Letter: Base excess and organic acidaemia

Organic acidurias: approach, results and clinical relevance

More than twenty-five inherited organic acidurias have been identified during the last fifteen years. This remarkable development is due mainly to the introduction of gas chromatography, and gas chromatography combined with mass spectrometry, in paediatric laboratories for metabolic disease. The chemical approach is determined mainly by physical properties of the acid, such as their extractability and volatility. Most progress has been made with extractable acids. The techniques used for derivatization are mentioned, such as trimethylsilylation, methylation and the preparation of asymmetric derivatives for the separation of optical enantiomers. Metabolite patterns may be so characteristic that the underlying enzyme defect can be deduced. Examples are the leucine degradation defects, all encountered in the authors' laboratory: branched-chain ketoaciduria; isovaleric acidaemia; 3-methylcrotonylglycinuria; 3-methylglutaconic aciduria; and 3-hydroxy-3-methylglutaric aciduria. These abnormalities are discussed. D-glyceric aciduria is shown as an example of a not yet fully understood organic aciduria. The clinical approach varies. Metabolic acidosis is an indication for organic acid analysis in urine and plasma, but in many defects there is no acidosis, or only a transient one caused by secondary metabolites, such as lactic and 3-hydroxybutyric acids. Gas chromatography is an obligatory routine investigation in screening programmes for inborn errors of metabolism, especially for the examination of acutely ill neonates and premature babies.

Metabolic acidosis in the critically ill

The processes that lead to increased blood lactate concentration in shock and critical illness are complex. Although it is widely believed that lactic acidosis develops because of tissue hypoxia and reduced aerobic metabolism, a number of workers have shown that lactate oxidation rate is actually increased. Lactate production is also increased and this is probably the cause of increased blood lactate concentration. The clinical usefulness of lactate estimation in critical illness is also obscure. Suggestions that whole blood lactate concentration is an accurate predictor of the outcome of an episode of shock are unconvincing. At best this method may differentiate between groups of patients. However, the rate of improvement in lactate concentration with intensive resuscitation probably correlates better with survival, and further investigation of this possibility is suggested.

Association between blood lactate and acid-base status and mortality in ventilated babies

AIM

To investigate the relation between common acid-base parameters and blood lactate concentrations and their prognostic importance in sick, ventilated neonates.

METHODS

Two hundred and seventy eight serial simultaneous measurements of arterial acid-base status and blood lactate concentrations were carried out in 75 mechanically ventilated neonates with indwelling arterial catheters (gestational age and birthweight, median (range) -29 (23-40) weeks, and 1340 (550-4080) g, respectively).

RESULTS

There were no correlations between arterial blood lactate and pH and base excess within subjects (r = 0.07 and r = -0.06, respectively) and only weakly positive but clinically irrelevant positive correlations between subjects (r = 0.28 and r = 0.27) in this group. Even in those infants who had not received any bicarbonate before their initial measurements (n = 48), there were no correlations between initial blood lactate concentrations and pH (r = 0.27), base excess (r = 0.17), or serum bicarbonate concentrations (r = -0.18). There was no relation between peak lactate concentration (PLC) and base excess (r = 0.16), and only a weak correlation between peak lactate concentration (PLC) and pH (r = 0.28). Negative base excess was an insensitive indicator of raised lactate concentrations. Only two out of 33 (6%) instances of hyperlactataemia (lactate > 2.5 mmol/l) would have been identified with a base excess < -10 mmol/l as a cutoff. Lower cutoff values of base excess or pH performed no better. Raised lactate concentrations were associated with increased mortality at all levels. While six of 53 (11%) infants with a PLC < 2.5 mmol/l died, this proportion increased to four of 15 (27%) with a PLC between 2.5-5.0 mmol/l, and four of seven (57%) with a PLC > 5.0 mmol/l. Infants showing little rise or a substantial fall in blood lactate fared better than those with persistently raised values. A clinically important increase in blood lactate preceded the development of clinical markers of deterioration and complications in six infants.

CONCLUSIONS

Contrary to popular belief, pH or base excess cannot be used as proxy measures for blood lactate concentration, and independent measurement of the latter are needed. Blood lactate concentrations may provide an early warning signal and important prognostic information in ill, ventilated neonates. In this regard, serial measurements of blood lactate are more useful than a single value.

Effects of thiamine in a patient with a variant form of branched-chian ketoaciduria

A boy with the intermediate variant of branched-chain ketoaciduria was studied. Treatment with an amino acid mixture was discontinued at the age of 7.5 years. Reintroduction of normal protein-containing foods precipitated the biochemical abnormalities, characteristic of MSUD, which were relieved by 10 mg thiamine/day. Adaptation to this regiment developed and thiamine intake was increased to 100 mg/day, later to 1 000 mg/day. The patient developed well and had no attacks of ketoacidosis. 1-14C-leucine degradation in intact fibroblasts was 15% of controls and did not increase upon addition of thiamine to the incubation medium.

A diagram to facilitate the understanding and therapy of mixed acid base disorders

A diagram based on in-vivo relationships between arterial hydrogen ion activity (H+) and carbon dioxide tension (PCO2) in primary abnormalities of acid base homeostasis is presented. It is designed to facilitate the interpretation of pH data by including the 95% confidence limits described in patients with simple metabolic and respiratory acid base disorders. These bands have been formulated from observation of simple acid base abnormalities and indicate the appropriate respiratory or renal compensatory response to the primary pH defect. A plot which falls outside these limits therefore indicates the presence of a mixed acid base disorder. The diagram presents a physiological approach to clinical disorders of pH regulation demonstrating maintenance of intra-cellular fluid homeostasis during primary extracellular fluid disturbances. Diagnostic and therapeutic advantages are further illustrated and discussed in six case reports.

Organic acidaemia in intensive care unit patients

Forty ICU patients were studied as to the prevalence and importance of organic acidaemia. It was observed that organic acidaemia was extremely common, secondly that a persistence of such an acidaemia heralded a poor prognosis and finally that the acidaemia rarely needed treatment in its own right.