The Stewart analysis: Pour new wine into old bottles

Abstract

The interpretation of acid-base status can be achieved by two approaches: the classic bicarbonate-centred one and the modern one based on Stewart approach, in which bicarbonate is considered as a dependent variable. The classic analysis is based on the simultaneous interpretation of plasma bicarbonate and the partial pressure of carbon dioxide (PCO₂). The Base Excess (BE) allows quantifying the metabolic contribution to the acid-base disorder. The plasma anion gap (TAP, corrected by the albumin) aims to detect the presence of non-dosed anions. The so-called modern Stewart analysis is based on four main principles of physics and chemistry: the equilibrium dissociation of water, the law of dissociation of weak acids, electroneutrality and conservation of mass. It identifies three independent variables: the strong ion difference (SID), the total concentration of weak acid and PCO₂. Strong ions are plasma ions completely dissociated: strong acids (Cl⁻ and lactate, pKa < 4) and strong bases (Na⁺, K⁺, Ca⁺⁺ and Mg⁺⁺, pKa > 12). The SID physiologically equals 40 mEq/l, corresponding to the whole weak acids buffers. The latter is quantified by the sum of albuminate, phosphate and bicarbonate, a term that is closely related to the BE of the classical approach. Stewart defines a strong ion gap (SIG) equivalent to TAP which is subtracted by bicarbonatemia. Low SID acidosis with normal SIG is used instead of hyperchloremic acidosis with normal anion gap. This review attempts to demonstrate that these two approaches look at the complex phenomena of equilibration between buffers through different prisms.