Apnea Testing: The Effects of Insufflation Catheter Size and Flow on Pressure and Volume in a Test Lung

Discussion

The 1995 Report of the Quality Standards Subcommittee of the American Academy of Neurology states, “place a cannula at the level of the carina.”² The word “cannula” is ambiguous in this context and may have led some clinicians to mistakenly use nasal cannula tubing to administer the oxygen during the apnea test.^7,8 Although the subsequent 2010 Report of the Quality Standards Subcommittee of the American Academy of Neurology revised the wording to “place an insufflation catheter through the endotracheal tube,”¹ the guideline does not indicate the IC size, even though the IC size affects the risk of pneumothoraces.^3,9

We included oxygen tubing because it is common practice in the apnea test in many locations. The use of oxygen tubing or nasal cannula tubing poses several concerns. First, oxygen tubing and nasal cannula tubing do not have length markings, so the clinician could advance the tubing past the end of the ETT and into a smaller airway, thus occluding the airway.^8,10 Furthermore, cut tubing could have a sharp tip that could perforate or damage bronchial tissue.⁷

IC/ETT ratios > 0.89 caused the test lung to continue inflating without reaching a plateau. If the test lung fully inflated (peak pressure > 22 cm H₂O and volume > 1,100 mL) we disconnected the test lung from the manikin, to prevent damage to the test lung, and therefore did not obtain sustained end-point pressures and volumes for IC/ETT ratios > 0.89.

At 6 L/min there were statistically significant differences between the mean pressures and volumes for IC/ETT ratios ≥ 0.7, so our null hypothesis was rejected. We therefore recommend that in the apnea test the outer diameter of the IC should not exceed 70% of the inner diameter of the ETT. For instance, we recommend an IC of ≤ 12 French when performing the apnea test through a 6.5 mm inner diameter ETT, ≤ 14 French in a 7.5 mm inner diameter ETT, and ≤ 16 French in a 8.0 mm inner diameter ETT.

Increasing the oxygen flow above the American Academy of Neurology recommend 6 L/min increased the pressures and volumes in the test lung, which could cause complications. As seen in Tables 2 and 3, the pressures and volumes were minimized at 6 L/min. We therefore conclude that 6 L/min is the appropriate flow during the apnea test.

Our results differ from those of Olguner et al, who used a 2.3-L anesthesia bag as a lung model¹⁴ and performed the apnea test with ETTs of 7.0 to 8.0 mm inner diameter, ICs of 8 to 14 French, and flows of 6, 8, and 10 L/min.¹⁴ Olguner et al calculated the IC/ETT ratio by dividing the ETT inner diameter by the IC outer diameter,¹⁴ which is the inverse of the traditional ratio calculation.^11–13 Olguner concluded that ETT/IC ratio of < 1.75 was significantly different and associated with higher pressure in their lung model than diameter-based ETT/IC ratios > 1.75.¹⁴ We converted our IC/ETT ratios to ETT/IC ratios to compare our findings to those of Olguner et al.

Our findings may differ from Olguner's study for several reasons. First, Olguner used an anesthesia bag as a lung model, and the compliance of the anesthesia bag was not determined. Second, Olguner used a pressure transducer that was calibrated for air, but then used 100% oxygen in the tests. Third, Olguner tested only 3 ETT sizes, whereas we tested 9 ETT sizes.¹⁴

Our data suggest that as the IC/ETT ratio increases, the pressures and volumes also increase. Lang and Heckmann stated that barotrauma may be prevented during the apnea test by limiting the oxygen flow, using an appropriate IC/ETT ratio, and not advancing the IC past the ETT tip.⁹ Our study supports that statement, and we further recommend an IC/ETT ratio of < 70%, to prevent excessive pressure and volume during the apnea test.