What is the most likely cause if a 12 Fr. suction catheter is difficult to pass through an 8.0-mm ET tube?

In this scenario, the difficulty experienced while passing a 12 Fr. suction catheter through an 8.0-mm endotracheal (ET) tube can often be attributed to the size compatibility between the two instruments. The 12 Fr. designation refers to the outer diameter of the catheter, which is relatively large when compared to the internal diameter of an 8.0-mm ET tube.

When considering the sizes, an 8.0-mm ET tube generally has an internal diameter that may not adequately accommodate a 12 Fr. catheter due to its dimensions. The larger the catheter, the more resistance it will encounter within the narrower space of the tube, leading to difficulty in passage. Therefore, the correct inference here is that the suction catheter is too large for the ET tube, which directly correlates to issues with placement and function.

While the amount of lubricant applied can impact maneuverability, the more critical concern in this context is the size mismatch, which explains the primary mechanical challenge being faced.