The Science Journal of the American Association for Respiratory Care

Original Contributions

September 2002 / Volume 47 / Number 9 / Page 994

Relationship of Neonatal Endotracheal Tube Size and Airway Resistance

Melisa J Oca MD, Michael A Becker RRT, Ronald E Dechert RRT, and Steven M Donn MD

BACKGROUND: Infants receiving mechanical ventilation require narrow-lumen, small-diameter endotracheal tubes. OBJECTIVE: Compare the resistances of endotracheal tubes used in the neonatal intensive care unit. METHODS: Endotracheal tubes of internal diameter 2.5, 3.0, 3.5, and 4.0 mm were tested with a standard neonatal ventilator and a test lung. An endotracheal tube of each diameter was cut to 12 cm and connected to a flow transducer at one end and the test lung at the other. Serial measurements of resistance were made at various flows (6, 8, 10, and 12 L/min) and ventilator rates (30-90 breaths/min) encompassing the ranges of clinical practice. Analysis of variance was performed for each tube size, comparing resistance to flows and ventilator rates. RESULTS: Resistance was significantly higher with the 2.5 mm tube than with the others. There was also a consistent trend, in all the tube sizes, towards higher resistance as flow was increased. CONCLUSIONS: The higher resistance of the 2.5 mm tube may be detrimental to extremely low birthweight infants kept on mechanical support merely "to grow." The higher resistance may increase the work of breathing and thus increase caloric expenditure and impede growth.
Key words: neonatal, endotracheal tube, airway resistance, mechanical ventilation, ventilator.
[Respir Care 2002;47(9):994–997]

Introduction

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.

The entire text of this article is available in the printed version of the September 2002 RESPIRATORY CARE.

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