Abstract

1. Abstract The metabolic rates of critically ill patients can vary drastically from typical metabolic rates. This can cause problems for health care providers when determining the correct amount to feed the patient. Matt O’Brien, RRT, performs metabolic tests on these critically ill patients to determine their metabolic rate and food intake needs. This is done with indirect calorimetry, measuring the concentration of oxygen of the inspired and exhaled air of the patient. In order to stabilize the inspired oxygen level produced by the ventilator, our final design is an air mixing chamber placed within the series of air flow from the ventilator to the patient.

2. Problem Statement To develop a mixing chamber to help stabilize oxygen percentage delivered from mechanical ventilators to critically ill patients; this chamber would allow for increased accuracy of metabolic measurements.

3. Research Greater metabolic rate of critically ill patients and malnutrition can lead to longer hospital stays and worsened conditions. Indirect calorimetry measures the dimensionless respiratory quotient, RQ, which is the CO2 produced divided by the O2 consumed (1) RQ depends on the amount and type of food consumed, therefore indicates if patient is receiving too few or too many calories (2) Two factors can cause inconsistent RQ values or values outside normal range (1.3- 0.67) : inadequate mixing of the gasses or changes in gas pressure between ports (3) FIO2 must be measured on a breath-to-breath basis during these metabolic tests and the inconsistencies from gas concentration and pressure must be made as small as possible in order to have appropriate RQ values (3)

4. Motivation Our client, Matt O’Brien, is a Registered Respiratory Technician at the UW Hospital who performs metabolic measurements and is interested in having more accurate and reliable data in the tests he performs. He believes that the inadequate mixing of oxygen and air could be to blame for some inconsistent RQ measurements he has made. Thus, he has requested our team to design and build him a small mixing chamber to sufficiently mix the gas in hopes for a more stable FIO2 value and thus logical RQ data. This will aid in a more accurate nutritional assessment of mechanical ventilator-dependent patients and help prevent over and underfeeding of these critically ill patients.

5. Design Specifications • Stabilize FIO2 measurements through improved mixing of gases • Reduced size than existing chamber dimensions: 15.9 x 5.1 x 8.3 cm • The inlet and outlet ports: outer diameter of 22 mm and inner diameter of 15 mm • Easily sanitized yet maintain an airtight seal • Chamber constructed of a transparent material • To withstand use approximately once a week

6. Design Alternative - 1 • A grid or filter to create turbulent flow • Tube and filter dimensions dependent on theory of Reynolds's number, predict behavior of air flow, laminar or turbulent (4) • Advantages: Design simplicity, ease of manufacture, low cost • Disadvantages: Mixing completely dependent on the grid turbulence created, induced turbulence can quickly die out

7. Design Alternative - 2 • Turbulence created by air flow obstacles: holes, wall heights, etc. and small pressure difference between the entrance and exit • Have same effect as electrically powered fan • Advantages: Simplicity, easily cleaned, deals with large and small scale mixing problems • Disadvantages: Pressure differences could affect ventilation, difficulty of construction

8. Design Alternative - 3 • Three stationary turbines each oriented in opposite rotation direction to the one preceding it • Advantages include simplicity, small size, ease of dismantling and sanitation • Design also minimizes areas of leakage due to airtight cylinder being purchased

9. Final Design • Plexiglas box with air path obstructions - variation of design 2 • Air ports placed on opposite sides of chamber • 12.0 x 12.0 cm length and width, 2.0 cm deep

10. Further Research • Gathered new information from Mayo Clinic about behavior of air and mixing in a ventilator • Using filter as mixing chamber could interfere with ventilation function • Air heater and humidifier could be source of leaks causing fluctuations • Given a prototype design of mixing chamber found to be most effective by Mayo Clinic

11. Testing • Tested Mayo Clinic chamber, filter attachments and client’s chamber • Several variables were tested -Ventilators – Servo 300a vs. Servo-I -Several ventilation modes -Artificial lung vs. test patient (group member) -With or without humidifier -Position of chamber within line of ventilation tubing • Tested interference of chamber with ventilation function

12. Results • None of chambers affected ventilation function • Mayo Clinic mixing chamber most effective, FIO2 fluctuation <0.1% • Client’s mixing chamber second most effective (+-0.2%) • Filter least effective (+-1.0%) • Newer ventilator, Servo-i, produced smaller fluctuations in FIO2 • Pressure regulated ventilation modes created the most fluctuation in FIO2 values for all chambers • Humidifier had little to no affect on consistency of FIO2, not causing leaks in system • Position of chamber made little to no difference

13. Final Design Specifications • Modeled in proportion to Mayo Clinic prototype, scaled down to meet client’s request • Input and outlet ports placed on opposite sides of chamber for easier attachment and integration into tubing circuit • Chamber lid clamps down, creating airtight seal against rest of chamber with silicone

14. System Diagram

15. Ethical Issues • Effective testing should be done with critically ill patients • Can be tested on several healthy people before use on the critically ill • If risks to patient found, must be weighed against benefits to patient • Protocol need for review prior to testing on patients

16. Future Works • Test effectiveness of various lid clamps and other air tight seals • Testing our 1/3 scale model with the client’s ventilator and equipment • Contacting client’s company to manufacture our design • Researching other techniques and materials for manufacturing to create most durable and air tight junctions of Plexiglas

17. References 1. Disease Management with Gas Exchange. Medical Graphics Corporation. 2002. 2. Harris, C.L. “Weaning With Indirect Calorimetry.” Clinical Window. 2003(12). 3. “American Association of Respiratory Care Clinical Practice Guideline.” Respiratory Care Journal. 1994:39(12):1170-75. 4. Smart Measurement. 2002. Fluid Mechanics: Overview [Online] http://www.efunda.com/formulae/smc_fluids/ov erview.cfm.