Dead space is reduced in tracheostomy by

Airflow resistance of the normal upper airway is substantial, constituting up to 80% of total airway resistance during nose breathing and 50% during mouth breathing.Theoretically, tracheostomy tubes should decrease airflow resistance, but in fact, this does not occur because of the smaller radius (inner diameter 7-8 mm) of the tubes. Tracheostomy tubes may reduce dead space by up to 100 mL -150 mL, 30-50% when compared to spontaneous breathing. This occurs because the tubes are small and bypass the glottic and supraglottic spaces.