Effective and gentle support for coughing up!

Secretion transport by means of a cough assistant

The cough reflex is an important physiological mechanism for the clearance of airways from excess secretions and foreign bodies ("airway clearance"). It begins by triggering chemical and mechanical receptors located in the respiratory tract mucosa.

Patients with a limited coughing less than 160 l / min flow can no longer cough up their secretions on their own. For this they need outside support to produce an efficient flow (air blast) of more than 160 l / min. The technical implementation is easily realized by mechanical cough support (mechanical insufflation / exsufflation) using cough-support devices.

Basic principle

By means of these Coughsupport devices, air volume is transported by positive pressure into the lungs. When the target pressure is reached, the devices quickly switch to a negative pressure and thus suck the air volume out of the lungs again. Due to the corresponding turbine performance and the interaction of the fast switching, a high flow of more than 160 l / min is generated. This is illustrated in the publication by Chatwin et al., Eur Respir J 2003. In this publication, cough without support (UA), cough with manual support (MAC), cough with support of a volume-controlled ventilator (NVAC), cough with support in Exhaled and Mechanical Insufflation / Exsufflation (MI-E). By far the best result in increasing the flow was the MI-E.

This high flow is similar to a natural cough that is able to transport the secretion into the upper respiratory tract or pharynx. There it can be sucked off or coughed off and spat out.

What happens to the secretion during insufflation in the lungs?

One would think that this is transported during insufflation into the depth of the bronchi and branches of the lung. But that is not the case, as physics counteracts this. At rest (after an exhalation), the lungs are to be regarded as a hollow body, filled with secretions and air, with the same pressure against the environment. The pressure conditions in the lungs remain outside. Consideration of insufflation: The coughsupport device creates a positive pressure against the level of pressure in the lungs and the air volume flows through the patient interface through the airways into the lungs. The increasing pressure expands the bronchi and respiratory tract and increases the air volume. Stretching increases the cross-section of the airways and reduces the flow of air (Bernoulli Law: as the cross-section increases in size, the flow velocity decreases, the static pressure increases.). Due to the increasing positive pressure in the lungs, the air wants to escape from the hollow body lungs, since this is already filled with air. The resulting overpressure reduces the air flow and thus prevents the deeper penetration of the secretion into the bronchi. If the secretion blocks one or the other of the bronchi, the enlargement of the cross-section (increase in pressure), the blockage (cohesive and adhesive force) breaks up and air flows into the bronchia. In addition, these forces prevent a deeper penetration of the secretion.

After switching from insufflation to exsufflation, negative pressure is used to aspirate the air volume from the lungs. The necessary flow must exceed at least 160 l / min in order to transport the secretion upwards from the lungs. The negative pressure in the lungs causes the airways to contract, reduce the cross-section and increase the flow, which transports the secretion upwards. (Bernoulli law: With narrowing cross-section increases the flow velocity, the static pressure decreases.) Furthermore, it comes through increased speed of exhaled gas to a entrainment of the secretion. Bronchial secretion has the property of mechanical liquefaction (thixotropy) under the influence of kinetic energy. A transport of viscous secretions is therefore only possible with the help of a significant acceleration of the exhaled gas and the consequent change in the viscosity of the secretion.

The patient can now remove the oral nasal mask and remove the secretion from the pharynx. In the invasive patient, the secretion usually reaches the tracheal cannula and must be aspirated there immediately. In practice, many cycles are necessary to remove the stuck secretion.

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