OXYGEN AND CARBON DIOXIDE EXCHANGE MONITORING

The most direct method of measuring O₂ uptake (V̇O₂ ) and CO₂ elimination (V̇CO₂ ) is by analysis of inspired (I) and expired (E) gas concentrations. If the minute volume is measured along with inspired and expired O₂ and CO₂ concentrations, V̇CO₂ can be calculated by the following equation:

V̇CO₂ = V̇E · FECO₂ − V̇_I · FICO₂ (26)

In Equation 26, V̇E and V̇I are the expired and inspired respiratory minute volumes, respectively. FECO₂ and FICO₂ are the mixed expired and inspired CO₂ concentrations, respectively. Because the quantities of O₂ consumed and CO₂ eliminated are rarely exactly equal, V̇E and V̇I differ slightly. Because FICO₂ is usually zero, Equation 26 can be simplified to the following form:

V̇CO₂ = V̇E · FECO₂ (27)

Similarly, an equation can be written for the calculation of O₂ consumption:

V̇O₂ = V̇I · FICO₂ − V̇E · FECO₂ (28)

In practice, it is difficult to measure minute ventilation with the necessary degree of accuracy to be able to distinguish small differences between V̇I and V̇E. Because the exchange of inert gas (i.e., gas in the breathing mixture that is neither CO₂ nor O₂ ) generally equals zero, V̇I can be calculated as follows:





FI inert = 1 − FIO₂ (30)

FE inert = 1 − (FEO₂ + FECO₂ ) (31)

Equation 28 can be rewritten as follows:

V̇O₂ measurement using this equation becomes progressively less accurate as the inspired O₂ fraction is increased above 50%. At high FIO₂ values, because FI inert becomes small, errors in the measurement of FI inert result in correspondingly large errors in O₂ . At FIO₂ = 1, this equation breaks down completely.

Open-circuit measurement of V̇O₂ and V̇CO₂ under anesthesia using this method has been described by Viale and coworkers.^[182] Self-contained analyzers have been designed.^{[183] [184]} These instruments have proved to be extremely satisfactory for gas exchange monitoring in patients in the critical care environment. One practical problem with systems incorporating zirconium oxide O₂ sensor is extremely sensitive to small concentrations of fluorinated anesthetic gases, which may cause severe malfunction. V̇O₂ and V̇CO₂ can be measured on a breath-by-breath basis by integrating FIO₂ - FEO₂ with respect to exhaled volume.^[185] A method of measurement of V̇O₂ by using rapid response temperature and humidity analyzers to make appropriate corrections may allow breath-by-breath monitoring even with high FIO₂ values.^[186]

An alternative technique (i.e., reversed Fick) is to calculate the O₂ consumption as the product of the arteriovenous O₂ content difference and the cardiac output derived by thermodilution:

V̇O₂ = (CaO₂ − Cv̄O₂ ) · T (33)

In Equation 33, CaO₂ and Cv̄O₂ are the arterial and mixed venous O₂ contents (mL/L), respectively. T is cardiac output. Blood O₂ content may be calculated using Equation 10; when using Equation 31, the O₂ content values calculated using Equation 10 must be multiplied by 10 to obtain the O₂ in the correct units.

The reversed Fick technique is reasonably satisfactory for clinical purposes, although it becomes less accurate in the setting of high cardiac output and low arteriovenous O₂ content differences. It tends to underestimate the V̇O₂ measured directly by 30 to 60 mL/min.^{[187] [188] [189]} This difference has been attributed to pulmonary O₂ consumption, which is not measured by the reversed Fick method. Use of continuous thermodilution cardiac output versus intermittent bolus injection may reduce the error.^[190]