September 2002 / Volume 47 / Number 9 / Page 994
Relationship of Neonatal Endotracheal Tube Size and Airway Resistance
A number of factors contribute to the determination of airway resistance, including the air flow velocity, the length of the conducting tubes, the properties of the gas, and most importantly, the diameter of the airway. According to Poiseuille's law, resistance is inversely proportional to the fourth power of the radius. Thus, small changes in airway diameter can have a profound effect on airway resistance. In vivo studies have demonstrated that the resistance of the respiratory system and diaphragmatic activity falls by 30-40% when ventilated infants are extubated. It has also been shown that resistance is considerably higher when continuous positive airway pressure (CPAP) is applied through an endotracheal tube (ETT) than when applied through a chamber. Air flow resistances and respiratory mechanics of ETTs have been studied in vitro, and all the studies indicate that the smaller the diameter of the ETT the higher is its resistance.
The development of microprocessor-based flow sensor technology for neonatal intensive care has enabled easy measurement of airway pressure, flow, and volume, on a breath-to-breath basis. The present study was designed to measure the resistance of ETTs commonly used in clinical practice and the effects on resistance of varying flow and mechanical ventilator rate.