Blood Gases

Basics of Blood Gases – Lexi Haugh

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)

  • PAO2 = FiO2 (Pbarom - PH2O) - PaCO2/R
    • FiO2 = 0.21 on room air otherwise obtain from ventilator
    • Pbarom = 760, PH2O = 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)

 

Primary Disorder

pH

pCO2

Respiratory Acidosis

< 7.35

>45

Respiratory Alkalosis

> 7.45

<35

Metabolic Acidosis

< 7.35

<45

Metabolic Alkalosis

> 7.45

>35

 

  • 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
     

 

Respiratory Acidosis

Respiratory alkalosis

Acute

10:1

10:2

Chronic

10:3.5

10:4

 

For every of 10 in pCO2,  HCO3  by 1 or 3.5

For every of 10 in pCO2 HCO3 by 2 or 4

 

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”