Biomedical Engineering Reference
In-Depth Information
of engineering and environmental controls. The
consensus among all concerned parties in mid-
March 2003 was to adopt the isolation room
requirements for
Mycobacterium tuberculosis
(TB)
[15]. Immediate ventilation improvement works
were thus designed and implemented in the SARS
wards to provide sufficient air change for dilution
and removal of contaminants; to create nega-
tive pressure and achieve directional airflow from
the clean to the infected areas; and to incor-
porate local source control to extract contami-
nants via the shortest possible path and within the
shortest time. In retrospect, we strongly believe
that this decision had been pivotal to our success in
combating SARS and minimizing its nosocomial
spread, since air-borne transmission of this disease
had been raised by Hong Kong [16] and Canadian
[14] researchers to be a real possibility.
In response to the demands of subsequent clin-
ical situations, FMT utilized the same principles to
create a negative-pressure OR with unidirectional
flow for a SARS patients requiring laparotomy, and
temporarily converted the positive-pressure cardiac
catheterization laboratory on two occasions for
diagnostic and interventional procedures in SARS
patients [17,18].
All ventilation improvement works were
executed in phases. In Phase 1, frequency of air
change was immediately increased on March
13, 2003 from the usual 5 ACH to 6 ACH
in all medical wards, and to 7-8 ACH in
the SARS cohort ward. Window-type exhaust
fans (Figure 23.3) with sound-absorbing hoods
were installed, and folding doors were added to
segregate the nurse station from patient cubicles.
Unidirectional airflow was thus achieved from the
corridors outside the SARS ward, through the
clean nurse station to patient areas and finally
to the window exhaust. Returned air dampers
in the HVAC system were closed to achieve
100% fresh air intake. In Phase 2, fan cowls
were added to discharge outlets of exhaust fans
to reduce fluctuation in performance caused by
strong winds (Figure 23.4). In Phase 3, window-
type exhaust fans were replaced by heavy-duty
ducted exhaust fans with non-return air dampers
to achieve 12 ACH. High efficiency particulate air
(HEPA) filters were installed at exhaust outlets.
Exhaust air ducts were next installed at low level
(Figure 23.5) in Phase 4 to create unidirectional
airflow from high level supply diffusers through
the breathing zone of HCW before passing on
to infected patients and then to the low-level
exhaust. Air grilles with double deflection supply
were adopted to control airflow patterns and
Figure 23.4
Ventilation improvement works (Phases 2-3):
Fan cowl covering discharge outlet of window-type exhaust
fan to counteract effect of strong winds. The exhaust fan was
connected via an air duct to low level exhaust in the de-gowning
area (not shown) in the SARS ICU.
Figure 23.3
Ventilation improvement works (Phases 1-3):
Window type exhaust fan and ventilation duct from low
level exhaust installed in de-gowning area (not shown) in the
SARS ICU.
