When a nasal cannula with two separate prongs, one for oxygen delivery and one for CO2 sampling, was used, it was the only one able to both reasonably sample ETCO2 at high fresh gas flows as well as provide the highest level of pharyngeal oxygen concentration. (Image source: Thinkstock)
When a nasal cannula with two separate prongs, one for oxygen delivery and one for CO2 sampling, was used, it was the only one able to both reasonably sample ETCO2 at high fresh gas flows as well as provide the highest level of pharyngeal oxygen concentration. (Image source: Thinkstock)

Basic anesthesia care includes monitoring exhaled end-tidal carbon dioxide (ETCO2) concentration in patients undergoing sedation. The challenge is to measure end-tidal gas concentration through a cannula that is also tasked with delivering supplemental oxygen. In their article published in this month’s issue of Anesthesia & Analgesia titled “The Effectiveness of Oxygen Delivery and Reliability of Carbon Dioxide Waveforms: A Crossover Comparison of 4 Nasal Cannulae,” Dr. Thomas J. Ebert, Department of Anesthesiology, Medical College of Wisconsin and VA Medical Center, Milwaukee, Wisconsin, and coauthors compare the ability of four different nasal cannula to provide an increased fraction of inspired oxygen as well as to accurately detect ETCO2.

Forty-five healthy volunteers between the ages of 18 and 35 were tested, each using all four of the nasal cannula designs. Monitoring included EKG, posterior pharynx oxygen sampling through a Hauge™ Airway, and nasal cannula end-tidal carbon dioxide. Eleven of the volunteers had arterial lines placed from which to draw samples of blood that were analyzed by an i-STAT® System to provide partial pressures of oxygen and carbon dioxide (PaO2, PaCO2). Data were collected while the volunteers were breathing room air (RA), as well as at 2, 4, and 6 liters per minute of supplemental oxygen.

The four models of nasal cannula included:

  1. the Hudson bifurcated cannula (Hudson Softech® Bi-Flo™), which both delivers oxygen and samples carbon dioxide within each nasal prong,
  2. the Medline oxygen blow-by dual vents, which provides “blow-by” oxygen from a “cloud” outside the nares via small holes in the main part of the nasal cannula while both prongs are used to sample carbon dioxide,
  3. the Oridion oxygen blow-by via multiple vents, whose design is similar to the Medline oxygen cannula described above, and
  4. the Salter Labs (sponsor of the study) separate oxygen delivery/CO2 sampling cannula in which oxygen is delivered through one prong while CO2 is sampled through the other prong.
The authors found the  bifurcated nasal prong design resulted in inaccurate ETCO2 readings as oxygen was delivered.  The multi-vented nasal cannula using the blow-by oxygen cloud did not provide the increase in FiO2 that the other models were able to provide. The Salter Labs nasal cannula with separate prongs, one for oxygen delivery and one for CO2 sampling, was the only one able to reasonably measure ETCO2 while providing high levels of oxygen delivery. Consistent with this, it also provided the highest pharyngeal oxygen concentration.

Patients in the study had no comorbidities, including no incidences of deviated septum or unilateral nare obstruction. Also, the patients in the study were not sedated, and the potential inaccuracy of ETCO2 data collection in patients during hypoventilation, low tidal volume breathing or mouth breathing, states that would be most applicable to clinical usage, is well known. Lastly, the cannula that performed best is the one manufactured by the Salter Labs, the study sponsor. The study design appears to be entirely objective, but the findings should be replicated by a study not funded by the manufacturer to be confident that subtle aspects of the study design did not favor the sponsor’s product.

Anesthesiologists are constantly refining techniques for making the operating room safer. Because vigilance is so important, the American Society of Anesthesiologists added monitoring of end-tidal carbon dioxide to the ASA Standards for Basic Anesthesia Monitoring in July of 2011. Studies like this can help us learn how to do this most effectively.