Resistance to gas flow

Intubation of the airway, whether via a tracheostomy or by oral or nasal intubation, causes a narrowing of the upper airway. This increases the resistance to gas flow

(Habib. 1.9.89.). Figure 1 demonstrates how the increase in resistance is critically dependent on the size of the airway tube. The use of an airway tube with an internal diameter of 9 mm (French 39) raises the resistance only modestly at flow rates that are commonly used during artificial ventilation. Thus the resistance does not exceed 2 cmH2O/l/s at flow rates up to 1 l/s. However, there is a rapid increase in resistance with increasing flow rate. If the inside diameter of the airway tube is reduced to 6 mm (French 24), the resistance rapidly increases to typically 6 cmH 2O/l/s at a flow rate of 1 l/s. This compares with the airway resistance seen in moderate asthma. Moreover, the addition of a humidifier adds further to the resistance, so that very high values can be reached ( Fig 1 ). While this is of little importance during mechanical ventilation, it may make weaning off the ventilator more difficult. Moreover, expiratory resistance will also be increased with a similar rise in expiratory work of breathing. Another aspect is the possible effect of increased expiratory resistance on end-expiratory lung volume.

FKW (l/î)

Fig. 1 Pressure-flow curves for French 24 and 39 tracheostomy tubes (internal diameters, 6 mm and 9 mm) with and without a humidifier. Note the considerable increase in pressure required for a given flow when the tube size is decreased or the humidifier is added. Recordings were made in model experiments with flow in the 'inspiratory' direction. (Reproduced with permission from H°lM..,§Lai (1,985))

Although the tracheal tube may exert an increased resistance to gas flow compared with a normal airway, it is important to point out that inspiratory resistance in the spontaneously breathing non-intubated, but sedated or anesthetized, subject may be increased because of intraction of the upper airways during inspiration

(Wheatley eLaL 1991). During this condition the muscle tone in the pharynx and upper airway seems to be reduced; an upper airway obstruction may cause a reduction of the anteroposterior diameter of the upper airways to 20 per cent of baseline at an intraluminal pressure of -15 cmH 2O. This imposes a dramatic increase in resistance to inspiratory gas flow. Treatment directed towards increasing the tone of the tongue and upper airways, including the pharynx, may be the proper approach, but in the absence of such selective treatment intubation of the airway appears to be the only alternative.

The recording of flow resistance of the tracheal tube may appear to be a simple task, but it is more complicated than generally considered. This is due to difficulty in measuring the correct pressure at the distal end of the airway tube. The introduction of a catheter into the airway lumen will itself exert a certain resistance, both by reducing the airway lumen and by promoting turbulent flow. More important is the fact that a catheter positioned so that its tip is below the distal end of the tube may measure a pressure that is affected not only by the pressure drop in the tube but also by dilatation of the airway distal to the tube. This dilatation reduces the pressure further and causes an overestimation of the calculated tube resistance. The ideal solution is to have a separate channel for recording pressure in the tube wall, with the opening inside the tracheal tube close to the distal end. Three to four openings around the distal end of the tube will compensate for any inhomogeneity in gas flow propagation.

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