Hidden hazards and dangers associated with the use of HME/filters in breathing circuits. Their effect on toxic metabolite production, pulse oximetry and airway resistance

Abstract

Current airway dust ®lter technology emanates from the USA mining industry during the 1930s. It was further developed and improved by the USA military during World War II to prevent workers from inhaling very ®ne radioactive particles in the nuclear industry. These later ®lters we now recognize as high ef®ciency particulate air ®lters (HEPA).The ef®ciency of ®lters varies according to the number and size of particles they capture. Filters may be classi®ed commonly as 95, 99.95, and 99.97% ef®cient (95, 99 and 100, respectively). The higher the ef®ciency the higher the classi®cation number used. The ef®ciency of ®lters may also be affected by volatile chemicals which may be inhaled with the particles. Industrial ®lters which are categorized by the above system may also have a pre®x of N, R, or P; N for Not resistant to oil, R for Resistant to oil, and P for oil Proof; thus, a ®lter would be classi®ed as for example a P100. Requirements for military and environmental biohazard ®lters are based on these classi®cations and surprisingly may only achieve 95% ef®ciency.A generally perceived increased threat from biological hazards that may be inhaled, including bioterrorism threats, especially in hospital environments, has increased interest in respiratory ®lters. 94 Protecting personnel from inhaled biological hazards requires a ®lter ef®ciency of several orders of magnitude greater than the industrial dust ®lters used in respirators alluded to above. As few as 10 inhaled smallpox viruses may be suf®cient to infect patients with the disease. 94 This has resulted in the sub-classi®cation of HEPA ®lters into true HEPA ®lters and HEPA type ®lters. A further sub group has also developed. ULPA ®lters (Ultra Low Penetration Air) and`absolute' ®lters are designed for industrial applications such as microelectronic clean rooms, but have too high a resistance for medical breathing apparatus.All HEPA ®lters are manufactured from glass ®bre materials supported on a rigid frame. In order to reduce resistance to air¯ow and increase ef®ciency, the surface area is increased by pleating. Filtration is achieved for larger particles (>0.3 m) by inertial impaction and interception; smaller particles are captured by Brownian diffusion. The size of particles used to test ®lters is measured in microns. Microns are units used for particles that can be seen with a light microscope. A micron is a thousandth of a millimeter, which is in turn a thousandth of a meter (1 m=1000 nm or 0.001 mm). The variation in ®ltration ef®ciency is tested by the British BS3928 Sodium Flame method and the USA Hot DOP method. The most dif®cult particle size to capture by ®ltration is a particle of 0.3 m as at this size the effects of inertial impaction, interception and Brownian motion are least effective. Particles of 0.3 m are also most likely to be deposited in the lungs if inhaled. The di-octyl-phthalate (DOP) test, used for testing the ef®ciency of ®lters, takes advantage of the properties of DOP. In particulate form, DOP has a constant mean diameter of 0.3 ...

Clinical evaluation of a screen pneumotachograph as an in-line filter

“…The controversy over using in-line filters for lung function testing resembles the one over the use of filters in mechanical ventilation [28]: it is a balance between the advantages and disadvantages. Several issues must be considered; some, but not all of which are related to hygiene.…”

Impact of Humidification and Nebulization During Expiratory Limb Protection: An Experimental Bench Study

“…10,21 The increase in the resistance of an expiratory filter over time is due mainly to the humidification circuit type, rather than to nebulization. If expiratory limb filtration is mandatory during mechanical ventilation, it may require the use of specifically dedicated filtering devices, and/or heated expiratory limbs.…”

“…Problems related to the use of filters with nebulized drugs are another part of the problem. 3,10 The performance of filters depends not only on the type, and bacterial or viral filtration efficiency may be inadequate or highly misleading. 1,9 Nebulization and humidification tend to be similar processes in that both are methods to convert liquid (medication or water) into mist or vapor for humidification and particles that remain suspended within a gas for a period of time for nebulization, and both can be inhaled into the lower respiratory tract.…”

“…Humidification of the inspiratory gases by a heated humidifier may therefore have a great impact on the mechanical properties of expiratory filters, thus modifying filtration properties and leading to potentially lethal ventilator dysfunctions. [10][11][12][13][14][15][16][17] This effect may be enhanced by temperature variations within the circuit, thus generating condensation. Whatever the device used, nebulization particles are usually of higher size than water mist and may thus increase saturation of filters, especially in case of "sticky" buffer like colistin or several other pharmaceutical drugs.…”

Use of filters in anaesthesia: Is it warranted?

“…[1] Cases of filter obstructions have been reported, generally due to excessive secretions, blood in trauma patients or excessive water condensation. [2] This can lead to high-airway pressures and inadequate ventilation. We report a case of high efficiency particulate air (HEPA-) filter causing obstruction of anaesthesia circuit despite a normal pre anaesthesia machine check.…”