Venous Blood Gas

ABCs and Vitals

  • If unstable, call for assistance
  • Treat acid-base disorders by addressing the underlying cause
  • Consider mechanical ventilation for severe respiratory/metabolic acidosis
  • Optimize ventilation settings in intubated patients
  • Correct electrolyte imbalances

STEPWISE APPROACH

Evaluate blood pH
Normal: 7.35–7.45

Evaluate pCO₂
Normal: 33–45 mmHg

Evaluate HCO₃⁻
Normal: 22–28 mEq/L

ACIDOSIS PH < 7.35

Respiratory Acidosis

↓ pH ↑ pCO₂

Acute compensation
HCO₃⁻ ↑ by 1 mEq/L per 10 mmHg ↑ pCO₂ (above 40 mmHg)

Chronic compensation
HCO₃⁻ ↑ by 4–5 mEq/L per 10 mmHg ↑ pCO₂

Causes

  • Acute: Pneumonia, pulmonary edema, asthma/COPD exacerbation, CNS depression (opiates, benzos), head trauma, post-ictal state
  • Chronic: COPD, asthma, neuromuscular weakness, OSA

Signs of CO₂ retention

  • Decreased consciousness
  • Flapping tremor (asterixis)
  • Bounding pulse

Management

  • Call respiratory technician to adjust BiPAP settings
  • Address underlying cause

Metabolic Acidosis

↓ pH ↓ HCO₃⁻

Step 1: Calculate Anion Gap (AG)
AG = Na⁺ – (Cl⁻ + HCO₃⁻)
Normal AG: 6–12 mEq/L

High AG Metabolic Acidosis (MUDPILES)

  • Methanol
  • Uremia
  • DKA
  • Propylene glycol
  • Iron/INH
  • Lactic acidosis
  • Ethanol
  • Salicylates

Normal AG Metabolic Acidosis

  • Diarrhea
  • Renal tubular acidosis
  • Addison’s disease
  • Acetazolamide
  • Saline or spironolactone

    ALKALOSIS pH > 7.45

    Respiratory Alkalosis

    ↑ pH ↓ pCO₂

    Acute compensation
    HCO₃⁻ ↓ by 2 mEq/L per 10 mmHg ↓ pCO₂

    Chronic compensation
    HCO₃⁻ ↓ by 4–5 mEq/L per 10 mmHg ↓ pCO₂

    Causes

    • Pulmonary: Pneumonia, PE, pulmonary edema, aspiration, ILD
    • CNS: Stroke, infections, neurogenic hyperventilation
    • Others: Anxiety, pain, sepsis, anemia, liver failure, hyperthyroidism, salicylates, theophylline, mechanical overventilation

    Metabolic Alkalosis

    ↑ pH ↑ HCO₃⁻

    Assess volume status and vitals

    Causes

    • GI losses: Vomiting, NGT suctioning
    • Renal losses: Diuretics
    • Exogenous alkali: Calcium, laxatives

    ESCALATION

    • If unsure, call for help and plan next steps
    • Always identify why the VBG was ordered
    • Rule out life-threatening conditions
    MOC