Nearly fatal metabolic acidosis: septic or toxic?

Calcium-Citrate Anticoagulation during Continuous Renal Replacement Therapy in Patients with Metformin Intoxication: A Case Series, Mathematical Estimation of Citrate Accumulation, and Literature Review

INTRODUCTION

Metformin intoxication causes lactic acidosis by inhibiting Krebs' cycle and oxidative phosphorylation. Continuous renal replacement therapy (CRRT) is recommended for metformin removal in critically ill patients. According to current guidelines, regional citrate anticoagulation (RCA) is the first-line strategy. However, since metformin also inhibits citrate metabolism, a risk of citrate accumulation could be hypothesized. In the present study, we monitored the potential citrate accumulation in metformin-associated lactic acidosis (MALA) patients treated with CRRT and RCA using the physical-chemical approach to acid-base interpretation.

METHODS

We collected a case series of 3 patients with MALA. Patients were treated with continuous venovenous hemofiltration (CVVH), and RCA was performed with diluted citrate solution. Citrate accumulation was monitored through two methods: the ratio between total and ionized plasma calcium concentrations (T/I calcium ratio) above 2.5 and the strong ion gap (SIG) to identify an increased concentration of unmeasured anions. Lastly, a mathematical model was developed to estimate the expected citrate accumulation during CVVH and RCA.

RESULTS

All 3 patients showed a resolution of MALA after the treatment with CVVH. The T/I calcium ratio was consistently below 2.5, and SIG decreased, reaching values lower than 6 mEq/L after 48 h of CVVH treatment. According to the mathematical model, the estimated SIG without citrate metabolism should have been around 21 mEq/L due to citrate accumulation.

CONCLUSIONS

In our clinical management, no signs of citrate accumulation were recorded in MALA patients during treatment with CVVH and RCA. Our data support the safe use of diluted citrate to perform RCA during metformin intoxication.

Continuous extracorporeal clearance in metformin-associated lactic acidosis and metformin-induced lactic acidosis: a systematic review

INTRODUCTION

Metformin poisoning with lactic acidosis is an uncommon yet clinically serious condition related to the inhibition of normal aerobic metabolism. Toxicity may occur after an acute overdose although it is much more common after a systemic insult, such as acute kidney injury, in the setting of chronic use. Hemodialysis is currently the preferred extracorporeal treatment modality (Grade 1D evidence) although some patients may be too hemodynamically unstable to tolerate it. Continuous renal replacement therapy is considered an alternative if hemodialysis is unavailable but an evaluation of survival amongst this specific treatment class is lacking.

OBJECTIVES

To assess overall survival and provide an updated review of the toxicokinetic elimination parameters of patients receiving continuous renal replacement therapy for metformin poisoning.

METHODS

A comprehensive search was performed using the EMBASE and MEDLINE libraries from inception until November 30, 2021. Data was extracted and findings were summarized. Toxicokinetic parameters were analyzed and confirmed for accuracy when data permitted.

RESULTS

Eighty-three reports met inclusion criteria. These consisted of only low-quality evidence including 75 case reports, four case series, and four descriptive retrospective reviews. Overall survival among patients suffering from metformin toxicity who received continuous extracorporeal treatment was 85.8%. When stratified between metformin-induced lactic acidosis and metformin-associated lactic acidosis, survival was 75.0% and 87.4%, respectively. Available continuous renal replacement therapy toxicokinetic parameters were quite heterogeneous. Errors in previously published toxicokinetic calculations were noted in only two instances. The overall average and median peak metformin concentrations were 70.5 mg/L and 41.9 mg/L, respectively. The average and median extracorporeal clearance rates were 39.0 mL/min and 42.1 mL/min (range 9.0-58.7 mL/min). The average and median elimination half-life parameters were 27.5 h and median 23.0 h. Elimination half-life ranged from seven to 74 h. There was no meaningful relationship between peak metformin concentration and continuous extracorporeal treatment half-life at lower concentrations, though at very high concentrations (over 200 mg/L), there was a trend towards a half-life below 20 h. There is insufficient data to robustly evaluate overall survival in relation to the extracorporeal clearance rate. Finally, there was no relevant relationship between maximal lactate concentration and survival, nor nadir pH and survival, for patients with either type of metformin toxicity.

CONCLUSIONS

This retrospective systematic analysis of published cases treating metformin related lactic acidosis with continuous renal replacement therapy notes an overall slightly greater survival percentage compared to previous publications of individuals requiring any modality of renal replacement therapy. Because of publication bias, these results should be interpreted with caution and serve as hypothesis generating for future research. Prospective study focusing on the most clinically meaningful endpoint - survival - will help elucidate if continuous modalities are non-inferior to intermittent hemodialysis in metformin toxicity.

The Association between Metformin Therapy and Lactic Acidosis

INTRODUCTION AND OBJECTIVES

There is increasing evidence to suggest that therapeutic doses of metformin are unlikely to cause lactic acidosis. The aims of this research were (1) to formally evaluate the association between metformin therapy and lactic acidosis in published case reports using two causality scoring systems, (2) to determine the frequency of pre-existing independent risk factors in published metformin-associated lactic acidosis cases, (3) to investigate the association between risk factors and mortality in metformin-associated lactic acidosis cases, and (4) to explore the relationship between prescribed metformin doses, elevated metformin plasma concentrations and the development of lactic acidosis in cases with chronic renal impairment.

METHODS

A systematic review was conducted to identify metformin-associated lactic acidosis cases. Causality was assessed using the World Health Organisation-Uppsala Monitoring Centre system and the Naranjo adverse drug reaction probability scale. Compliance to dosing guidelines was investigated for cases with chronic renal impairment as well as the association between steady-state plasma metformin concentrations prior to admission.

RESULTS

We identified 559 metformin-associated lactic acidosis cases. Almost all cases reviewed (97%) presented with independent risk factors for lactic acidosis. The prescribed metformin dose exceeded published guidelines in 60% of cases in patients with impaired kidney function. Metformin steady-state plasma concentrations prior to admission were predicted to be below the proposed upper limit of the therapeutic range of 5 mg/L.

CONCLUSIONS

Almost all cases of metformin-associated lactic acidosis reviewed presented with independent risk factors for lactic acidosis, supporting the suggestion that metformin plays a contributory role. The prescribed metformin dose, on average, exceeded the dosing recommendations by 1000 mg/day in patients with varying degrees of renal impairment but the predicted pre-admission plasma concentrations did not exceed the therapeutic range.