Introduction: The benefits of using endotracheal tubes with cuff are well-known, although this practice has an impact on the work of breathing during pediatric anesthesia; however, it´s intended to study the physical consequences of variation in the length of the tubes to compensate for the loss flow, based on the law of Hagen-Poiseuille. Methodology: An experimental study was conducted in vitro, in which repeated measurements of flow were performed, varying the length and diameter thereof, with pediatric endotracheal tubes of different sizes (from 3.5mm to 6.5mm), with lengths 20 cm, 15 cm, 10 cm and its original length. Data were analyzed to measure the impact on flow. Results: Although the results show significant differences statistically (p 0,000), the modification of the length of the endotracheal tubes has less effect on flow, compared to the impact that the change in diameter has on it. Discussion: Although the change in length of endotracheal tubes can help to reduce the death space and CO2 retention, the impact that it has on flow in minimal. When it comes about reducing the work of breathing of a child in spontaneous ventilation during general anesthesia, physicians must choose the proper diameter according to the age.