Lactate

Journals Reviewed: Critical Care Medicine, J. Trauma.
Abstracted by: Dr S. Robertson; MB.BCh. (Registrar, University of the Witwatersrand)

Summary of abstract

Out of the vast literature on lactate and critical care, we pull out a few articles, and savage others! (You may wish to briefly browse our editorial comment ).


Lactate and Cardiogenic Shock

A modest study that compared healthy volunteers with patients in cardiogenic shock following cardiac surgery. There were seven patients in each group. The authors address the question: "Is hyperlactataemia in such patients due to increased peripheral production of lactate, or decreased liver removal of lactate?"

The assessment of lactate metabolism appears to have been fairly rigorous, as the authors used both a pharmacokinetic approach (measuring lactate level decay on lactate loading), and an isotope dilution technique. Glucose turnover was also determined using labelled glucose.

Arterial lactate levels were impressive in the patients (a mean of 7.8 mmol/L). The key result is that although lactate production in the patients was markedly increased compared with controls (over three times as great, on average), lactate removal was the same as in controls!

It is unfortunate that inotropic support in these patients was with adrenaline (or noradrenaline), as this may have confounded the issue. In addition, the controls and the patients differed markedly in age (the patients tended to be older). Age-matched, and even disease-matched controls (or possibly even determining glucose and lactate kinetics later on in survivors) might have been preferable. Despite these limitations, the authors make a valuable contribution to our knowledge of what happens to lactate metabolism in shock - the liver keeps on doing its job, with increased lactate levels originating in the peripheral tissues. It's interesting that in this study, glucose production was also dramatically increased (threefold), with increased glucose turnover. The authors make the important point that treating hyperglycaemia in such patients may lessen the tendency of peripheral tissues to produce lactate, yet another reason to maintain meticulous glycaemic control in the critically ill!

The authors contrast their results with reports of lactate metabolism in severe sepsis , where impaired clearance seems extremely important (See: Levraut et al. Am J Resp Crit Care Med 1998 156 1-6). The reason for such differences is not known. An accompanying editorial (p 3932) resurrects the unfortunate term "Type A lactic acidosis".
 

Article 1: Effects of cardiogenic shock on lactate and glucose metabolism after heart surgery.
Crit Care Med 2000 28(12) 3784-3791.
Article type: Clinical Study
Authors: Chioléro, et al. (Lausanne, Switzerland)

Lactate and Trauma

First, a pilot study (retrospective) was undertaken to assess the incidence of elevated serum lactate in trauma patients, and to correlate that with outcome. 31 patients were enrolled. In this group, there were 4 deaths, 6 cases of MODS and 13 cases of ARDS/ALI. All of the patients with these complications were found to have a persistently raised serum lactate within the first 4 hours of admission to ICU. On this basis, the prospective study was performed...

85 patients were admittd to ICU over a four month period. Inclusion criteria were:

  • Must survive the first 24 hours
  • Stable vital signs (pulse < 120 , systolic blood pressure > 100 , urine output > 1.0 ml/kg/hour)
  • Injury severity score greater than 20
Exclusions: 6 patients with untreatable raised intracranial pressure

In the whole group:

  1. 21 patients in the total group had normal serum lactate on admission and within the first 24 hours. In the group there were no deaths, no cases of MODS and no cases with Respiratory Complications
  2. 58 patients had elevated serum lactate on admission.
The second subgroup were then aggressively resuscitated: The first resuscitation measure was to give an additional bolus of fluid and/or blood product If the serum lactate failed to normalize after this, a PA catheter and arterial line were placed, the PCWP raised to 12-15, the haematocrit raised to 30%, and inotropic agents were considered. This second group was then divided into two groups, those who normalized and those who failed to normalize their serum lactate
  1. 44 patients serum lactate normalized within 24 hours. In this group, there were no deaths, 3 cases of MODS, and 14 cases of respiratory complications
  2. 14 patients filed to normalize their serum lactate within 24 hours. In this group there were 6 deaths, 5 cases of MODS, and 7 cases of respiratory complications

The data was analysed using the Students t test and the Chi-Square test. It was found that there was a statistically significant difference between all groups. Both initially elevated and persistently elevated serum lactate levels were associated with a higher morbidity and mortality. The authors' conclusion was that early identification and aggressive resuscitation, aimed at normalizing serum lactate (as evidence of occult hypoperfusion) improves survival and decreases morbidity
 

Article 2: The Golden hour and the Silver day: Detection and Correction of Occult hypoperfusion within 24 hours Improves Outcome in Major trauma
Journal of Trauma 1999 Vol 47 No 5
Article type: Clinical Study
Authors: Blow et al

Lactate Clearance following Injury

A fair-sized study of seventy-six patients looked at oxygen delivery, lactate levels and survival over a forty-eight hour period following ICU admission.

It's interesting to contrast the "supranormalisation" tenor of this study with current practices, but out of fairness to the authors, even in 1993 they raise several questions about the validity of "fixed end-points" in resuscitation.

The authors report no difference in cardiac index, oxygen delivery, or oxygen consumption when comparing survivors and those who died! Forty percent of survivors attained "supranormalisation criteria", compared with 25% of survivors. (This is the sort of study that began to give supranormalisation a bad name)!

In the group of survivors (n=51) all but three normalised their blood lactate levels in the first 48 hours; 19 of the 25 who died failed to achieve normal lactate levels by this time. One of the many studies attesting to the prognostic value of serial blood lactate levels.
 

Article 3: Lactate clearance and survival following injury
J. Trauma 1993 (October) 584-8.
Article type: Clinical Study
Authors: Abramson D, et al.


Editorial - All that glisters is not gold!

"Statistics - the shortest route between an unfounded hypothesis and a foregone conclusion"

Many articles have been published on the relationship between blood lactate and outcome. It does seem likely that failure to clear lactate is associated with a high mortality. Unfortunately, the studies examining this issue are sometimes less than perfect.

Take the second article. Two hypotheses are proposed, the first that "..elevation of serum lactate levels, in the absence of clinical signs of shock, correlates with an increased incidence of death, MSOF [multiple system organ failure], and RC [respiratory complications] after severe trauma. The second is that "early identification and correction of [occult hypoperfusion] improves survival and decreases the incidence of MSOF and RC that follows severe trauma".

Unfortunately neither hypothesis is substantiated by the article! Many small criticisms can be levelled at the study. The "pilot study" was merely a retrospective chart review. The study group was heterogeneous. Hypotension was defined as a systolic blood pressure of 100, but in patients with a mean age of ~ 45 ± ~ 20 years, there could have been a substantial number of patients who had baseline hypertension, and were thus under-resuscitated at such levels; likewise, patients with extremely low diastolic pressures may not have had adequate organ perfusion. It is not clear whether anyone received adrenaline, which can result in high lactate levels without necessarily implying a poor prognosis. Patients with (one deduces) any one of (a) a systolic blood pressure under 100 mmHg, (b) a heart rate over 120/min, or (c) urine output under 1 mL/kg/hour, were excluded from analysis as they were considered to be "in shock". Someone with their usual baseline blood pressure of 90/60 (and a heart rate of 60/min), or a patient with a heart rate of 130/min (due to pain, with say a blood pressure of 170 systolic and a urine output of 2ml/kg/min), might have been inappropriately excluded! However all of these 'minor' criticisms pale into insignificance when regarded in the light of the study design.

There was no randomisation. Groups to be compared were defined according to how long it took blood lactate levels to become normal, but management depended on lactate levels. Swan-Ganz catheter placement was as follows:

Pulmonary Artery Catheter Placement
Group (number) Deaths "MSOF" PAC Placement
1 (21) 0 0 0
2 (19) 0 0 7
3 (11) 0 0 3
4 (14) 3 0 9
5 (14) 5 6 8

In addition, the decision to allocate patients into the five groups (no lactic acidosis, or clearance of lactate by 6, 12, 18 or 24 hours) was only decided on analysis, ie post hoc . So in a very real sense, the study is retrospective !

The authors assert that the mortality rate in group five was "significantly higher than in all other groups", but we learn that "To analyse differences in outcome and categorical variables, the chi-squared test was used". There is a problem with this usage - the chi-squared test is only valid for large samples. A rule of thumb is that the test should be avoided if any cell in the table of expected values contains a number under five (unless the total for that group is over forty). Constructing our table of expected values for deaths, noting that overall, eight of seventy-nine patients died, we get:

Chi-squared expected values
Group: 1 2 3 4 5 Total
Lived 18.8 17.1 9.9 12.6 12.6 71
Died 2.1 1.9 1.1 1.4 1.4 8
Group Total 21 19 11 14 14 79

Using our above criterion, a Chi-Squared test is wholly inappropriate! (Also note that if we use our Chi-Squared test to compare multiple groups, the test does not indicate which group differs from the others, merely that there is an overall difference ). Some homework - which test should the authors have used?

Reconstructing the scene of the crime, we can conclude that the authors eyeballed the results, thought "Hmm, if we lump the patients according to how long it took the acidosis to clear then all those who died had persistently high lactate levels. Okay, let's now find some statistics to substantiate this contention" !

The study epitomises statistical mis-application. It is effectively not prospective but retrospective, as groups were chosen post-hoc . In view of the lack of randomisation into a treatment and control group, we certainly cannot support the authors' conclusion that "early identification and treatment of 'occult hypoperfusion' directly impacts on patient survival and patient morbidity". We merely observe in passing (before we consign the article to the statistical rubbish tip) that those patients most fiddled with, appear to have done worst. We should probably resist the temptation to re-analyse the autors' data. There is a point in some resuscitations where one has to simply stand back and say "the patient is too far gone to continue our attempts to revive him"!

Ed      

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