• Beta-lactams (penicillins, cephalosporins, carbapenems, monobactams) - inhibit cell wall synthesis
• Aminoglycosides (gentamicin, amikacin) - inhibit protein synthesis (30S)
• Macrolides (azithromycin, erythromycin) - inhibit protein synthesis (50S)
• Fluoroquinolones (ciprofloxacin, levofloxacin) - inhibit DNA gyrase
• Tetracyclines (doxycycline, tigecycline) - inhibit protein synthesis (30S)

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Classification of Antibiotics

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1. INHIBITORS OF CELL WALL SYNTHESIS

A. Beta-Lactams

i. Natural Penicillins

ii. Penicillinase-Resistant Penicillins (Anti-staphylococcal)

iii. Broad-Spectrum (Aminopenicillins)

iv. Extended-Spectrum (Antipseudomonal) Penicillins

v. Beta-Lactam + Beta-Lactamase Inhibitor Combinations

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B. Cephalosporins

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C. Carbapenems

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D. Monobactams

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E. Glycopeptides

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F. Other Cell Wall Inhibitors

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2. INHIBITORS OF PROTEIN SYNTHESIS (30S Ribosome)

A. Aminoglycosides

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B. Tetracyclines

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3. INHIBITORS OF PROTEIN SYNTHESIS (50S Ribosome)

A. Macrolides

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B. Ketolides

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C. Lincosamides

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D. Chloramphenicol

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E. Oxazolidinones

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F. Streptogramins

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G. Pleuromutilins

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4. INHIBITORS OF DNA SYNTHESIS / REPLICATION

A. Fluoroquinolones (Quinolones)

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B. Nitroimidazoles

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C. Nitrofurans

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5. INHIBITORS OF RNA SYNTHESIS

Rifamycins

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6. INHIBITORS OF CELL MEMBRANE FUNCTION

A. Polymyxins

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B. Lipopeptides

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7. ANTIMETABOLITES (Inhibitors of Folate Synthesis)

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8. ANTI-MYCOBACTERIAL AGENTS (Special Group)

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Summary Table by Mechanism

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📋 Venous Blood Gas (VBG) - Full Teaching Analysis

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Step 1: What Is a Venous Blood Gas (VBG)?

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Step 2: Read Every Value - What Is Normal and What Is Not?

BLOOD GAS

pH 6.88 is a life-threatening value. Normal cellular enzyme function requires pH 7.35-7.45. Below 7.0, cardiac arrhythmias, vasodilation, and cellular death occur.

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ELECTROLYTES / METABOLITES

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DERIVED VALUES - Explained One by One

🔴 HCO₃⁻ (Bicarbonate) = 6.0 mmol/L (normal ~22-26)

• Bicarbonate is the body's main blood buffer.
• Normal reaction: H⁺ + HCO₃⁻ → H₂CO₃ → CO₂ + H₂O
• When massive acid accumulates (like lactate here), HCO₃⁻ is consumed to neutralize it → it falls dramatically.
• HCO₃⁻ of 6.0 means the buffer is nearly exhausted - catastrophic.

🔴 Standard Bicarbonate (cHCO₃⁻ P,st) = 7.2 mmol/L

• Standardized to normal pCO2 (40 mmHg) to isolate the metabolic component.
• Confirms this is a primary metabolic acidosis, not just a respiratory issue.

🔴 Base Excess = -27.4 mmol/L (normal: -2 to +2)

• Base excess (BE) tells you how much base (or acid) needs to be added to restore normal pH.
• Negative BE = base deficit = the patient needs base.
• -27.4 means the patient is 27.4 mmol/L short of base - this is incompatible with survival without urgent intervention.
• Normal BE = 0. Mild acidosis = -5. Moderate = -10. Severe = -15. This patient is at -27 - extreme.

🔴 Anion Gap = 27.5 mmol/L (normal <12)

• Formula: AG = Na⁺ - (Cl⁻ + HCO₃⁻)
• Patient: 133 - (99 + 6) = 28 (matches ~27.5 as calculated by machine)
• Normal AG = 6-12 mmol/L. This patient's AG = 27.5 → massively elevated.
• High AG means unknown negative ions (acid anions) are filling the gap - in this case, lactate (14.4 mmol/L) is a major contributor.

🔴 Anion Gap with K⁺ = 33.6 mmol/L

• Same calculation but includes potassium: (Na + K) - (Cl + HCO3)
• = (133 + 6.18) - (99 + 6) = 139.18 - 105 = 34.18 ≈ matches machine value.
• Even higher because of the elevated K⁺.

Ionized Ca²⁺ corrected to pH 7.40 = 0.73 mmol/L

• True calcium at normal pH.
• Very low - suggests significant hypocalcemia independent of pH effects (acidosis artificially raises ionized Ca²⁺, so correcting back to 7.40 reveals the true deficit is even worse: 0.73 vs normal 1.15-1.30).

Total CO₂ (ctCO₂) = 6.4 mmol/L (normal ~23-27)

• Includes dissolved CO₂ + HCO₃⁻ + carbamino compounds.
• Reflects the profoundly low bicarbonate. Critically low.

sO₂e (Estimated O₂ Saturation) = 77%

• For a venous sample, this is actually not unexpected (venous O2 sat is normally 60-80%).
• However, the pO2 of 66 mmHg is high for venous - this may indicate reduced oxygen extraction by tissues (cells cannot use O2 due to mitochondrial failure from severe acidosis/shock), OR the sample is partially arterial.

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Step 3: Putting It Together - Diagnosis

🔴 PRIMARY DISORDER: Severe High-Anion Gap Metabolic Acidosis

• HCO₃⁻ = 6 (very low) → Primary metabolic acidosis
• pCO2 = 32.2 (low) → Respiratory compensation (the lungs are trying to blow off CO2 to reduce acid - this is called Kussmaul breathing)

Expected pCO2 = 1.5 × HCO₃⁻ + 8 ± 2 = 1.5 × 6 + 8 = 17 ± 2 mmHg

• There is a concurrent respiratory acidosis (lungs cannot compensate fully, perhaps due to respiratory muscle fatigue or lung disease), OR
• The patient is tiring and losing their compensatory hyperventilation - a very ominous sign.

Diagnosis: Type A Lactic Acidosis (tissue hypoperfusion/shock state)

According to Tintinalli's Emergency Medicine: "In critical illness, an oxygen debt develops when oxygen delivery is inadequate... This results in global tissue hypoxia, anaerobic metabolism, and lactate production."

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Step 4: The Electrolyte Abnormalities - Explained

Hyperkalemia (K⁺ = 6.18)

• In acidosis, excess H⁺ ions enter cells to be buffered.
• To maintain electrical neutrality, K⁺ exits cells into the blood.
• For every 0.1 unit drop in pH, K⁺ rises approximately 0.5-0.7 mmol/L.
• pH dropped from 7.4 to 6.88 = 0.52 units → expected K⁺ rise of ~2.6-3.6 mmol/L.
• This explains most of the hyperkalemia here - it is redistributional (not total body excess in most cases), but still causes dangerous cardiac arrhythmias.

Hypocalcemia (Ca²⁺ = 0.97)

• May occur in septic shock, rhabdomyolysis, or pancreatitis.
• The corrected Ca²⁺ at pH 7.40 is only 0.73 (very low) - worsens cardiac function.

Hyponatremia (Na⁺ = 133)

• Mild; may reflect fluid shifts, or dilutional in resuscitation.

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Step 5: Summary Table of All Abnormalities

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Step 6: What Is Killing This Patient? - Clinical Correlation

• Profound metabolic acidosis (pH 6.88, HCO3 = 6, BE = -27)
• Massive lactic acidosis (lactate 14.4 - 9x normal)
• Hyperkalemia from acidosis
• Failed respiratory compensation (pCO2 higher than expected for the degree of acidosis)

1. Septic shock (most common cause of lactic acidosis in ICU)
2. Cardiogenic shock (low output → tissue hypoperfusion → anaerobic metabolism)
3. Hypovolemic shock (hemorrhage, severe dehydration)
4. Mesenteric ischemia (gut hypoperfusion gives massive lactate)
5. Toxic/drug-induced (cyanide, metformin toxicity in renal failure, etc.)

"When lactic acidosis accompanies low-flow disease states such as sepsis, mortality increases by almost threefold." - Tintinalli's Emergency Medicine

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Step 7: Memory Aid - How to Read Any VBG/ABG

1. pH < 7.35 = Acidosis  |  pH > 7.45 = Alkalosis
2. Primary cause: HCO3 ↓ or pCO2 ↑ → Acidosis
                  HCO3 ↑ or pCO2 ↓ → Alkalosis
3. Compensation: Metabolic acidosis → lungs blow off CO2 (pCO2 ↓)
                 Respiratory acidosis → kidneys retain HCO3 (HCO3 ↑)
4. Anion Gap = Na - (Cl + HCO3)   Normal < 12
   High AG causes: GOLD MARK
   G = Glycols (ethylene glycol)
   O = Oxoproline
   L = L-lactate (THIS PATIENT)
   D = D-lactate
   M = Methanol
   A = Aspirin (salicylates)
   R = Renal failure (uremia)
   K = Ketoacidosis (DKA)
5. Base Excess < -2 = base deficit = metabolic acidosis
6. Lactate > 2 mmol/L = hyperlactatemia; > 4 = severe lactic acidosis

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⚠️ Clinical urgency: A patient with pH 6.88, lactate 14.4, and base excess -27 requires immediate ICU-level resuscitation - airway management, identification and treatment of the underlying cause (sepsis workup, echo for cardiac function), IV sodium bicarbonate consideration, and urgent correction of hyperkalemia.

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🩸 This Patient's VBG - Plain Language Explanation

The Big Picture in One Sentence

This patient's blood is dangerously acidic because their tissues are not getting enough oxygen, causing massive lactic acid buildup - this is a medical emergency.

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The 5 Most Critical Findings

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🔴 1. pH = 6.88 (Normal: 7.31–7.41)

• pH 7.4 = normal blood
• pH 7.2 = mild-moderate acidosis
• pH 7.0 = severe, life-threatening
• pH 6.88 = extreme acidosis - below the machine's reportable range (the ‡ symbol means "too low to reliably measure")

• Heart muscle stops contracting properly → cardiac arrest risk
• Blood vessels dilate → blood pressure collapses
• Enzymes in every cell stop working
• Brain function deteriorates → coma

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🔴 2. Lactate = 14.4 mmol/L (Normal: 0.5–1.6)

Normal:   Glucose + O₂ → Energy + CO₂ + H₂O  (clean)
In shock: Glucose → Energy + LACTIC ACID      (dirty, acidic)

• Normal lactate < 1.6 mmol/L
• Mild elevation: 2-4 (concerning)
• Severe: > 4 (organ failure territory)
• This patient: 14.4 = nearly 9× the upper limit of normal

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🔴 3. Bicarbonate (HCO₃⁻) = 6.0 mmol/L (Normal: 22–26)

HCO₃⁻ + H⁺  →  H₂CO₃  →  CO₂ + H₂O
(buffer)   (acid)          (breathed out)

• Normal = 22-26 mmol/L (full extinguisher)
• This patient = 6.0 mmol/L (extinguisher nearly empty)

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🔴 4. Base Excess = -27.4 mmol/L (Normal: -2 to +2)

• If base excess = 0 → perfect
• If base excess = -5 → mildly short of base
• If base excess = -10 → moderate deficit
• If base excess = -27.4 → catastrophically short of base

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🔴 5. Anion Gap = 27.5 mmol/L (Normal: < 12)

Anion Gap = Sodium - (Chloride + Bicarbonate)
           = 133 - (99 + 6) = 28 mmol/L

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The Compensation Attempt

• Patient's pCO₂ = 32.2 mmHg (below normal 41-51)
• This confirms the lungs ARE trying to compensate

Expected pCO₂ = (1.5 × HCO₃⁻) + 8 = (1.5 × 6) + 8 = 17 mmHg

• Concurrent lung disease limiting ventilation, OR
• The patient is tiring and losing their ability to hyperventilate - an ominous sign of impending respiratory failure

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The Electrolyte Problems

K⁺ = 6.18 (High - Dangerous)

Ca²⁺ = 0.97 (Low)

Na⁺ = 133 (Mildly Low)

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Overall Diagnosis

pH 6.88 + HCO3 6.0 + Lactate 14.4 + High Anion Gap
         ↓
SEVERE HIGH ANION GAP METABOLIC ACIDOSIS
CAUSED BY MASSIVE LACTIC ACIDOSIS
         ↓
Underlying cause = SHOCK (tissue not getting oxygen)
Most likely: Septic shock / Cardiogenic shock / Severe hemorrhage

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Visual Summary

NORMAL PATIENT:
pH:       |--------7.40--------|  ✅
HCO3:     |------24------------|  ✅
Lactate:  |1.0|                    ✅
BE:       |0|                      ✅

THIS PATIENT:
pH:      6.88|----------------------7.40  🔴 Catastrophically acidic
HCO3:    6|---------------------------24  🔴 Buffer nearly gone
Lactate: |----14.4-----------------1.6|  🔴 9× normal
BE:      -27.4|---------------------0|   🔴 Massive base deficit

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What Needs to Happen Immediately

1. ICU admission - this patient cannot be on a ward
2. Identify the cause of shock - blood cultures (sepsis?), echo (cardiac?), CT (source of bleeding or ischemia?)
3. Restore tissue perfusion - IV fluids, vasopressors if needed
4. Manage airway - if the patient tires from Kussmaul breathing, they need intubation
5. Treat hyperkalemia - calcium gluconate (protects heart), insulin + glucose, Kayexalate
6. Consider sodium bicarbonate - controversial, but when pH < 7.1 and patient is deteriorating, IV sodium bicarbonate may be given as a bridge
7. Serial lactate measurements - lactate clearance (dropping by ≥25% in 6 hours) is a sign of recovery

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Bottom line: This is one of the most critically abnormal blood gases you can see. The pH of 6.88 with lactate of 14.4 and base excess of -27 represents a patient in severe shock with end-organ failure. Without urgent diagnosis and treatment of the underlying cause, survival is very unlikely.