Airflow oscillations have been shown to have a positive effect on air-mucus interaction and enhance mucus movement both in vitro and in vivo (King et al. 1984, Freitag et al. 1989, Tomkiewicz et al. 1994). Experimental studies by Ragavan et al. (2010) have shown that the viscoelastic properties of mucus have a strong effect on efficiency of the simulated cough in its clearance through a rigid tracheal model.  In these studies cough is simulated as a pulse with constant flowrate though a D-shaped rigid Plexiglas tracheal model with a mucus simulant aliquot prepared by cross-linking locust bean gum solution with borax. Their results also have shown that superimposed flow oscillations at the same average airflow rate of simulated cough greatly enhances mucus clearance, which may be attributed to sharp decrease in viscosity and slight increase in elastic modulus of the mucus simulant with increasing frequency.  Our studies using similar set up showed that there exists an optimum frequency where the displacement of mucus simulant aliquot is maximum (Tuna et al. 2018a).   In these studies a clinically approved oscillator device, Quake, with controllable frequency is used. Simulant viscoelastic behavior is checked against native mucus collected from patients with chronic obstructive pulmonary diseases by our group (Tuna et al. 2018b). Our studies also showed that the optimal frequency and resulting displacement of the aliquot increases with increasing elastic modulus of the mucus simulant.  This implies that thicker mucus, seen in many pulmonary illnesses such as COPD results in increased efficiency of cough clearance, while it may be very difficult, even impossible, to clear with ciliary beating.  Second part of our recent studies included exploring the effect of repeated rectangular pulses, 1-6 times, without oscillation with the same average velocity and total volume of air (Tuna et al. 2020). The results showed that the increased number of pulses increases the displacement of the mucus aliquot for the range considered. In other words coughing in the form of interrupted pulses, huff coughing, rather than single pulse, increases the cough efficiency to clear mucus. Our current study explored the wait time between repeated pulses for 3-pulse case, 25-150 ms each.   The results show that there is a clear optimum time between pulses, 100 ms for mucus simulant studied.  Comparison of our results showed that, for the devices used, Quake oscillator to generate airflow oscillations, on-off valve to generate pulses, oscillatory pulse is more effective compared to repeated pulses, but it requires higher pressure, about 5 bar, compared to repeated pulses, about 4 bar, to achieve the same average velocity.  Also, one may consider the fact that repeated pulses, huff coughing, can be achieved without an external device. 

References

Freitag, L., Bremme, J. and Schroer, M., 1989. High Frequency Oscillation for Respiratory Physiotherapy, Br. J. Anaesth.63, 44-46.

  
King, M., Phillips, D., M., Zidulka A. and Chang, H., K. 1984. Introduction, Tracheal Mucus Clearance in High-Frequency Oscillation. Am Rev Respir Dis, 130, 703-706.

  
Ragavan, A.J.,  Evrensel, C.A., Krumpe, P.  2010 Interactions of Airflow Oscillation, Tracheal Inclination and Elasticity Significantly Improve Mucus Clearance during Simulated Cough, Chest, Vol. 137, No. 2, pp. 355-361.  
Tomkiewicz, R., P., Biviji, A. and King, M.1994. Effects of oscillating airflow on the rheological properties and clearability of mucous gel simulants. Biorheology,31(5), 511-520.

Tuna, D. L., Unal, A., Ari, F. , Krumpe, P., Evrensel, C. A., 2018a, Optimum Frequency Range for Enhancing Cough Clearance through Flow Oscillations International Mechanical Engineering Congress and Exposition (IMECE2018), Pittsburgh, PA. 

Tuna, D., L., Ünal, A., Şen, E., Ceyhan K., Çiftci, F., Özer, H., B., Saryal, S., Oktay, Ş. U.  Evrensel, C., A. 2018b. Rheological characterization of native human mucus, 8th World Congress of Biomechanics, 8-12 July, Dublin, Ireland

Tuna, D. L., Ünal, M. A., Arı, F., Krumpe, P., and Evrensel, C. A. 2020. "Enhancing Mucus Clearance by Cough Through Repeated Pulses." ASME. ASME J of Medical Diagnostics; 3(3): 031004. https://doi.org/10.1115/1.4047188  
 


Mucus Clearance in Simulated Trachea by Repeated Pulses and Airflow Oscillations

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