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October 16, 2006
9:00 AM - 11:00 AM
Room Hall E, Area F
Nasal Cannula Oxygen: Are You Giving What You Think You Are Giving?
Anthony Kovac, M.D., R. Dean Crist, B.S.
Anesthesiology, Univ of Kansas Med Cntr, Kansas City, Kansas
Introduction: Oxygen (O2) by nasal cannula is frequently administered during sedation analgesia or regional anesthesia monitored anesthesia care (MAC). O2 can be given directly via the fresh gas flow (FGF) outlet, or a separate flowmeter attachment. On anesthesia machines without the flowmeter attachment, O2 occasionally may be given via the Y piece connector of the anesthesia circle system. However, using the tubing of the anesthesia circuit has the possibility for error because less O2 may be delivered to the patient depending on the FGF setting due to the anesthesia system, circuit tubing distensibility and airway pressure limiting (APL) valve settings. This could affect patient safety. Purpose: To experimentally measure the flow delivered to the Y piece and nasal cannula via the anesthesia circuit and circle system using various FGFs and APL valve settings as may occur during administration of MAC anesthesia.

Methods: Two anesthesia machines and their circle system circuits were evaluated in this bench experimental study: (1) Modulus 2 Plus, (2) Aestiva 3000/5. The instrument used to measure delivered FGF was a Timeter Series RT-200 manufactured by Timeter, Lancaster, PA. Measurements at flowmeter settings of 2,3,4 and 5 L/min were determined at APL valve settings of 0 (minimum), 20, 30 and 40 (maximum) cmH2O. Per clinical practice, the FGF was first set on the flowmeter followed by adjustments of the APL valve. Five measurements of delivered O2 gas were made at each flowmeter and APL valve setting. Data values (Figures 1 and 2) were evaluated by ANOVA and t test. A p<0.05 value was set for statistical significance.

Conclusion: When connecting the tubing for a nasal catheter using the Y piece and circle system tubing of the anesthesia machine, the two machines and absorbers tested produced different rates of O2 flow when delivered at different flowmeter settings. The clinician changes the APL setting depending on the anesthesia bag distension. This causes shunting of FGF out the APL valve to the scavenger and away from the patient. The two machines produced different results because of different APL valves and different types of CO2 absorbers specific to each anesthesia machine. The Aestiva 3000/5 machine had less than expected O2 flow through the nasal catheter at all tested flowmeter settings (Figure 1). The Modulus 2 Plus had close to expected O2 flow at flowmeter settings < 2 L/min, but less than expected O2 flow occurred with flowmeter settings >2 L/min (Figure 2).[figure1][figure2]

Anesthesiology 2006; 105: A933
Figure 1

Figure 2