Encyclopedia of capnograms
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|
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.
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).