Sir,
Heat and moisture exchanger filters (HMEFs) are widely used during general anaesthesia to humidify gases and protect the breathing system from expired infective droplets. One of the major advantages includes the prevention of microbial contamination in the breathing circuit which holds a major significance during this evolving coronavirus pandemic. However, they can alter the ventilatory parameters in a mechanically ventilated patient.[1] Moreover, these can alter the capnography waveform, particularly in paediatric patients and can cause the capnogram to disappear altogether. We hereby wish to highlight an unusual case scenario wherein the HMEF resulted in the disappearance of capnography waveform in a neonate under general anaesthesia.
A 4-day old neonate weighing 3 kg was scheduled for excision of occipital encephalocoele. General anaesthesia was induced and endotracheal intubation was done with an uncuffed endotracheal tube (ETT) of internal diameter 3 mm. Initially, we tried 3.5 mm tube but it could not be inserted. The correct position was confirmed by auscultation, visible chest rise and capnography. ETT was connected to Drager Primus anaesthesia workstation and ventilated with volume control mode with tidal volume 30 mL and frequency 30 breaths/min. The baseline values for end-tidal carbon dioxide (EtCO2) of 30 mmHg, airway pressures of 20 cm water and oxygen saturation (SpO2) 98% were noted. Prone positioning was done to aid in the surgical field. A fresh paediatric HMEF was connected and CO2sampling line was attached to the side port of HMEF. Sudden disappearance in capnogram was observed within a few seconds after connecting HMEF but no change in heart rate, SpO2and tidal volume was noticed. The patient was immediately shifted to the manual mode of ventilation. Paediatric circuit F was attached to Drager machine and the machine provided the ventilator parameters. Bag compliance was found to be normal. Auscultation of the chest revealed equal breath sounds bilaterally and chest rise was adequate. Other possibilities such as kinking of ETT and circuit kink were ruled out. Attempts were made to change the sampling line of EtCO2and installation of another fresh HMEF, but no effect on capnogram was observed. Throughout the surgery, SpO2was between 97–98% with a fraction of inspired oxygen (FiO2) of 0.5 although a slight increase in airway pressure was noted. Eventually, after removing the HMEF, we attached the EtCO2 sampling line between the ETT and breathing circuit which resulted in the appearance of normal capnograph within 1–2 min and airway pressure returned to normal. The problem was diagnosed as excessive dead space in comparison to delivered tidal volume due to HMEF which hampered EtCO2reading.
HMEF is of great importance in intubated patients where the normal function of the upper respiratory tract is compromised, especially in neonates and infants. However, it has certain drawbacks like breathing circuit disconnection owing to filter weight or moisture condensation.[2] Therefore, we used HMEF after prone positioning. These were electrostatic filters of dead space 30 mL, low resistance, weighing 20 g and having filtration efficiency of 99.99%.
In the present case, after induction of anaesthesia, the position of ETT was confirmed by EtCO2.[3,4] However, after putting HMEF, EtCO2decreased and eventually disappeared. HMEF is routinely used for both adult and paediatric patients but we never encountered this problem earlier. Further, Costigan and Snowdon reported a case where the capnography waveform could not be detected with the use of HMEF and documented that accuracy and implementation of anaesthesia monitors are inevitably affected by the use of HMEF.[5]
To conclude, although HMEF adds to significant dead space volume in neonates, one should be vigilant enough to remove it and directly connect the CO2sampling line to the ETT connector. One should not panic and rather be vigilant in monitoring the ventilatory parameters.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
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Conflicts of interest
There are no conflicts of interest.
REFERENCES
- 1.Wikes AR. Heat and moisture exchangers and breathing system filters: their use in anaesthesia and intensive care. Part 2- practical use including problems and their use with paediatric patients. Anaesthesia. 2011;66:40–51. doi: 10.1111/j.1365-2044.2010.06564.x. [DOI] [PubMed] [Google Scholar]
- 2.Kwon MA. The effect of a paediatric heat and moisture exchanger on dead space in healthy paediatric anaesthesia. Korean J Anesthesiol. 2012;62:418–22. doi: 10.4097/kjae.2012.62.5.418. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Abhishek C, Munta K, Rao M, Chandrasekhar CN. End-tidal capnography and upper airway ultrasonograpgy in the rapid confirmation of endotracheal tube placement in patients requiring intubation for general anaesthesia. Indian J Anaesth. 2017;61:486–9. doi: 10.4103/ija.IJA_544_16. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.D'Mello J, Butani M. Capnography. Indian J Anaesth. 2002;46:269–78. [Google Scholar]
- 5.Costigan SN, Snowdon SL. Breathing system filters can affect the performance of anaesthetic monitors. Anaesthesia. 1993;48:1015–6. doi: 10.1111/j.1365-2044.1993.tb07501.x. [DOI] [PubMed] [Google Scholar]