Proximal and tracheal airway pressures during different modes of mechanical ventilation: an animal model study.

OBJECTIVE

To determine the differences between the mean proximal and tracheal airway pressures during 3 different modes of mechanical ventilation (MV) in an animal model of acute cardiac failure (CF) and respiratory failure (RF).

DESIGN

Prospective, randomized, cross-over design.

SETTING

University research laboratory.

SUBJECTS

Twelve young pigs weighing 10-16 kg.

INTERVENTIONS

The experimental protocol consisted of 3 stable 30 min conditions: when ventricular and pulmonary function was normal (control), after the induction of acute cardiac failure by a beta-blocking agent and after respiratory failure induced by repeated lung lavages. Modes of MV included controlled mechanical ventilation (CMV), high-frequency oscillation (HFO), and high-frequency jet ventilation (HFJV).

MEASUREMENTS AND RESULTS

The tracheal mean airway pressure (P(aw)) was measured at the distal port of the Hi-lo jet tube using an air-filled pressure transducer. The mean transpulmonary P(aw) increased significantly from 0.41 +/- 0.14 kPa during the control period to 1.15 +/- 0.17 kPa (P < 0.0001) during the RF period. In all study periods both the proximal and tracheal P(aw) were lowest during HFJV. There was no difference between the proximal and tracheal P(aw) during CMV and HFJV throughout the protocol. In the cardiac and respiratory failure periods the proximal P(aw) (CF, 1.45 +/- 0.08 kPa; RF, 3.13 +/- 0.27 kPa) was significantly higher than the tracheal P(aw) (CF, 1.04 +/- 0.09 kPa, P < 0.01; RF, 2.18 +/- 0.3 kPa, P < 0.01) with HFO. When ventilated by HFO, the mean external oscillatory amplitude was 4.33 +/- 0.14 kPa and the intratracheal oscillatory amplitude was only 0.49 +/- 0.06 kPa (P < 0.0001).

CONCLUSION

HFJV provides adequate respiratory support at lower P(aw) than CMV and HFO. Proximal P(aw) closely reflects tracheal P(aw) during CMV and HFJV. However, with HFO great pressure differences between the proximal and tracheal airways are evident. Therefore, additional intratracheal airway pressure monitoring seems to be very useful for optimizing ventilator settings during HFO.