Arterial blood gas

Question 1

ABG measurements

Answer 1

• Measures: PO2, O2 sat, PCO2 (PACO2), pH
• Can calculate HCO3, base excess
• Can also measure carboxyHb (COHb), and metHb

Question 2

Indication for an ABG

Answer 2

• Assess blood oxygenation
• Assess ventilation
• Determine acid-base status
• Determine COHb or MetHb levels

Question 3

Arterial O2 content (CaO2)

Answer 3

• CaO2 = (Hb) (SaO2) (1.34) + (PO2x.003)
• Since majority of O2 in blood is bound to Hb, the PAO2 is essentially negligible
• Therefore [Hb] has a large impact on CaO2
• O2 sat (SaO2) is measured and can also be calculated by the eqn
• It can also be estimated by the PAO2 using a sat curve
• Calculated SaO2 will not be accurate if COHb or MetHb is present

Question 4

Alveolar gas eqn

Answer 4

• PAO2 = (pressure - PH20)xFiO2 - (PaCO2/.8)
• Pressure is usually 760 (1 atm), PH20 is 47, and if breathing room air FiO2 is .21
• After substituting in constants, the equation is PAO2= 150- (PaCO2/.8)
• A normal value is around 100 (normal PaCO2 is 40)

Question 5

Causes of hypoxemia

Answer 5

• V/Q (ventilation/perfusion) mismatch, R-L shunt, hypoventilation, diffusion defects, and altitude
• A-a O2 difference is widened when there is a shunt (R-L or V problem), increased dead space (Q problem), or a diffusion problem
• Therefore A-a increased in V/Q, R-L, and diffusion defect
• A-a normal in hypoventilation, altitude, and diffusion defects
• To distinguish V/Q from R-L, use O2 supplementation
• V/Q will partially recover the A-a once on O2, but R-L shunting will not show improvement in A-a on O2
• Diffusion defect will look like V/Q (A-a wide and responds to supplemental O2)

Question 6

Minute ventilation

Answer 6

• MV = RR x TV
• Most accurate measurement of lung ventilation is PaCO2 and pH
• As ventilation increases PaCO2 decreases, and vice versa
• Hypoventilaiton is most common cause of elevated PaCO2
• But other things can cause elevated PaCO2 (depends on both ventilation and CO2 production)
• Things that increase CO2 production include excessive caloric intake, fever, shivering, seizures