Analog Computer for Base Excess and HCO3 - from pH and PCO2 Electrodes

This communication reports a new mathematical and analog computation of in vivo or extracellular fluid (ECF) base excess (BEe) of man derived from blood pH and PCO2. BEe is defined here as BE of blood diluted fourfold with extracellular fluid, as estimated by reading Siggaard-Andersen's alignment nomogram at 3 gm hemoglobin (Hb). An equation for his nomogram, which can be solved analogically but not analytically, is: BEe = 0.61 × 10^π - 24.4 - 3.8 (1.2 - π)+ where π = log P_CO2 + f(pH - 7.4), and f = Δ log P_CO2/ΔpH, a function of Hb and BE. At Hb = 3, f = 11.5/(BEe + 37.5) + 0.95. "+" indicates that negative values of the final term are counted zero. The multiplier "f" is generated in a FET in which source-drain resistance is a hyperbolic function of gate voltage, controlled by feedback from the computed BEe. The π summing amplifier drives both an antilog circuit to obtain 10^π and, through a diode, a linear subtraction of the final term, which applies only when -30 3 - is obtained by setting f = 1.044 and dropping the two final terms.