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Chapter 21: The Anesthesia Machine and Ventilators

You are planning on intubating a patient in respiratory distress with a 7.0 mm internal diameter standard endotracheal tube. In order to decrease the resistance associated with the tube during the anticipated prolonged mechanical ventilation, the most efficient maneuver is to:

(A) cut the tube by 4 cm to decrease its length

(B) use an 8.0 mm rather than 7.0 mm internal diameter endotracheal tube

(C) lubricate the inside of the tube with silicone spray

(D) warm the tube in a jug of hot water prior to insertion

(E) position the tube so the cuff is immediately below the vocal cords

The answer is B. Assuming that flow is laminar in an endotracheal tube (an assumption that is NOT correct in the presence of kinks, secretions, sharp bends with RAE tubes, or very high gas flows), resistance is proportional to the length of the tube and viscosity of the gas, and inversely proportional to the radius of the internal diameter to the 4th power. Cutting the tube is a maneuver that is sometimes performed in an effort to reduce the resistance to airflow, or to prevent the external portion of the tube from becoming kinked due to traction from the circuit. However, given that most endotracheal tubes are approximately 30 cm long, a reduction in length of 4 cm only provides about 15% less resistance. In contrast, increasing the internal radius by 0.5 mm (from 3.5 mm to 4 mm), the resistance is reduced by over 40%.

Large tubes also improve pulmonary toilet during mechanical ventilation by allowing for improved suctioning of secretions and improved work of breathing during weaning off ventilation. It has been advocated by some that during anesthesia, a size 6.0 mm to 7.0 mm is appropriate, whereas 7.5 mm to 8.5 mm is preferred during long-term ventilation in the critical care unit (0.5–1.0 mm smaller for nasal intubation).

Lubrication with silicone spray, warming, or altering position within the trachea is not effective at reducing resistance to airflow.

Ref: Tobin MJ. Principles and practice of mechanical ventilation, 3rd ed. New York, NY: McGraw Hill; 2013.

Rebreathing of expired gas occurs when:

(A) carbon dioxide absorbent is 50% saturated

(B) carbon dioxide absorbent is 97% saturated

(C) fresh gas flow is less than minute ventilation

(D) the I:E ratio is 1:3

(E) auto-PEEP exceeds 10 ...

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