Spuriously low end tidal carbon dioxide in capnometry: Nafion tube malfunction in end tidal carbon dioxide module blamed for near mishap!

Dear Editor,

We often fail to understand the technical complexity and related malfunction of newer sophisticated anesthesia workstations when encountered. Over the years there are many monitoring devices evolved for patient safety but malfunctioning of such devices gives a false sense of security and may adversely affect patient safety.1 Anesthesia is like flying an aircraft, and most mishaps are during takeoff and landing. Like the flight data recorder, the anesthesia workstation records important information which can be analyzed later to find out the technical fault to ensure further patient safety.

A 38-year-old male patient was planned for clipping of middle cerebral artery aneurysm under general anesthesia. The pre-induction vitals were blood pressure 144/96 mmHg, heart rate 88 beats per minute. After uneventful induction, the patient was intubated and put-on mechanical ventilation. The ventilatory parameters in the workstation (GE Datex Ohmeda, Avance CS2’, Wipro GE Healthcare Pvt Ltd., Kolkota, West Bengal, India) were tidal volume of 500 mL, respiratory rate of 14/min, inspiratory to expiratory ratio of 1:2, and fractional inspired oxygen of 50%. The anesthesiologist noticed an initial end tidal carbon dioxide (ETCO2) of 17 mmHg on the patient monitor (). The tidal volume was reduced to 450 mL and respiratory rate to 12, but these changes in ventilatory parameters failed to correct the ETCO2. Few minutes later, the patient had profuse sweating and tachycardia. Arterial blood gas analysis revealed pH 7.22, partial pressure of carbon dioxide (PaCO2) 56 mmHg, partial pressure of oxygen 117 mmHg, bicarbonate 23.4 mM. A gradient between PaCO2 and ETCO2 of 39 seemed inappropriate. As PaCO2 was 56 mmHg, we increased the tidal volume to 550 mL and respiratory rate to 16/min. The repeat Arterial blood gas analysis displayed pH 7.40, PaCO2 34.3 mmHg, partial pressure of oxygen 128 mmHg, HCO3 23.7 mM. There was no sweating and heart rate returned to baseline value. The potential technical sites causing an erroneous ETCO2 value were analyzed as (1) external sampling tube (2) D-fend and (3) capnometer module. The D-fend is present in front of Datex-Ohmeda capnographs. It is a disposable water trap and filter that prevents moisture or humidity from entering the monitor. The external tube and D-fend were changed with no favorable result. So, we suspected a malfunction in the ETCO2 module or workstation. The intraoperative ventilatory parameters were monitored by repeat arterial blood gas analysis from arterial cannulation site. Rest of the perioperative course was uneventful. At the end of the surgery the patient was sent to intensive care unit where the patient was extubated later. A complaint was logged with GE Health Care (GE Healthcare Pvt Ltd., Kolkota, West Bengal, India) for the technical evaluation. A technical expert from the GE Health Care retrieved the intraoperative events data and we found that the ETCO2 was low from the initiation of mechanical ventilation. The technical expert thoroughly evaluated all the components and found the defective nafion tube inside the module (Carescape Respiratory Module E-sCAiO) was the cause of false low ETCO2 ().

Ventilator control panel and display depicting the capnography with low end tidal carbon dioxide.

Note: Nafion tube (double red arrows) in the respiratory module.

Capnography works on the principal that carbon-dioxide absorbs infrared rays and the principle of Beer-Lambert law.2 Infrared waves of specific wavelengths passing across the sampling gas are absorbed by the carbon-dioxide in it which is directly proportional to ETCO2. The remaining is detected by a sensor. The sampling tube in the respiratory module is made of nafion. Nafion is a sulfonated tetrafluoroethylene-based polymer highly permeable to water.2 It equilibrates the gas for analysis with ambient air and removes excess water vapor if present. In our case, a defective nafion tube led to excessive clogging probably allowing insufficient gas to pass, resulting in erroneously low ETCO2.3

ETCO2 is a mandatory safety parameter in the perioperative period. In neurosurgical patients the ETCO2 must be monitored for maintenance of cerebral autoregulation4. Both hypercapnia as well as hypocapnia correlates with worse outcome. Hypocapnia as in our case alarmed us as it results in vasoconstriction decreasing cerebral blood flow with a resultant risk of cerebral ischemia.4 This might have an additive effect on vasospasm induced decreased cerebral blood flow caused by subarachnoid haemorrhage. Low ETCO2 values may be caused by states of low cardiac output, hyperventilation, and pulmonary embolism. However, when pathologies are ruled out, technical glitches like damage to capnometry module, sampling line breakage, temperature or pressure effects and fault in ventilator itself must be looked into.5 shows step wise approach to low ETCO2 value in patient capnometry.3 The abnormal ETCO2 values should be clinically correlated, and blood gas analysis must be done. Regular periodic maintenance of anesthesia workstations is essential to prevent this type of malfunction. We need to acquire some knowledge regarding the technical details of the machines used on a regular basis rather than waiting for a mishap to happen.

Flow diagram showing step wise approach to a low ETCO.

Note: ETCO2: End tidal carbon dioxide.

Written informed consent was obtained from the patient.