An accurate knowledge of the ratio of physiologic dead space to tidal volume (VD/VT) is useful in managing mechanically ventilated patients. When collecting the sample of exhaled gases, one must separate the gas compressed in the ventilator circuit during inspiration from the gas exhaled by the patient. This is best accomplished by placing a separate gas-collection valve at the patient Y-piece, which will allow the patient's exhaled volumes to be collected separately. We designed a gas-collection valve for use with Siemens-Elema Servo 900B and 900C ventilators that performs this function over a wide range of ventilatory conditions. Our system has several distinct advantages over the option provided by the manufacturer: Its economical cost represents significant savings, and it is more versatile than the Siemens gas analyzer. A larger variety of gas analyzers can be used for the measurement, and more accurate CO2 production values can be provided, along with directly determined VD/VT values. (Respir Care 1986;36:1096-1098.)
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