Can capnography detect bronchial flap-valve expiratory obstruction?

Area of Science:

• Pulmonary physiology
• Mechanical ventilation
• Respiratory mechanics

Background:

• Bronchial flap-valve obstruction leads to sequential lung emptying, previously detected by prolonged, low tracheal expired flow (V).
• Clinical ventilation circuits can mimic this by causing prolonged, low exhaled V, potentially confusing diagnosis.
• A biphasic tracheal capnogram was hypothesized as a distinguishing marker for flap-valve obstruction due to sequential CO2 unloading.

Purpose of the Study:

• To investigate if bronchial flap-valve obstruction creates a biphasic tracheal capnogram.
• To differentiate flap-valve obstruction from normal ventilation circuit resistance.
• To identify reliable markers for sequential lung emptying during obstruction.

Main Methods:

• A dual mechanical test lung was ventilated, with a flap-valve creating expiratory resistance in the left main-stem bronchus.
• Airway parameters including expired flow (V), pressure (P), and CO2 (PCO2) were monitored.
• Measurements were compared with and without the flap-valve obstruction.

Main Results:

• Left bronchial V was delayed during obstruction, mixing dead space CO2 with normal exhalate, depressing the capnogram's early phase.
• Severe obstruction reduced left lung perfusion and alveolar PCO2, further altering the capnogram's plateau.
• Low magnitude bronchial V from the obstructed lung had limited effect on the overall tracheal capnogram.

Conclusions:

• Bronchial flap-valve obstruction alters tracheal capnograms by delaying CO2 unloading from the affected lung.
• Late expiratory flow changes, independent of total tidal volume, appear to be the most reliable indicator of sequential lung emptying.
• Capnography combined with expired flow analysis can help differentiate bronchial flap-valve obstruction from other ventilation-induced flow alterations.