ABG analysis

• Format: pH/PaCO2/PaO2/HCO₃, e.g. 7.38/40/106/25
• Simple SpO2 doesn't tell you anything about pH/CO₂ - need ABG to further evaluate acid-base status
• Hypoxia = Type I respiratory failure = PaO₂ < 55 (normal 55-80)
• PaO₂ of 55 = SaO₂ of 90%
• PaO₂ of 45 = SaO₂ of 75%
• A-a gradient = [FiO₂ * (760-47)] - PaCO₂/0.8 - PaO₂ (ABG)
• Normal = Age/4 + 4 = 5 to 20
• Normal gradient = low oxygen supply (hypoventilation, low FiO₂)
• Increased gradient = V/Q mismatch (COPD/asthma, cardiac R-L shunt - PFO, alveolar collapse, pneumonia, pulmonary edema)
• Hypercapnea = Type II respiratory failure = PaCO₂ > 55
• Increased CO₂ production: fever, sepsis, injury
• Decreased CO₂ perfusion (dead space): atelectasis, ARDS, lung injury, PE

VBG analysis

• pH and HCO₃ correspond fairly well with VBG
• CO₂ can vary 20-45 mmHg from ABG
• If normal (35-40), decent negative predictive value for hypercapnea
• PvO₂ will obviously be low - no utility unless it is a ScvO₂/SvO₂ (from central line) in the setting of sepsis
• ScvO₂ measured from central line, SvO₂ measured from PA catheter
• ScvO₂ < SvO₂ by a few % because brain has higher O2 extraction and goes to SVC. ScvO₂ measures mostly SVC blood while SvO₂ measures mixed SVC + IVC blood
• Sepsis: target ScvO₂ > 70%
• Normal O₂ extraction 25-30%, so normal ScvO₂ will be > 65%
• < 65% = impaired oxygenation
• > 80% = impaired oxygen extraction (e.g. severe sepsis/mitochondrial dysfunction) 

Normal values

• pH: 7.36 - 7.44
• pCO₂: 40 mmHg
• PaO2: 55-80mmHg
• HCO₃: 24 mEq/L

• Respiratory compensation for metabolic processes occurs over hours; metabolic compensation occurs over days
• If compensation is not what is expected, there is a mixed disorder
• "Normal" pCO₂ + tachypnea may = impending resp. failure

Metabolic acidosis: calculate anion gap (Na - [Cl + HCO₃]) (normal 10-14)

• Low albumin (unmeasured anion) - for every 1 mg/dL below 4, add 2.5 to anion gap
• Elevated anion gap (AGMA): check urine ketones (DKA), Cr/BUN (uremia), tox screen, osmolar gap
  • Methanol
  • Uremia
  • DKA
  • Paraldehyde
  • INH
  • Lactate
  • Ethylene glycol, ethanol
  • Salicylates, sepsis
• AGMA - check osmolar gap (measured osmolarity - calculated osmolarity)
  • Calculated: (Na x 2) + glucose/18 + BUN/2.8 + EtOH/4.6
  • > 10: possible toxic ingestion (EtOH, ethy glyc, MeOH)
• AGMA - check delta gap to look for second metabolic acid-base disorder
  • Corrected HCO₃ = [HCO₃] + (AG - expected AG)
  • > 24: concomitant metabolic alkalosis
  • < 24: concomitant NAGMA
• NAGMA (anion gap 10-14) (eg hyperchloremic)
  • Hypertonic IVF with low HCO₃ (eg NS)
  • Acetazolamide (carbonic anhydrase inhibitor -> renal HCO₃ loss
  • Renal tubular acidosis
  • Diarrhea
  • Ureterosigmoidostomy
  • Pancreatic EC fistula
• NAGMA - check urinary anion gap (UNa + UK) - UCl
  • Reflects NH4 (unmeasured anion) urinary excretion
  • +: renal HCO₃ loss
  • -: extrarenal (eg GI) HCO₃ loss

Metabolic alkalosis:

• Saline responsive
  • Volume contraction (urine Cl < 20)
  • GI loss (emesis, NGT sxn, villous adenoma)
  • Diuretics
  • Insensible losses
  • Post-hypercapnia
• Saline resistant
  • Hypervolemic (urine Cl > 20)
  • HypoK, hypoMg
  • Excess mineralocorticoid (hyperaldo, Cushing's, exogenous)
  • Exogenous alkali
  • Bartter's (like loop), Gitelman's (like thiazide -> Mg wasting)
  • Licorice

Respiratory acidosis:

• Neuro: CNS depression, neuromuscular disorders
• Structural: airway obstruction, laryngospasm, OSA, COPD, bad asthma, pneumonia; flail chest, hemo/pneumothorax

Respiratory alkalosis (increased RR)

• CNS disorder
• Hypoxia
• Anxiety, pain
• Mechanical ventilator
• Progesterone (pregnancy)
• Salicylates, sepsis