Separate circuit nasal cannulae for end-tidal CO₂ monitoring may lead to hypoxia in patients with unilateral nasal airway obstruction

Dear Editor,

Monitored anesthetic care in the form of moderate or deep sedation is ubiquitous in anesthesia practice. Given that most sedative agents affect respiration, hypopnea and apnea (secondary to airway obstruction and/or central nervous system depression) is of constant concern, as it may lead to significant hypoxia and potentially respiratory or cardiovascular collapse. In this setting, nasal cannulae are commonly used to provide low-flow supplemental oxygen to patients undergoing procedural sedation. The utilization of capnography monitoring has been shown to decrease the incidence of hypoxia compared to supplemental oxygen alone, likely due to earlier detection of ventilatory inadequacy allowing prompt intervention prior to the development of critical hypoxia.¹,² Therefore, capnography monitoring is deemed mandatory for all patients undergoing general anesthesia or procedural sedation by many renowned anesthesiology societies worldwide.³,⁴,⁵,⁶ Hence, nasal cannulae with combined side-stream sampling ports for waveform capnography monitoring are commonly used by anesthesiologists.

Differences in nasal cannula design, however, can influence their ability to deliver O₂ while concurrently and accurately sampling end-tidal CO₂ (EtCO₂). For instance, expired CO₂ can be washed out by high oxygen flows within standard nasal cannulae where oxygen is delivered into both nares through bilateral prongs (Fig. 1A). Conversely, a “separate circuit” design - whereby oxygen is delivered through one (single) nare and CO₂ is sampled from the other/contralateral nare (Fig. 1B) - can be more effective in such instances.

Figure 1
Alternatives to provide supplemental oxygen while concurrently monitoring end-tidal CO₂ (EtCO₂) during procedural sedation. (A) Standard nasal cannula. (B) EtCO₂-measuring nasal cannula with independent/separate circuits for oxygen delivery and EtCO₂ sampling. The impermeable barrier between circuits at the level of the nasal prongs is denoted with a red arrow. Note the oneway oxygen inflow and CO₂ outflow (white arrows) through the independent/separate circuits. (C) Facemask with adapted EtCO₂ sampling tubing using an intravenous cannula.

Indeed, this “independent/separate circuit” cannula design is particularly beneficial when higher oxygen flows are required as it prevents the expired gas mixture from being washed out by the oxygen flow, thereby significantly improving accuracy of waveform capnography monitoring. However, delivering oxygen to a single nare is not ideal and can, in some cases, present a safety risk. For instance, patients with acute and/or chronic unilateral nasal obstruction may become hypoxic if oxygen is delivered to the obstructed nare. Additionally, increasing oxygen flow rate is unlikely to improve the hypoxemia in such instances. Notably, (unilateral) nasal airway obstruction is a common symptom and can occur due to a variety of processes, including sinonasal mass, acute sinusitis following upper respiratory tract infection, chronic sinusitis, allergic rhinitis, nasal polyps, and septal deviation.⁷

Anesthesiologists should be aware of the specific type of EtCO₂-monitoring nasal cannulae used at their institution, while also being aware of potential new equipment they may be faced with minimal or no forewarning due to product shortages commonly seen in this post-pandemic era. Consideration should be given to assessing patients for nasal patency prior to using nasal cannulae, particularly when independent/separate circuit cannulae are the only option available. In such instances, if a patient with unilateral nasal obstruction becomes hypoxic, the cannula can be turned 180 degrees thus delivering oxygen to the opposite (patent) nare which may improve oxygenation. Finally, in patients with known nasal airway obstruction, consideration should be given to utilizing a facemask for oxygen delivery. Notably, EtCO₂ monitoring can also be adapted to a facemask to allow for both adequate oxygen delivery and capnography monitoring (Fig. 1C).

Both supplemental oxygen delivery and capnography monitoring are essential for patient safety and should, therefore, be routinely incorporated into monitored anesthetic care. Given the current paucity of data surrounding safety and efficacy of different capnography-monitoring devices commonly used in procedural sedation, future research is needed to ensure that devices can accurately measure EtCO₂ without compromising oxygen delivery (i.e., regardless of the adopted [higher] oxygen flows). This, however, may become increasingly difficult with new oxygen delivery devices, such as high-flow nasal cannulae. Nevertheless, familiarity with the various options for capnography-monitoring devices and their respective potential limitations is advisable for practicing anesthesiologists.

• Funding
  Departmental and institutional resources. No external funding was acquired for the current work.

References

• ¹
  Askar H, Misch J, Chen Z, Chadha S, Wang HL. Capnography monitoring in procedural intravenous sedation: a systematic review and meta-analysis. Clin Oral Investig. 2020;24:3761–70.
• ²
  Saunders R, Struys MMRF, Pollock RF, Mestek M, Lightdale JR. Patient safety during procedural sedation using capnography monitoring: a systematic review and meta-analysis. BMJ Open. 2017;7:e013402.
• ³
  Dobson G, Chau A, Denomme J, et al. Guidelines to the Practice of Anesthesia: Revised Edition 2023. Can J Anaesth. 2023;70:16–55.
• ⁴
  Klein AA, Meek T, Allcock E, et al. Recommendations for Standards of Monitoring During Anaesthesia and Recovery 2021. Anaesthesia. 2021;76:1212–23.
• ⁵
  Hinkelbein J, Lamperti M, Akeson J, et al. European Society of Anaesthesiology and European Board of Anaesthesiology Guidelines for Procedural Sedation and Analgesia in Adults. Eur J Anaesthesiol. 2018;35:6–24.
• ⁶
  Practice Guidelines for Moderate Procedural Sedation and Analgesia 2018. A Report by the American Society of Anesthesiologists Task Force on Moderate Procedural Sedation and Analgesia, the American Association of Oral and Maxillofacial Surgeons, American College of Radiology, American Dental Association, American Society of Dentist Anesthesiologists, and Society of Interventional Radiology. Anesthesiology. 2018;128:437-79.
• ⁷
  Villwock JA, Kuppersmith RB. Diagnostic algorithm for evaluating nasal airway Obstruction. Otolaryng Clin North Am. 2018;51: 867–72.

Publication Dates

• Publication in this collection
  23 Oct 2023
• Date of issue
  2023

History

• Received
  02 May 2023
• Accepted
  03 July 2023
• Published
  09 July 2023