How does this information relate to the clinical situation? Since fully developed flow regimens probably rarely occur in the tubes we use, the equations described form only an approximation of the truth. However, they indicate that the longer the tube, the greater the resistance—although one study in infant endotracheal tubes reported that the relationship of resistance to tube length was not linear, as one would have predicted from the equations for fully developed flow regimens outlined above. Nevertheless, resistance is reduced by shortening the tube. These equations also predict that the smaller the diameter of the tube the greater the resistance to flow (Fig 4). Tube diameter is the most important variable, since the pressure drop down the tube is inversely related to the fourth power of the radius for fully developed laminar flow and to the fifth power for fully developed turbulent flow. Measurements of endotracheal tube resistance demonstrate the importance of tube diameter.
The placing of an endotracheal tube will increase the upper respiratory resistance. Pressure drops across endotracheal tubes are increased compared with the normal upper airway (Fig 4). The relationship between pressure drop and flow rates for tubes of the same length (24 cm) and different internal diameters is such that as flow rate increases, the pressure drop increases linearly until turbulent flow occurs. Then the pressure drop increases in a curvilinear fashion. Thus, the use of lower flow rates will reduce the pressure drop across the endotracheal tubes.
The effect of tube diameter on work and pressure drop through endotracheal tubes explains the importance of maintaining the tube clear from secretions or concretions. In addition to the effect secretions have on tube diameter is the effect they have on the tube roughness, which will increase the Moody friction factor. Yung and Snowden showed how concretions in the tube affect the pressure flow characteristics of tracheostomy tubes. Humidification and tracheal toilet are quite important in the care of intubated patients. A similar effect on the resistance of the tube will occur if the patient bites down on the endotracheal tube or if the tube becomes kinked, not uncommon occurrences.
Figure 4. Schematic representation of the pressure drop across the normal airway and endotracheal tubes of different sizes as a function of flow rate.
Tags: artificial airways, breathing, endotracheal tube