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A bizarre end-tidal carbon dioxide waveform

Pittsburgh, Pennsylvania

To the Editor:

A 37-yr-old woman was scheduled to undergo partial gastrectomy for gastric tumour. Anesthesia was induced using fentanyl, thiopental and succinylcholine and maintained with isoflurane, 50% oxygen in air, morphine and cisatracurium. Mechanical ventilation settings consisted of 10 mL•kg^-1 tidal volume, respiratory rate of 10, I: E ratio of 1:2 and fresh gas flow of 2 L. The ETCO₂ waveform (Figure) was thought to be the patient starting to breathe and cisatracurium was administered. There was no change in the peak inspiratory pressure. Correct placement of the endotracheal tube (ETT) was again verified by listening to breath sounds and observing for bilateral chest movement. The ETT was suctioned to ensure there was no kinking or secretions that may have caused partial obstruction. Fibreoptic bronchoscopy ruled out movement of the ETT endobronchially during expiration (theoretically, both conditions could produce the observed waveform). The bellow of the ventilator was moving appropriately (a colleague suggested that a similar waveform is produced if the respiratory bellow stops at the middle of the expiratory cycle for few instants). There was no visible obstruction in the ETCO₂ sampling port or in the anesthesia circuit. To eliminate a potential problem in this area, the ETCO₂ side port sampling line and the capnograph (Datex Capnomac Ultima) were changed. At that time, it was noted, however, that the distal end of the ETCO₂ sampling line (luer-lock-fitting) attached to the capnograph was slightly loose. Upon tightening, the ETCO₂ waveform returned to normal. The abnormal oxygen waveform also returned to normal. Upon loosening again, the same ETCO₂ and oxygen waveform were reproduced. We were able to duplicate the same ETCO₂ waveform on a different patient.

View larger version (52K): [in this window] [in a new window]   FIGURE The initial, small rise in ETCO2 (1st arrow) during expiration results from entrainment of room air at the loose connection between the distal end of the ETCO2 sampling line and the port of the capnograph machine. This air dilutes the expired carbon dioxide resulting in a smaller than normal rise in the ETCO2.1 The secondary upsloping of the ETCO2 (2nd arrow) is noted at the onset of the next positive pressure ventilation (PPV). We hypothesize that less entrainment of room air occurs at that time because of increased pressure secondary to PPV. Less entrainment of air results in less dilution of carbon dioxide and, therefore, the ETCO2 rises.

Reference

1 Vender JS, and Gilbert, HC. Monitoring the anesthetized patient. In: Barash PG, Cullen BF, Stoelting RK (Eds.). Clinical Anesthesia, 3^rd ed. Philadelphia: Lippincott-Raven, 1992: 623.

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