kit by Hospira)with an operating pressure range of -50 to 300 mmHg was used to monitor the pressure in the test lung.Figure 2 shows the logic flow diagram used to write the LabVIEW program. The front panel is shown in Figures3, 4 and 5 while the block diagram is shown in Figures 6, 7, 8 and 9. The program was designed to inflate the teslung when the pressure falls below 9mmHg and deflate the test lung when the pressure rises above 20 mmHg to maintain a stable pressure of 15mmHg. This stabilization occurs when the automatic control option is selected. A manual control option was also implemented to allow us to inflate or deflate the test lung regardless of the pressure.

RESULTS

Prior to completing validation activities, we developed a calibration curve for the pressure sensor using a pressure

cuff to determine the correlation between the output voltage and pressure in mmHg. This curve is shown in Figure

10. To validate the functionality of the system, we simulated both low and high pressure conditions using the

manual interface on the LabVIEW program. We then switched the system to automatic mode and observed the

response. Table 1 shows that the actual results match the expected results for each simulation meaning our project

reached the proposed goal. In addition, the system is reliable because 5 validation trials resulted in low standard

deviations, 0.2 to 0.3 mmHg, for stabilized pressure.

We set acceptance criteria to ensure that the pressure we detected during validation was accurate. We determined

that achieving accuracy to within 10% error of a standalone pressure calibration unit was sufficient to accept the

functionality of the system. Table 2 displays the average of 7 trials of pressure readings from the sensor and the

calibration unit. The error for each validation was less than 5% meaning we satisfied the acceptance criteria. The

data for all 7 trials of validation are shown in Table 3.

DISCUSSION AND CONCLUSIONS

The goal of our project was to design a system that can take accurate measurements of intra-abdominal pressure

during laparoscopic surgery and use these measurements to control insufflation. While we were successfully able

to demonstrate proof of concept, our system needs further improvement before it can be successfully integrated

with laparoscopic surgery.

The main source of error in our system came from the pressure sensor. While we were able to remove most of the

noise from the signal by using a low pass filter, the pressure still varied by about 1mm Hg introducing error in the

system. Any movement in the sensor also introduced measurement errors. This was minimized by ensuring that

the sensor remained stationary during inflation/deflation through proper mounting. Further, the signal obtained

through pressure sensor was in the range of millivolts and had to be amplified by a factor near 40,000 for

conversion to mmHg. This amplification increased the DC offset that had to be then subtracted from the signal in

LabVIEW. Another source of error was the slight leakage of gas from our system which introduced some inherent

variation in our measurements. Our intended pressure range also had to be slightly modified due to system

constraints. For example, pressure less than 3 mmHg was difficult to detect and pressure greater than 27 mmHg

automatically shut off the pressure regulator (insufflator unit).

To improve the system, a more precise measurement system needs to be implemented. A more accurate pressure

sensor could be utilized to give a steadier signal. Further, the solenoid valve was a bit noisy and could be replaced

by a solenoid that operates more smoothly and has smaller excitation /de-excitation times.

APPLICATION

For a beta prototype, a heated and 95% humidified CO

2 system could be introduced. The goal would be to

accurately monitor and stabilize the gas to around 37±0.5ºC and 95±0.5% humidity. This tight control could be

implemented using a similar algorithm as presented in this paper. The improved system would decrease

postoperative pain and risk for the patient [1].