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 obstru ction on the delivered tidal volume and selected parameters of pulmona ry mechanics during transtracheal jet ventilation (TTJV) in a dog mode l. Design: Laboratory study in which seven dogs were anesthetized, par alyzed, and placed within a volume plethysmograph with the head and ne ck externalized. Interventions: Ventilation was performed using TTJV a t 45 psi and a frequency of 15 beats per minute. The upper trachea was occluded progressively using a Foley catheter balloon to induce trach eal pressure levels of approximately 150%, 200%, 250%, and 300% of the tracheal pressure obtained during TTJV-c. Tidal volume, tracheal pres sure, transpulmonary pressure, airflow, arterial blood pressure, centr al 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. Res ults: Application of graded upper-airway obstruction during TTJV yield ed mean tracheal pressures of 130% (level 1), 190% (level 2), 220% (le vel 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 obstr uction except between levels 3 and 4(spontaneous breathing, 506+/-72 m L; 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 obst ruction level 1, transpulmonary pressure was not significantly higher than either spontaneous breathing or TTJV-c, but did significantly inc rease during higher levels of obstruction. The mean arterial Pco2 sign ificantly increased minute ventilation, with a concomitant increase in arterial pH. There was no signficant difference seen in the quasistat ic compliance of the lungs among spontaneous breathing, TTJV-c, or TTJ V at any level of upper airway obstruction. Conclusion: Partial upper- airway obstruction increases the delivered tidal volume, minute ventil ation, and transpulmonary pressure of the lungs during TTJV, with cons equent decreases in the arterial Pco2 as the amount of obstruction inc reases. No significant changes were seen in the quasistatic compliance of the lungs, pulmonary flow resistance, or alveolar:arterial gradien t, lending support to the position that TTJV is a safe technique under conditions of partial upper-airway obstruction. However, due to signi ficant 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.