One of the common questions asked by residents is what happens to PETco₂ and capnogram during endobronchial Endobronchial Intubation (EBI)?.

Capnography is not a sensitive diagnostic tool of endobronchial intubation (EBI). A study comparing capnography with pulse oximetry in children, showed that pulse oximetry provided the first diagnostic clue in 13 of 14 episodes of EBI; the one event diagnosed by capnography had concurrent desaturation.1

In a 1997 publication from the Australian Incidence Monitoring study, accidental bronchial intubation occurred in 154 of 3947 (3.7%) incidents, and capnography remained normal or unremarkable during 88.5% of the episodes. One-third of the cases were associated with head or neck surgery and possible flexion of the patient’s head.2

Lower as well as higher end-tidal Pco₂‘s, however, have been reported during EBI. Riley and Marcy presented a case report where they were able to diagnose endobronchial intubation when there was a rise in end-tidal co₂ from 4.1% to 6.1%.3 The rise in end-tidal Pco₂ was the result of reduced ventilation consequent to leakage of tidal volumes around a non-cuffed tube migrating into the bronchus, thereby increasing the resistance to ventilation.3 On the other hand, Chang et al 4, have reported a decrease in ETco₂ when the endotracheal tube migrated into the bronchus in dogs ventilated under anesthesia and IPPV.4 During EBI with a cuffed tube, and volume cycled ventilation, there is a doubling of alveolar ventilation of one lung resulting in a decrease in PAco₂ of the ventilated lung with a significant drop in arterial PO2 with a minimal increase in Paco₂. This results in a fall in PETco₂.4-6

If a pressure cycled ventilation is used, then PETco₂ may rise consequent to hypoventilation. Occasionally, the author has observed an obstructive pattern on the capnogram (with prolonged phase II and steep phase III), during cesarean section general anesthesia. This is due to a partial occlusion of the endobronchial airway, most probably as a result of the endotracheal tube abutting against the wall of the bronchus. The capnogram reverted to a normal shape as soon as the endobronchial tube was repositioned within the trachea.

Gilbert and Benumof7 have reported a biphasic capnogram during endobronchial intubation in a patient with no known lung disease who was found to have a right main-stem bronchial intubation.

Right main stem intubation	Endotracheal tube pulled back into the trachea

This capnogram probably occurred as the tip of the endotracheal tube changed while moving in and out of the right main bronchus during the respiratory cycle. This resulted in a partial obstruction to gases from the left lung, thereby prolonging expiratory time from the left lung. The initial peak is due to the carbon dioxide from the well-ventilated right lung. The second peak is most likely due to the prolonged expiratory time of the poorly-ventilated left lung.

Therefore, in summary, capnography is a non-sensitive diagnostic tool to detect EBI. PETco₂ can increase, decrease, or remain unaltered, depending on the circumstances.

A normal PETco₂ and normal capnogram will not rule out EBI. On the other hand, an increased PETco₂, reduced PETco₂, or an abnormal capnogram should encourage one to consider using EBI in the differential diagnosis of hypoxia or increased peak inspiratory pressures.

References:

1. Rolf N, Cote CJ. Diagnosis of clinically unrecognized endobronchial intubation in peadiatric anaesthesia; which is more sensitive, pulse oxymetry or capnography? Paediatr Anaesth 1992;2:31-5.

2. McCoy EP, Russell WJ, Webb RK. Accidental bronchial intubation: an analysis of AIMS incident reports from 1988 to 1994 inclusive. Anaesthesia 1997;52:24-31.

3. Riley R, Marcy J. Unsuspected endobronchial intubation – Detection by Continuous Mass Spectrometry. Anesthesiology 1985;63:203-4.

4. Chang PC, Reynolds FB, Lang SA, Ha HC. Endobronchial intubation in dogs. Can J Anaesth 1990;37(suppl);S44.

5. Gandhi SK, Mushi CA, Kampine JP. Early warning sign of accidental endobronchial intubation: A sudden drop or sudden rise in PAco₂. Anesthesiology 1986;65:114-5.

6. Benumof JL. Monitoring. Anesthesia for Thoracic Surgery. Philadelphia, W.B.Saunders Company;p250.

7. Gilbert D, Benumof JL. Biphasic carbon dioxide elimination waveform with right mainstem bronchial intubation. Anesth Analg 1989;69:829-32 (with permission).