In July 2015, I wrote that though this topic has never been examined before, "it would be worthwhile to apparoach it in a "critically evaluate lactate monitoring" sort of way". Then, in Question 8 from the first paper of 2016 the college asked the candidates to "critically evaluate the role of monitoring blood lactate levels in the critically ill". It was easy to guess that this was going to come up at some stage. Probably the best published resource for a quick overview of this topic is this review article by Bakker et al (2013).
Rationale for lactate monitoring in critical illness
- Lactate is a product of anaerobic metabolism
- Anaerobic metabolism in human tissues is an abnormal physiological state, be it of poor tissue perfusion or impaired oxygen utilisation.
- Lactate clearance is rapid in the presence of normal hepatic function
- Serum lactate may therefore be useful as a biomarker, a measure of anaerobic metabolism.
- Conditions which result in a raised lactate may not be easy to identify in critically ill patients (eg. ischaemic gut, ischaemic limbs, sepsis, etc) and so lactate levels may be the only feature of such conditions
Advantages of lactate monitoring in critical illness
- A simple test, widely available, and can be performed as a part of blood gas analysis
- Venous and arterial lactate levels correlate conveniently
- Lactate is cheap to test - there may be a saving in healthcare costs
- Its clearance by kidneys and dialysis is proportionally poorer than hepatic clearance, which means as a biomarker it is neither obscured by dialysis nor falsely elevated in the presence of renal failure.
- It offers a quantitative marker to evaluate the success of resuscitation
- It is thus an effective surrogate endpoint in the evaluation of therapies aimed to improve tissue perfusion
Errors of lactate measurement
- Storage delay: metabolism by blood cells can cause an elevated lactate
- Collection error: A sample accidentally mixed with compound sodium lactate will cause a spuriously elevated lactate level.
- Measurement error: a "lactate gap" is generated when the lactate-sensing electrode in the b;lood gas machine confuses lactate with ethylene glycol
Errors of lactate interpretation
- Tissue perfusion may be normal, and lactate clearance may be impaired (as in liver disease)
- Lactate may be raised due to poor tissue oxygen utilisation, rather than poor perfusion (eg. in septic shock)
- Lactate may be raised in the absence of acidosis (eg. exogenous compound sodium lactate administration).
- Lactate may be raised because of increased production by abnormal metabolic processes in neoplastic tissue
Evidence for and against the use of lactate as a biomarker
In the college answer to Question 8 from the first paper of 2016, various statements were made regarding the evidence for lactate monitoring. Below, I have tried to track down the references from where those statements came from.
Lactate as a prognostic marker:
- It correlates well with mortality in sepsis: patients with a presenting lactate over 4.0mmol/L were almost five times more likely to die (Mikkelsen et al, 2009)
- High levels may reveal underlying "occult" sepsis in otherwise stable patients (Rivers et al, 2001).
- It is associated with a poor prognosis in trauma. Specifically, failure to normalise the lactate over the first 2 days is associated with worse organ failure and increased mortality (Manikis et al, 1995)
- It predicts poor recovery from cardiac arrest. Survivors and patients with good neurological outcome had lower lactate levels at 0, 12 and 24 hours (Donnino et al, 2015 is the 100-patient study mentioned by the college)
- It acts as a trigger for liver transplatation: lactate predicts non-survivors from paracetamol toxicity earlier than the King's College Criteria (Bernal et al, 2002). However, somehow the effect of adding lactate to the King's College Criteria actually decreases their diagnostic odds ratio slightly, from about 27 to about 26. (Craig et al, 2010)
Lactate as a guide to therapy:
- It is non-inferior to mixed venous saturation as a goal of early sepsis therapy (Jones et al, 2010).
- It can be used to guide therapy in cardiac surgical patients (though neither Pölönen et al in 2000 nor Shrestha et al in 2015 were able to demonstrate a statistcially significant mortality benefit, owing to underpowered study design)