An evaluation of the effectiveness of the Sinapi Chest Drain's flutter valve in comparison to a water seal as found in an UWD to resolve pneumo- or hemothorax Prof AH Basson, Pr Eng March 2010 Departement Meganiese en Megatroniese Ingenieurswese Department of Mechanical and Mechatronic Engineering
Table of Contents 1. Introduction... 1 2. Mechanical description of the Sinapi Chest Drain... 1 3. Comparison of the Sinapi Chest Drain to the Under Water Drain... 1 3.1. Opening pressure... 1 3.2. Water traps... 2 3.3. Backflow causing hysteresis... 2 3.4. Dead space... 3 3.5. Dependence on orientation and mobility... 4 3.6. Further functions... 4 4. Conclusions... 4 Copyright 2010 University of Stellenbosch
1. Introduction This report evaluates the effectiveness of a one-way flutter valve, as found in the Sinapi Chest Drain, when compared to a water seal, as found in an Under Water Drain (UWD). A one-way valve is used to evacuate gas and liquid from the pleural space to resolve a pneumothorax (presence of air in the pleural space), hemothorax (presence of blood in the pleural space) or a combination of the two. Prof Basson is a registered Professional Engineer, and holds a PhD in Aerospace Engineering from the Pennsylvania State University. He has been a Professor of Mechanical Design at Stellenbosch University since 1996. His assessment in this report is limited to the mechanical and fluid-mechanics aspects of the Sinapi Chest Drain. 2. Mechanical description of the Sinapi Chest Drain The device is primarily composed of a transparent thermoplastic container, a flutter valve that admits gasses and liquids into the container, a vent pipe on the same side as the flutter valve and a drain pipe on the opposite side. The flutter valve and the vent pipes contain design features to enable each to fulfil more than one function. The flutter valve is a one-way valve. It terminates in a flexible, flattened tube that has no openings when under zero pressure differential. The valve allows gas and liquid to flow only into the container. The one-way valve has a diaphragm at the root of the flexible portion, constructed in such a way that the major part of the tube deflects sideways when the upstream pressure is lower than the pressure in the container. It therefore serves as a visual indicator of pressure below ambient in the upstream tube. The vent pipe contains a reed that deflects when air flows out through the pipe. The reed serves as an indicator of airflow from the patient through the valve. 3. Comparison of the Sinapi Chest Drain to the Under Water Drain 3.1. Opening pressure One of the mechanical properties that affects one-way valve efficiency, is the "opening pressure", i.e. the pressure above ambient pressure that must be reached at the drain inlet before gas or liquid will pass through the valve. 1
For an UWD to function correctly, the end of the tube leading from the patient must be below the surface of the liquid in the bottle. The depth that the tube exends below the surface of the water, is equal to the static opening pressure (measured in mm H 2 O). Inertial effects can increase this effect, but the relative magnitude of the inertial effects has not been analysed. In a single bottle UWD, any liquid that is expelled into the bottle will raise the water level in the bottle, and thus increase the opening pressure. Adjustment of the depth of tube end is required to correct the opening pressure. To overcome this effect, a triple bottle UWD has a liquid accumulator upstream of the drain. The flutter valve in the Sinapi Chest Drain has a constant opening pressure, independent of the amount of liquid passed and the orientation of the valve. 3.2. Water traps Due to the size of an UWD and the reliance of its operation on maintaining an upright orientation, the bottle is normally placed on the floor. The tube between the patient and the UWD therefore has to be long enough to accommodate the movement of the patient and to place the UWD where it will not be in the way of the patient and the nursing staff. If there is any liquid accumulation in a bend of the tube itself, this liquid will form a water trap and can cause a substantial increase in the opening pressure. Since the Sinapi Chest Drain is small and light, and its operation is not dependent on orientation, the length of tubing between patient and device can be much shorter than with an UWD. This significantly reduces the likelihood of having liquid trapped in a bend of the tube. 3.3. Backflow causing hysteresis Before an UWD can capture any gas from the tube coming from the patient, the water column inside the tube extending into the fluid has to be pushed out of the tube (the phenomenon that also leads to the opening pressure). When the upstream pressure decreases below the opening pressure, the water column flows back into the tube. The equivalent volume of air therefore has to flow towards the patient. As the lung expands, larger negative pressures are evident in the pleural space, causing the water column in an UWD to rise, which results in backflow. When positive pleural pressures force air out of the system, the liquid column has to move downwards again before any air/gas can pass through the valve. With an UWD this effect of hysteresis is evident with every breath and would decrease the efficiency of the valve. 2
The effect of hysteresis will be significantly less when using a flutter valve during chest drainage. The flutter valve of the Sinapi Chest Drain allows very little backflow, associated with the deformation of the flutter valve when opening, unless the seal is disturbed by solid or semi-solid matter. If the matter is enough to disrupt the sealing action, it should be visible through the tube. Since the valve can open to a crosssectional area greater than the upstream tube, matter that has passed through the upstream tube will also be able to pass through the valve. 3.4. Dead space The dead space is the space between the patient's lung and the one-way valve. For a single bottle drain and the Sinapi Chest Drain, the tube leading from the patient to the valve creates the dead space. In the case of a three bottle UWD, the large volume of air in the first bottle is also part of the dead space. The dead space reduces the flow of liquids through the valve due to air's expansion and compression when the pressure changes. For the small pressure changes evident in these applications (approximately equal to the opening pressure of the one-way valve), the amount by which the dead volume has to change before the opening pressure is reached (i.e. before liquid passes through the valve), is given by Volume change Total volume Change in pressure Absolute pressure (Note that the pressures referred to elsewhere in this document, e.g. 5 cm H 2 O, are gauge pressures. Gauge pressure added to atmospheric pressure, typically 1032 cm H 2 O, gives the absolute pressure.) The volume change required before liquid is expelled through the valve, over and above that required to overcome the backflow, is therefore directly proportional to the total volume. For example, if the total volume is 200 ml and the change in volume in the pleural space is 50 ml, the pressure increase at the valve will be 33%. On the other hand, if the total space is 1200 ml and the change in volume still 50 ml, the pressure increase at the valve is now only 4%. This illustrates a significant disadvantage of the three-bottle UWD. Even when compared to a single bottle UWD, the Sinapi Chest Drain has a smaller dead space because the tube between the patient and the flutter valve can be shorter. During the latter part of a pneumo- or hemothorax, when the volume in the pleural space is already much reduced, the volume in the tube constitutes a significant dead 3
space. The shorter tube required for the Sinapi Chest Drain will therefore lead to substantially increased efficiency during these latter phases. 3.5. Dependence on orientation and mobility UWD's rely on gravity and the maintenance of a water level above the tube end for their operation. They can therefore only be used in an upright orientation and in circumstances where accelerations are small. The flutter valve of the Sinapi Chest Drain does not rely on gravity or on the maintenance of a liquid level. It is therefore independent of orientation and acceleration. Although vigorous movements may cause spillage of liquid from the Sinapi Chest Drain's container, the operation of the flutter valve is not effected. The design of the Sinapi Chest Drain is such that moderate movements, e.g. those a person's body makes when walking, will not cause spillage of liquid as long as the volume of liquid in the container itself is less than about 175 ml. 3.6. Further functions The Sinapi Chest Drain can perform the following functions, which are also possible with an UWD, but not with some competing products: Vacuum can be applied. There is no backflow after removal of the vacuum in the Sinapi Chest Drain, but in UWD's there will be some backflow as the liquid level in the tube rises. The "tidal effect" of the water level in the UWD's tube is reproduced by the reciprocating deflection of the flutter valve. Air flow from the patient to the atmosphere, which can be observed in an UWD as bubbles leaving the tube's end, can be seen in the Sinapi Chest Drain by the deflection of the reed in the vent pipe. 4. Conclusions The efficiency of the Sinapi Chest Drain's flutter valve, used in the treatment of a pneumo- or hemothorax, is substantially better than that of an UWD for the following reasons: The Sinapi Chest Drain's opening pressure is independent of the volume of liquid that has passed through it. This can be achieved in a three bottle UWD arrangement, but only with regular attendance in a single bottle UWD. Since the pipe between the drain and the patient can be much shorter than with an UWD, there is a much reduced chance of water traps forming. 4
The volume of backflow of the Sinapi Chest Drain is substantially less than that of an UWD. The dead space when using the Sinapi Chest Drain is less than with a single bottle UWD, and an order of magnitude less than with a three bottle UWD. The reduced dead space has a pronounced effect on improving the valve efficiency, particularly during the later phases of a pneumo- or hemothorax. The Sinapi Chest Drain's flutter valve operation is independent of the orientation of the valve and of movements that it is exposed to. It can therefore remain in operation when a patient walks around. In summary: The Sinapi Chest Drain can perform the same mechanical functions as an UWD. It has a one-way valve (which also replicates the tidal effect of an UWD) and an indicator for air flow. Vacuum can also be applied with an Sinapi Chest Drain. 5