The primary objective of breathing filters is to stop bacteria
and viruses entering/leaving the patient. Electrostatic filters
achieve this by holding onto micro-organisms through the static
electric charge in the medium. The pleated paper filter also known
as a mechanical filter achieves this by tightly packed layers of
mixed strands of glass fibres physically preventing bacteria and
viruses from passing through.
HME's use the patients own moisture and humidity from expiration
to humidify anesthetic gases during inspiration. When endotracheal
tubes are used, the upper airway that usually warms inspired air is
bypassed and for patients that are anticipated to be under
anesthesia for long periods or ventilated patients, it is
recommended that an HME/HMEF be used as prolonged dry gas exposure
can have adverse effects.
The descriptions and diagrams below show a detailed example of
Electrostatic Filters and Mechanical Filters.
Electrostatic
Filters
Electrostatic filters use a medium constructed of permanently
bipolar charged rectangular split fibres which are able to capture
airborne particles. The nature of this fibre material provides good
strength and guarantees a long service life without significant
degradation. This type of filter is recommended for applications
where high efficiency and low pressure drop is required. It offers
excellent efficiency for anesthetic breathing filters and pulmonary
function filters. |
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Mechanical Filters
Pleated paper filters use tightly packed layers of mixed strands
of glass fibre filter paper. This non-uniform method of packing
produces a thick layer of material. Particles are trapped in the
tortuous path created by the crisscross mat. In principle, the
medium used acts as a sieve, holding back unwanted particles while
allowing anaesthetic gases and humidified air to travel through
with minimum obstruction. |
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Efficiency Testing
The effectiveness of a filter is stated as a percentage (%)
efficiency and is defined as:
cfu or
pfu collected without BSF in place - cfu or pfu with BSF in place
x100
cfu or pfu without BSF in place
cfu = colony forming units, pfu = plaque forming units, BSF =
breathing system filter
Efficiency is measured by microbial penetration value which is
defined as the number of colony forming units or plaque forming
units passing through the filter per 10,000,000 challenge
micro-organisms.
To test a filter for bacterial efficiency the filter is subjected
to a preconditioning of 24 hours under relative humidity of
96%
or more and temperature of 34° ± 1 at a tidal volume of 500ml at
20 breaths per minute with I/E ratio of 1:2. Following this
preconditioning the filters are challenged with microbial aerosols
using the Henderson Apparatus to deliver a challenge of over
10,000,000 Bacillus Subtillis Var.Niger Spores (NCTC10073). For
viral efficiency a RNA-Phage (MS-2 Coliphage) which is similar to
the smallest human viruses is to be used to test the efficiency of
the filter. The test results certify the proven high efficiency of
Flexicare filters.
The efficiency of the Flexicare and HepaShield filter is tested by
the Health Protection Agency at Porton Down, Salisbury, UK. Testing
was done for bacterial and viral efficiency using Bacillus
Subtillis and MS-2 Coliphage.
The diagram below highlights the size of test particles used in
evaluating the efficiency of the Flexicare Filter.
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Independent tests confirm
that the Flexicare range of breathing filters prevent bacteria and
viruses from passing between patients and ventilator
equipment.
Dead Space
This is the internal air filled volume of the filter or HMEF. Dead
Space contributes to the compliance of the breathing system as
filters are placed between breathing systems and the patient.
Moisture Output
This may be an important requirement for some patients who are
being intubated over a long period of time. The unfortunate
consequence of intubation is that the upper airway is bypassed and
results in a loss of humidification of inspired air. Dry gases can
have adverse effects and thus to prevent causing the patient trauma
HMEF's play the role of acting as the humidifier in lieu of the
upper airway. However, humidification can cause rain-out in either
direction. Flexicare Filter medium has a double sided hydrophobic
membrane which repels water and helps to maintain effective
performance of the filter during its use.
Construction
The diagram below shows the construction of the Adult
Electrostatic Breathing Filter with HME
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