The discussion on the so called lactic acidosis and its causes has become increasingly interesting over the last couple of years as several biochemical explanations have been challenged. A big confusion persists on the various relationships between lactate, lactic acid and metabolic acidosis.
Most clinicians continue to refer to the classical understanding of impaired tissue oxygenation causing increased lactate production, impaired lactate clearance and therefore resultant metabolic acidosis. Just recently we had a discussion on our ward round on this topic when I was presented the most recent article of UpToDate online on the causes of lactic acidosis. The authors state that ‘Lactic acidosis is the most common cause of metabolic acidosis in hospitalised patients’ and that ‘Lactic acidosis occurs when lactate production exceeds lactate clearance. The increase in lactate production is usually caused by impaired tissue oxygenation…’… finally suggesting that lactate is no good!
These statements support the classical understanding that:
– Hyperlactatemia is caused by tissue hypoxemia, and
– This in turn then leads to a metabolic acidosis called lactic acidosis
This biochemical understanding has persisted for decades but there are some good reasons to strongly challenge this classical aspect on the ‘bad’ lactate. Lactate turns out to be by far more complex in its characteristics and functions, so I decided to try and make a short but comprehensive overview on this molecule.
What is lactate?
Lactate is a small organic molecule with the chemical formula CH3CH(OH)CO2H and structurally looks like on the image to the left. It is produced in the cytoplasm of human cells largely by anaerobic glycolysis by the conversion of pyruvate to lactate by LDH. This chemical reaction normally results in a blood lactate to pyruvate ratio of about 10:1. And while lactate is produced, NAD+ also is incurred and this actually can accept protons itself, so does not result in acidosis itself.
Lactate arises from the production of energy by consuming glycogen and glucose.
Where does it come from?
Typically most people think of muscles first as an origin of lactate. As a matter of fact lactate originates from many other organs, including our red blood cells. Red blood cells always produce lactate as they lack the mitochondria required to regenerate NAD+ needed for glycolysis. In general you can say that tissues with lots of LDH are the main producers of lactate. Around 20mmol/kg/day of lactate are produced under normal circumstances.
Lactate is not only produced in skeletal muscle.
What happens with it?
Lactate is not just for nothing. After its production by anaerobic glycolysis lactate is reutilised, for instance in the liver and the cortex of the kidneys. As an example: under the influence of cortisol it is used for gluconeogenesis in hepatocytes and restores glucose and glycogen. Also it is a part of oxidative phosphorylation in the liver, kidney, muscles, the heart and the brain. Like this lactate helps conserve glucose levels in our blood.
Lactate actually serves as a fuel for oxidation and glucose regeneration and therefore is a source for energy itself.
How does hyperlactatemia develop?
In general you can assume that there is a balance between lactate production and its consumption or usage. The classical understanding that tissue hypoxia leeds to overproduction and underutilisation by impaired mitochondrial oxidation is basically correct.
The key point though is that lactate is also produced via aerobic glycolysis as a response to stress. This happens in septic patients, asthmatic exacerbations, trauma and other critical conditions. In these situations the trigger for lactate production is adrenergic stimulation and NOT tissue hypoxia. There are also several other reasons for hyperlactatemia other than tissue hypoxia:
Sepsis: Adrenergic drive
Asthma: Adrenergic drive
Trauma: Adrenergic drive
Cardiogenic and haemorrhagic shock: Adrenergic drive
Pheochromocytoma: Adrenergic drive
Inflammation: Cytokine drive
Alkalosis, antiretroviral medication and others
Also, there is good evidence showing that organs like the lungs are an important producer of lactate during stress. And of course in all these conditions hypoxic and non-hypoxic hyperlactatemia might also co-exist.
In critically ill patients often other reasons than tissue hypoxia are responsible for hyperlactatemia (e.g. adrenergic drive).
Is lactate harmful?
In contrast to the classical understanding of lactate and lactic acidosis more and more evidence comes up indicating that lactate during stress actually serves as a fuel for energy production. Various tissues, e.g. the myocardium increase their lactate uptake during stress significantly. Also our brain consumes more lactate during stress which is used for oxidation. Research has shown that lactate infusions improve cardiac output in pigs and even in patients with heart failure.
Experimental work on isolated muscles suggests that circulating catecholamines and development of acidic conditions during exhaustive exercise may improve muscles’ tolerance to elevated K+ levels. This implies that during high-intensity activity with high extracellular K+ and adrenaline, lactate actually serves as a performance-enhancing chemical, rather than being the cause of muscle fatigue.
Lactate is not harmful for our organism. On the contrary, recent compelling evidence actually suggests that lactate might actually be beneficial, rather than detrimental, during high-intensity activity and to force development in working heart and skeletal muscle.
Why do critically ill patients with hyperlactatemia die more often then?
In critical care hyperlactatemia indeed is a marker of illness severity and a strong indicator of mortality. This is especially true for patients with sepsis. However, as described above, hyperlactatemia often doesn’t indicate hypoperfusion or tissue hypoxia. Hyperlactatemia rather reflects the severity of illness by representing the degree of our body’s activation to stress. A fall in lactate concentration following treatment of critically ill patients is due to an attenuation of the stress response rather than to correction of oxygen debt.
Hyperlactatemia reflects severe disease and the patients response to stress. Patients die due to their illness, not because of a high lactate.
What about Ringer’s lactate?
Ringer’s lactate (RL) is not harmful in patients with hyperlactatemia.
As a matter of fact RL turns out to be superior compared to normal saline in hyperlactatemia, acidotic patients and patients with hyperkalemia.
The bottom line
– Lactate is an indicator of stress, a marker of illness severity and a strong predictor of mortality, but not harmful as a molecule itself.
– Lactate is helpful as an important source of energy and an important fuel for oxidation and glucose generation.
– During conditions like septic shock there is no proof that lactate is produced only due to tissue hypoxia. In fact well ventilated lungs produce a large amount of lactate during sepsis. Lactate in sepsis and other critical conditions is mostly not due to hypoxemia or hypoperfusion.
– Ringer’s lactate contains sodium lactate, but not lactic acid. Lactate itself, as mentioned above, is actually beneficial in severe disease. Therefore RL remains the fluid of choice during severe disease like for instance septic shock.
– Ringer’s lactate is superior to normal saline in patients with metabolic acidosis, hyperlactatemia and also hyperkalemia.
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