Blood lactate: biochemistry, laboratory methods, and clinical interpretation.

Volume 28, Issue 4 (1991)

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Blood Lactate: Biochemistry, Laboratory Methods, and Clinical Interpretation

Critical Reviews in Clinical Laboratory Sciences Downloaded from informahealthcare.com by Freie Universitaet Berlin on 05/02/15 For personal use only.

John G. Toffalefti

ABSTRACT With the renewed awareness of blood lactate as an indicator of circulatory impairment, there has been much interest in the use of lactate measurements to determine the overall state of oxygenation of patients in critical care. This review begins by covering the areas of lactate homeostasis and biochemistry, both of which are essential for fully understanding the interpretation of lactate measurements. Then, the clinical interpretation of lactate measurements includes sections on the causes and treatments of hernodynamic deficits leading to lactic acidosis, the classification of lactate abnormalities, and the use of lactate measurements in critical care monitoring, including surgery. Both the principles and the latest developments in lactate methodology are covered, including the new whole blood analyzers. This review concludes with reference intervals and guidelines to the interpretation of results. Key Words: blood lactate, hypoxia, glycolysis, oxygen delivery.

1. INTRODUCTION An understanding of the biochemistry and clinical interpretation of blood lactate concentrations might at first seem simple and easily grasped. While there is only one reaction that produces or consumes lactate (pyruvate to lactate and vice versa), the biochemistry of lactate production and regulation is not at all simple. Historically, the discovery of the biochemical steps in the production of lactate from glucose during glycolysis, and the reconversion of lactate to glucose during gluconeogenesis are among the greatest achievements of biochemistry during the 20th century. As with the apparent simplicity of the biochemistry of lactate, the clinical causes of excess lactate are mostly due either to tissue hypoxia or to vigorous muscle activity. However, the pathophysiology of tissue or cellular hypoxia is complex, often representing a great challenge to the critical care physician. Even during rest, skeletal muscle metabolizes glycogen to lactate in order to produce ATP. Since both blood flow and tissue oxygenation are balanced during exercise, increased blood lactate does not result only from hypoxia, but rather to a greatly enhance metabolism of glycogen to lactate. Although it will not be the focus of this review, in studies of exercise and sports physiology, lactate measurements are becoming an

r-

J. G . Toffaletti, Ph.D., Associate Professor of Pathology, Director of the Blood Gas and Duke North Clinical Laboratories, Duke University Medical Center, Durham, NC 27710.

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Critical Reviews in Clinical Laboratory Sciences

important tool for determining the level of effort that optimally balances the utilization of glycogen and fatty acids in the production of ATP. Even though the conversion of pyruvate to lactate does not produce ATP, lactate production is not merely a dead-end side reaction that serves as a marker for glycolysis. It is an important reaction that ensures maximal ATP production during “anaerobic” glycolysis, both by removing pyruvate and by producing NAD+. This will be detailed in Section ILI on biochemistry. Perhaps the clinical area of greatest current interest is the use of a blood lactate as a marker for hypoxia caused by circulatory impairment. As such, a rise in the lactate concentration of blood should be among the earliest markers of impaired oxygen delivery and or oxygen utilization. While some regard a rise in blood lactate as an indicator of impending death, in many critical situations lactate measurements serve as an early warning of the need for medical intervention.’ However, the proper utilization of lactate for this purpose requires that lactate results be provided both very rapidly and on small volumes of whole blood, especially since pediatric applications may be among the most important. As such, lactate results may ultimately need to be provided in the same time frame and perhaps together with blood gas results.

II. LACTATE HOMEOSTASIS The concentration of lactate in the extracellular fluid is normally about 1 mmol/l. This concentration represents a dynamic steady state that results from approximately 1200 to 1500 mmol of lactate being both produced and consumed daily by a 70 kg person.’ Lactate, which can only be formed from pyruvate as a product of anaerobic glycolysis, is produced mostly in tissues with a high rate of glycosis such as skeletal muscle, gut, brain, skin, and red blood cells. While each of these tissues produce lactate at a basal rate, strenuous exercise can increase the production of lactate in skeletal muscle by at least tenf01d.~While the basal rate of lactate production is estimated to be about 1.0 mrn~l/kg/h,~ the maximal rate of lactate production, as estimated from running long distances of perhaps 5000 m, may be up to 400 rnm~l/kg/h.~ Under basal conditions, the lactate produced by all tissues is reutilized at a nearly constant rate by the liver, which resynthesizes glucose from lactate in the process of gluconeogenesis. This process converts more than half of the lactate back to glucose, which provides a continual source of glucose for glycolytic tissue. Of the remaining lactate, most enters the Krebs cycle and then is metabolized to CO, and H,O by oxidative phosphorylation. While complete removal of the kidneys (nephrectomy) appears to slow lactate removal by about 30%,5 renal failure per se is rarely associated with lactic acidosis, because under basal conditions the liver is readily capable of increasing its rate of lactate removal to accommodate the increased load. In addition, under certain conditions, resting skeletal muscle can be converted from a lactate-producing to a lactate-consuming t i s s ~ e . ~ Lactic acid per se is not a product of glycolysis. The lactate anion is produced during glycolysis, while the hydrogen ion is produced by hydrolysis of ATP. On the other hand, when two molecules of lactate anions are converted to glucose by gluconeogenesis, two hydrogen ions are consumed. Either gluconeogenesis or complete oxidation of glucose to CO, and H,O will consume about 1200 to 1500 mmol of hydrogen ions per day. Thus, both gluconeogenesis and oxidative phosphorylation are important processes in acid base balance, being quantitatively more important than even the kidneys, which excrete about 60 to 75 mmol of hydrogen ions in a 70 kg person. Therefore, the liver plays a pivotal role in acid base balance because, during gluconeogenesis, for each two lactate anions utilized for glucose synthesis, two hydrogen ions are consumed.6


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111. BIOCHEMISTRY


The glycolytic pathway is essentially the same under either aerobic or anaerobic conditions. Glycolysis is but one of the three stages involved in the complete oxidation of glucose to carbon dioxide and water, with oxygen as the ultimate electron acceptor. For the net reaction: glucose

+ 60, -+

6C0,

+ 6H,O

Three stages are involved: 1. 2. 3.

Glycolysis in the cytosol (Embden-Meyerhof pathway) Tricarboxylic acid (TCA) cycle in the mitochondria (also called either the citric acid cycle or the Krebs cycle) Oxidative phosphorylation in the mitochondria (electron transport)

The fate of pyruvate is part of the key to whether a moderate or a very large amount of lactate is produced. If the oxygen supply is adequate, oxidative phosphorylation produces sufficient NAD to continue the conversion of pyruvate to acetyl CoA that enters into the TCA cycle. At least two situations can prevent conversion of pyruvate to acetyl CoA for entry into the TCA cycle: 1.

2.

Hypoxia No or few mitochondria in cells

Since some cell types have no or relatively few mitochondria, such as erythrocytes and some types of muscle cells (the so-called white or fast-twitch cells), the cells without mitochondria must rely heavily on glycolysis to produce ATP, a relatively inefficient process. It is well known that the aerobic oxidation of glucose produces far more ATP (38 mol per mol of glucose) than does the anaerobiq metabolism of glucose to lactate (2 mol of ATP per mol of glucose). This is not surprising considering that the oxidation state of carbon in lactate is nearly the same as carbon in glucose, while carbon in CO, is essentially completely oxidized at a much higher oxidation state. Several tissues (muscle, brain, skin, erythrocytes, and gut) produce lactate at a basal rate, with most lactate being reconverted to glucose in the liver by gluconeogenesis. Especially with strenuous exercise, production of lactate by skeletal muscle increases enormously, even with good tissue perfusion, both because muscle tissues have large supplies of glycogen available, and because lactate production is a major route by which some types of muscle cells produce ATP. Therefore, an understanding of lactate metabolism requires familiarity with glycolysis, gluconeogenesis, the TCA cycle, and oxidative phosphorylation. A basic question is, What biochemical events favor synthesis of lactate? When the oxygen supply to cells with mitochondria is reduced, NAD production by electron transport ultimately ceases. This results in the buildup of both pyruvate and NADH, conditions that favor the production of lactate. Even though no additional ATP is produced when pyruvate is converted to lactate, this reaction is very important, both because pyruvate is removed, and because NAD is produced. While the removal of pyruvate ensures favorable conditions for the previous reactions to proceed, the conversion of NADH to NAD is a most important step in the production of ATF’, since NAD is usually in short supply and is therefore the rate limiting factor in glycolysis. The NAD produced when pyruvate is converted to lactate is essential to maintain the conversion of glyceraldehyde-3-P04 to 1,3 diphosphoglycerate, which is then dephosphorylated to produce ATP from ADP (see Figure 1).

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Critical Reviews in Clinical Laboratory Sciences no ATP

GLUCOSE ATP

4

tiexoklnaae

It

/c

Glucoae-&phoaph.L.ae

Glycogen pho.phorylase

Glucos.4-phoaphate 4Phoophog’ucomutsret Glucoaephwphate Iaomerue Fructose 6- phosphate

1t

Glycogen mynlhetese

Glucoae 1-phoaphale

Fructose-blaphomphataae + no ATP

ATP -b bPhosphofructoklnau


Glycogen

Fructose, 1.6-blmpho~Ph*~

Dlhydroxyacetone Glyceraldehyde %phosphate phoaphale Trloaephoaphate Glyceraldehyde-phosphate Iaomera8e dehydrogenase

-r

3-Phomphoglyceroyl phosphate (2) ATP

klnate

(2) NAD

3-PhoaphoglyCerate

--

Phosphoenolpyruvate urboxyklnase (2)GTP Oxaloacetate

NAD-f malate

1

Phoaphoglyceromutase

2-Phosphoglycerste

5

Enolase

Phosphocnolpyruv.te

(2) ATP

-1

Pyruvate klnase

Lactate LACTATE

PYRUVATE

Bloaynthellc pathways: Fatly Acids. Ketone bodks Choleaterol, Sterold hormones., Acetylchollne

Bound hydropen atom. for the reaplratoly chaln

0xld.Uve Phoaporylatlon

-

ATP

FIGURE 1. Metabolic pathways of glycolysis and gluconeogenesis in relation to the TCA cycle and oxidative phosphorylation. Side arrows indicate the reactions in which NAD or ATP (GTP) are produced or consumed. In the cytosolic reactions, a (2) indicates a net gain or loss of 2 molecules since two 3-carbon reactions occur for each 6-carbon molecule of glucose. (Redrawn from Madias, Kidney Int., 29, 754, 1986.)

While the glycolytic production of 2 mol of ATP during hypoxia may seem a trickle compared to the 38 mol formed by the combined glycolytic, TCA, and oxidative phosphorylation systems, this trickle is extremely important since it maintains cell viability three to four times longer than if no ATP were formed. The process of glycolysis occurs in the cytosol and is diagrammed in Figure 1. While about ten enzymatic reactions are necessary to convert glucose to lactate, because several “forward” reactions are irreversible, other enzymes are required for reverse reactions. Therefore, gluconeogenesis (lactate + glucose) is not simply a reversal of glycolysis. Glycolysis is largely controlled by three rate-limiting irreversible reactions: those catalyzed by hexokinase, 6-phosphofructokinase (PFK), and pyruvate kinase. As shown in Figure 1, three different enzymes are needed for the reverse reactions. Even though PFK consumes ATP in its reaction, PFK is allosterically inhibited by high concentrations of ATP. Therefore, a low concentration of ATP promotes glycolysis at this step.



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A key step in glycolysis is the conversion of glyceraldelyde-3-phosphate to 3-phosphoglyceroyl phosphate, in which NAD is converted to NADH. Therefore, more NAD must be generated in order to maintain glycolysis. With an adequate supply of oxygen, aerobic tissues regenerate NAD via oxidative phosphorylation in the mitochondria. Under anaerobic conditions, tissues regenerate NAD in the cytosol by converting pyruvate to lactate. In aerobic tissues, pyruvate is transported across the mitochondrial membrane where it is decarboxylated to acetyl-CoA, a reaction that requires a continual supply of NAD. The resulting acetyl-CoA can enter the TCA cycle that produces reduced substances for electron transport in oxidative phosphorylation. While NAD is initially reduced to NADH in the TCA cycle, NAD is regenerated in the early stages of oxidative phosphorylation, thus providing the necessary NAD to continue production of acetyl CoA from pyruvate. Gluconeogenesis is a process that occurs exclusively in the liver and renal cortex, and is especially active during recovery from intense muscular activity.’ While we focus on the conversion of lactate to pyruvate, there are other pathways that produce pyruvate, for example, alanine can be converted to pyruvate by the enzyme alanine aminotransferase (ALT). As stated earlier, gluconeogenesis by the liver is the major mechanism for removal of lactate, with blood lactate homeostasis during basal conditions being largely a balance between lactate production by many tissues and lactate consumption by gluconeogenesis. Gluconeogenesis is not simply a reversal of glycolysis, since several different enzymes are involved, some of which are in the mitochondria. A key step in gluconeogenesis is the conversion of pyruvate back to phosphoenolpyruvate (see Figure 1). Since pyruvate kinase cannot function in the “reverse” direction, the phosphorylation of pyruvate is achieved by an alternate pathway through a somewhat roundabout sequence of reactions that require the cooperation of enzymes in both the cytosol and mitochondria. Pyruvate enters the mitochondria and is carboxylated by pyruvate carboxylase, then eventually becomes phosphoenolpyruvate in the cytosol, consuming both ATP and GTP in the process. The two additional irreversible steps in gluconeogenesis are catalyzed by fructose bisphosphatase and glucose6-phosphatase, neither of which produce ATP. The net process of gluconeogenesis consumes six high-energy phosphate compounds: 2 pyruvate

+ 4ATP + 2GTP + 2NADH + 2H’ + 7H,O + glucose + 4ADP + 2GDP + 2NAD + 6P0,

Even though the conversion of pyruvate back to glucose consumes more ATP, this process conserves reduced carbon substrates when energy supplies are adequate for a later time when energy supplies may be low. Of the several control points for gluconeogenesis and glycolysis, one of these is the regulation of the enzyme pyruvate carboxylase by the mitochondrial concentration of acetyl-CoA. Excess acetyl-CoA promotes the conversion of pyruvate to phosphoenolpyruvate, which eventually becomes glucose. Therefore, whenever the cell has ample supplies of ATP, acetyl-CoA, citrate, or NADH, glycolysis is inhibited and gluconeogenesis is promoted. On the contrary, when energy fuels (ATP, etc.) are low, glycolysis is accelerated and gluconeogenesis is inhibited.

IV. CLINICAL CONSIDERATIONS AND INTERPRETATION A. Causes of Hemodynamic Defects Leading to Lactic Acidosis In this review, since we emphasize the use of lactate measurements as an indicator of circulatory shock, a brief description of the hemodynamic defects in various classes of circulatory shock is given, as nicely summarized by Rackow and Weil.g


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The primary characteristic of circulatory shock is acute perfusion failure, in which oxygen metabolism is impaired by decreased availability of oxygen to tissues. Several categories of hemodynamic deficits are currently recognized as causes of circulatory shock leading to inadequate oxygen delivery: hypovolemia, cardiac failure, vascular obstruction, and distributive defects. Hypovolemia, which accounts for most cases of acute circulatory failure, results from the loss of blood, plasma, and or electrolyte fluids. When the volume of blood within the intravascular space is reduced to less than two thirds of normal, immediate survival is threatened. Although not necessarily a hypovolemic condition, anemic hypoxia is related to this category. Cardiogenic shock represents primary pump failure in which cardiac output is critically reduced. Some causes may be from cardiac arrhythmias or ventricular failure. Obstructive defects are caused by an impediment or obstruction to the mainstream of blood flow. Examples of this cause are pulmonary embolism, clot formation, or pericardial tamponade . Distributive defects represent impairment in the distribution of blood flow or blood volume. This is the least-understood type of circulatory defect in that it is often caused by septic shock, which may cause a complex spectrum of circulatory derangements. Perhaps the most likely mechanism in humans with septic shock is that a bacterial endotoxin causes a maldistribution of blood flow in which there is high perfusion to tissues with low oxygen needs and low perfusion to tissues with high oxygen needs." Other proposed mechanisms of distributive defects are peripheral shunting of blood from arterial to venous circulation without adequate exchange of oxygen, and expansion of the venous space such that arterial blood volume is critically r e d u ~ e d . ~ . ' ~ The development of circulatory shock can be assessed by the use of either systemic or local indices of tissue perfusion. Being an indicator of tissue anaerobiosis, arterial lactate concentrations are a systemic indicator of circulatory shock, as are several other parameters: mixed venous oxygen tension and saturation, oxygen delivery, and oxygen consumption. The measures of local indices include: transcutaneous PO,, toe temperature, and muscle oxygen histograms. Because it is a relatively easily measured indicator of tissue hypoxia, blood lactate may serve as a useful guide to detect, monitor, and treat a variety of conditions of circulatory shock.

B. Treatment of Lactic Acidosis Treatment of lactic acidosis may involve one or more techniques of mechanical, pharmacologic, surgical, respiratory, or transfusion interventions. Because of the importance of lactate measurements in monitoring oxygen delivery and consumption, correction of lactic acidosis is usually aimed at correcting the cardiac and or circulatory impairment. An excellent textbook reference for the pharmacologic support of critically ill patients is that by Chemow. I I The management of hypovolemic shock aims at controlling fluid loss and administering fluid or blood as guided by hemodynamic measurements. If anemia is present, either whole blood or packed red cells may be administered. Oxygen delivery may be enhanced by increasing the oxygenation of blood by using positive pressure ventilation, hyperventilation, or enriching the oxygen content (FIO,) of the inspired air. The treatment of cardiogenic defects must correct the alteration in cardiac performance. This is usually by administering either cardiotropic (inotropic) or antiarrhythmic agents. Cardiotropic agents include epinephrine, dopamine, dobutamine, and calcium; antiarrhythmic agents include lidocaine, quinidine and magnesium. To decrease the cardiac "afterload" effects (factors that oppose venticular contraction), or when the response to blood volume repletion is inadequate,I2vasodilators are often used. Depending on the type given, vasodilators may lower arterial pressure, venous pressure, or



259

both. Common vasodilators used in critical care include nitroprusside, hydralazine, and calcium channel blockers such as nifedipine. In patients with obstructive shock, removal of the physical impediment by either surgical or pharmacologic (thrombolytic) intervention is the primary goal. Among the more common drug groups given are aminoglycosides, penicillin derivatives, and cephalosporins. An interesting experimental drug that has been reported to lower concentrations of both lactate and hydrogen ions while increasing bicarbonate is dichloroacetate.l 3 Dichloroacetate stimulates the activity of pyruvate dehydrogenase, the enzyme that converts pyruvate to acetyl CoA (see Figure 1). Increased conversion of pyruvate to acetyl CoA, by decreasing pyruvate, increases utilization of both lactate and alanine. Remarkably, dichloroacetate also increases arterial systolic blood pressure, with the effect lasting from several minutes to several hours. Despite these apparently beneficial effects, in one study the eventual mortality of patients given this drug decreased only slightly.l3

C. Classification of Lactate Abnormalities Lactic acidosis as a clinical syndrome was described by Huckabee in 1961.I4 The simplest description of lactic acidosis is that it is due either to overproduction or to underutilization of lactate. As we have seen previously, the large amounts of lactate ordinarily produced and consumed indicate that an imbalance of either process could rapidly lead to increased lactate concentrations in blood. As proposed by Cohen and Woods in 1976,15 causes of lactate acidosis have been categorized into four classes: A, B1, B2, and B3. Type A, the most common, is from poor tissue oxygenation, such as from shock due to ventricular failure, hypovolemia, sepsis, carbon monoxide poisoning, acute asphyxia, acute pulmonary edema, or severe congestive heart failure. The use of lactate measurements in the critical care management of these conditions will be discussed later. Type B disorders are not associated with poor tissue oxygenation, except perhaps as a terminal event. Type B disorders are divided into three groups. The first are patients with disorders such as diabetes, liver disease, and kidney disease. The second group includes patients with lactic acidosis caused by some drugs or poisons. In the third group are patients with rare hereditary enzyme defects in either the glycolytic or Krebs cycles.’6 As a general rule, the type B disorders represent more chronic disease processes in which the diagnosis of lactic acidosis does not require a rapid turnaround time for the lactate result. The type B1 patients include those with diabetes, renal failure, liver disease, infections, some malignancies, and convulsions. Clinically significant lactic acidosis occurs in only about 10% of the patients with diabetes mellitus.” While the mechanism of lactate accumulation is not certain, pyruvate dehydrogenase (see Figure 1 ) may be inhibited. In insulindependent diabetic patients, even though lactate production by leg muscles is increased by 180 to 500% above normal subjects,I8the excess lactate produced by the muscles is removed by the liver, preventing a significant rise in blood lactate concentrations. Because the liver has such a large capacity to remove lactate, renal failure per se rarely leads to lactic acidosis. Patients with severe liver disease, especially fulminant hepatitis, can develop severe lactic acidosis,I9 presumably because there is insufficient functional liver tissue to convert lactate, produced from normal metabolism, to glucose. Certain leukemias and other malignant conditions are sometimes associated with a chronic increase of blood lactate,16-20 which often subsides when cytolytic therapy causes regression of the neoplastic process. The increase in lactate may be attributed to a reduction in hepatic extraction secondary to the administration of parenteral nutrition in neoplastic conditions.*lGrand mal seizures cause lactic acidosis,’6.22 both from acute asphyxia during laryngospasm and from severe muscular hyperactivity during the seizure. While lactic acidosis often occurs with bacteremia, the mechanism may be complex, as described under hemodynamic shock, with circulatory impairment either from


260


inappropriate distribution of blood, shunting from arterial to venous circuits, or expanded venous space. However, the explanation is apparently not because of excessive production of lactate by bacteria. l6 The group B2 classification includes drugs or intoxicants such as phenfomin or other biguanides and ethanol. Phenfonnin inhibits lactate gluconeogenesisby inhibiting the activity of pyruvate carboxylase (see Figure 1). Both lactate and hydrogen ion regulation are impaired, Since phenformin was removed from the U.S. leading to hyperlactatemia and market in 1977 because it caused lactic acidosis, it should not be a factor, except in countries where biguadines are still prescribed to control diabetes. It is not clear why ethanol causes lactic acidosis in some but not most alcohol abusers. The mechanism may be related to either an undiagnosed seizure disorder or to an increased ratio of NADH to NAD. Since oxidation of ethanol to acetaldehyde produces NADH, this would promote reduction of pyruvate to lactate. l6 The group B3 category includes various rare genetic enzyme defects associated with impaired mitochondria1 oxidation of pyruvate. D. McArdles Disease Lactate measurements are important in the diagnosis of McArdles disease, in which an absence of a rise in blood lactate after mild exercise is an important criterion in the diagnosis. McArdles disease, also known as type V glycogen storage disease, occurs predominantly in males and is characterized by exercise intolerance in which muscle aching and stiffness occur soon after beginning exercise. While pain and stiffness disappear as exercise is continued, prolonged exercise eventually results in muscle necrosis and myoglobinuria. The laboratory diagnosis is made by having the person exercise their forearm after applying a tourniquet to prevent blood flow. Blood collected 1 min after beginning exercise will show no rise in blood lactate. The definitive diagnosis is made by measuring phosphorylase in a muscle biopsy .24

E. Critical Care Monitoring As mentioned earlier, lactate measurements in critical care monitoring may be both their most exciting and their most controversial use. While there is little doubt that a rise in blood lactate is a sensitive measurement of cardiovascular failure in critically ill patients, there is controversy about whether this laboratory data can guide medical intervention that will benefit the patient or whether it simply predicts impending death. We believe the controversy arises because of the wide diversity of patients who may be under critical care. In patients with multiple disease processes (e.g., respiratory, renal, or liver failure, sepsis, etc.) who are already undergoing supportive care, a definite rise in lactate indicates a very poor prognosis. At the other extreme, are generally younger patients who have undergone major surgery, especially cardiovascular surgery. An elevated lactate following surgery or a rise in lactate during recovery are used to guide the level and type of care required and are by no means certain indicators of death. Broder and Weil in 1 9 a Z 5were among the earliest to use lactate measurements to indicate the severity of shock. They studied 56 patients with shock caused by hypovolemia, sepsis, cardiac failure, and vascular obstruction. They found the following rates of survival were related to “excess” blood lactate concentrations*: 1 mmoYl or less 82%; 1 to 2 mmol/l 60%; 2 to 4 mmoYl 26%; above 4 mmol/l 11%. Thus, any excess lactate above 4 mmoYl predicted a grave prognosis.

*

Excess blood lactate is a defunct term that essentially referred to the concentration of lactate in excess of the basal concentration. The concentration of pyruvate was also included in the calculation.



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In a study of 11 patients with septic shock, a decrease in blood lactate after therapy predicted survival, whereas an increase in blood lactate after initiating therapy (blood volume support, hyperbaric oxygen, etc.) predicted death.26 In a study of patients with endotoxic or septic shock, Blair et al. found the mortality rate to be 100% when blood lactate was over 3 m m ~ Y l . ~ ’ A large study of 410 critically ill patients surveyed over 31 d used several indices to predict survival: blood lactate, blood pressure, heart rate, arrhythmias, spontaneous respiration, urine volume, body temperature, age, and a five-point subjective rating system by nurses. While urine volume had significant predictive value for patients with hypovolemia, they found the lactate concentration was the most significant variable relating to survival in patients with cardiogenic or other forms of shock. When lactate was above 2.7 mmol/l, survival was 50%.’* Henning et al. ,29 in a study of 28 patients with acute myocardial infarction (MI) complicated by cardiogenic shock, compared serial measurements of blood lactate with established hemodynamic, respiratory, and metabolic measurements as prognosticators of survival. Compared to cardiac output, pH, PCO,, PO,, oxyhemoglobin saturation, arterial and venous 0, contents, stroke work index, peripheral arterial resistance, and heart rate, blood lactate concentration was the only early and consistently useful prognostic indicator of survival of patients with both MI and myocardial pump failure. Patients with blood lactate persistently greater than 4 mmol/l for 12 h or more, all expired, regardless of the therapy. In patients whose lactate concentrations rapidly declined and remained at or below 2 mmol/l, all survived. Cowan et al. ,30 in a study of 30 patients admitted consecutively to an intensive care unit with shock of mostly septic origin, compared blood lactate, mean arterial pressure, urine production, and core:peripheral temperature ratios, before and during the course of treatment. While they concluded that lactate measurements were less valuable than the hemodynamic variables in predicting outcome, their data show that lactate decreased significantly in the survivors, then remained nearly the same during the first 24 h after admission. While Cowan et al. concluded that the complexity of hemodynamic and metabolic disturbances modified lactate independently of circulatory status,30Sch~ster,~’ in a review of the literature on critically ill patients, concluded that blood lactate was of considerable value for metabolic monitoring. Furthermore, he concluded that hyperlactatemia helped detect superimposed complications in the critically ill. He emphasized the importance of serial lactate measurements, particularly persistently high or progressively rising concentrations. In a study of pyruvate and lactate changes during open-heart surgery, Schiavello et al. ,32 found that as plasma pyruvate fell during extracorporeal circulation, lactate increased remarkably during the extracorporeal circulation, then gradually declined in the postoperative period. They point out that, while blood flow and perfusion pressure may be high and constant during extracorporeal circulation, localized hypoperfusion of certain areas occurs frequently, which would elevate blood lactate concentrations. Serial measurements of lactate following pediatric cardiac surgery have become part of routine monitoring during recovery at our hospital. The physicians use rapid measurements of lactate as a guide to the intensity of cardiotropic support, volume support, and blood pressure regulation. Generally a higher initial lactate following surgery indicates a more intensive degree of care will be needed. A persistently elevated lactate or a rise in lactate at any time is taken as a serious sign and warrants medical intervention. In these patients, either an initially very high lactate or a rise in lactate do not necessarily indicate a fatal outcome. Rashkin et al.33 observed that the overall patient data on blood lactate correlated with both survival and oxygen delivery. Even in adults with respiratory distress syndrome, patients with oxygen deliveries of 8 ml/kg/min or more had both low lactate and a survival rate of 55%. Vincent et al.34 showed that when measured every 20 min during the frst hour of


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therapy for shock, changes in blood lactate concentrations provided an early and objective evaluation of the patients response to therapy. Vincent et al. studied 17 patients with noncardiogenic shock (hypovolemic, septic, obstructive), five patients following successful CPR for cardiac arrest, and five patients after grand ma1 seizures. This study has importance for several reasons: (1) samples were collected prospectively at defied intervals (20 min); ( 2 ) serial results on individual patients were clearly presented; (3) an initially high lactate did not necessarily predict a poor response; (4) both a protocol for obtaining lactate measurements and interpretive guidelines were presented, as follows: 1.

2. 3.

Measure lactate 1 h after initiating aggressive therapy. A decreasing lactate of more than 10% predicts clinical improvement. If no decrease in lactate is observed, a change in therapy should be actively considered.

Vincent et al. also showed that lactate decreased gradually in the five patients following grand ma1 seizures, and in the five patients who had successful CPR for cardiac arrest. In a retrospective study of 31 patients with circulatory shock of septic origin and of 19 patients with circulatory shock of nonseptic origin (hypovolemic, cardiogenic, obstructive), Groeneveld et al.l0 reported that the arterial blood lactate of these two groups responded differently to therapy that increased arterial oxygen content, cardiac output, and, therefore, oxygen delivery. In cases of nonseptic shock, blood lactate decreased as oxygen delivery increased. However, in cases of septic shock, despite having similar increases in oxygen delivery, blood lactate declined in 12 patients and increased in 19 patients. Groeneveld believed this may have been due to differences in individual tissue oxygen needs, as only three patients died of the 12 who had declining lactate, compared to 16 patients dying of the 19 who had increasing lactate. Despite apparently adequate oxygen delivery to the tissues, peripheral shunting of blood between arterial and venous circuits without adequate oxygen exchange may have led to inadequate oxygen consumption in the tissues of some patients with circulatory shock of septic origin. Since lactic acidosis unrelated to tissue hypoxia has been described in patients with liver disease, Kruse et al.35retrospectively studied the utility of arterial lactate concentrations as an indicator of tissue hypoperfusion in critically ill patients with both parenchymal liver disease and total bilirubin of over 2 mg/dl. Because a lactate concentration of over 2.2 mmoVl was significantly associated with mortality, they concluded that elevated lactate in critically ill patients with liver disease is associated with both clinical evidence of shock and increased mortality.

F. Labor and Delivery Lactate measurements have been used in the evaluation of fetal stress during labor. In a study of 132 liveborn infants, Suidan et al.36 observed that, in both mothers and fetuses, blood lactate increased during labor and reached maximal concentrations at vaginal delivery. If fetal hypoxia was present, they observed a substantial rise in umbilical blood lactate concentrations. They concluded that fetal lactate measurements were better than base excess measurements for evaluating fetal stress during labor. Although the placenta is a net producer of lactate, most placental lactate is discharged into maternal cir~ulation.~’ measured both pH and lactate in fetal scalp blood during In this same area, Smith et labor along with fetal heart rate patterns as indicators of intrauterine hypoxia. They found that both lactate and pH were equally effective and comparable to continuous electronic fetal heart monitoring at predicting the condition of the infant at birth. While over 200 fetuses were studied during labor, only four were born with severe asphyxia, thus indicating a need for studying a greater number of these cases. They recommended that fetal heart rate


263

monitoring should be the usual protocol, with fetal blood lactate or pH reserved for patients with ominous heart rate patterns.


V. METHODOLOGY Methods for measuring lactate in blood have been developed since the early years of this century. Many of the older methods used either permanganate or manganese dioxide to oxidize lactate to acetaldehyde and either CO, or CO. Acetaldehyde could then be detected by spectrophotometry , by titration, or by gas chromatography. Carbon monoxide could be detected by titration or gasometric analysis, and carbon dioxide could be quantitated ga~ o m e t r i c a l l yWhile . ~ ~ some of these methods could be reasonably accurate, both the labor and time required prevented any practical use in a clinical laboratory. For the past 20 years enzymatic methods have been used in most clinical laboratories. Most enzymatic methods are based on the biochemical reaction of lactate and NAD to form pyruvate and NADH, with the NADH detected either by kinetic or end-point spectrophotometry at 340 nm.40.41The lactate method adapted to the duPont aca4' is in especially wide use in clinical laboratories, according to surveys by the College of American Pathologists.39 Although sold in a prepackaged reagent test pack, by virtue of its accuracy, precision, reliability, and established use, the lactate method on the aca is the method to which others must be compared. Briefly, the aca method uses lactate dehydrogenase to catalyze conversion of lactate to pyruvate with the concomitant conversion of NAD to NADH. To force the reaction to completion, hydrazine is present to trap pyruvate. The absorbance of NADH is determined at 340 and 383 nm. The pH is buffered at 37°C by Tris at pH 8.55. The method requires 40 pl plasma, about 7 min to complete, and is linear up to about 20 m m ~ l / l . ~ ' Recently, a dry multilayered reagent slide methodology for lactate has become available on the Kodak@Ektachem. The reagent slide requires 10 p,l of sample and uses both lactate oxidase (LO) as the converting enzyme, and peroxidase (PO) to develop a colored product measured at 540 nm:42

LO L( )lactate + 0, + pyruvate H,O,

+

2H,O,

+ 4 aminoantipyrine +

+

PO 1,7 dihydroxynaphthalene -+ red dye

The method is reported to be linear to nearly 15 m o Y l and gives results in about 6 min. While both the aca and Ektachem methods are fast, both methods require plasma, which adds about 5 to 10 min to the overall analysis time due to centrifugation. An exciting new development in lactate analysis has come with the development of electrochemical sensors for lactate. They offer the potential to measure lactate in whole blood within 2 to 3 min. Lactate results available this quickly could be most useful in critical care monitoring, perhaps along with blood gas results. Using an improved version of an electrochemical-enzymatic sensor developed earlier by Williams et al. ,43 Racine et a1.& developed an automated lactate analyzer that measures lactate in 50 p1 of diluted blood, serum, or CSF within 3 min. The measurement was based on the oxidation of lactate to pyruvate in the presence of cytochrome b, and hexacyanoferrate (111) ion:

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-

CYt b2

+ 2Fe(CN)c3 Pyruvate + 2 F e ( c N ) ~+~ 2H’ lactate


The hexacyanoferrate (D) ion is then reoxidized: 2Fe(CN)i4 + 2Fe(CN)i3

+ 2e-

The current generated in the last reaction is related to the lactate concentration in the solution. The sensor consists of a platinum anode coated with a thin layer of dissolved enzyme that is covered by a semipermeable membrane. This membrane allows low-molecular-weight substances to pass through, such as lactate and hexacyanoferrate. Geyssant et al.45recently adapted this technology to a commercial analyzer. The analyzer required blood diluted 1:lO (samp1e:buffer) into a buffer provided in special tubes supplied by the manufacturer. The method was linear to 9 mmol/l and compared moderately well to a spectrophotometric enzyme method. A lactate sensor was developed by Clark et al.46that measured lactate directly in whole blood in less than 1 min on 10 p1 of sample. The sensor was a polarographic enzyme electrode that developed a current linearly related to lactate concentration. The sensor was lactate oxidase (LO) bound to a membrane that catalyzed the following reaction:

LO

L-lactate

+ 0, -+

pyruvate

+ H202

The H,O, was then detected by the polarographic electrode. The enzyme is held between a special membrane made of cellulose esters and polycarbonate that immobilize the enzyme next to the platinum reducing surface. This membrane excludes interferents such as catalase, ascorbic acid, and acetaminophen. This electrode was also adapted to a commercially available analyzer that was evaluated by Weil et al.47 This analyzer requires 25 pl of blood, which is diluted by an unspecified amount of buffer within the analyzer. While the analyzer gave precision on the order of 10%and linearity to around 10 mmol/l, a negative bias of about 5% was noted in comparison to a spectrophotometric enzymatic method. This bias was not felt to be clinically significant since the analyzer was reported to be portable and potentially capable of bedside monitoring. Apparently, this same analyzer was evaluated by Wandrup et al.48Both the dilution (1:14 sample: diluent) and the electrode reactions were described more clearly than in the previous rep01-t.~’ Flavine adeninedinucleotide (FAD) was now listed as an ingredient in the reaction catalyzed by lactate oxidase (LO): L-lactate + 0,

FAD +

H,O,

+ pyruvate

LO The hydrogen peroxide is then oxidized at a platinum anode held at to the silver reference electrode: H,O,+

2H+

+ 0.70 v, compared

+ 0, + 2e-

The circuit is completed by a reaction at the silver reference electrode:



2AgCl

+ 2e-

+ 2Ag

265

+ 2C1-

Precision of this analyzer was acceptable, with CVs of less than 2% over most ranges of concentration. Linearity was reported to be up to 15 mmoVl. The principal drawback to the analyzer was that electrodes required replacement about every 4 to 5 d, with each new electrode needing about 1 h to stabilize.* Although the above analyzer represents a useful advance in lactate testing, the analyzer uses diluted whole blood. While the use of diluted whole blood may or may not have problems such as variation due to hematocrit or comparability to plasma results, Mullen et al.49 developed an enzyme electrode that measures lactate in undiluted whole blood. This electrode reportedly overcomes the drawback inherent in lactate oxidase, which, due to a low Michaelis constant, has a restricted range of linearity requiring dilution of lactate concentrations above 1 mmoY1. They developed a special organosilane-treatedmicroporous membrane over the enzyme layer that limits the diffusion of lactate to the sensor electrode. This reportedly extends the linear range to 18 mmolfl for direct measurements in undiluted samples. Both Mascini et aL50 and Weaver et al.51have developed on-line type electrodes capable of measuring lactate in flowing streams. While still considered research tools, these may potentially allow reagentless continuous extracorporeal lactate monitoring.

VI. REFERENCE RANGES AND GUIDE TO INTERPRETATION References ranges reported for blood lactate are approximately 0.3 to 1.5 mmoVl for arterial blood, and somewhat higher, up to about 1.8 mmoVl for venous blood. Weil et al.52 have demonstrated that lactate measurements in venous blood sampled either from a pulmonary artery or from a central venous catheter yield lactate concentrations essentially equivalent to those in arterial blood. Reference ranges for different types of samples, as well as lactate concentrations in healthy people during vigorous exercise, are shown in Table 1. However, reference ranges have only minor use in monitoring patients potentially with circulatory shock. While a single elevated blood lactate result in a patient suspected of having circulatory shock probably indicates a circulatory defect is present, assuming no muscle exertion or nitropmsside toxicity, an isolated lactate result cannot indicate whether the condition is improving or deteriorating. Therefore, following the trend of serial lactate measurements appears to be of most clinical value, with rising or persistently elevated results indicating no improvement, and with decreasing results indicating a favorable prognosis. For general monitoring of patients with circulatory shock, the work of Vincent et al.34 provides the clearest guidelines. For patients who responded favorably to therapy, lactate decreased during the first 2 h of treatment by an average of - 1.3 mmoYl per hour (range -0.8 to -2.7). For the nonresponders, except for one patient with a lactate over 20 mmol/l, the average lactate change was + 0.3 mmoYl per hour (range -0.4 to 1.6). As a monitor of hemodynamic stability during recovery from pediatric surgery, the pediatric cardiac surgeons at our institution obtain a lactate result immediately after surgery, then about every 4 to 8 h during the critical period of recovery. The results are used as guidelines in the following manner: 1. *

An initial lactate result serves as an indicator of the magnitude of the oxygen debt. Note from J.T.:We had very similar experiences with an older YSI lactate analyzer in OUT laboratory. However, we have been using a new completely redesigned YSI model 2300 that has much longer electrode stability.

266


Table 1 Reference Ranges of Various Fluids Reported in the Literature Reference range Critical Reviews in Clinical Laboratory Sciences Downloaded from informahealthcare.com by Freie Universitaet Berlin on 05/02/15 For personal use only.

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