Effects of graded upper-airway obstruction on pulmonary mechanics during transtracheal jet ventilation in dogs.

STUDY OBJECTIVE

To quantify the effects of graded upper-airway obstruction on the delivered tidal volume and selected parameters of pulmonary mechanics during transtracheal jet ventilation (TTJV) in a dog model.

DESIGN

Laboratory study in which seven dogs were anesthetized, paralyzed, and placed within a volume plethysmograph with the head and neck externalized.

INTERVENTIONS

Ventilation was performed using TTJV at 45 psi and a frequency of 15 beats per minute. The upper trachea was occluded progressively using a Foley catheter balloon to induce tracheal pressure levels of approximately 150%, 200%, 250%, and 300% of the tracheal pressure obtained during TTJV-c. Tidal volume, tracheal pressure, transpulmonary pressure, airflow, arterial blood pressure, central venous pressure, and arterial blood gases were measured during all conditions of ventilation. Quasistatic compliance curves of the lungs were measured at the conclusion of spontaneous breathing, TTJV-c, and TTJV (at all levels of obstruction). Minute ventilation and pulmonary flow resistance were calculated for each condition of ventilation.

RESULTS

Application of graded upper-airway obstruction during TTJV yielded mean tracheal pressures of 130% (level 1), 190% (level 2), 220% (level 3), and 230% (level 4) of that obtained during TTJV-c (10.9 +/- 2.0 cm H2O). Tidal volume significantly increased with each level of obstruction except between levels 3 and 4 (spontaneous breathing, 506 +/- 72 mL; TTJV-c, 446 +/- 69 mL; level 1, 663 +/- 139 mL; level 2, 780 +/- 140 mL; level 3, 931 +/- 181 mL; and level 4, 944 +/- 135 mL). During TTJV at obstruction level 1, transpulmonary pressure was not significantly higher than either spontaneous breathing or TTJV-c, but did significantly increase during higher levels of obstruction. The mean arterial PCO2 significantly decreased at all levels of obstruction due to significantly increased minute ventilation, with a concomitant increase in arterial pH. There was no significant difference seen in the quasistatic compliance of the lungs among spontaneous breathing, TTJV-c, or TTJV at any level of upper airway obstruction.

CONCLUSION

Partial upper-airway obstruction increases the delivered tidal volume, minute ventilation, and transpulmonary pressure of the lungs during TTJV, with consequent decreases in the arterial PCO2 as the amount of obstruction increases. No significant changes were seen in the quasistatic compliance of the lungs, pulmonary flow resistance, or alveolar:arterial gradient, lending support to the position that TTJV is a safe technique under conditions of partial upper-airway obstruction. However, due to significant increases in tidal volume and functional residual capacity and decreases in mean arterial blood pressure, concerns still exist during near-total or total upper-airway obstruction.