Monitoring blood lactate level helps to monitor tissue oxygenation during critical illness.
Below 2 mmol/L is normal.
Above 4 mmol/L is abnormal.
Between 2 & 4 mmol/L is gray zone.
Reasons for elevated Lactate level:
1) Anerobic metabolism is activated - presumably due to oxygen demand exceeding oxygen delivery at tissue level. In other words, VO2 is less than the metabolic rate. (Treatement will be to either decrease metabolic demand - Sedation/stopping feeds or increase VO2 above 160 ml/min/m2.
2) Hepatic insufficiency (Impaired clearance)
3) Gram negative sepsis with endotoxemia. Endotoxin inhibits pyruvate dehydrogenase, the enzyme that initiates pyruvate oxidation in mitochondria. So, endotoxemia causes hyperlactatemia without cellular oxygen deprivation in gram-negative sepsis.
3) Thiamine deficiency (Blocks pyruvate entry into mitochondria)
4) Alkalosis (both metabolic and respiratory) causes increased activity of pH-dependent enzymes in glycolytic pathway that promotes lactate production. pH has to exceed 7.6 for this effect to show. However, in the presence of liver dysfunction, the lactate clearance will be less than normal and therefore, this effect may result in hyperlactatemia at lower pH. (Alkalosis induced hyperlactatemia is an undesirable consequence of alkali therapy for lactic acidosis).
5) Other causes of hyperlactatemia:
(a) Epinephrine infusion (enhanced glycogenolysis with increased production of lactate)
(b) Nitroprusside infusion (from cyanide accumulation, sign of cyanide intoxication and therefore, is an ominous sign)
(c) Acute asthma (possibly increased lactate production from respiratory muscles)
(d) Seizures (transient)
6) Increased production by enteric microbes (D-lactic acid). ...usual lactate the we talk about is L-isomer of lactate (levo). D-lactate is not measured in usual lab tests. Therefore, this will manifest as widened Anion gap with normal L-lactate level. Some labs can perform D-lactate estimation upon request.
From: The ICU book. Paul Marino (2nd Ed. 1997)