ABSTRACT
Acid-base status is assessed, as a first step, by direct evaluation of blood acidity. The level of blood acidity is determined by the prevailing ‘‘respiratory component’’ (i.e., carbonic acid concentration, [H2CO3], a function of carbon dioxide tension, PCO2) and ‘‘metabolic component’’ (i.e., actual bicarbonate concentration, [HCO3
]), as stipulated by the Henderson-Hasselbalch equation (Chapter 26). Although only blood pH and the ‘‘respiratory component’’ (PaCO2) are usually directly measured, the physicochemical constraints of the Henderson-Hasselbalch equation obligate a unique level of [HCO3
]. Thus, [HCO3
] stands alone among the various estimates of the ‘‘metabolic component’’ (i.e., standard bicarbonate and base excess, see below) as the only one whose level derives from direct measurements of the patient’s blood sample. Further, plasma [HCO3
] can almost be equated to the directly measured level of total CO2 content in plasma (Chapter 26).
