Add-on Mixing
Chamber for
Mechanical Ventilator
March 5, 2004
University of Wisconsin - Madison
Biomedical Engineering Design Courses
INTELLECTUAL PROPERTY STATEMENT
All information provided by individuals or Design Project Groups during this or
subsequent presentations is the property of the University and of the researchers
presenting this information. In addition, any information provided herein may
include results sponsored by and provided to a member company of the
Biomedical Engineering Student Design Consortium (SDC). The above information
may include intelle ctual property rights belonging to the University to w hich the
SDC may have license rights.
Anyone to w hom this information is disclosed:
1) Agrees to use this information solely for purposes related to this review;
2) Agrees not to use this information for any other purpose unless given w ritten
appr oval in advance by the Project Group, the Client / SDC, and the Advisor.
3) Agrees to keep this information in confidence until the University and the
relevant parties listed in Part (2) above have evaluated and secured any
applicable intelle ctual property rights in this information.
4) Continued attendance at this presentation constitutes compliance with this
agreement.
Team Members
Missy
Haehn (BWIG)
Laura Sheehan (BSAC)
Ben Sprague (Communications)
Andrea Zelisko (Team Leader)
Client
Matt
O’Brien, RPT
Advisor
Professor
John Webster
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.
Background
Critically-ill patients on mechanical ventilators
have higher metabolic rates than healthy
individuals
 This higher rate leads to changes in caloric
needs in the patient diet
 The patient’s caloric intake must be precise in
order to prevent malnutrition and because it
affects CO2 production which can deteriorate
respiratory muscles and lead to difficulty in
weaning off the ventilator

Background cont.
Indirect calorimetry is a method of metabolic
measurement used to determine the nutritional
needs of a patient
 Method utilizes the Respiratory Quotient (RQ)
which is the CO2 produced divided by the O2
consumed
 RQ determines if patient is being over or
underfed carbohydrates, lipids, proteins, etc.
 Breath-by-breath measurements are made using
a pneumotach to measure flow and a gas
sample line to measure concentration of the gas
mixture

Motivation
Client performs metabolic measurements and is
interested in having more accurate and reliable
data
 Believes inadequate gas mixing could be to
blame for inconsistent RQ values measured
 Mixing chamber should better mix the gasses
and result in a more stable FIO2 value
 This in turn will result in more accurate
metabolic measurements for critically-ill patients

Client/Design Requirements





Improved method of gas mixing
Chamber reduced in size from existing model
retaining specific inlet and outlet port
dimensions
Ideally able to be sanitized while maintaining
airtight seal
Made of transparent material
Used approximately once a week
Design #1: Grid





Reynolds number determines what kind of flow
is present: R= ρVD/μ
For an open container, Reynolds number above
4000 indicates turbulent flow while Reynolds
number less than 2000 indicates laminar flow
Our calculated Reynolds number for an open
container was 724 and turbulence would not be
created in tube alone
By inserting a grid, grid turbulence can be
created
For grid turbulence to occur a Reynolds number
of 10 to 100 is needed
Design #1 cont.
With a Reynolds number of approx
40 when gasses enter from ventilator
turbulence will be created
 Turbulence will be fully mixed 30 to
50 rod diameters downstream in the
design
 Advantages include design simplicity, ease of
manufacture, low cost
 Disadvantages include that mixing is completely
dependent on the grid turbulence created, induced
turbulence will quickly dies out

Design #2: Walls/Holes





By taking advantage of the small pressure
difference between the entrance and exit
turbulence can be created by logically placing
holes, wall heights, etc
This would have the same effect as a electrically
powered fan
Advantages include simplicity, easy to clean,
deals with large and small scale mixing problems
No way to measure effectiveness of mixing prior
to testing
Possible pressure drop and errors in construction
with this design
Design #2 cont.
Improvement on existing chamber’s features
 Design consists of three dividers: the first with
slits at top and bottom, the second with holes of
increasing size, the third a screen


Chamber dimensions: 13.7 x 7.8 x 5.0 cm
Design #3: Turbines
Three stationary turbines oriented in opposite
rotation direction to the one preceding it
 The turbines will be approximately 2.5 cm in
diameter and 3 cm apart
 Turbines in cylinder tube of approximate length
10 cm and radius 3 cm

Design #3 cont.
Will be acting strictly as an obstacle to the air
flow, forcing it to travel in one direction as it
passes the first turbine, in the opposite direction
to pass the second, and then back in the first
direction to pass the third
 Advantages include simplicity, small size, ease of
dismantling and sanitation
 Design also minimizes areas of leakage due to
airtight cylinder being purchased
 Disadvantages include difficulty to gauge the
effectiveness of the design without future testing

Design Matrix
Feasibility
Mixing
Effectiveness
Design 1 –
Grid
Highly
Feasible,
Simple
High, Supported
by Calculations
Design 2 –
Walls and
Holes
Slightly
Feasible,
Design
Constraints
Design 3 –
Turbines
Moderately
Feasible,
Availability
of Supplies
Undetermined,
Need to Test
Undetermined,
Need to Test
Integration
with
Ventilator
Ability to
be
Cleaned
Reliability/
Durability
Cost
Moderate,
Grids
Expensive
Easy to
Integrate
Moderately
Easy
Medium
Durability,
Grid
Replacement
Easy to
Integrate
Easily
Cleaned,
Removable
Dividers
Low
Durability,
Construction
Constraints
Low, No
Specialized
Supplies
Moderately
Easy
High
Durability,
Airtight
container
Moderate,
Propellers
Expensive
Easy to
Integrate
Final Design: Grid
Simple construction
 Effectiveness supported by calculations
 Easy to clean
 Effective airtight seal
 Ability to withstand pressure drop

Potential Problems
Integration of mixing chamber is vital and it is
yet unknown if putting it in series with the air
input line will affect pressure
 A precise, reliable way of determining the mixing
abilities of each present prototype must be
determined
 Main problem: Will any of the prototypes mix the
gasses well enough to reach a consistent
percent oxygen delivered?

Future Work
Determine materials most suitable for design
 Self-constructed or purchased products
 Ensure that the design will maintain airtight
seal
 Extensive testing of each design to determine
quality of mixing
 Possible design alternative of a filter to
replace the grid

References






Campbell, N.A., Reece, J.B. Biology. San Francisco:
Benjamin Cummings. 2002.
Madama, Vincent C. Pulmonary Function Testing and
Cardiopulmonary Stress Testing, Second edition. Delmar
Learning, 1997.
Harris, C.L. “Weaning With Indirect Calorimetry.”
Clinical Window. 2003(12).
Disease Management with Gas Exchange. Medical
Graphics Corporation. 2002.
“American Association of Respiratory Care Clinical
Practice Guideline.” Respiratory Care Journal.
1994:39(12):1170-75.
The Hard Facts About Burning Calories (Metabolism). 17
Feb. 2004. LifeChek.
http://www.lifechek.com/HardFactsPage.php
References

MedGraphics Direct Connect preVent. 2 Feb. 2004. Medical
Graphics Corporation.
http://216.122.201.195/datasheet_direct_connect.html
 Easson, William. 2001. Fluid Mechanics 4. [Online]
http://www.eng.ed.ac.uk/~will/teaching/Fluids4/index.html.
 Easson, William. “Re: Questions about a Biomedical
Engineering Design.” E-mail to the author. 27 Feb. 2004.
 Ghandhi, Professor Jaal B. Personal Interview. 6 February
2004.
 Rutland, Professor Chris J. Personal Interview. 18 February
2004.
 Smart Measurement. 2002. Fluid Mechanics: Overview
[Online]
http://www.efunda.com/formulae/smc_fluids/overview.cfm.
 Chesler, Professor Naomi. Personal Interview. 26 Feb. 2004.
 Shedd, Professor Timothy A. Personal Interview. 27 Feb.
2004.