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 ...

Manual inflation of the lungs should be avoided (before tracheal intubation) when rapid sequence induction is performed. This teaching is applied to all patients at risk from the dangers of a full stomach, particularly in obstetric practice. Unless hypoxia supervenes, manual ventilation is contraindicated on two grounds. First, if cricoid pressure has been inadequately applied (a not uncommon situation (Howells et al., 1983)), silent regurgitation may occur and gastric contents be forced into the lungs. Second, and more importantly, it has been stated that manual inflation of the lungs can lead to gaseous distension of the stomach and further predispose to vomiting or regurgitation, or both (Snow, 1963). With the use of adequate preoxygenation and a rapidly-acting depolarizing neuromuscular blocking agent, manual ventilation of the lungs before intubation is not usually required. Under certain circumstances, however, the patient's oxygen reserves may be insufficient as, for example, during failed or delayed intubation-particularly when the oxygen reserves are decreased (low FRC) (Nunn, 1977), or when oxygen consumption is increased as during pregnancy (Archer and Marx, 1974), thyrotoxicosis or fever. Similarly, when suxamethonium is contraindicated, the speed of onset of a non-depolarizing agent may be adequate to allow intubation before the exhaustion of the oxygen reserves. Recently, the "priming" principle has been described (Gergis et al., 1983; Foldes, 1984; Miller, 1985; Schwarz et al., 1985). A nondepolarizing agent is used as an alternative to suxamethonium when this agent is contraindicated for rapid sequence induction. The principle E. G.

An investigation was carried out into the relationship between the inflation pressures of normally compliant lungs and the airways pressure necessary to produce the insufflation of gas to the stomach. This relationship was examined during manual ventilation with a mask, using a rapid sequence induction technique. In the absence of cricoid pressure the lungs of all the patients could be ventilated "gently" satisfactorily by hand without gas entering the stomach. In only half of the patients could gas be redirected to the stomach when maximal inflation pressures were generated. It was not possible to cause gas to enter the stomach in any patient with a patent airway when cricoid pressure was applied.

The incidence of pulmonary aspiration in a group of patients who did not respond to cardiopulmonary resuscitation (CPR) was assessed at autopsy and found to be 29%. This figure is undoubtedly an underestimate of the total problem, and some indication of the potential for aspiration during CPR is revealed by the fact that 46% of the patients studied had full stomaches at autopsy. Clearly this fact has implications for CPR methods as suggested by Cummings and Eisenberg. The problem could be reduced by incorporating the use of cricoid pressure into the techniques of Basic CPR but this will require modification of current teaching.

An instrument is described which, when used during the accelerated induction technique, ensures that consistent and adequate cricoid pressure can be applied. Mothers undergoing general anaesthesia for elective Caesarean section were studied in order to illustrate the clinical application of the instrument. The consequences to intubating conditions of applying adequate cricoid pressure, and an assessment of the instrument's control over the incidence of regurgitation during operation were investigated.