Category : Correspondence

A normal capnograph trace does not rule out circuit disconnection

Santhana Kannan, MD, FRCA

Consultant anesthesiologist & critical care specialist, City Hospital, Birmingham (UK).

Correspondence:Dr. S. Kannan, Department of Anesthesia, City Hospital, Dudley Road, Birmingham B18 7QE.(United Kingdom); Phone: 0044 121 5074343; Fax: 0044 121 5074349; E-mail: kannan.gas@gmail.com

Loss of a capnograph trace is usually taken as one of the indicators of disconnection in the anesthetic circuit and vice versa.1 The following report shows that presence of a normal capnograph trace does not rule out circuit disconnection.

A 65-year old woman was scheduled for ankle surgery. Apart from controlled hypertension, she was otherwise healthy. Anesthesia was induced in the anesthetic room with a combination of midazolam, fentanyl and propofol. A size 3 laryngeal mask airway was inserted and anesthesia maintained with 40% oxygen, nitrous oxide and 1% isoflurane under spontaneous ventilation. Femoral and sciatic blocks were given using a nerve stimulator and 20 ml of bupivacaine 0.25% into each site. Patient was then transferred to the operating room. Routine monitors and the circle anesthetic circuit were reconnected and the settings of the gas flows and isoflurane vaporizer checked. Patient had a heart rate of 60 beats/min, blood pressure 100/48 mmHg, oxygen saturation of 97% and end expiratory carbon dioxide concentration of 5.8% with a normal capnograph trace. About five minutes later, it was noticed that the end tidal isoflurane concentration was reading 0.2% although the inspired concentration was set at 1%. The N2O concentration was reading 20%. The total gas flow was about 6 L at this stage. The vaporizer was not empty. The measured tidal volume was reading around 0.3 L and had not changed. The reservoir bag was not filling adequately but was not collapsed. The capnograph trace was unchanged. There was no change in the patient’s vital signs. It was then noticed that there was a disconnection at the common gas outlet. The circuit was reconnected and the gas concentrations improved. Further anesthetic course was uneventful. The incident was discussed with the patient postoperatively. There was no evidence of any intraoperative awareness.

The previous patient had received a spinal anesthetic with supplementary oxygen. The common gas outlet had been disconnected to connect the facemask oxygen tubing. However, when that operation finished, it was not reconnected. The side-stream port for sampling tube for spirometry and gas analysis was situated between the filter and the Y-piece of breathing circuit. Since the patient was breathing spontaneously, the capnograph was showing a normal waveform despite the disconnection at the common gas outlet. This also ensured that the measured expired tidal volume was unchanged. The recirculation of gases within the circle system contributed to a slower rate of fall in the concentration of isoflurane and N2O. The presence of the regional nerve blocks ensured analgesia and little change in vital signs.

This report shows that a normal capnograph trace does not rule out circuit disconnection during spontaneous ventilation. It also reinforces the need to briefly re-check the anesthesia machine after every case.

Reference

Moon RE, Camporesi EM: Respiratory Monitoring, Anesthesia, 5th Edition, Miller RD (Editor). Philadelphia, Churchill Livingstone, 2000, pp 1255-95