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J Thorac Cardiovasc Surg 2003;125:1178-1179
© 2003 The American Association for Thoracic Surgery
Letters to the Editor |
Department of Cardiothoracic Surgery and Anesthesiology, Division of Medical Engineering, Department of Medical Laboratory Science and Technology, Karolinska Institute, Stockholm, Sweden
To the Editor:
In an effort to improve deairing in cardiac surgery, Martens and colleagues
1 used a gauze sponge to reduce the outflow velocity of carbon dioxide delivered from a thin, open-ended tube. The proposed method is no doubt both simple and inexpensive. Their study included patients operated on through a complete median sternotomy or through a right anterolateral minithoracotomy (7-cm skin incision). Gauze sponges were used in combination with carbon dioxide insufflation through a standard perfusion line or a Veress needle. In the standard cardiothoracic wound cavity, they at best reached a mean carbon dioxide content of 52% (range 10%-85 %) with the open-ended perfusion line and of 81% (range 35%-100%) with the gauze sponge added. In the minithoracotomy the mean carbon dioxide content at best reached 97% (range 95%-100%) when applied with a Veress needle together with the gauze sponge.
Apparently, the gauze sponge improved the carbon dioxide deairing substantially, especially in the minimally invasive wound. In the standard chest wound the carbon dioxide content was lower and had a much wider range. The lower carbon dioxide levels in the standard chest wound may have been due to a greater diffusion rate between carbon dioxide and ambient air. The wound opening was wider, and according to Fick's law of diffusion, the diffusion flux is proportional to the area through which the diffusion occurs. Because of the smaller dimensions of the minimally invasive wound, the distance between the carbon dioxide sampling device and the carbon dioxide supply may also have been smaller, which could have influenced the readings. However, these distances are not reported in the study. The use of rough suction impairs carbon dioxide deairing, 2 because its effect of 20 to 25 L/min is several times greater than the inflow of carbon dioxide. Because the time and duration of the carbon dioxide measurements were not presented, however, and the conditions regarding suction thus may have varied, these influences cannot be evaluated.
There is another and more important possible explanation for the wide range and lower means of the carbon dioxide levels measured in the standard open chest wound. Because of their hydrophilic properties, gauze sponges are used to absorb fluids. When a gauze sponge gets wet, as it easily does in the cardiothoracic wound, its structure collapses, and it loses its function as a diffuser. In a not yet published study, we have shown that a diffuser must be positioned inside the wound cavity, several centimeters below the wound edge, to provide a high degree of deairing. In contrast, carbon dioxide delivered above the wound opening and falling down by gravity into the wound will rapidly become diluted with ambient air through diffusion. Thus the gauze sponge may be exposed to blood and other liquids during surgery.
After having read Martens and colleagues' report, 1 we tested a gauze sponge for carbon dioxide deairing of a cardiothoracic wound model positioned in a fully ventilated operating theater. Suction was not used. A dry gauze sponge (in front of a small tube), positioned near the diaphragm, provided a mean carbon dioxide content of 97.5%, as measured at the topmost part of the right atrium (8 cm from the gauze sponge). When the gauze sponge was wetted with water, however, the measured mean carbon dioxide content immediately decreased to 30%, at which level it remained. We have designed a gas diffuser, 2 which may solve this problem. It is a thin tube (2.5 mm inner diameter) with a diffuser at the end. The diffuser is a small piece (18 x 14 mm) of soft polyurethane plastic foam with open cells. We tested the gas diffuser along with the gauze sponge. The gas diffuser, positioned as the gauze sponge, produced carbon dioxide contents greater than 99.5% not only when dry but after having been completely dipped in water. The foam material in our gas diffuser is hydrophobic. Because of its elastic properties, the foam does not collapse even if it is soaked, and as the gas is blown through the diffuser large parts of its cell structure will remain clear. If the gas diffuser gets partly covered with tissue or blood, the carbon dioxide gas will, according to the law of least resistance, automatically be redirected inside the diffuser foam to exit through a clear part, and its function will remain. Thus the hydrophobic and elastic properties of the gas diffuser enable it to function well even if it is in direct contact with the wet inner wound. Although the foam material is hydrophobic, the foam may absorb and store a liquid by capillary forces if it is completely drowned and at the same time compressed in the liquid, just as a car wash sponge would. If the gas diffuser unexpectedly becomes soaked through with blood or water, a short compression with the tip of a finger will evacuate the fluid and restore full function. The same does not hold true for a wet gauze sponge because of its lack of hydrophobic and elastic properties.
To sum up, the gauze sponge has inherent properties that make it unsuitable for carbon dioxide deairing. It stops functioning as a gas diffuser when it gets wet.
References
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