Transient perioperative metabolic acidosis in a patient with ileal bladder augmentation

Das Stewart-Modell

About twenty years ago, Peter Stewart had already published his modern quantitative approach to acid-base chemistry. According to his interpretations, the traditional concepts of the mechanisms behind the changes in acid-base balance are considerably questionable. The main physicochemical principle which must be accomplished in body fluids, is the rule of electroneutrality. There are 3 components in biological fluids which are subject to this principle: a)Water, which is only in minor parts dissociated into H+ and OH-, b)"strong", i.e. completely dissociated, electrolytes, which thus do not interact with other substances, and body substances, such as lactate, and c)"weak", i.e. incompletely dissociated, substances. Peter Stewart strictly distinguished between dependent and independent variables and thus indeed described a new order of acid-base chemistry. The 3 dependent variables (bicarbonate concentration [Bic(-)], pH, and with this also hydrogen ion concentration [H(+)]) can only change if the 3 independent variables allow this change. These 3 independent variables are: 1. Carbon dioxide partial pressure, 2.the total amount of all weak acids ([A-] (Stewart called these ATOT), and 3.strong ion difference (SID). [A(-)] can be calculated from the albumin (Alb) and the phosphate concentration (Pi): [A(-)]=[Alb x (0.123 x pH - 0.631)] + [Pi x (0.309 x pH - 0.469)]. An apparent SID (or "bedside" SID) can be calculated using measurable ion concentrations: SID=[Na(+)] + [K(+)] - [Cl(-)]-lactate. Regarding the metabolic disturbances of acid-base chemistry, according to Stewart's terminology, changes in pH, [H(+)], and [Bic(-)] are only possible if either SID or [A(-)] itself changes. If, for example, SID decreases (e.g. in case of hyperchloremia), this increase in independent negative charges leads to a decrease in dependent negative charges in terms of [Bic(-)] resulting in acidosis (and vice versa). Therefore, according to Stewart, the decrease in SID during hyperchloremic acidosis results from the increase in serum chloride concentration and is the causal mechanism behind this acidosis. Contrary for example, a decrease in [A(-)] (e. g. during hypoalbuminemia) leads to an increase in [Bic(-)] and therefore to an alcalosis (and vice versa). Thus, by Stewart's approach, completely new acid-base disturbances, like "hyperchloremic acidosis" or "hypoalbuminemic alcalosis" (which, of course, can also exist in combination) can be detected, which had been unrecognised by the classic acid-base concepts. Consequently, Stewart's analysis can lead to a better understanding of the mechanisms behind the changes in acid-base balance.

[Hyperchloremic acidosis druing plasma volume replacement]

OBJECTIVES

Review of the physiological and clinical consequences of hyperchloraemic acidosis observed during plasma volume replacement using crystalloids and colloids.

DATA SOURCES

Data were searched in the Medline database after 1990 using the following key words: metabolic acidosis, crystalloids, colloids, albumin, gelatin, hydroxyethyl starch.

DATA EXTRACTION

Publications before 1990 were selected for their historical value. Most of articles published after 1990 and all types including case report were accepted.

DATA SYNTHESIS

Large volume infusion of isotonic solution can cause hyperchloraemic acidosis. Colloid plasma substitutes using saline solvent may be responsible for the same kind of acidosis with acidaemia. The anion gap is not modified in this case because of chloride increase. Physiological mechanism may be described using the Henderson-Hasselbach equation or the strong ion difference decrease (Stewart concept). Excessive chloride infusion is a major factor in this acid-base disorder and the term hyperchloraemic acidosis should be preferred to dilutional acidosis. When perioperative acidosis occurs, careful and complete analysis of acid-base disturbance should be made. The association of a normal anion gap, normal lactatemia, hyperchloraemia and acidaemia does not need specific treatment. Acidosis corrects spontaneously and slowly following chloride normalization. But any factor that may increase acidosis should be avoided.

CONCLUSION

The use of balanced solution like lactated-Ringer solution instead of isotonic saline solution for fluid resuscitation, except for specific contra-indication as intracranial hypertension, may avoid hyperchloraemic acidosis. Potential risk of this acidosis led to the conception of a new colloid using balanced crystalloids solution as the solvent (Hextend).

[Ischemia of a transplanted kidney and hyperlactatemia]

The authors report four cases of patients presenting with hyperlactatemia following renal transplantation. The post-transplantation course of three patients who underwent renal transplantation was complicated by occult haemorrhage. Excessive blood loss was not evident, the patients were haemodynamically stable and their blood pressure, pulse rare and filling pressure (central venous pressure) were unremarkable. Late examinations revealed an early increase in arterial lactate concentrations a non-aniograp acidosis or lactic acidaemia occurred. Surgical decompression was carried on in all patients. An increase in the intraabdominal pressure might have caused renal impairment in the absence of haemodynamic disturbance, and retroperitoneal haematoma a change in the distribution of intrarenal blood flow. Another patient developed a partial renal venous thrombosis associated with hyperlactatemia. During this re-operation, a renal lactate production was measured. The renal cortex is a site of lactate clearance. Impaired renal perfusion should result in decreased lactate clearance and when the kidney is hypoperfused a lactate production was occur. In the absence of any signs of clinical shock, patient at risk of retroperitoneal haematoma or presenting with oliguria should benefit from lactate measurements, which could help diagnosing severe hypoperfusion of the graft.