Blood Gases

Did you order an ABG or VBG?

• If you need to assess oxygenation (PaO2), then must order an ABG. An ABG is also the most accurate way to assess PaCO2 and pH
• If you want to assess hypoventilation, a VBG is informative and usually similar, but it may be inaccurate in some cases such as shock or severe hypercapnia
• The pH on a VBG usually correlates with an ABG (i.e. venous pH is ~0.05 lower than arterial pH), but this is less accurate in shock or severe acid-base disturbance
• ABGs are obtained by respiratory both at VUMC and the VA. VBGs can be ordered the same as any other lab. Alternatively the nurses can obtain a VBG (+lactate) with the iSTAT in the ICU at either VUMC or VA, which will return considerably faster
  • To order iSTAT at VUMC, order "Respiratory Lab Panel" and choose selections
  • To order VBG at VA, order POC CG4+ (standard), POC CG8+ (loaded VBG), or simply enter a free text order "please obtain VBG"

Assessing Oxygenation:

On an ABG, can use the arterial PaO2 (measurement of O2 dissolved in plasma) to assess oxygenation and calculate the A-a gradient.

A-a gradient = PAO2 (alveolar O2) – PaO2 (arterial O2)

• P_AO₂ = FiO₂ (P_barom - P_H2O) - PaCO₂/R
  • FiO2 = 0.21 on room air otherwise obtain from ventilator
  • P_barom = 760, P_{H2O =} 47
  • R (respiratory quotient) = 0.8
  • PaCO₂ = arterial CO2 measured on ABG
  • Normal A-a gradient = (Age +10)/4
• Common Reasons for an Increased A-a Gradient:
  • V/Q Mismatch: PE, atelectasis, PNA, pulmonary edema, pneumothorax, ILD, COPD
  • Diffusion limitation: interstitial lung disease
  • RàL Shunt Physiology:
    • Cardiac: PFO, VSD, ASD, AVMs
    • Pulmonary: blood, pus, water(edema), cells, protein in the Alveoli 

Assessing Acid/Base Status and Ventilation:

• Look at the pH (normal = 7.35 – 7.45)
  • pH < 7.35 = acidosis
  • pH > 7.45 = alkalosis
    • Look at the pCO2 (normal = 35-45 mmHg)

• Is the primary disorder acute vs. chronic? 
• Is the primary disorder appropriately compensated?
• If not appropriately compensated, what additional process is present?
   

Primary Respiratory Acidosis:

• Assess patient’s history: Is this acute or chronic?
  • Acute respiratory acidosis and hypercarbia will often present with somnolence or AMS
  • Similarly, if a PCO2 is 80, and the patient is talking to you, it is most likely chronic
• Common Acute Causes: Decreased respiratory drive (opiates, intoxication) or respiratory muscle weakness (i.e. myasthenia gravis exacerbation)
• Common Chronic Causes: COPD, sleep apnea
• Appropriate metabolic compensation?
  • If acute, every 10 mmHg increase in pCO2 above normal (40) should raise HCO3 by 1
    • Expected HCO3 = 24 + ((pCO2 -40)/10))If chronic, every 10 mm Hg ↑ in pCO2 above normal (40) should raise HCO3 by 3.5
    • Expected HCO3 = 24 + 3.5 ((pCO2-40/10)).
• If HCO3 higher than expected à additional metabolic alkalosis
• If HCO3 lower than expected à additional metabolic acidosis
   

Primary Respiratory Alkalosis

• Assess patient’s history: Is this acute or chronic?
  • Common Acute Causes: Mechanical ventilation, anxiety/panic attack, pain, PE
  • Common Chronic Causes: Pregnancy, CNS disorder, hormones (thyroid, progesterone)
• Appropriate metabolic compensation?
  • If acute, every 10 mmHg change in pCO2 should decrease the HCO3 by 2
    • Expected HCO3 = 24 – 2*((40-pCO2/10)
  • If chronic, every 10 mmHg change in pCO2 should decrease the HCO3 by 4
    • Expected HCO3 = 24 – 4*((40-pC02)/10)
• If HCO3 higher than expected à additional metabolic alkalosis
• If HCO3 lower than expected à additional metabolic acidosis
   

Primary Metabolic Alkalosis

• Appropriate Respiratory Compensation?
  • Expected pCO2 = 0.7*(HCO3) + 20 (+/-5)
• Etiologies: Majority of causes are associated with high aldosterone (either appropriate or inappropriate)
  • Chloride Responsive = appropriate hyperaldosteronism, can usually be fixed with NaCl administration/ volume resuscitation
    • Etiologies: Volume contraction i.e vomiting, over-diuresis which stimulates kidneys to reabsorb Na (and Cl absorbed with Na)
    • Urine Cl <20
• Chloride Unresponsive = inappropriate hyperaldosteronism, cannot fix with NaCl administration/ volume resuscitation as already likely volume overloaded
  • • Etiologies: Steroids, Cushing’s, Conn’s, RAS, CHF, cirrhosis
    • Urine Cl >20.
       

Primary Metabolic Acidosis

• See nephrology section, “Metabolic Acidosis”