In vivo measurements of tissue pH of the heart, brain, liver, spleen, and skeletal muscle following therapy of severe metabolic uraemic acidosis with infusion of bases: an experimental study on the limitation of the Mellemgaard-Astrup equation.

The intracellular tissue pH of the heart, brain, liver, spleen, and skeletal muscle of nephrectomized rats was determined in vivo from the distribution of 14C-labelled DMO (5,5-dimethyl-2-4-oxazolidinedione) following acute variation of arterial plasma pH (pHe) by CO2 inhalation or sodium bicarbonate administration in the range of pHe 6.9-7.7, or following correction of severe metabolic uraemic acidosis with sodium bicarbonate or Tris (hydroxymethyl) aminomethane (THAM) infusion according to the Mellemgaard-Astrup equation. It was found that: (a) during severe uraemic acidosis administration of sodium bicarbonate according to the Mellemgaard-Astrup equation leads to a stable correction of the extracellular disturbance while a comparable amount of THAM has a less pronounced effect; (b) in the first 60-120 min following therapy of severe chronic uraemic acidosis there is an overcorrection of intracellular tissue pH when compared to acute acidosis of the same degree. PHi of the investigated tissues increased more following sodium bicarbonate administration than following THAM; (c) the amount of base required for the correction of the overall acid-base status of the organism cannot be determined from the extracellular measurements alone; (d) when the normal acid-base correlation between blood and tissues is disturbed, measurements of arterial acid-base parameters can give misleading information about the whole body acid-base status and may result in failure of therapy.