Relation between blood pH and ionized calcium during acute metabolic alteration of the acid-base balance in vivo

Abstract

We induced metabolic alkalosis and acidosis in 10 healthy volunteers in order to analyse in vivo relation between pH and ionized calcium (cCa²⁺). In the alkalinization test, 2.7 mol/kg NaHCO₃ was injected. In the acidification test, volunteers took 4 mmol/kg NH₄ Cl. Blood pH and cCa²⁺ (mmol/l) mean values (SD) baseline, after alkalinization and acidification tests, were: 7.363 (0.018), 7.456 (0.031), 7.244 (0.031), 1.27 (0.03), 1.14 (0.03) and 1.38 (0.04). Mean slope of regression log cCa²⁺/pH was -0.39 (SD 0.11). Such a slope differs after in vivo or in vitro changes, due to the in vivo rapid restoration of equilibrium between the plasmatic and interstitial compartments following changes in water and electrolyte concentrations. The type of acid-base alteration—respiratory or metabolic—influences pH changes, and consequently the regression slope. The in vivo slope for log cCa²⁺/pH in normal subjects (-0.21) is much the same as in acute respiratory alterations (-0.17), whereas it differs in acute metabolic alterations (present study). Bicarbonates play different roles: the same changes in pH cause greater changes in cCa²⁺ after acute metabolic rather than respiratory alterations. Ca²⁺ homeostasis is maintained in acute respiratory acid-base imbalance, despite wide shifts in pH, whereas in acute metabolic alterations even small pH changes have striking repercussions on cCa²⁺. The experimental angular coefficient for in vivo acute metabolic acid-base alterations differs from the theoretical one calculated by Thode's differential equation (-0.25).

Key Words:

• acid-base imbalance
• bicarbonate
• blood pH
• ionized calcium
• metabolic acidosis
• metabolic alcalosis