Effects of erythrocytes, bicarbonate, temperature and albumin on in vitro ionized calcium variations with pH

Abstract

Using a heart lung machine as an in vitro model, the relation between ionized calcium (cCa²⁺) and pH has been shown to depend on several variables such as erythrocytes, temperature and albumin as well as the total calcium and bicarbonate concentrations. The respiratory acid-base disturbances were simulated by changing the gas flow between 1.0 and 2.4 1 min^-1 by adding CO₂ to the machine at a concentration of 0 to 17%. When pCO₂ was used to alter pH, cCa²⁺ varied from 0.16 mmol l^-1 per pH unit to 0.52 mmol l^-1. The regression slope of cCa²⁺ on pH was made steeper by decreasing erythrocyte volume fraction and by increasing temperature and the concentrations of HCO_-3, calcium or albumin. The metabolic acid-base alterations were produced by HC1 or NaHCO_-3 at a constant gas flow. cCA²⁺ changes per pH unit were 0.70 mmol 1^-1 in plasma and 1.04 mmol l^-1 in whole blood. The different results found in plasma and in erythrocyte fluids may be explained by their different buffering capacity. Haemoglobin may buffer hydrogen ions, and the formation of HCO is catalysed by carbonic anhydrase from the red cells.

Key Words:

• heart lung machine
• metabolic acid-base change
• plasma ionized calcium
• respiratory acid-base change
• whole blood ionized calcium