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Respiratory protection

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Respiratory protection involves 4 levels of infection control:

  • administrative control
  • environmental control
  • source control
  • personal protective measures

Adminstrative control

This consists of mechanisms to identify and isolate potentially infectious patients. In epidemics it may not be possible to isolate each potentially infectious patient and in these circumstances cohorting of patients should be used.

Environmental control

  • adequate space between beds in open areas
  • negative pressure isolation rooms with ante-rooms
  • ventilation
    • 12 air changes per hour, preferably with no recirculation of air. Air should be passed through a HEPA filter if it is recycled
    • air exhaust separated from air intake
    • smoke tests to ensure that there are no areas of stagnation
    • appropriate air flow within ICU
    • minimize air turbulence (eg sliding rather than swinging doors)

Source control

Minimize spread of infectious particles from patient. In the intubated patient the following may help:

  • high efficiency bacterial-viral heat and moisture exchange filters at the Y-piece of ventilator circuit
  • high efficiency bacterial-viral filter at expiratory port of ventilator
  • scavenging of exhaust gas
  • turn ventilator into standby mode before breaking circuit
  • closed suction devices

Other measures:

  • avoid aerosol generating procedures if possible (eg high flow oxygen, non-invasive ventilation, bronchoscopy)
  • two person technique for bag-mask ventilation
  • high efficiency bacterial-viral filter between mask and valve of bag-mask ventilation device

Personal protective measures

Hand cleansing is an important part of these measures

Aerobiology

Coughing and aerosol generating procedures produce droplets which are potential vehicles for spreading infection. The largest droplets fall to the ground almost immediately. Smaller ones travel a few feet before falling to the ground but some of these droplets may undergo sufficient evaporation as they fall to become very small and remain airborne. These airborne particles will then spread throughout the room in which they are generated. When these airborne particles are inhaled they are rehydrated and therefore become bigger and deposit in the lungs. The ability of an infectious organism to be spread by the airborne route depends, in part, in its ability to survive the dessication and rehydration process.

Personal protective equipment (PPE)

Respiratory PPE can be divided into equipment designed to protect against droplet spread and measures to prevent airborne spread. It is also important to consider eye protection as the conjunctivae serve as a portal of entry for many respiratory viruses.

Droplet

Surgical masks are thought to be an effective method of preventing droplet spread. Use of multiple masks increases their effectiveness but even 5 surgical masks used together are much less effective than a N95 mask

Airborne

  • Masks of N95 standard or its equivalent are the minimum standard. The effectiveness of the filtering material is negated if air is entrained around the mask and therefore it is essential that the mask fits the face tightly. This requires each person to be individually fit tested to find a brand and model of mask that achieves a satisfactory fit. This testing should be repeated annually or if the subject's body weight changes by 10%
  • Powered air purifying respirators (PAPR) draw air through a HEPA filter and then pump it through a full face mask or hood at a high flow rate. The high flow rate prevents entrainment of unfiltered ambient air. While these systems provide a higher level of protection they are critically dependent on proper functioning of the pumps and their design impedes hearing and thus limits verbal communication

Risk factors for airborne infection

Probability of infection estimated as:

where

I = no. of infectious source patients
q = no. of infectious quanta produced per hour
p = minute ventilation of healthcare worker x 60
t = exposure time
R = respirator leak ratio
Q = room ventilation rate (room volume x air changes per hour)
 

An infectious quanta is the number of organisms that need to be inhaled to produce infection. For tuberculosis this is estimated to be 1.

 


©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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