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Designed, Produced, and maintained (Edition 10, March 2019) by 

Bhavani Shankar Kodali MD

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LMA and Capnography

PETCO2 measured via LMA or ETT correlate well with PaCO2 during mechanical ventilation in children as well as in adults breathing spontaneously

 PETCO2  measured via ET tube                                PETCO2 measured via LMA
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During spontaneous ventilation in adults, the mean difference between PaCO2 and PETCO2 measured via LMA is similar to that measured via endotracheal tube.1 However, in children, PETCO2 measured via LMA does not accurately reflect PaCO2 in spontaneously breathing children.2 On the other hand, infants and children weighing less than 10 kg who are mechanically ventilated via the LMA, PETCO2 is as accurate an indicator of PaCO2 as when ventilated via LMA.3

References:
1. Hicks I, Soni N, Shephard J. Comparison of end-tidal and arterial carbon dioxide measurements during anaesthesia with laryngeal mask airway.  Br J Anaesth 1993;71:734-5.
 
2. Spahr-Schopfer IA, Bissoonnette B, Hartley EJ. Capnometry and the pediatric laryngeal mask airway. Can J Anaesth 1993;40:1038-43.

3. Chhibber AK, Kolano JW, Roberts WA. Relationship between end-tidal and arterial carbon dioxide with laryngeal mask airways and endotracheal tubes in children. Anesth Analg 1996;82:247-50.]
 
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Venous CO2embolism

End-tidal CO2 monitoring is essential during laparoscopy, as it may help in the early detection of venous CO2 embolism (accidental insufflation of CO2 into the veins).  In addition, CO2 is also absorbed from abdominal cavity.  A transient but rapid rise in PETCO2, has been suggested as a useful early sign of venous CO2 embolism.
However, when the CO2 embolism is large enough so as to cause pulmonary outflow obstruction resulting in decreased cardiac output, PETCO2 decreases thereby increasing the alveolar dead space as well as (a-ET)PCO2 difference.

Reference:

Shulman D, Aronson HB. Capnography in the early diagnosis of carbon dioxide embolism during laparoscopy.  Can Anaesth Soc J 1984;31:455-9.
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Endobronchial Intubation

The characteristic features of capnogram in endobronchial intubation are dependent on the extent of endobronchial intubation. The ETCO2 can increase if the consequent one lung ventilation results in hypoventilation. On the other hand, if the tube is far down resulting in over all reduced ventilation (small tidal volumes), then ETCO2 can decrease (not adequate ventilation). However, if the endobronchial intubation results in a airway obstruction, the capnogram may have prolonged phase II and phase III with increased slope. The author has observed several such capnograms during cesarean section general anesthesia (because of intubation circumstances of cesarean section) and capnograms revert to normal once adjustment of the endotracheal tube are made (The tube probably too close to carina).

A dual waveform capnogram has also been observed in endobronchial intubation.

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This animation shows how end-tidal PCO2 changes with air embolism. Alveolar dead space is increased.
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