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The British guidelines on asthma management and other recent reviews have widely advocated the use of spacer devices for asthmatic patients.1-3 Spacer devices allow aerosol to be inhaled through a valved mouthpiece during a single inspiratory breath or as is often recommended for children during tidal breathing.2During the expiratory phase of a tidal breathing manoeuvre the valve closes. Although this action prevents leakage of aerosol from the spacer and ingress of moisture into the holding chamber, there is an inevitable increase in the expiratory resistance. We have measured the resistance to expiratory airflow in two commonly used spacer devices: the Volumatic (GlaxoWellcome) and the Nebuhaler (Astra-Zeneca) and found it to be high. Expiratory resistance was measured by passing air through the mouthpiece using a sealed connection. Two flow rates were chosen and measured using a Gap Rotameter: 30 l/min designed to mimic airflow during quiet expiration and 100 l/min to mimic more active expiration. Pressures were measured at the mouthpiece using a Hewlett Packard differential pressure transducer and resistances calculated. In an attempt to reduce the resistances the spacers were then modified by drilling four holes of 5 mm diameter proximal to the valve around the mouthpiece. Each hole was covered with a strip of latex rubber attached to the mouthpiece with tape so as to function as a “blow-off valve”. Expired air could then flow out through the “blow-off valves” during expiration but not during inspiration (fig 1). Pressure measurements were repeated and resistances (in cm H2O/l/s) were calculated (table 1). The valves of the modified spacers continued to function by closing during expiratory flow despite the lower resistances.
The results indicate high expiratory resistance in both unmodified spacer devices at both low and high flow rates. The values are well in excess of the minimal recommended values of expiratory resistance for diagnostic spirometry (1.5 cm H2O/l/s at flow rates of <840 l/min).4 Introducing blow-off valves significantly lowered the expiratory resistance. The high resistances may explain both our observations and those of others that some patients find it difficult to expire through some types of spacer devices.5Over a series of several breaths, tidal volume may reduce while the functional residual capacity increases as the chest becomes progressively hyperinflated. Simple modification of the mouthpiece allows marked reduction in the expiratory resistance. This should facilitate the correct use of spacer devices and enhance drug deposition in the lungs. We therefore urge manufacturers to look at the possibility of modifying their spacer devices. It is likely that such modification would facilitate the correct use of spacer devices leading to enhanced drug deposition in the lungs.
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