The PPVL Isolation Room: Infection Risk and Policy Implications

1
The PPVL Isolation Room Infection Risk and
Policy Implications

• Dr Cath Noakes, Dr Louise Fletcher, Dr Andy
  Sleigh
• University of Leeds
• William Booth, Blanca Beato-Arribas
• BSRIA Ltd
• Nigel Tomlinson
• Department of Health October 2009

2
Content

• Isolation rooms and need for study
• Tracer methodology
• Leeds Chamber Results
• Application to the PPVL mock-up
• Infection Risk Analysis
• Implications for Build and Operation

3
Isolation Rooms

• Aim to contain most infectious patients OR
  protect those most at risk
• Pressure differential controls transmission with
  outside world
• Negative for infectious patients
• Positive for immunocompromised
• Who sets operation? Is it safe?
• What about the airflow within the space?
• Essential understand risk outside and inside
  space
• under normal and failure conditions

4
The PPVL concept
5
How do we measure performance?

• What is the risk of airborne transmission to or
  from a healthcare worker in room?
• What is the risk of transmission to those outside
  the room?
• How does the risk change with events?

Development and Validation of CFD models and
tracer techniques to mimic the behaviour of
airborne microorganisms
Application to PPVL mock-up to evaluate physical
behaviour
Combine with risk models to understand exposure
Set performance requirements and operation
validation method
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Tracer Study Objectives

• Compare the performance of 4 tracer techniques in
  a controlled chamber environment
• Nitrous Oxide gas tracer, Inert NaCl particles,
  Bioaerosols containing B subtilus or S aureus
  bacteria, CFD model
• Establish how well inert tracer techniques can
  represent the behaviour of small bacteria
  particles
• Understand behaviour of tracers across three
  ventilation regimes under transient and steady
  state conditions
• Apply inert tracer techniques to PPVL mock-up to
  establish mixing, decay rate and leakage

7
Leeds Chamber Set-up
Wall supply (Low High)
Nebuliser port (A)
Wall extract (Low High)
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Tracers

• Nitrous Oxide
• Constant release from cylinder
• Sampled with gas analyser
• NaCl Particles
• 5 NaCl solution in collison nebuliser
• Samples 0.5-5mm with laser particle counter
• Bioaerosols
• Bacterial culture in collison nebuliser
• Anderson sampler to collect on TSA then
  incubation and enumeration

• Three ventilation regimes at 10 ACH.
• All tracers released via tubing to the DIN man
  (70W)
• Measurements made at extract during build up and
  decay
• Measurements made at 5 sample locations during
  steady state
• Room sealed and unoccupied during tests
• Mechanical air change rate measured with
  balometer for all ventilation set-ups

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Leeds Chamber CFD model

• Model built using Fluent
• Unstructured tet grid with 360000 cells
• Steady state simulations at 10 ACH
• Passive scalar released from DIN man to represent
  pathogen
• K-e turbulence model
• Boussinesq model with heat flux on DIN man

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Leeds Chamber NaCl Tracer Results
Ventilation regime 2 High wall supply, low wall
extract
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Leeds Chamber Tracer Decay

• Presented relative to room air change rate as a
  k-factor
• Similar rates across 3 techniques - best
  comparison with 3-5 mm particles
• Short-circuiting in regimes V1 and V3
• Regime V2 showed good mixing and stable airflow

12
Leeds Chamber Steady State Distribution

• Ventilation regime 2 high wall supply, low wall
  extract
• Normalised concentration around 1 for all tracers
  good mixing in space
• Bioaerosol behaviour similar to inert tracers
• Best comparison with 3-5 mm particles

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Leeds Chamber CFD ResultsV1 Low wall supply,
high wall extract
2.90 2.60 2.30 2.00 1.70 1.40 1.10 0.90 0.60 0.30
0.00
Vertical Plane shows plume and significant
gradient
low
Horizontal Plane shows low concentration and
limited mixing
high
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Leeds Chamber CFD ResultsV2 High supply, low
extract
2.90 2.60 2.30 2.00 1.70 1.40 1.10 0.90 0.60 0.30
0.00
Vertical Plane shows plume and slight gradient
high
Horizontal Plane shows good mixing - similar to
experiment
low
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Tracer Methodology Summary

• Good comparison between all tracer techniques
  suggests that inert particles or gas tracers can
  represent bioaerosols
• Best comparison in 3-5mm particle range
• Inert particle tracers offer realistic and safe
  tracer technique for assessing risk
• CFD results gave similar mixing and insight into
  vertical concentration gradients
• Ventilation regime strong effect on mixing across
  all tracers
• General implications for isolation room design
  and risk to healthcare workers

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PPVL Tracer Study

• Compare N2O and Salt tracers in PPVL mock up
• Use techniques to assess various scenarios
• Release - patient, lobby, heat loads
• Room challenges - fan failure, effect of doors
• Combine with CFD, smoke tests and anemometry -
  quantitative data for risk evaluation

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Decay Results

• Tracer measured at extract
• Divided by mechanical ACH to determine k-factor
• Good comparison between Salt and N2O
• K-factors typical of well mixed room

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Steady State Behaviour
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PPVL Tracer Summary

• Same good comparison between tracer techniques in
  PPVL
• Best in 1-5mm particle range
• Results consistent within these tests and with
  previous N2O tests
• Both techniques offer realistic method of
  physically modelling small airborne pathogen
  transport in ventilated rooms
• Both techniques can give quantitative data on
  steady state and transient performance of a space

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Infection Risk Scenarios

• Normal operation (doors closed)
• Good mixing inside room with k factors 0.8-0.9
• Source dilution 3 log reduction
• Protection to outside gt 5 log reduction
• Transient mode (doors open)
• Mixing inside room remains good
• 7 air escapes
• Impact (hence risk) depends on duration
• Complete failure (eg. fan fail)
• Leakage determined by air tightness
• Room recovers protection when normal operation
  resumed

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Healthcare Worker Risk

• Three zone mixing model to show effect of
  internal airflow
• Assume clean supply air and no backflows
• Patient releases infectious material at 60
  quanta/hour over 1 minute period
• HCW inhaled dose based on breathing rate of 8
  l/min
• K-factors of 0.8 and 0.4 to model good and poor
  mixing

Bulk room air
En-suite
Near patient zone
Lobby
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HCW Pathogen Concentration
Well mixed room (k 0.8) - PPVL
Poorly mixed room (k 0.4)
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HCW Inhaled Dose
Well mixed room (k 0.8) - PPVL
Poorly mixed room (k 0.4)
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Transient Performance
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Implications for Build

• When built and operated correctly
• Designed to be low risk even under transient and
  failure modes
• Designed to offer high level of protection
  without significant user intervention
• Requires high quality construction - air
  tightness at least 5 m3/(h.m2)
• Pressure stabilizer crucial to air mixing and
  lobby pressurisation
• Smoke testing can indicate performance
• Tracer testing can quantify and validate
  performance

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Implications for Safe Operation

• Infection risk can never be zero
• Actual risk will depend on pathogen - Airborne
  focus
• Infectious diseases TB, SARs, pandemic influenza
  (?) plus unknown conditions
• Immunocompromised Hospital plus outdoor
  (Aspergillus)
• Inherent environmental protection can be
  compromised by poor operation
• PPE and good user operation further reduce risks
• Transmission to/from outside Good hygiene
  practices, routine operation with doors closed,
  training to react in event of failure
• Transmission within room PPE (eg.
  mask/respirator) appropriate to pathogen and
  activity

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Thank You for ListeningAny Questions?

• Dr Cath Noakes
• Pathogen Control Engineering Institute
• University of Leeds
• C.J.Noakes_at_leeds.ac.uk