Where along the tracheobronchial tree is the isothermic saturation boundary (ISB) located in relation to its normal position in a patient who has an endotracheal tube inserted?
A. The ISB shifts upward when a person is intubated.
B. The ISB shifts downward when a person is intubated.
C. The location of the ISB fluctuates when a person is intubated.
D. The ISB has no influence in relation to endotracheal intubation.
?ANS: B
A. Incorrect response: See explanation B.
B. Correct response: The isothermic saturation boundary (ISB) is the point along the tracheobronchial tree where the inspired gas assumes 100% saturation at body temperature. Normally, the ISB resides in the 4th to 5th generation of subsegmental bronchi. The location of the ISB often shifts in the presence of lung disease, and the patient's fluid status. Above the ISB, the patient's respiratory mucosa renders heat and moisture to the inspired gas. Below the ISB, the exchange of heat and moisture are essentially in equilibrium.
The insertion of an artificial airway lowers the location of the ISB. Artificial airways do not provide heat and moisture to the inspired gas. In fact, an artificial airway causes the air to bypass the normal heat and moisture exchange mechanism by preventing contact between the inspired gas and the respiratory mucosa where the tube is located. Consequently, when an artificial airway is inserted in a patient, the therapist must ensure that the inspired air is sufficiently heated and humidified to prevent drying of the patient's respiratory mucosa.
C. Incorrect response: See explanation B.
D. Incorrect response: See explanation B.
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