the patient was a 13-day - old male infant 1,350 g at birth , who was born by cesarean delivery . pulmonary surfactant was administered via an endotracheal tube twice because of respiratory distress syndrome and the newborn was provided with mechanical ventilation for 35 hours . the patient received oxygen by hood until the sixth day of life , and received 2.5 mg injections of aminophylline every 12 hours because of apnea diagnosed on the eighth day of life . gavage feeding was discontinued and bile excretion was noted . before the surgery , the vital signs were stable . there were no abnormalities on the ecg or chest x - ray . on the day of surgery , standard monitoring ( electrocardiogram , noninvasive blood pressure and peripheral pulse oxygen saturation ) was performed . injection of 500 ml of 10% dextrose water + nacl 15 meq + heparin 500 iu 8 ml / hr was carried out via the internal jugular vein using a 24 gauge catheter and was maintained at 10 - 0 ml / hr during the surgery . an 8 vol% of sevoflurane was administered and then maintained with a 3 vol% of sevoflurane . fresh gas flow was maintained with o2 1 l / min and air 4 l / min . injection of 1 mg of rocuronium was administered because there was no difficulty with mask ventilation . two minutes later , intubation with a 2.0 mm i d ( internal diameter ) endotracheal tube was attempted . the tube was checked and intubation attempted again with the same endotracheal tube , two minutes later . the tube was fixed at 7 cm the tube was cut from 12 cm to reduce the dead space . after cutting the tube , there was no change in the depth ; ventilation could not be provided and there were no breath sounds . the endotracheal tube was removed and mask ventilation was provided again . the tube was fixed at 7 cm and cut from 12 cm to reduce the dead space . after cutting the tube , one - lung ventilation was suspected , so manual ventilation was attempted , changing the depth of the endotracheal tube . after the intubation , ventilation was successful and breath sounds were detected bilaterally , in addition , to regular capnography of the etco2 . the tube was fixed at 7 cm and cut from 12 cm to reduce the dead space . however , this time the ventilation was successful . injections with 0.05 mg of pyridostigmine and 0.01 mg of glycopyrrolate were provided and spontaneous ventilation occurred . after surgery , the duration for anesthesia was 2 hr 30 min and the operation was 2 hr . there was no specific change in the bp or hr . two hours after surgery , the endotracheal tube was removed because the respirations were stable . however , rigid bronchoscopy was not performed for confirmation , because there were no specific symptoms at that time . cough , inspiratory stridor , and recurrent pneumonia are symptoms associated with tracheomalacia ; the most severe symptom is apnea . in most cases , symptoms improve spontaneously and patients require conservative care . however , if severe apnea develops , recurrent pneumonia , or extubation failure , surgery is needed . tracheomalacia is frequently associated with other congenital anomalies such as an esophageal stricture or tracheoesophageal fistula . it is also associated with tumors that can press on the trachea , connective tissue disorders and long - term high pressure ventilator care . common symptoms associated with a tracheoesophageal fistula include dyspnea during feeding , cyanosis , and aspiration pneumonia caused by regurgitation of food or saliva ; this can be diagnosed by failure of attempts to insert a nasogastric tube . the diagnosis is usually made immediately after birth or within 15 days of life . the h - type of tracheoesophageal fistula , which is not associated with an esophageal stricture , may have no specific symptoms until adulthood . however , in this case , a tracheoesophageal fistula was ruled out because the patient had gavage tube feeding and there were no specific signs on the gastrograffin test . ventilation was not successful with a 2.0 mm i d tube or a cut 2.5 mm i d tube in this case . the relationship between the flow , the tube and gas in a straight tube is explained by the hagen - poiseuille equation . the hagen - poiseuille equation is as follows : v = pr4/8ul ( v = flow and  = 3.1416 , p = pressures , r = radius of tube , l = length of tube , u = viscosity of gas ) . a tube with a smaller radius has greater flow ; the flow was greatest for the 2.0 mm i d tube and slowest with the 3.0 mm i d tube . according to the bernoulli effects , pressure decreases as the flow increases , at narrow points . turbulent flow occurs at the end of the tube where the radius of the tube changes . the relationship between the flow and the pressure is as follows : v  p . it is assumed that the obstruction of the airway was caused by the pressure outside of the tube . flow increases with the 2.0 mm i d tube more than with 3.0 mm i d tube . the faster flow passes to the end of the tube and the pressure inside of the tube decreases when the faster flow passes the narrow point caused by the tracheomalacia . at that moment , obstruction of the airway occurs due to pressure outside of the tube . in this case , ventilation was not successful immediately after the tube was cut to reduce the dead space when a 2.5 mm i d tube was used . ventilation failure after cutting the tube was caused by obstruction of the airway due to a decrease in the pressure . the decrease in pressure was caused by shortening of the length of the tube which increases the flow rate ; then , faster flow passes through the narrow segment . reported that with a 4.0 mm i d tube , there is a 22% decrease in the resistance when the length of tube was changed to 11.3 cm from 20.7 cm . the decrease of the pressure by 22% was associated with shortening of the length of the endotracheal tube . for endotracheal tubes smaller than the 4.0 mm i d tube , the decrease in the radius is associated with a rapid increase in the pressure . an increase in the gas flow from 5 l / min to 10 l / min causes an increase of pressure from 81.2 h2o / l / sec to 139.4 h2o / l / sec in a 2.5 mm i d tube while the pressure changes from 3.1 h2o / l / sec to 4.6 h2o / l / sec in a 6.0 mm i d tube . in this case , the 2.5 mm i d tube length was changed from 18 cm to 12 cm and the 3.0 mm i d tube length was changed from 19 cm to 12 cm . the flow in the tube was not laminar flow but turbulent flow , which is not directly proportional to the pressure or length . the decrease in pressure was 18% and 19% , each , according to the relationship between the length of laminar flow and the pressure . disappearance of the parabola form , with turbulent flow , can lead to an increase in the pressure . it is assumed that the decrease of pressure is lower with turbulent flow than with laminar flow . during manual ventilation , 5 l / min was used ; however , the fresh gas flow rate was briefly more than 10 l / min . this patient received mechanical ventilator care and oxygen hood treatment before surgery ; there were no abnormal symptoms associated with respiration . reported that 95% of patients with tracheomalacia have no symptoms until two to three months after birth . thus , when obstruction of the airway occurs during induction of general anesthesia , tracheomalacia should be suspected . it is important to select the radius and length of a tube carefully when endotracheal intubation is performed in a patient with tracheomalacia .