Peripheral neuropathy in genetic mitochondrial diseases

Peripheral neuropathy is an underrecognized but common occurrence in genetic mitochondrial disorders. To gain insight into the frequency and clinical presentation of this complication, nerve conduction studies were performed on 43 subjects with congenital lactic acidosis enrolled in a controlled clinical trial of oral dichloroacetate. Median and peroneal motor conduction studies and median and sural sensory conduction studies were performed on each patient. The mean amplitude of the peroneal motor nerve (P < 0.001) and the conduction velocities of the median (P < 0.001) and peroneal (P < 0.001) motor nerves were uniformly lower in our subjects than in healthy literature control subjects. There were no significant differences in sensory nerve conduction studies. A generalized reduction in motor nerve conduction velocity was the dominant electrophysiological abnormality in the patients in this study and was independent of age, sex, or congenital mitochondrial disorder. We postulate that cellular energy failure is the most likely common cause of peripheral neuropathy in patients with genetic mitochondrial diseases, owing to the high demand for adenosine triphosphate via aerobic carbohydrate metabolism by nerve tissue.

Lactic acidosis: from sour milk to septic shock

Lactic acidosis is frequently encountered in the intensive care unit. It occurs when there is an imbalance between production and clearance of lactate. Although lactic acidosis is often associated with a high anion gap and is generally defined as a lactate level >5 mmol/L and a serum pH <7.35, the presence of hypoalbuminemia may mask the anion gap and concomitant alkalosis may raise the pH. The causes of lactic acidosis are traditionally divided into impaired tissue oxygenation (Type A) and disorders in which tissue oxygenation is maintained (Type B). Lactate level is often used as a prognostic indicator and may be predictive of a favorable outcome if it normalizes within 48 hours. The routine measurement of serum lactate, however, should not determine therapeutic interventions. Unfortunately, treatment options remain limited and should be aimed at discontinuation of any offending drugs, treatment of the underlying pathology, and maintenance of organ perfusion. The mainstay of therapy of lactic acidosis remains prevention.

Low-glycemic-index treatment: A liberalized ketogenic diet for treatment of intractable epilepsy

The ketogenic diet is often effective for intractable epilepsy, but many patients have trouble complying with the strict regimen. The authors tested an alternative diet regimen, a low-glycemic-index treatment, with more liberal total carbohydrate intake but restricted to foods that produce relatively little increase in blood glucose (glycemic index < 50). Ten of 20 patients treated with this regimen experienced a greater than 90% reduction in seizure frequency.

Clinical review: reunification of acid-base physiology

Recent advances in acid-base physiology and in the epidemiology of acid-base disorders have refined our understanding of the basic control mechanisms that determine blood pH in health and disease. These refinements have also brought parity between the newer, quantitative and older, descriptive approaches to acid-base physiology. This review explores how the new and older approaches to acid-base physiology can be reconciled and combined to result in a powerful bedside tool. A case based tutorial is also provided.

Propylene Glycol–Induced Lactic Acidosis in a Patient with Normal Renal Function: A Proposed Mechanism and Monitoring Recommendations

OBJECTIVE:

To report a case of severe propylene glycol–induced lactic acidosis not attributable to renal dysfunction that was secondary to administration of high-dose intravenous lorazepam.

CASE SUMMARY:

A 24-year-old female with community-acquired pneumonia presented with severe acute respiratory distress syndrome. To maintain adequate sedation and ventilation and reduce airway pressure, several relaxation strategies were used including high-dose intravenous lorazepam, fentanyl, and cisatracurium. After 18 days of high-dose continuous infusion of lorazepam (maximum dose 50 mg/h), the patient developed severe lactic acidosis secondary to propylene glycol toxicity, the main diluent of lorazepam. The acidosis temporarily resolved with bicarbonate administration and discontinuation of lorazepam. Her renal function remained stable for a time (serum creatinine 0.5 mg/dL, blood urea nitrogen 10 mg/dL, urine output 100–200 mL/h). However, after several more days, the patient's condition deteriorated, and she ultimately died.

DISCUSSION:

Previous cases of propylene glycol toxicity secondary to high-dose lorazepam infusion have occurred in patients with compromised renal function. Our patient's renal function remained stable throughout the hospital course, which caused us to look further for an explanation for the propylene glycol–induced lactic acidosis. Based on the Naranjo probability scale, propylene glycol was determined to be the probable cause of lactic acidosis. Since this case occurred, our intensive care unit has instituted recommendations for the prevention of lorazepam-associated propylene glycol toxicity.

CONCLUSIONS:

Our case highlights the development of propylene glycol–induced lactic acidosis secondary to high-dose lorazepam infusion not associated with renal dysfunction.

Reviews: The Current State of Nonuremic Applications for Extracorporeal Blood Purification

Despite the commonly accepted indications for hemodialysis and extracorporeal depuritive techniques, some clinicians have come to rely on blood purification for clinical states where the targeted substance for removal differs from uremic waste products. Over the last decade, a number of studies have emerged to help define the application of extracorporeal blood purification (ECBP) to these "nonuremic" indications. This review describes the application of extracorporeal blood purification in clinical states including sepsis, rhabdomyolysis, congestive heart failure, hepatic failure, tumor lysis syndrome, adult respiratory distress syndrome, intravenous contrast exposure, and lactic acidosis. Additional comments are provided to review existing literature on thermoregulation and osmoregulation, including acute brain injury.

Lactic acidosis stimulates ganglioside and ceramide generation without sphingomyelin hydrolysis in rat cortical astrocytes

Acidosis is a ubiquitous feature of cerebral ischemia, and triggers a cascade of biochemical events that results in neuronal injury. The purpose of this study was to evaluate the effects of lactic acidosis on the ganglioside composition, the ceramide and sphingomyelin (SM) levels in rat cortical astrocytes. Primary astrocyte cultures were exposed to lactic acid (pH 5.5) for 2, 5 and 17 h, and cell death was evaluated at each time point. Gangliosides, ceramides and SM were analyzed by high-performance thin layer chromatography. Lactic acidosis caused a progressive increase of both GM3 and GD3 gangliosides up to 5 h of treatment. However, at 17 h of acidosis, GM3 tented to return to the normal level whereas GD3 accumulated. Additionally, ceramides were gradually generated, whereas no significant decrease of SM occured for 17 h of acidosis. These results suggest that ceramides were not produced by the breakdown of SM and may be served as metabolic precursor for the biosynthesis of GM3 and GD3. Since these lipids are important messengers of the adaptative responses to stress, accumulation of sphingolipids triggered by lactic acid exposure of astrocytes might play an important role in determining the outcomes of injurious processes.

NaHCO3-induced alkalosis reduces the phosphocreatine slow component during heavy-intensity forearm exercise

During heavy-intensity exercise, the mechanisms responsible for the continued slow decline in phosphocreatine concentration ([PCr]) (PCr slow component) have not been established. In this study, we tested the hypothesis that a reduced intracellular acidosis would result in a greater oxidative flux and, consequently, a reduced magnitude of the PCr slow component. Subjects (n = 10) performed isotonic wrist flexion in a control trial and in an induced alkalosis (Alk) trial (0.3g/kg oral dose of NaHCO3, 90 min before testing). Wrist flexion, at a contraction rate of 0.5 Hz, was performed for 9 min at moderate- (75% of onset of acidosis; intracellular pH threshold) and heavy-intensity (125% intracellular pH threshold) exercise. 31P-magnetic resonance spectroscopy was used to measure intracellular [H+], [PCr], [Pi], and [ATP]. The initial recovery data were used to estimate the rate of ATP synthesis and oxidative flux at the end of heavy-intensity exercise. In repeated trials, venous blood sampling was used to measure plasma [H+], [HCO3-], and [Lac-]. Throughout rest and exercise, plasma [H+] was lower (P < 0.05) and [HCO3-] was elevated (P < 0.05) in Alk compared with control. During the final 3 min of heavy-intensity exercise, Alk caused a lower (P < 0.05) intracellular [H+] [246 (SD 117) vs. 291 nmol/l (SD 129)], a greater (P < 0.05) [PCr] [12.7 (SD 7.0) vs. 9.9 mmol/l (SD 6.0)], and a reduced accumulation of [ADP] [0.065 (SD 0.031) vs. 0.098 mmol/l (SD 0.059)]. Oxidative flux was similar (P > 0.05) in the conditions at the end of heavy-intensity exercise. In conclusion, our results are consistent with a reduced intracellular acidosis, causing a decrease in the magnitude of the PCr slow component. The decreased PCr slow component in Alk did not appear to be due to an elevated oxidative flux.

[Blood lactate concentration as prognostic marker in critically ill children]

OBJECTIVE

To assess the use of lactate as a marker of tissue hypoperfusion and as a prognostic index in critically ill patients.

METHODS

Prospective, longitudinal, observational study of 75 patients admitted to the pediatric ICU of Hospital de Clínicas of Universidade Federal do Paraná, between November 1998 and May 1999. According to the lactate level on admission, patients were divided into group A (lactate > or = 18 mg/dl) and group B (lactate < 18 mg/dl). In terms of outcome, patients were classified into survivors and nonsurvivors. In group A, the clinical evaluation and the collection of arterial blood samples were performed on admission, at 6, 12, 24, 48 hours, and every 24 hours after that. In group B, they were carried out in the same way, but interrupted 48 hours after admission.

RESULTS

Groups A and B consisted of 50 and 25 patients, respectively. Group A presented more clinical signs of hypoperfusion (24/50). There was a statistically significant difference regarding the mean lactate levels on admission between those patients who died within 24 hours of admission (95 mg/dl) and those who died 24 hours after admission (28 mg/dl). The lactate level at 24 hours of admission revealed better sensitivity (55.6%) and specificity (97.2%) as a predictor of death.

CONCLUSIONS

Most patients with lactate levels > or = 18 mg/dl showed clinical signs of hypoperfusion on admission. The normalization or reduction of lactate levels at and after 24 hours of admission was significantly related with higher chances of survival.

Clinical review: Acid-base abnormalities in the intensive care unit -- part II

Acid-base abnormalities are common in the critically ill. The traditional classification of acid-base abnormalities and a modern physico-chemical method of categorizing them will be explored. Specific disorders relating to mortality prediction in the intensive care unit are examined in detail. Lactic acidosis, base excess, and a strong ion gap are highlighted as markers for increased risk of death.

Investigations of D-lactate metabolism and the clinical signs of D-lactataemia in calves

Five clinically healthy calves received an intravenous injection of 25 g sodium D-lactate (223 mmol) in 100 ml sterile water and five control calves were given the same volume of 0.9 per cent sodium chloride. Two clinical examiners who were blinded to the status (test or control) of the calves observed that between eight and 40 minutes after the injections the calves that had received sodium-D-lactate could be distinguished with certainty from the control calves on the basis of their clinical signs, for example, an impaired palpebral reflex, somnolence and a staggering gait. One-compartment and two-compartment analyses of the changes in the plasma concentration of D-lactate, and its renal clearance, indicated that the calves metabolised considerable amounts of D-lactate.

Is lactic acidosis a cause of exercise induced hyperventilation at the respiratory compensation point?

OBJECTIVES

The respiratory compensation point (RCP) marks the onset of hyperventilation ("respiratory compensation") during incremental exercise. Its physiological meaning has not yet been definitely determined, but the most common explanation is a failure of the body's buffering mechanisms which leads to metabolic (lactic) acidosis. It was intended to test this experimentally.

METHODS

During a first ramp-like exercise test on a cycle ergometer, RCP (range: 2.51-3.73 l x min(-1) oxygen uptake) was determined from gas exchange measurements in five healthy subjects (age 26-42; body mass index (BMI) 20.7-23.9 kg x m(-2); Vo(2peak) 51.3-62.1 ml x min(-1) x kg(-1)). On the basis of simultaneous determinations of blood pH and base excess, the necessary amount of bicarbonate to completely buffer the metabolic acidosis was calculated. This quantity was administered intravenously in small doses during a second, otherwise identical, exercise test.

RESULTS

In each subject sufficient compensation for the acidosis, that is, a pH value constantly above 7.37, was attained during the second test. A delay but no disappearance of the hyperventilation was present in all participants when compared with the first test. RCP occurred on average at a significantly (p = 0.043) higher oxygen uptake (+0.15 l x min(-1)) compared with the first test.

CONCLUSIONS

For the first time it was directly demonstrated that exercise induced lactic acidosis is causally involved in the hyperventilation which starts at RCP. However, it does not represent the only additional stimulus of ventilation during intense exercise. Muscle afferents and other sensory inputs from exercising muscles are alternative triggering mechanisms.

Effect of graded hypoxic and acidotic stress on contractile force of heart muscle from hypoxia-tolerant and hypoxia-intolerant turtles

Previous studies have shown that isometric contractile force of in vitro cardiac muscle from the anoxia-tolerant painted turtle, Chrysemys picta bellii, decreases when anoxic and when acidotic. This study sought to define the thresholds for these responses in the isolated ventricular strips of the painted turtle and in the anoxia-intolerant softshell turtles, Apalone spinifera. The ventricular strips were exposed to HCO3- Ringer's solution equilibrated at P(O2) 156, 74, 37, 19, and 0 mmHg (45 min at each grade), at both pH 7.0 and at pH 7.8. Strips were also exposed to graded lactic acidosis with intervals between pH 6.8 and pH 7.8 at P(O2) 156 mmHg (softshell) or 37 mmHg (painted). In painted turtle strips at pH 7.8, force remained at control levels until it decreased by 30% at P(O2) 19 mmHg. No further significant decrease occurred at P(O2) 0. In contrast, softshell turtle muscle force did not fall significantly until P(O2) reached 0. When graded hypoxia was imposed at pH 7.0, strips from both species were more sensitive to hypoxia, but the softshell force decreased at a higher P(O2) than the painted turtle (P(O2) 156 mmHg vs. 37 mmHg), its force fell to a lower level at P(O2) 0 (22 % of control vs. 40 % of control), and unlike painted turtle heart muscle, softshell muscle did not recover fully. In summary, these data indicate that ventricular strips of the painted turtle are no more tolerant of hypoxia alone than strips from the softshell turtle, but that when hypoxia is combined with acidosis, the painted turtle heart muscle functions significantly better during the exposure and recovers more fully after exposure.

Surviving extreme lactic acidosis: the role of calcium lactate formation in the anoxic turtle

During prolonged anoxia at low temperature, freshwater turtles develop high plasma concentrations of both lactate and calcium. At these concentrations the formation of the complex, calcium lactate, normally of little biological significance because of the low association constant for the reaction, significantly reduces the free concentrations of both lactate and calcium. In addition, lactate is taken up by the shell and skeleton to an extent that strongly indicates that calcium lactate formation participates in these structures as well. The binding of calcium to lactate thus contributes to the efflux of lactic acid from the anoxic cells and to the exploitation of the powerful buffering capacity of the shell and skeleton.

Lactic acidosis in HIV-I infected patients receiving antiretroviral therapy

Highly active antiretroviral therapy (HAART) has resulted in dramatic declines in morbidity and mortality in HIV-I infected patients in the developed world. However, with the availability of generic antiretroviral treatments (ART) in India, a large number of patients now receive ART. Increase in experience with ART has led to the detection of drug-related toxicities. We report herein potentially fatal side effects associated with the use of nucleoside analogues in HIV treatment--hyperlactatemia and lactic acidosis/hepatic steatosis.

Gas Exchange Responses to Constant Work-Rate Exercise in Patients with Glycogenosis Type V and VII

During constant work-rate exercise above the lactic acidosis threshold, oxygen consumption fails to plateau by 3 minutes, but continues to rise slowly. This slow component correlates closely with the rise in lactate in normal subjects. We investigated if oxygen consumption during constant work-rate exercise could rise after 3 minutes in the absence of a rise in lactate. We studied five patients with McArdle's disease, one patient with phosphofructokinase deficiency and six normal subjects. Subjects performed two 6-minute duration constant work-rate exercise tests at 40 and 70% of peak oxygen consumption. During low-intensity exercise, oxygen consumption reached steady state by 3 minutes in both groups. Lactate rose slightly in control subjects but not in patients. During high-intensity exercise, oxygen consumption rose from the third to the sixth minute by 144 (21-607) ml/minute (median and range) in control subjects and by 142 (73-306) ml/minute in patients (p = not significant, Mann-Whitney U test). Over the same period, lactate (geometric mean and range) rose from 2.68 (1.10-5.00) to 5.39 (2.70-10.00) mmol/L in control subjects, but did not rise in patients (1.20 [0.64-1.60] to 0.70 [0.57-1.20] mmol/L). We conclude that the slow component of oxygen consumption during heavy exercise is not dependent on lactic acidosis.

Asymptomatic hyperlactataemia: predictive value, natural history and correlates

BACKGROUND

The significance of asymptomatic hyperlactataemia is unknown.

METHODS

We measured serial lactate levels in a group of HIV-infected subjects. Objectives were to identify covariates associated with hyperlactataemia, and to explore the natural history of hyperlactataemia.

RESULTS

Overall, 1487 lactate measurements were performed on 396 subjects; 299 subjects had serial lactate testing, with a median of four performed per patient (range: 2-16). At study entry, lactate was >2.4 mmol/l in 6% of subjects and >3.6 mmol/l in 1%. A multiple logistic regression analysis revealed that cholesterol and current stavudine were the only predictors of hyperlactataemia. Lactate levels were highly reproducible both for short-term (within 3 months) and long-term (>1 year) follow-up. During the study period, 16 subjects were identified with sustained hyperlactataemia; 12/16 remained asymptomatic after a median of 210 days (30-585), and four developed symptoms suggestive of lactic acidosis syndrome. In three of the four, asymptomatic hyperlactataemia had preceded the onset of symptoms by 18-122 days. A longitudinal model showed a rapid rise of lactate levels from 0 to 12 months on nucleoside reverse transcriptase inhibitor (NRTI) and then stabilization. Lactate levels did not correlate with baseline or on-study development of lipoatrophy.

CONCLUSIONS

Even when optimal methods of collection are used, asymptomatic hyperlactataemia does exist and could be sustained in a subset of NRTI-treated subjects for as long as 585 days. A subset of these subjects became symptomatic, after as long as 122 days of asymptomatic hyperlactataemia. Asymptomatic hyperlactataemia is not predictive of lipoatrophy.

Edema and brain trauma

Brain edema leading to an expansion of brain volume has a crucial impact on morbidity and mortality following traumatic brain injury (TBI) as it increases intracranial pressure, impairs cerebral perfusion and oxygenation, and contributes to additional ischemic injuries. Classically, two major types of traumatic brain edema exist: "vasogenic" due to blood-brain barrier (BBB) disruption resulting in extracellular water accumulation and "cytotoxic/cellular" due to sustained intracellular water collection. A third type, "osmotic" brain edema is caused by osmotic imbalances between blood and tissue. Rarely after TBI do we encounter a "hydrocephalic edema/interstitial" brain edema related to an obstruction of cerebrospinal fluid outflow. Following TBI, various mediators are released which enhance vasogenic and/or cytotoxic brain edema. These include glutamate, lactate, H(+), K(+), Ca(2+), nitric oxide, arachidonic acid and its metabolites, free oxygen radicals, histamine, and kinins. Thus, avoiding cerebral anaerobic metabolism and acidosis is beneficial to control lactate and H(+), but no compound inhibiting mediators/mediator channels showed beneficial results in conducted clinical trials, despite successful experimental studies. Hence, anti-edematous therapy in TBI patients is still symptomatic and rather non-specific (e.g. mannitol infusion, controlled hyperventilation). For many years, vasogenic brain edema was accepted as the prevalent edema type following TBI. The development of mechanical TBI models ("weight drop," "fluid percussion injury," and "controlled cortical impact injury") and the use of magnetic resonance imaging, however, revealed that "cytotoxic" edema is of decisive pathophysiological importance following TBI as it develops early and persists while BBB integrity is gradually restored. These findings suggest that cytotoxic and vasogenic brain edema are two entities which can be targeted simultaneously or according to their temporal prevalence.

Unexpected survival from severe metformin-associated lactic acidosis

Lactic acidosis is a recognised complication of the antihyperglycaemic biguanide agent metformin, especially in patients with renal failure. We report a case of severe lactic acidosis and hypothermia due to metformin treatment and renal impairment. The favourable outcome despite extremely unfavourable clinical signs and symptoms for survival after admission and initial treatment was unexpected. Specific aspects of the clinical course are addressed.

Lactate sequestration by osteoderms of the broad-nose caiman, Caiman latirostris, following capture and forced submergence

Lactate accumulation in osteoderms of the broad-nose caiman, Caiman latirostris, was determined following capture and surgery and after a period of forced submergence and related to concurrent values in blood. Control samples of bone and blood were taken after recovery from surgery and before submergence. In addition, samples of osteoderm were incubated in a lactate solution to determine equilibrium concentration, and additional samples were analyzed for elemental and CO(2) concentrations. The composition of the osteoderms closely resembles that of typical vertebrate bone, with a high concentration of calcium and phosphate. Plasma and osteoderm lactate concentrations were both elevated following surgery and decreased significantly after 1 day of recovery. Submergence produced a typical lactate pattern in the plasma, with only a modest increase during the dive and then a sharp increase during recovery to a peak of 31.2+/-1.9 micromol ml(-1) after 1 h. When caimans were anesthetized 2 h after submergence, osteoderm lactate in the same animals was significantly increased to 14.8 micromol g(-1) wet mass. The ratio of the osteoderm:plasma lactate concentration after submergence was similar to the ratio observed in the incubated samples, suggesting that osteoderm lactate concentrations in vivo were equilibrated with circulating plasma levels. We conclude that caiman osteoderms sequester lactate during lactic acidosis and that the time course is fast enough to have benefit to these animals following normal anaerobic burst activity.

Lactic acidosis related to nucleoside therapy in HIV-infected patients

The incidence of nucleoside analogue-related lactic acidosis has been estimated in 0.57 - 8.5 cases/1000 person years of antiretroviral therapy. The onset of lactic acidosis is usually insidious and patients complain of nausea, vomiting, abdominal pain, fatigue and weight loss. When lactate levels are high enough, a catastropic clinical situation develops, resulting in multiorgan failure. The mortality rate of patients with lactic acidosis related to nucleoside therapy is very high, 33 - 60%. Although all nucleoside analogues have been implicated in lactic acidosis, most cases are associated with stavudine. At present, there are no controlled trials to evaluate the treatment of nucleoside-induced lactic acidosis. Therapy is based on supportive measures and discontinuation of all antiretroviral drugs. Administration of essential vitamin coenzymes, electron acceptors and L-carnitine may be useful in some cases.

Fatal lactic acidosis and mimicking Guillain-Barré syndrome in an adolescent with human immunodeficiency virus infection

We report a case of antiretroviral therapy-related fatal lactic acidosis occurring in a vertically infected HIV-positive 17-year-old patient. While receiving antiretroviral therapy with stavudine, didanosine, tenofovir and amprenavir, the patient developed severe acidosis and rapid neuromuscular and respiratory failure mimicking Guillain-Barré syndrome.

EMG signs of neuromuscular fatigue related to the ventilatory threshold during cycling exercise

We questioned whether electromyographic (EMG) signs of neuromuscular fatigue accompany the changes in respiratory variables measured at the ventilatory threshold (VTh) during exercise on a cycloergometer. This was based on the assumption that the activation of muscle afferents sensitive to accumulation of lactate and potassium is suspected to elicit both the EMG signs of fatigue and hyperventilation. In 39 subjects performing an incremental cycling, the EMG estimates of neuromuscular fatigue in vastus lateralis were a non-linear increase in root mean square (RMS), a decrease in median frequency (MF), a non-linear increase in low-frequency EMG energies (EL), and/or a decrease in high-frequency energies (EH). VTh was determined from a non-linear increase in VCO2 [VTh(VCO2 slope)] and an increased value of the respiratory equivalent for oxygen [VTh(VE/VO2)]. We measured a significant increase in venous blood concentration of lactate and potassium, and a significant pHv fall at VTh. One EMG estimate of fatigue was detected in 33/39 individuals and two EMG estimates in 17 subjects. Highly significant positive correlations were found between the oxygen uptakes corresponding to each EMG estimate and to each detection criterion of VTh. These observations suggest that the activation of muscle sensory pathways contribute to the mechanism of VTh.

Myocardial flow regulation in people with mitochondrial myopathy, encephalopathy, lactic acidosis, stroke-like episodes/myoclonic epilepsy and ragged red fibers and other mitochondrial syndromes

OBJECTIVE

This study tests the hypothesis that elevated levels of rest myocardial blood flow (MBF), indicative of inefficient aerobic metabolism, will be present in some patients with mitochondrial disorders but structurally normal hearts.

BACKGROUND

Regulation of MBF is a complex process closely linked to myocardial energy production. Aerobic metabolism in turn depends on normal mitochondrial function and so investigation of patients with mitochondrial disorders may provide important information regarding heritable mechanisms involved in regulation of myocardial flow.

METHODS

Rest and adenosine-stimulated MBF was measured by the positron emission tomography (PET) 13NH(3) technique in nine patients with mitochondrial disorders and compared with 15 age-matched control participants.

RESULTS

Basal heart rate (beats/min) and rate pressure product (mm Hg/min) were elevated in patients (76+/-13 and 9302+/-1910, mean+/-SD, respectively) compared with control participants (63+/-9 and 7411+/-1531, P<0.01 and P<0.05, respectively). However, rest and adenosine-stimulated MBF (ml/min per g) did not differ significantly between groups (patients, 1.13+/-0.52 and 4.17+/-0.84, respectively; control participants, 0.85+/-0.30 and 3.56+/-0.63, respectively). Normalization of rest MBF to rate pressure product, however, demonstrated three patients whose values exceeded that of all control participants (chi2=5.71, P<0.05, Fisher's exact test).

CONCLUSIONS

Elevated basal MBF, in some patients with mitochondrial disorders but structurally normal hearts, suggests the level of basal flow is responsive to efficiency of aerobic metabolism, which closely reflects mitochondrial function. Mitochondrial heteroplasmy with relative sparing of myocardial mitochondria may account for normal basal flow in others with these disorders.

[Metformin-associated lactic acidosis remains a serious complication of metformin therapy]

We report 4 cases of lactic acidosis in diabetic patients usually treated with metformin. For the first 3 patients, the clinical history was similar because lactic acidosis was precipitated by gastro-intestinal disorders whereas all of them were simultaneously treated with several nephrotoxic drugs. These 3 patients presented with acute renal failure on arrival at hospital. Their issue was fatal whereas any obvious cause of overproduction of lactate was found. The fourth case, which was due to a voluntary intoxication, was the only one presenting with a favourable evolution. The metformin plasma and red blood cell levels were performed for 2 of 4 patients and confirmed the overdose. These observations remind that metformin-associated lactic acidosis remains a serious complication, and that medical doctors must respect strictly contra-indications and guidelines for withdrawing metformin.

Metabolic abnormalities in patients with HIV infection

I would like to focus on metabolic alterations, and the problems associated with them such as dyslipidemia, insulin resistance, fat loss or lipoatrophy, and fat accumulation. There are also conditions such as bone disease, hyperlactatemia, and other complications. It is not clear whether these problems are just toxicities of the drugs, how they are related to HIV independent of treatments, and whether they belong in discussions of metabolic complications. For the purposes of this discussion, I would like to consider each problem individually while also recognizing overlapping elements in an effort to understand complex etiologic interrelationships.

Effects of epinephrine and norepinephrine on hemodynamics, oxidative metabolism, and organ energetics in endotoxemic rats

OBJECTIVE

To determine whether epinephrine increases lactate concentration in sepsis through hypoxia or through a particular thermogenic or metabolic pathway.

DESIGN

Prospective, controlled experimental study in rats.

SETTING

Experimental laboratory in a university teaching hospital.

INTERVENTIONS

Three groups of anesthetized, mechanically ventilated male Wistar rats received an intravenous infusion of 15 mg/kg Escherichia coli O127:B8 endotoxin. Rats were treated after 90 min by epinephrine ( n=14), norepinephrine ( n=14), or hydroxyethyl starch ( n=14). Three groups of six rats served as time-matched control groups and received saline, epinephrine, or norepinephrine from 90 to 180 degrees min. Mean arterial pressure, aortic, renal, mesenteric and femoral blood flow, arterial blood gases, lactate, pyruvate, and nitrate were measured at baseline and 90 and 180 min after endotoxin challenge. At the end of experiments biopsy samples were taken from the liver, heart, muscle, kidney, and small intestine for tissue adenine nucleotide and lactate/pyruvate measurements.

MEASUREMENTS AND RESULTS

Endotoxin induced a decrease in mean arterial pressure and in aortic, mesenteric, and renal blood flow. Plasmatic and tissue lactate increased with a high lactate/pyruvate (L/P) ratio. ATP decreased in liver, kidney, and heart. The ATP/ADP ratio did not change, and phosphocreatinine decreased in all organs. Epinephrine and norepinephrine increased mean arterial pressure to baseline values. Epinephrine increased aortic blood flow while renal blood low decreased with both drugs. Plasmatic lactate increased with a stable L/P ratio with epinephrine and did not change with norepinephrine compared to endotoxin values. Nevertheless epinephrine and norepinephrine when compared to endotoxin values did not change tissue L/P ratios or ATP concentration in muscle, heart, gut, or liver. In kidney both drugs decreased ATP concentration.

CONCLUSIONS

Our data demonstrate in a rat model of endotoxemia that epinephrine-induced hyperlactatemia is not related to cellular hypoxia.

Effects of Prior Exercise on Metabolic and Gas Exchange Responses to Exercise

'Warm-up' activity is almost universally performed by athletes prior to their participation in training or competition. However, relatively little is known about the optimal intensity and duration for such exercise, or about the potential mechanisms primed by warm-up that might enhance performance. Recent studies demonstrate that vigorous warm-up exercise that normally results in an elevated blood and presumably muscle lactate concentration has the potential to increase the aerobic energy turnover in subsequent high-intensity exercise. The reduced oxygen deficit is associated with a reduction in both the depletion of the intramuscular phosphocreatine stores and the rate at which lactic acid is produced. Furthermore, the oxygen uptake 'slow component' that develops during high-intensity, ostensibly submaximal, exercise is attenuated. These factors would be hypothesised to predispose to increased exercise tolerance. Interestingly, the elevation of muscle temperature by prior exercise does not appear to be implicated in the altered metabolic and gas exchange responses observed during subsequent exercise. The physiological mechanism(s) that limit the rate and the extent to which muscle oxygen uptake increases following the onset of exercise, and which are apparently altered by the performance of prior heavy exercise, are debated. However, these mechanisms could include oxygen availability, enzyme activity and/or availability of metabolic substrate, and motor unit recruitment patterns. Irrespective of the nature of the control mechanisms that are influenced, 'priming' exercise has the potential to significantly enhance exercise tolerance and athletic performance. The optimal combination of the intensity, duration and mode of 'warm-up' exercise, and the recovery period allowed before the criterion exercise challenge, remain to be determined.

Suppression of sodium pump activity and an increase in the intracellular Ca2+ concentration by dexamethasone in acidotic mouse brain

The effects of dexamethasone on adenosine 5'-triphosphatase (ATPase) activity and the intracellular Ca(2+) concentration ([Ca(2+)](i)) were investigated in acidotic mouse brain. Dexamethasone (3 mg/kg, i.p.) or vehicle was administered 3 h before decapitation ischemia, and the brain concentration of adenosine 5'-triphosphate (ATP) was determined 0.5-2 min after ischemia. The effects of dexamethasone (0.3-3 mg/kg, i.p.) on Na(+),K(+)-activated ATPase (Na(+),K(+)-ATPase) and Ca(2+)-ATPase activities were evaluated at pH 7.4 and 6.8. Changes in [Ca(2+)](i) in an acidic medium were determined in hippocampal slices by microfluorometry using rhod-2 acetoxymethyl ester as a Ca(2+) marker, and the effects of dexamethasone (240 microg/l) was evaluated. Decapitation ischemia for 0.5 and 1 min reduced the brain ATP contents to 32% and 16% of the basal level, respectively. Dexamethasone slightly suppressed the extent of the decrease in the ATP level. Although dexamethasone did not affect Na(+),K(+)-ATPase activity at pH 7.4, the activity was suppressed by dexamethasone (3 mg/kg) to 68% at pH 6.8. The activity of Ca(2+)-ATPase was not affected by dexamethasone at either pH 7.4 or pH 6.8. When the pH of the medium of the brain slices was changed from 7.4 to 6.8, almost no increase in [Ca(2+)](i) was observed in the control group. The dexamethasone treatment increased [Ca(2+)](i) in the CA1 field and dentate gyrus immediately after induction of the acidic medium, the effect being significant after 150 s. Because anaerobic glucose metabolism in the early stage of ischemia enhances intracellular lactic acidosis, the findings may suggest a mechanism for the aggravation of ischemic neuronal damage by glucocorticoids.

Endotoxin-induced hyperlactatemia results from decreased lactate clearance in hemodynamically stable rats

OBJECTIVE

To determine whether endotoxin-induced hyperlactatemia in hemodynamically stable animals is due to increased lactate production or decreased lactate clearance by measuring lactate turnover rate in the vascular compartment (LTRvc).

DESIGN

Prospective, controlled trial.

SETTING

Research laboratory in a university hospital.

SUBJECTS

Male Sprague-Dawley rats weighing 275-425 g with chronic vascular catheters.

INTERVENTIONS

Chronically catheterized rats were treated with 6 microg/kg endotoxin or saline. LTRvc was determined from the specific activity of carbon-14 [14C]lactate in aortic blood during a constant infusion of [14C]lactate into the inferior vena cava. The role of the splanchnic organs in lipopolysaccharide-induced alterations in LTRvc was determined from the splanchnic first-pass clearance of [14C]lactate infused into the superior mesenteric artery and direct measurements of blood lactate concentration gradients across the splanchnic organs.

MEASUREMENTS AND MAIN RESULTS

Despite a 260% increase in lactate concentrations after lipopolysaccharide treatment, the specific activity of [14C]lactate and the LTRvc did not change, indicating that lipopolysaccharide-induced hyperlactatemia is caused by decreased lactate clearance from the vascular compartment rather than increased lactate flux into the vascular compartment. In contrast, lactate clearance by the splanchnic system was increased. The specific activity of [14C]lactate in aortic blood decreased 33% after lipopolysaccharide treatment when the [14C]lactate was infused into the superior mesenteric artery, indicating increased first-pass clearance of [14C]lactate by the splanchnic organs. Furthermore, the hepatic venous-aortic concentration gradient of lactate became increasingly negative after lipopolysaccharide treatment, indicating increased vascular extraction of lactate by the splanchnic system (0.07 +/- 0.07 micromol/mL vs. -0.34 +/- 0.14 micromol/mL).

CONCLUSIONS

Lipopolysaccharide-induced hyperlactatemia in hemodynamically stable rats is caused by a net decrease in lactate clearance from the vascular compartment despite the fact that the clearance of lactate by the splanchnic system remains intact.

Muscle deoxygenation as related to work rate

PURPOSE

The kinetics of the decrease in venous O(2) content in response to constant work rate exercise below the lactic acidosis threshold (LAT) is very rapid, reaching a constant value by approximately 1 min. However, for work rates above the LAT, a slow further decrease in venous O(2) content takes place that is attributable to the Bohr effect rather than further decrease in end capillary PO. We hypothesized that similar differences, with respect to the LAT, will be observed in muscle deoxygenation kinetics when studied with near-infrared spectroscopy (NIRS).

METHODS

Twelve normal subjects performed three constant work rate tests from unloaded cycling at 60% of LAT, 80% LAT, each with four repetitions, and above LAT (LAT + 35% between LAT and VO(2max) three times, on a cycle ergometer for 6 min. We measured tissue deoxygenation with NIRS, with the probe over the vastus lateralis muscle, time-averaging the repetitions. Gas exchange and heart rate (HR) were measured breath-by-breath and beat-by-beat.

RESULTS

Tissue deoxygenation kinetics were significantly faster than VO(2) and HR at 60%- and 80%-LAT work rates. By 1 min of exercise, deoxygenation was constant for the work rate below the LAT. At 30 s, tissue deoxygenation was 70-95% complete, whereas VO(2) and HR were only 30-60% complete. For the work rate above the LAT, a steady state for muscle deoxygenation was not reached during the 6 min of exercise. After 1 min of above-LAT exercise, either one of two patterns of slow change in tissue oxygenation developed, deoxygenation or reoxygenation. It is postulated that these different responses might be due to effects of the exercise lactic acidosis. H accompanying lactate increase might cause further deoxygenation due to the Bohr effect, and acidosis-induced vasodilatation might cause reoxygenation after the initial deoxygenation.

CONCLUSION

1) The kinetics of tissue deoxygenation are significantly more rapid than VO(2) and HR kinetics at all work rates studied, and 2) steady-state in tissue deoxygenation is seen by 1 min of constant work rate exercise below the LAT, but this is much delayed for work rates above the LAT.

Metformin-associated lactic acidosis: case reports and literature review

BACKGROUND

Lactic acidosis is a widely recognized, though rare, side effect of metformin. This paper describes five patients admitted to Chang Gung Memorial Hospital from 1 September 1998 to 31 May 2001 suffering severe lactic acidosis caused by metformin, and reviews the literature.

PATIENTS

Five cases diagnosed as having meftormin-associated lactic acidosis (MALA) were discovered during the study period. Three had normal renal function before the onset of MALA and two had attempted suicide bytaking large amounts of metformin. One patient with end-stage renal disease developed MALA despite regularhemodialysis three times a week. One of the patients who had taken metformin to attempt suicide was not diabetic.

RESULTS

All patients suffered severe metabolic acidosis with a high anion gap and blood lactate level. Four developed profound hypotension, and three of these also suffered acute respiratory failure. Three patients received conventional hemodialysis and two continuous renal replacement therapy. A young non-diabetic female who had taken a large dose of metformin to commit suicide died from multiple organ failure despite aggressive treatment.

CONCLUSIONS

Lactic acidosis is a serious reaction to metformin, and hemodialysis (the treatment of choice) should be done urgently to prevent serious complications. MALA should be suspected in patients presenting with wide anion gap metabolic acidosis and high blood lactate, even when they are non-diabetic.

Continuous pH monitoring using the Paratrend 7 inserted into a peripheral vein in a patient with shock and congenital lactic acidosis

The authors present a 25-year-old woman who was admitted to the ICU for treatment of shock, respiratory failure, and acidosis related to congenital lactic acidosis from pyruvate dehydrogenase deficiency. To aid in ongoing management of the metabolic acidosis, the Paratrend blood gas monitoring sensor was inserted through a peripheral venous site to provide a continuous measurement of pH and partial pressure of carbon dioxide (Pco2). With the venous insertion of the Paratrend, a clinically useful correlation with arterial blood gas values was noted. Linear regression analysis of the pH values from the venous blood gas analyses and the Paratrend monitor revealed r2 = 0.71 with p = 0.001 and r2 = 0.78 with a p = 0.0003 for the Pco2 values. Our preliminary experience suggests that venous placement of the Paratrend monitor can be used to provide clinically useful, continuous measurement of pH and Pco2.

Metabolic acidosis in the intensive care unit

Metabolic acidosis is a common occurrence in critically ill patients. Understanding the pathological mechanisms underlying the generation of protons will enable the clinician to quickly recognize these disorders and establish an acceptable treatment strategy. This article presents a logical approach to metabolic acidosis.

Skeletal muscle acidosis correlates with the severity of blood volume loss during shock and resuscitation

BACKGROUND

Continuous assessment of tissue perfusion and oxygen utilization may allow for early recognition and correction of hemorrhagic shock. We hypothesized that continuously monitoring skeletal muscle (SM) PO2, PCO2, and pH during shock would provide an easily accessible method for assessing the severity of blood loss and the efficacy of resuscitation.

METHODS

Thirteen anesthetized pigs (25-35 kg) underwent laparotomy and femoral vessel cannulation. Multiparameter fiberoptic sensors were placed in the deltoid (SM) and femoral artery. Ventilation was maintained at a PaCO2 of 40-45 mm Hg. Total blood volume (TBV) was measured using an Evans blue dye technique. Animals were bled for 15 minutes, maintained at a mean arterial pressure (MAP) of 40 mm Hg for 1 hour, resuscitated (shed blood + 2 times shed volume in normal saline) and observed for 1 hour. Four animals served as controls (sham hemorrhage). Blood and tissue samples were taken at each time point.

RESULTS

Blood loss ranged from 28.5-56% of TBV. SM pH and SM PO2 levels fell rapidly with shock. SM PO2 returned to normal with resuscitation; however, SM pH did not return to baseline. SM PCO2 significantly rose with shock, but returned to baseline promptly with resuscitation. There was a significant correlation between SM pH and blood volume loss at end shock (r2 = 0.73, p < 0.001) and recovery (r2 = 0.84, p < 0.001). Animals (n = 2) whose SM pH did not recover to 7.2 were found to have ongoing blood loss from biopsy sites and persistent tissue hypercarbia despite normal MAP.

CONCLUSION

Continuous multiparameter monitoring of SM provides a minimally invasive method for assessing severity of shock and efficacy of resuscitation. Both PCO2 and PO2 levels change rapidly with shock and resuscitation. SM pH is directly proportional to lost blood volume. Persistent SM acidosis (pH < 7.2) and elevated PCO2 levels suggest incomplete resuscitation despite normalized hemodynamics.

Effects of dobutamine on critical capillary PO(2) and lactic acidosis threshold in patients with cardiovascular disease

BACKGROUND

Muscle capillary PO(2) has been found to reach a minimal value, ie, a critical capillary PO(2), in the midrange of work capacity in patients with cardiovascular disease. However, it is not known if the critical capillary PO(2) can be influenced by a change in blood flow response to exercise. This study was carried out to determine the effect of changing the blood flow response to exercise, using low-dose infusion of dobutamine, on muscle end-capillary PO(2) (as approximated by femoral vein PO(2)), lactate concentration, oxygen uptake (O(2)), and the relation among these variables.

METHODS

Eleven male patients with coronary artery disease performed an incremental exercise test on a cycle ergometer with and without continuous infusion of dobutamine, 6 microg/kg/min. Respiratory gas analysis was performed on a breath-by-breath basis; femoral vein blood was sampled every minute through a percutaneous catheter.

RESULTS

Dobutamine increased resting O(2) and O(2) at the lactic acidosis threshold (LAT) but not peak O(2). The femoral vein PO(2) rapidly decreased toward a minimal value with increasing work rate (O(2)) irrespective of the infusion of dobutamine. After reaching its nadir (critical PO(2)), femoral vein lactate began to increase without further decrease in PO(2). Infusion of dobutamine significantly increased femoral vein resting PO(2) (27.4 +/- 4.9 mm Hg vs 32.5 +/- 3.8 mm Hg) and critical PO(2) (20.5 +/- 1.5 mm Hg vs 21.9 +/- 1.7 mm Hg), but not the PO(2) at peak O(2) (22.1 +/- 3.3 mm Hg vs 22.0 +/- 2.9 mm Hg).

CONCLUSIONS

Infusion of dobutamine was found to raise the critical PO(2) and LAT but not peak O(2). These findings suggest that some of the acute increase in blood flow induced by dobutamine infusion benefits exercising muscle by increasing capillary PO(2), thereby delaying the onset of lactic acidosis.

Metformin-associated lactic acidosis in a low risk patient

Metformin is an oral hypoglycemic agent belonging to the class of biguanides that are commonly used in the treatment of type II diabetes mellitus. Lactic acidosis is a rare but severe adverse reaction that occurs primarily in patients with contraindications such as renal failure. The case of a 71-year-old woman with type II diabetes, in whom severe metformin-associated lactic acidosis was precipitated by acute renal failure in the absence of pre-existing chronic renal failure or other absolute contraindications to biguanide use, is presented. Aggressive correction of the acidosis and prolonged dialysis resulted in a favourable outcome despite severe acidosis. The present case report shows that metformin-associated lactic acidosis can occur in patients without pre-existing renal insufficiency. Metformin should be temporarily stopped when acute renal failure occurs or is anticipated.

Role of pH on the calcium ion dependence of the nitric oxide synthase in the carp brain

The role of pH on the calcium dependence of nitric oxide synthase (NOS) of Cyprinus carpio brain was investigated. This fish is known to survive prolonged periods of hypoxia. Under this condition, cerebral blood flow is no longer regulated by nitric oxide (NO). Nitric oxide synthase activity is pH dependent in the range of pH between 7.4 and 6.2 with a decrease when tissue acidifies. At acidic pH, the dependence of the NOS activity on the free Ca(2+) concentrations changes considerably and shows an EC(50) of 0.13 microM at pH 7.1 and of 5.1 microM at pH 6.2 for the soluble enzyme. The variation in the Ca(2+) dependence with acidification is greater for the soluble than for the particulate enzyme. This may be the main factor protecting sudden NO formation mainly during anoxic-normoxic transitions.

A SURF1 gene mutation presenting as isolated leukodystrophy

Mitochondrial respiratory chain defects are increasingly recognized in patients with leukodystrophy. We report the first case of leukodystrophy with systemic cytochrome oxidase deficiency caused by a loss of function mutation in the SURF1 gene in a 2-year-old girl presenting with failure to thrive, global neurodevelopmental regression, and lactic acidosis. Although all previously reported mutations in the SURF1 gene have been found in patients with cytochrome oxidase (COX)-deficient Leigh syndrome, the phenotype associated with SURF1 protein deficiency should be extended to include leukodystrophy.

Exercise intolerance in mitochondrial myopathy is not related to lactic acidosis

In a double-blinded, placebo-controlled, crossover study in seven mitochondrial myopathy patients (MM), we investigated whether lowering of lactate with dichloroacetate (DCA) can improve exercise tolerance and oxidative capacity in MM. DCA lowered plasma lactate at rest and during exercise (from 10.5 +/- 2.0 to 5.0 +/- 1.6 mM; p = 0.005) but did not improve maximal work load or VO2 in cycle exercise or phosphorous magnetic resonance spectroscopy (31P-MRS)-assessed indices of muscle oxidative metabolism. This indicates that lactate acidosis is not the primary cause of exercise intolerance in MM.