Secretion Clearance in the Endotracheal Tube Is Influenced by Expiratory Flow and Breath Delivery Profile

Background: Retention of respiratory secretions can be a complication in managing patients requiring mechanical ventilation. Mechanical ventilator settings that promote enhanced expiratory flows may enhance secretion clearance. Determining respiratory secretion movement patterns under different mechanical ventilation modes can provide clinicians guidance for implementing mechanical ventilation strategies when confronted with patients with a secretion burden. This bench study aimed to compare secretion clearance in volume control- continuous mandatory ventilation (VC-CMV) and airway pressure release ventilation (APRV). Methods: The bench model consisted of a commercially available (critical care) ventilator connected to a test lung (3 L) in an acrylic box via a horizontally secured ETT (7 mm). A CPAP machine/circuit was connected to the acrylic box to apply simulated pleural pressures. A commercially available in-line oscillatory device (OD) was utilized to assess for benefit in secretion clearance (Figure 1). Guar gum (1 g/100 mL water) was used as simulated sputum. Fixed VC-CMV settings included: frequency-16, VT 300 mL, inspiratory flow 40 L/min. Three different PEEP levels were evaluated during VC-CMV testing (5, 10, 15 cm H2O). An expiratory transpulmonary pressure of 0 (± 2) cm H2O was maintained during testing. Fixed APRV settings included: Thigh 4.0 s, Plow 0 cm H2O. Tlow was adjusted to maintain an expiratory flow termination of 75% (0.2-0.3 s). Three Phigh settings were evaluated during testing (20, 25, 30 cm H2O). Results: Under all PEEP settings in VC-CMV (without OD), there was a movement of the secretions within the ETT toward the lung (4.33 ± 2.87, 11.67 ± 7.41, 5.67 ± 0.94 mm/min) (Figure 2). The application of the OD did not change the movement direction of secretions in VC-CMV (6.0 ± 1.41, 9.0 ± 4.32, 7.0 ± 3.74). During all Phigh settings in APRV (without OD) there was movement of the secretions within the ETT away from the lung (4.33 ± 4.19, 6.33 ± 1.88, 14.33 ± 6.64) (Figure 2). Application of the OD during APRV inhibited secretion movement to the extent that secretions were moving toward the lung. Conclusions: Secretion movement within the ETT is influenced by expiratory flow and breath delivery profile. High expiratory flows achieved with APRV demonstrated secretion movement away from the lungs when compared to VC-CMV. Caution should be observed with this conclusion. Secretion movement was only observed within the ETT and the response of oscillations within the lung may yield different results.