Effect of patient head position on the aspirated volume of regurgitated clear fluid. A fresh human cadaver study

Introduction
The relationship between regurgitated and aspirated volume of clear fluids remains undetermined and may depend on anatomical factors and patient position. We aimed to assess whether head position (sniffing vs. extension position) affected this relationship in fresh human cadavers. We also determined the critical volume of water regurgitated that led to pulmonary aspiration of volume ≥ 0.8 mL kg-1 and ≥ 1.5 mL kg-1 for each head position.

Material and methods
Six volumes of water (40, 80, 100, 120, 150, and 200 mL) were injected each twice, in a randomised order, at a flow rate of 20 mL per second, into the oesophagus of seven fresh human cadavers lying in the supine position on a non-tilted table, with the head in the sniffing position and in the extension position. Aspirated volume was measured in the trachea, blindly to the volume injected.

Results
Overall, more than 85% of the regurgitated volume was aspirated into the trachea. The volume of aspirated water was significantly greater in the sniffing position than in the extension position. The cut-off volumes of water injected into the oesophagus leading to aspirated volume ≥ 0.8 mL kg-1 and ≥ 1.5 mL kg-1 were, respectively, 0.8 mL kg-1 and 1.5 mL kg-1 in the sniffing position, and 1.2 mL kg-1 and 1.8 mL kg-1 in the extension position.

Conclusions
These results confirm that most of the regurgitated clear fluid enters the trachea in humans lying in the supine position on a non-tilted table, especially when the head is in the sniffing position.