The PDU collection chamber is a closed volume, so that when the ambient pressure increases, vacuum/negative pressure will develop in the collection chamber unless this is relieved by inflow of gas or fluid. A water seal chamber indicates the negative pressure delivered on the patient side of the device, with bubbles through the water seal chamber indicating the rate of any air leakage present. In this device the degree of suction is controlled by an inbuilt regulator with the desired suction selected via a crude rotary dial. Under these conditions the water-seal was maintained and the column of water making the seal did not back flow into the collection chamber. Currently our institution predominantly uses the Pleur-EvacĀ® A-6000 PDU from Teleflex. Previous studies have shown that PDUs from different manufacturers show variations from the prescribed to the actual delivered intra-pleural pressure (IPP). Our group has previously tested the function of one PDU (Atrium Oasis Dry Suction 3600 Chest Drain) under hyperbaric conditions this device proved to be dramatically affected by pressure changes. However, safe use in an hyperbaric environment was possible if no suction was applied during pressurisation and if the rate of pressurisation was limited to 10 kPa To avoid potential complications arising from use of a proprietary PDU, many hyperbaric units and hyperbaric texts recommend that a PDU be disconnected with the substitution of a Heimlich valve during HBOT. To safely manage a patient with a PDU during HBOT, close observation is necessary and a number of modifications must be made to standard pleural drainage system care. Further, some hyperbaric scenarios result in a bubbling backflow of water-seal fluid towards the patient, for example during fast compression for CAGE, and this risks contamination of what should be the sterile side of the system.
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A key performance requirement for pleural drain systems is the avoidance of excessively negative intra-pleural pressure (IPP) as this may result in lung injury and potentially worsening of a trans-pulmonary air leak. Although these units have manual and automatic pressure relief systems designed to prevent excess negative pressure, the automatic relief settings are generally quite high.
The designs of PDUs incorporate multiple air-filled spaces on either side of a water trap and these are affected by hyperbaric exposure in ways that depend upon whether the unit is on suction or not, and whether the patient has a pleural air leak or not. On occasions, patients with an intercostal catheter (ICC) and a PDU will require hyperbaric oxygen treatment (HBOT) to manage a coexistent condition, such as decompression illness (DCI), cerebral arterial gas embolism (CAGE) or necrotizing fasciitis. Proprietary water-seal pleural drain units (PDUs) are the preferred method for chest drainage of pneumothorax, empyema, blood or fluid during in-hospital care.