Shapiro et al2 noted that the highest values for the work of breathing occur in the smallest endotracheal tubes. Moreover, they noted that at all levels of minute ventilation, work values were different between the different tube sizes tested. It could be argued that at low minute ventilations, this does not represent a clinically significant difference to the normal subjects evaluated. However, the clinical significance of this finding in patients in respiratory failure remains to be tested. Patients can often wean successfully with minute ventilations greater than 10 L/min. Fiastro et al showed that successful weaning could be achieved in a group of patients requiring prolonged mechanical ventilation at a time when the minute ventilation was greater than 10 U min. The contribution to the work of breathing at this level of minute ventilation may be clinically significant. Furthermore, Fiastro et al suggested that work of breathing in patients requiring prolonged mechanical ventilation may be a better reflection of a patients ability to wean than the standard weaning parameters that are often used.
All changes in the work of breathing cannot be ascribed only to the physical properties of the tubes, however. The endotracheal tube (or tracheostomy tubes) may cause reflex diffuse bronchoconstriction via irritant receptor stimulation. These tubes also may prevent dynamic compression of the airways by splinting the trachea against collapse which in turn will require increased flow rates in the trachea in order to achieve the linear velocities required to clear secretions. The effect of this is to cause secretions to accumulate at the area around the end of the tube. However, endotracheal tubes have been shown by Galu not to effect the development of cough pressures. Flows tend to be submaximal with the tube present, since the resistance at high flows becomes prohibitive. In addition, mucociliary clearance has been shown to be decreased with inflation of the endotracheal tube cuff.
It is possible that increased upper airways resistance caused by tubes may be physiologically advantageous, analogous to the pursed-lip breathing of adults with obstructive lung disease or the grunting respirations of infants in respiratory distress. To this end, it has been suggested that tracheostomy tubes should mimic the flow resistance characteristics of the upper airway. Tracheostomy and endotracheal tubes also decrease anatomic dead space. Cullen showed that tracheostomy breathing in patients with emphysema resulted in a reduction in minute ventilation compared to mouth breathing. The volume of the upper airway is approximately 0.5 ml/lb, and the volume of endotracheal tubes is approximately 3545 ml, although this will vary with tube length, diameter, and the connector used. This effectively reduces upper airway anatomic dead space by about 50%. The volume of tracheostomy tubes of similar internal diameter is not much less than this. These data suggest that proceeding from an endotracheal tube to a tracheostomy tube would not alter minute ventilation substantially.
Tags: artificial airways, breathing, endotracheal tube