this presentation explains the technical basis for high ... · this presentation explains the...

This presentation explains the technical basis for high-efficiency filtration and then explores the fundamentals of Pulmonary Function Testing, the procedure for which our

PFT Filters are used, and also describes the technical details of the Westmed BlockAide™ PFT filters.

by Michael McPeck, RRT FAARC Director, Clinical Educa1on Westmed, Inc. [email protected] 516.729.9989

BASIC TRAINING

Copyright © 2014 Westmed, Inc. All Rights Reserved


What is a filter?


Filter defined


Filtration …the process of separating the components of a fluid “stream.” The components may be the ingredients of the stream or accidental contaminants that need to be removed to purify the stream. The fluid “stream” can be a liquid or a gas. The two main types of filtration are:

•  particulate •  gas phase

In most medical applications, we are concerned with particulates or “patient droplets.”


What is efficiency?

Efficiency is the expression of a filter’s ability to to perform filtration, or to trap and remove components of a certain size from the stream.

Efficiency is expressed mathematically as … Output divided by Input:

Efficiency = Output

Input


Mechanisms of particle capture There are five primary mechanisms by which filtration of particles occurs …

Inertial impaction – the particle’s forward inertia slams it into the filter media.

Diffusion – exceptionally small particles “drift” into the media by Brownian motion.

Interception – the particle has too little forward inertia for impaction but not enough to completely bypass the media.

Sieving – the most commonly perceived mechanism; the particle is too large to pass through “pores” in the media.

Electrostatic attraction – differences in electric charge create attraction of the particle to the media. This may be used in conjunction with the other mechanisms to enhance them.


How is filter efficiency determined? Filters are challenged with 0.3 µμM (micron) particles in order to analyze the percentage of particles that have been retained. Why 0.3 µμM ?

For a variety of physical reasons, the 0.2 to 0.3 µM diameter particle is the most difficult to filter. For that reason, 0.3 µM has been termed the “MPPS,” Most Penetrable Particle Size and has become the standard by which filters are tested for efficiency.


What does HEPA mean?

HEPA is the acronym for …

High Efficiency Particulate Air So, going back to the definitions, we see that a HEPA filter is concerned with particulates and a gas stream (air) … … and it is especially concerned with efficiency.


HEPA defined

HEPA is a type of air filter. Filters meeting the HEPA standard have many applications, including use in medical facilities and equipment, automobiles, aircraft, and homes. The filter must satisfy certain standards of efficiency such as those set by the United States Department of Energy (DOE). To qualify as HEPA by US government standards, an air filter must remove (from the air that passes through) 99.97% of particles that have a size of 0.3 µμM or larger. A filter that is qualified as HEPA is also subject to interior classifications.

High-Efficiency Particulate Air or High-Efficiency Particulate Absorption

99.97% Efficient @ 0.3 µM


HEPA defined

HEPA media is similar to blo[ing paper, and made of a matrix of very thin, r a n d om l y a r r a n g e d overlapping fibers that create a tortuous pathway. The airflow pathway is made even more tortuous by pleating the media.

High-Efficiency Particulate Air or High-Efficiency Particulate Absorption

polyester glass fibers

plea1ng


Electrostatic Filters In electrostatic filters, 50% of the fibers are positively charged, and 50% negatively charged. Particles and droplets tend to carry either a positive or negative electrostatic charge. So, roughly half of the negative particles will be a[racted to positive fibers and vice versa. Upon reaching the fibers particles are held in place by the electrostatic charge. The electrostatic mechanism is utilized to enhance the fundamental physical particle capture mechanisms.

Electrosta1c par1cle aFrac1on mechanism

Nega1ve ( -‐ ) Par1cle

Posi1ve ( + ) Fiber


Electrostatic Filters Electrostatic filters are nearly as good as HEPA filters and are used in some similar applications where airflow resistance and resistance to we[ing may be critical factors. Features: •  Open structure •  Larger pore size •  Electrostatically charged polypropylene fibers •  Lower airflow resistance •  Lower weight •  Failsafe hydrophobic mechanism

(does not absorb water) •  Tend to be sold at lower price than HEPA •  Media of choice in global market for respiratory

applications 99.9% Efficient @ 0.3 µM


Electrostatic vs HEPA

Electrosta?c HEPA

10,000 10,000

10 3

99.9 % Efficient 99.97 % Efficient

Theoretical mean efficiencies predicted for electrostatic and HEPA filters based upon challenge with 0.3 µμM test particles.

10 particles / 10,000 3 particles / 10,000


How small/large is a micron (µμM)? Can you really appreciate how small 1 micron actually is?

Human Hair

1 micron (µμM)

2.5 microns (µμM) (average aerosol particle created by nebulizer)


How small/large is a micron (µμM)? Can you really appreciate how small 1 micron actually is?

Human Hair

1 micron (µμM)

Relative Sizes •  Filters are challenged with

0.3 µμM spherical particles.

•  P. aeruginosa is a rod, ~1 to 5 X 0.5 to 1 µμM.

•  M. tuberculosis is a rod, ~2 to 4 X 0.2 to 0.4 µμM.


Purpose of the PFT filter PFT filters serve one purpose: to protect the patient from inhaling micro-‐‑organisms and aerosols from the PFT equipment, and to keep aerosols and droplets containing micro-‐‑organisms from entering the PFT equipment.

However, they must do their job without inhibiting flowrates or causing high resistance or creating a gas leak in the PFT system.

Morgan PFT filter shown


For Pulmonary Function Testing, electrostatic filters are preferred for the 3 principle reasons:

•  Lower resistance than HEPA filters •  Be[er moisture resistance •  Lower cost than HEPA filters.

Use of an electrostatic filter instead of a HEPA filter allows for the manufacture of an effective filter, with airflow resistance as low as possible, at a cost acceptable to the market. Use of an electrostatic filter instead of a HEPA filter does not increase either the machine contamination risk or the patient infection risk because: (1)  the theoretical efficiencies between the devices are statistically similar, and (2)  particles encountered in pulmonary function testing are almost always

large “patient droplets,” many microns in size, not individual microorganisms.*

Electrostatic vs HEPA for PFTs

* Microorganisms do not exist in free form, they are aFached to, or a part of, some suitable substrate that tends to form larger droplets or par1cles.


Westmed BlockAide™ PFT Filters

The sales flyer is very important because, in addition to describing the product, it also contains important specifications to help match the correct filter to the wide variety of PFT devices currently in use.


Elliptical vs Round Pulmonary function labs often like to have different shapes of mouthpieces available to accommodate patient preferences. Some patients, particularly children, have small mouths and therefore tend to do be[er with the elliptical mouthpiece.

Studies where high expiratory flowrates are anticipated (spirometry and flow/volume loops) require filter mouthpieces with large inside diameters that do not impose high resistance or impede the flow.


Flanged vs Straight Rubber flanged mouthpieces that fit in the mouth with the flange situated between the lips and the gums tend to do a be[er job of making a gas-‐‑tight seal for tests that require a long breathing period, such as gas dilution lung volumes and diffusing capacity. They are also useful for plethysmography where the flange helps hold the mouthpiece in place during the panting breathing technique. Straight mouthpieces are more frequently used for spirometry procedures where forced expiratory flowrates are very high.

Be aware that our mouthpieces are cost-effective substitutes for OEM filters (shown here) and can be used either with or without a flanged rubber mouthpiece.

Morgan PFT filter shown


Basic Specifications

Common customer questions might include:

• What is the filter efficiency? • What is the filter resistance to flow? • What is the filter deadspace?


Filter Efficiency A common customer question might be: What is the filter efficiency?

Efficiency is the expression of a filter’s ability to perform filtration, or to trap and remove components of a certain size from the stream. Efficiency is expressed mathematically as …

Output divided by Input: Efficiency = Output Input

The Westmed BlockAide™ PFT Filter has an bacterial/viral efficiency of 99.9% of particles 0.3 µM in size. 0.3 µM is the industry standard test size for filters because 0.3 µM represents the MPPS, or Most Penetrable Particle Size.


Resistance to Flow A common customer question might be: What is the filter resistance to flow?

How is filter resistance determined? Very simply, a number of samples of filters are tested to determine their pressure drop (△P) at many different flowrates across their expected range of use. One way of doing this would be to subject the filter to increasing flowrates at 1 L/sec increments, up to a maximum of 14 L/sec, and then measuring the △P at each flowrate. The resulting data set might look like the table on the left below. Next, the data would be plo[ed in a spreadsheet to create a graph showing the data points on the chart on the right, below.

Finally, the equation for the slope of the △P /Flowrate line is solved by linear regression to reveal the filter resistance (y); in this case, 0.46 cmH2O/L/sec.


Resistance to Flow A common customer question might be: What is the filter resistance to flow?

Our filter resistance specification, 0.46 cmH2O/L/sec indicates that, for every 1 L/sec of increasing flowrate, the pressure drop (△P) across the filter increases by 0.46 cmH2O. The spec also indicates that the filter was tested up to a maximum flowrate of 14 L/sec which is slightly more than we would expect even the healthiest patient to achieve.


Deadspace A common customer question might be: What is the filter deadspace?

“Deadspace” is simply the amount of gas-‐‑containing volume enclosed within the body of the filter. Obviously, the larger the filter, the greater the deadspace. Deadspace is important because it often represents the amount of exhaled CO2-‐‑containing “rebreathed volume” that a patient may have to inhale when a[ached to an apparatus through a filter. The smaller the patient, the smaller the tidal volume and, hence, the more critical that deadspace becomes.

S o m e d e g r e e o f d e a d s p a c e i s unavoidable because of the large diameters that are required to reduce airflow resistance. However, the shape of our PFT filter is designed specifically to reduce deadspace as much as possible while maintain the requisite diameter for low flow resistance.

Round = 54 mL Ellip?cal = 51 mL


Connector Sizes

One way of selecting the correct filter would be to match the inside and outside diameter of the PFT machine-side of the filter to the actual diameter on the mouthpiece port of the PFT machine or device.

Inside (ID) &Outside (OD) Filter Diameters in millimeters (mm)


Connector Sizes

Another way of selecting the correct filter would be to look up the brand and model of the PFT device on the chart on the flyer. Sizes given are in millimeters (mm).


Conclusion •  The American Thoracic Society (ATS), issues clinical practice guidelines

that are widely accepted by pulmonary practitioners and manufacturers of pulmonary diagnostic equipment. ATS has specified a maximum allowable PFT filter resistance of 1.5 cmH2O/L/sec. Obviously, the Westmed Blockaide™ PFT Filters, at 0.46 cmH2O/L/sec, are well below that maximum.

•  Statements by other manufacturers of filters, especially by the companies that make the PFT equipment, that suggest our filter resistance is too high, or that the high limit is impractical for pediatric patients, are merely sales tactics that are designed to confuse the customer and cast doubt on the quality of our filter. They are trying to sell their filters, with a high markup, but which are no be[er than ours.

•  The fact is the Westmed Blockaide™ PFT filters are high quality filters that are well within the maximum resistance specified by the ATS. The high testing limit of 14 L/sec is assurance that the filter resistance is linear up to that value; it does not affect whether or not the filter can be used on pediatric patients. It can.

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