Lactic acidosis full details for md exams 10 marks answer

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Lactic Acidosis - Complete MD Exam Answer (10 Marks)

1. Definition

Lactic acidosis is defined as the accumulation of lactic acid in the blood resulting in metabolic acidosis, characterized by:

Serum lactate > 5 mmol/L (some sources use >4 mmol/L)
Arterial pH < 7.35
Serum HCO3- < 20 mEq/L
Elevated anion gap (AG)

Normal serum lactate: 0.67 to 1.8 mmol/L. Levels of 2-3 mmol/L are termed hyperlactatemia (without frank acidosis).

2. Biochemistry and Pathophysiology

Normal Lactate Metabolism

Glucose undergoes anaerobic glycolysis in the extramitochondrial cytoplasm to yield pyruvate. Pyruvate has two fates:

Aerobic conditions: Pyruvate enters mitochondria → oxidized by pyruvate dehydrogenase (PDH) complex → acetyl-CoA → Krebs/TCA cycle → CO2 + H2O (no H+ produced)
Anaerobic conditions: High NADH/NAD+ ratio drives lactate dehydrogenase (LDH) reaction:

Pyruvate⁻ + NADH + H⁺ ↔ Lactate⁻ + NAD⁺

The normal lactate:pyruvate ratio is approximately 10:1.

Why Does Acidosis Occur?

Lactic acid does NOT itself generate H+ by its formation. The acidosis occurs because:

Under hypoxic conditions, ATP hydrolysis exceeds ATP production: ATP → ADP + H⁺ + Pi
Lactate buildup is therefore a surrogate marker for ATP consumption during hypoxic states

The Cori Cycle

Normal lactate production is ~1300 mmol/day, yet serum lactate stays <1 mmol/L because:

Liver and kidneys convert lactate back to pyruvate → gluconeogenesis (Cori cycle)
Both increased production AND/OR decreased utilization can cause lactic acidosis

Rate-Limiting Factors

Increased production: enhanced phosphofructokinase activity (triggered by ischemia, catecholamines, alkalosis, seizures)
Impaired hepatic clearance: poor liver perfusion, altered redox state, enzyme defects, severe acidosis, alcohol

3. Classification (Cohen & Woods, 1976)

Type A - Tissue Hypoxia/Hypoperfusion (More Common)

Imbalance between O2 supply and O2 demand:

Cause	Mechanism
Shock (septic, cardiogenic, hypovolemic)	Reduced tissue O2 delivery
Cardiac arrest	No perfusion
Severe arterial hypoxemia (PaO2 <30 mmHg)	Reduced O2 availability
Severe anemia (Hb <5 g/dL)	Reduced O2-carrying capacity
Carbon monoxide poisoning	Displaces O2 from Hb
Vigorous exercise, generalized seizures	O2 demand exceeds supply

Type B - No Overt Tissue Hypoxia

O2 delivery is normal but oxidative phosphorylation is impaired:

Type B1 - Associated with diseases:

Liver failure (reduced lactate clearance)
Leukemia/lymphoma, large solid tumors (malignant cells rely on aerobic glycolysis - Warburg effect; can cause hypoglycemia + hypophosphatemia + lactic acidosis)
Uncontrolled diabetes mellitus
Sepsis (also has Type A component)
Malaria
Thiamine (B1) deficiency - cofactor for PDH; without it, pyruvate cannot enter TCA cycle

Type B2 - Drugs and Toxins:

Biguanides (metformin, phenformin - withdrawn 1977): inhibit mitochondrial complex I
NRTIs (zidovudine, stavudine, didanosine, lamivudine): inhibit mitochondrial DNA polymerase-γ → mitochondrial toxicity
Salicylates, cyanide (blocks cytochrome a/a3), propofol (prolonged high-dose)
Linezolid, isoniazid, cocaine
Catecholamines (stimulate glycolysis), sorbitol/fructose, nitroprusside
Ethanol (altered redox state NADH/NAD+)
Beta-2 agonists

Type B3 - Inborn Errors of Metabolism:

Pyruvate dehydrogenase deficiency
Pyruvate carboxylase deficiency
Mitochondrial myopathies
Glucose-6-phosphatase deficiency (von Gierke disease)
Fructose-1,6-bisphosphatase deficiency

D-Lactic Acidosis (Special Type)

Occurs in patients with short bowel syndrome or jejunoileal bypass
Carbohydrates not absorbed in small intestine → colonic bacteria metabolize them → produce D-lactate
D-lactate is NOT metabolized by human LDH
Manifests as: episodes of encephalopathy (confusion, slurred speech, ataxia) after high carbohydrate meals
Routine lactate assay does NOT detect D-lactate (specific assay required)
Treatment: low-carbohydrate diet, antibiotics (neomycin, metronidazole)

4. Clinical Features

Lactic acidosis typically presents with:

Non-specific symptoms: weakness, malaise, nausea, vomiting, abdominal pain
Kussmaul breathing (deep, rapid) - compensatory hyperventilation
Altered mental status, confusion → stupor → coma (with severe acidemia)
Cardiovascular: myocardial depression, vasodilation, hypotension, arrhythmias (at pH <7.1-7.2)
Laboratory findings:
- Serum lactate > 5 mmol/L
- Elevated AG (>12 mEq/L): AG = Na⁺ - (Cl⁻ + HCO3⁻)
- Low bicarbonate (<20 mEq/L)
- Arterial pH <7.35
- Leukocytosis, hyperphosphatemia, hyperuricemia are common
- Hyperkalemia may occur (H⁺/K⁺ exchange)
- Hypoglycemia may occur (especially with malignancy or liver disease)

Important note: There is poor correlation among arterial pH, uncorrected AG, and serum lactate. ~25% of patients with lactate 5-9.9 mmol/L have pH >7.35; up to 50% may have AG <12 mEq/L.

5. Diagnosis

Serum lactate level (arterial preferred; tourniquet-free collection; immediate processing)
- Normal: 0.67-1.8 mmol/L
- Lactic acidosis: >4-5 mmol/L
Arterial blood gas - low pH, low PaCO2 (compensation), low HCO3-
Serum electrolytes - calculate AG
Blood glucose, BUN, creatinine
Liver function tests (type B1)
Toxicology screen if poisoning suspected
Lactate:pyruvate ratio - normally ~10; in hypoxic type A: >25; in enzymatic defects (type B3): >25
D-lactate assay if short bowel syndrome suspected

6. Prognosis

Lactic acidosis is the most serious and most rapidly developing form of metabolic acidosis
Mortality is approximately 50% overall; approaches 80% when lactate exceeds 9-10 mmol/L
As lactate increases above 4 mmol/L, survival probability decreases precipitously
Lactate clearance is prognostically important: patients who reduce lactate by ≥50% within 18 hours of resuscitation have significantly better survival
Inability to clear lactate = surrogate marker for organ dysfunction

7. Treatment

A. Treat the Underlying Cause (Most Important)

Correct tissue hypoperfusion: fluids, vasopressors, inotropes
Goal: central venous O2 saturation ≥70%, MAP 65-70 mmHg, HR <100 bpm, Hb >7 g/dL
Treat infection/sepsis aggressively
Stop offending drugs (metformin, NRTIs, linezolid)

B. Sodium Bicarbonate

Controversial; considered when:

Arterial pH <7.0
Acidemia causing decreased cardiac inotropy or vasodilation/shock
Give as isotonic NaHCO3 in 5% dextrose/water (NOT hypertonic bolus)

Arguments against NaHCO3:

May paradoxically worsen intracellular acidosis (CO2 crosses cell membranes faster than HCO3-)
May cause volume overload, hypernatremia, alkalosis rebound
Does not improve outcome in controlled trials

C. Thiamine

Give empirically when thiamine deficiency is possible (alcohol use, prolonged parenteral nutrition, malnutrition)
Thiamine is required as cofactor for PDH (pyruvate → acetyl-CoA step)

D. Dialysis / Renal Replacement Therapy

Consider if metformin-associated lactic acidosis (MALA) - removes metformin
Useful for volume overload and severe refractory acidosis
Bicarbonate can be provided via dialysate

E. Vasopressors / Mechanical Circulatory Support

For cardiogenic or septic shock with tissue hypoperfusion

F. Experimental Therapies

Dichloroacetate (DCA): activates PDH → enhances pyruvate oxidation → lowers lactate; proven to lower lactate in trials but has NOT shown mortality benefit and causes peripheral neuropathy
Tromethamine (THAM): a non-CO2 generating buffer; useful when CO2 generation from NaHCO3 is undesirable
Carbicarb: equimolar mixture of Na2CO3 + NaHCO3; generates less CO2 than NaHCO3 alone

8. Metformin-Associated Lactic Acidosis (MALA) - High-Yield

Metformin inhibits mitochondrial complex I (NADH-ubiquinone oxidoreductase) → impairs oxidative phosphorylation
Risk factors: CKD (contraindicated when eGFR <30; caution 30-45), hepatic failure, heart failure, dehydration, contrast dye administration
Should be stopped several days before contrast media administration
Treatment: stop metformin, supportive care, hemodialysis (removes metformin)

9. D-Lactic Acidosis vs L-Lactic Acidosis

Feature	L-Lactic Acidosis	D-Lactic Acidosis
Source	Mammalian cell metabolism	Gut bacteria
Setting	Hypoxia, drugs, diseases	Short bowel syndrome, jejunoileal bypass
Neurological sx	Coma, stupor	Encephalopathy, ataxia, confusion
Routine assay	Detected	NOT detected (specific D-lactate assay needed)
Treatment	Treat cause, NaHCO3	Low-carb diet, oral antibiotics

10. Summary Table - Key Points for Exam

Parameter	Value
Definition	Lactate >5 mmol/L + pH <7.35
Normal lactate	0.67-1.8 mmol/L
pKa of lactic acid	3.8
Normal lactate:pyruvate	~10:1
Daily lactate production	~1300 mmol/day
Type A	Tissue hypoxia/hypoperfusion
Type B	No hypoxia; diseases/drugs/enzyme defects
NaHCO3 threshold	pH <7.0
Prognosis indicator	Lactate clearance at 18 hours
DCA mechanism	Activates PDH; lowers lactate but no survival benefit

Sources: Brenner and Rector's The Kidney (9th Ed.) | Goldman-Cecil Medicine | NKF Primer on Kidney Diseases (8th Ed.) | Comprehensive Clinical Nephrology (7th Ed.)

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