Sunday, May 8, 2016

"Effect of positive pressure ventilation on lymphatic drainage"

The lymphatics subserve at least two functions, drainage and defence. The lung has a massive surface area continuously exposed to a large volume of poorly filtered air. Fluid is drained from the interstitium by the reticuloendothelial system that removes extraneous material before it reaches the circulation. The lung lymphatics consist of thin, single cell, conduits within the interstitium with valves to assist unidirectional flow. During inspiration, the negative pressure in the lung is transmitted to the interstitium and to the lymphatics and a gradient allows fluid to drain into the lymphatics. This is highly efficient and lymph flow can increase considerably if needed.

 Positive pressure ventilation will push fluid from alveolus towards the interstitium and potentially towards and into the lymphatics. The positive pressure on the interstitium may compress some peripheral lymphatics potentially aiding flow, but it may also compress these thin-walled vessels impeding flow. A second effect is that high CVPs associated with ventilation will form a significant hydrostatic barrier to flow, given that the lymphatic pressure is usually in single figures.Lymph flow is again impeded. During expiration, lower intrathoracic pressure and decreasing CVPs may allow resumption of flow, but the use of PEEP, especially high PEEP, may obviate this recovery.

Positive pressure ventilation increases lung water as does PEEP. PEEP helps remove fluid from alveoli, but the reduction in thoracic duct drainage results in fluid retention in the interstitium. Airway pressure of 15 mm Hg or higher compresses thin-walled collapsible lymphatics. The lymphatics drain into the central veins characterized by the superior vena cava (SVC). If the SVC pressure is high, 12 mm Hg in this example, drainage will be impeded. Hence both airway pressures and venous pressures will potentially impede drainage. In the deliberately injured lung, PEEP increases lymph production but impairs lymph flow.

 The net effect of impaired drainage over time would be fluid accumulation in the lung and pleural spaces and potentially increased susceptibility to lung infection. In the lung, there is no direct evidence linking impaired lymphatic drainage to infection risk but elsewhere, such as with chronic lymphoedema there is both fluid sequestration and predisposition to infection."

Read full article:  N. Soni and P. Williams: Positive pressure ventilation: what is the real cost?: Br. J. Anaesth. (2008) 101 (4): 446-457.


No comments:

Post a Comment