The key components of all open-circuit systems are accurate volume-measurement systems and the individual O2 and CO2 analyzers. These systems must be capable of measuring small changes in gas concentrations in normoxic and hyperoxic environments. The simplest method is to analyze aliquots of mixed inhaled and mixed exhaled gases in a conventional blood gas machine. Many other factors must be considered to ensure accuracy of measurements in the clinical setting.
Elevated levels of FiO2 have an undesirable effect on open-circuit measurements. Errors occur when FiO 2 approaches 1.0 and the denominator of the Haldane equation approaches zero. Also, any error in the measurement of gas concentrations is amplified as FiO 2 increases because Vo2 becomes a small difference between two large numbers, and because of variations in FiO2. In most ventilators, FiO2 can vary slightly during a single breath. This is often the case with ventilators that utilize on-line mixing of air and O2 with proportional valves instead of an external blender and/or reservoir to dampen fluctuations in FiO 2. Therefore FiO2 must be measured in mixed inspired gas at the same time as FeO2 is measured in exhaled gas.
The presence of anesthetic gases or other vapors can complicate open-circuit techniques, specifically by negating the assumptions made in the Haldane equation. Also, water vapor that is not removed by desiccants, temperature manipulations, or special tubing can adversely affect the function of O 2 and CO2 sensors. During measurements, the ventilator or breathing system circuit must be free from leaks. Loss of gas to the environment or entry of room air into the system will obviously invalidate measurements of Ve. This is also the case in patients with leaking endotracheal tube cuffs, leaking chest tubes, and bronchopleural fistulas, in which Ve and FeCO2 are decreased and subsequently Vo2, VCO2, and resting energy expenditure are underestimated. The Vo2 measured by open-circuit technique is the volume at ambient temperature and pressure saturated with water vapor (ATPS), which must be converted to standard temperature and pressure under dry conditions (STPD) for metabolic calculations.
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