Acid-base disturbances in poisoned patients

Poisoning by drugs and non-medicinal chemicals can cause a variety of disturbances to acid-base homeostasis, so that the initial emergency room (ER) assessment of poisoned patients often includes arterial blood gas analysis.

This recently published clinical study of acid-base disturbances in poisoned patients was designed to discover if there is an association between acid-base disturbance, at the time of admission to ER, and mortality, following acute poisoning.

The study fulfils its stated aims, and in so doing provides a wealth of data that illuminates in a more general way, acid-base disturbance in acute poisoning.

Retrospective in nature, the study was conducted at a Tehran hospital that is a specialist referral center for poisoning. Close to half of all those admitted to the ER of this hospital are poisoned patients.

Investigators retrieved the medical records of all poisoned patients who had blood gas analysis at the time of admission to ER, during a 6-month period. This study population comprised 1167 patients with a median age of 25 years (range 12-90 years). Exclusion criteria determined that no study patient had suffered cardiopulmonary arrest, which itself causes gross acid-base disturbance.

The admission blood gas results (pH, pCO₂, bicarbonate and base excess) of each study patient were reviewed retrospectively to determine acid-base status, and if present, classify the type of acid-base disturbance, the severity of the disturbance and the presence or absence of compensation for the disturbance.

The majority (81.5 %) of study patients had a disturbance of acid-base status of one kind or another; just 216 (18.5 %) had normal acid-base status (i.e. pH, pCO₂, bicarbonate, and base excess all within their respective reference range) at admission.

All four types of acid-base disturbance (respiratory acidosis, respiratory alkalosis, metabolic acidosis, metabolic alkalosis) were evident in the studied population, but the most common was mixed respiratory alkalosis and metabolic acidosis with normal pH, which was present in 333 (28.5 %) patients.

Metabolic acidosis (either with or without respiratory compensation) was the second most common disturbance, being present at admission in 261 (22.4 %) patients.

Of the 1167 patients, 212 (17.1%) required admission to intensive care and 98 did not survive. The overall mortality rate for this study cohort then was 8.4 %. Mortality of subgroups defined by admission acid-base status revealed marked differences.

For those with normal acid-base status mortality was 4.6 %. The acid-base disturbance associated with lowest mortality (just 3 %) was the most common, i.e. mixed respiratory alkalosis and metabolic acidosis with normal pH.

The two acid-base disturbances associated with highest mortality were mixed respiratory alkalosis and metabolic acidosis with reduced pH (mortality 21.7 %) and metabolic acidosis with respiratory compensation (mortality 18.8 %).

Multivariate regression analysis revealed severe metabolic acidosis (Odds Ratio OR 6.016; 95 % Confidence Interval CI 1.647-21.968; P=0.007) and primary respiratory alkalosis (OR 5.579; 95% CI 1.353-23.00; P=0.017) to be independent risk factors for death.

Authors of the study are able to conclude that they have demonstrated association between acid-base disturbance and mortality in poisoned patients, and thereby that admission blood gas results may well be clinically useful in helping to identifying those poisoned patients who are most at risk.