trometamol (or THAM or tromethamine)

this substance acts as a "weak base" under physiologic conditions, that is to say, it is incompletely dissociated in the pH range of 6 to 8. it is a small aminoalcohol molecule, which by joining with an H+ can convert into a monovalent kation.
its pK is 7.8, which at pH 7.4 translates to 72.5% of it existing as the "protonated" kation, while the remainder remains in the electroneutral form of the aminoalcohol.

you can calculate its ionisation (and the negative ionisation of albumin and phosphate) for different pH values by following this link!.
the US National Library of Medicine offers information about trometamol. citing from this source:
Tromethamine also acts as an osmotic diuretic, increasing urine flow, urinary pH, and excretion of fixed acids, carbon dioxide and electrolytes. A significant fraction of tromethamine (30% at pH 7.40) is not ionized and therefore is capable of reaching equilibrium in total body water. This portion may penetrate cells and may neutralize acidic ions of the intracellular fluid.
The drug is rapidly eliminated by the kidney; 75% or more appears in the urine after eight hours. Urinary excretion continues over a period of three days.

detailed practical information can be found in Nahas et alii, reference (1), quoting from this review article:

"Following an intravenous bolus or short term infusion, THAM rapidly distributes into a volume approximating the extracellular space; at steadystate, it distributes into a volume slightly in excess of total body water."
"However, renal drug elimination occurs rapidly, in contrast to the slow rate at which THAM permeates most cells" (with the exception of liver cells and erythrocytes.)
"The primary mechanism by which THAM produces an immediate intracellular alkalinising effect is the reduction of capillary and interstitial pCO2, which causes the rapid diffusion of carbon dioxide out of the cell.[86,90,91] This is probably the only significant mechanism by which THAM increases intracellular pH in organs such as the brain, and skeletal or cardiac muscle, into which tissue uptake of THAM is very slow."

our scripts thus tend to slightly overestimate its serum concentration, but ought to be reasonably correct within an hour or two after administration.

at least in Scandinavia its most popular application form seems to be tribonate®; this solution contains:
trometamol 300mmol/l
Na+ 195mmol/l
HCO3- 155mmol/l
phosphate 20mmol/l
acetate 200mmol/l

its pH is 8.1 and its osmolality 800mosmol/kg H2O .
(all these data according to the Swedish pharmacopoeia FASS®)

to my knowledge there is no literature about trometamine from the point of view of the "Stewart approach", but three articles can be recommended:
(1) - Nahas et alii, Drugs 1998 Feb; 55 (2): 191-224 , Review article: Guidelines for the Treatment of Acidaemia with THAM
(2) - Holmdahl et alii, Acta Anaesthesiol Scand 2000; 44: 524-527, The place of THAM in the management of acidemia clinical practice
(3) - Kallet et alii, Am J Respir Crit Care Med 2000; 161: 1149-1153, The Treatment of Acidosis in Acute Lung Injury with Tris-Hydroxymethyl Aminomethane (THAM)

consult the glossary for other aspects of acid-base equilibria and the rules and mathematics behind our website:     Glossary