pH shifts
severely ill patients tend to have more than one or even two concomitant acidbase derangements. in this programme we differentiate into
- unknown anion metabolic acidosis
- lactate metabolic acidosis
- chloride metabolic acidosis or alkalosis
- albumin metabolic acidosis or alkalosis
- respiratory acidosis or alkalosis
according to the physiocochemical or Stewart approach, the pH of body fluid depends on only
3 independent quantities:
1- the "strong ion difference" or SID
the difference between the completely dissociated or "strong" positive ions (kations)
and their negative counterparts (anions).
on the kation side in the vast majority of cases there are only four well-defined protagonists:
sodium and potassium make up the dominating bit, with calcium and magnesium playing a secondary role. all of these kations are often
routinely measured in an ICU context.
the dominating strong anions can be differentiated into:
- chloride, which is a major and often neglected contributor
and
- lactate.
2- the weak acids Atot
these are substances that in solutions with a pH between 6 and 8 are incompletely dissociated.
they are virtually all acids, with by far the largest contribution coming from albumin with its
more than 200 dissociating subgroups and as a second, but minor, contributor phosphate.
you can calculate the negative ionisation of albumin and phosphate for different pH values
by following this link! - with a lot more information about the calculations.
3- carbon dioxide partial pressure, PCO2
as this is regulated by respiration interacting with its metabolic production,
and as these quantities are
far larger than anything produced or consumed by acidbase processes,
its value is independent as a contributor to acidbase balance.
(the unknown anions, XA can be weak or strong anions and their concentration is calculated based on the value of the other components.
read more!
)
