Objective: We measured back pressures present in a gas delivery circuit for 100% oxygen and 70% helium:30% oxygen (70:30 heliox) at physiological temperature (37�C) using various sizes of nasal cannulae.

Design and Setting: A U-tube manometer was used to measure the back pressure in a Fisher-Paykel® MR850 gas delivery circuit that consisted of a gas source (either 100% oxygen or 70:30 heliox) flowing through a respiratory humidifier to various sizes of nasal cannulae. For each experiment, four trials were conducted.

Intervention: Experiments were conducted at 1 liter per minute (LPM) and 5 LPM of 100% oxygen and 1 LPM and 5 LPM of 70:30 heliox. Four trials were conducted: 1) A standard adult, pediatric, infant, and neonate nasal cannula was connected to the U-tube manometer. 2) A pediatric, infant, and neonate cannula with the nose piece removed was connected to the U-tube manometer. 3) A pediatric nasal cannula was connected to a pediatric, infant, and neonate nose piece, then connected to the U-tube manometer. 4) A pediatric, infant, and neonate ear tube was shortened to a length of 10 cm then connected to the U-tube manometer.

Results: Back pressures in the circuit were higher at 5 LPM flow rates than 1 LPM for both 100% oxygen and 70:30 heliox. At 5 LPM, back pressures were higher for 100% oxygen than for equivalent flow rates of 70:30 heliox. At 5 LPM, back pressures were higher for smaller nasal cannulae for the control, modified nose piece, and short ear tube trials. Decreases in back pressure were seen by using a pediatric nasal cannula and by shortening the length of the ear tubes.

Conclusion: Gas flow rates of 5 LPM produce back pressure in gas delivery circuits. Back pressure in gas delivery systems may be decreased by modifying the nasal cannulae and shortening ear tube length.