9/10- Cases 1: ABGs/Pulmonary Function Flashcards
What is A-a gradient?
Difference between alveolar oxygen and arterial oxygen
- Alveolar oxygen content is found from the alveolar gas equation (using PaCO2 from ABG)
- Arterial oxygen content is found from the ABG
What should someone’s A-a gradient be?
P(A-a) = 2.5 + 0.21 (age)
What is the alveolar gas equation?
PAO2 = PIO2 - (PaCO2)/R
where PIO2 (inspired O2) = FiO2 (PB - PH2O)
on room air, PIO2 = 0.21
barometric P as sea level is 760 mmHg
PH20 = 47 mmHg at 37’C
R = respiratory quotient (ratio of oxygen uptake to CO2 exhaled); normal R = 0.8
Overall: PAO2 = 0.21 (760 - 47) - PaCO2/0.8
On room air at sea level, the first term = 150
Typically PAO2 ~ 100
What are the only 2 situations where you can accurately calculate the A-a gradient?
- On room air (FiO2 = 0.21)
- On a ventilator (FiO2 = 0.5)
Not when a pt is on a nasal canula!
In what cases of hypoxemia is the A-a gradient normal?
- Decreased PIO2 (low barometric pressure or fractional oxygen)
- Hypoventilation (increased PCO2)
In what cases of hypoxemia is the A-a gradient low?
- Right to left cardiac shunt
- Right to left intrapulmonary shunt
- Diffusion barrier
- Low mixed venous oxygen content
- V/Q mismatch …
What is the H-H equation for bicarb?
pH = pK + log [HCO3]/[H2CO3] where [H2CO3] = ?
T/F: body fluids are electroneutral?
True; this is the principle behind being able to calculate the anion gap
What is the equation for anion gap?
What is a normal value?
Na - (Cl + HCO3)
Normal ~ 10 +/-4
dAG ~ dHCO3 under what conditions?
Simple AG metabolic acidosis (if it’s not, it’s a sign of another process going on)
What is happening when dHCO3 >> dAG?
Fall in HCO3 is > rise in AG
Mixed AG acidosis + non-AG acidosis
What is happening when dAG >> dHCO3?
Rise in AG is > fall in HCO3
Mixed AG acidosis + primary metabolic alkalosis
Questions/process for acid-base cases
- Is pt acidemic or alkalemic
- What is the primary disorder (how are pCO2 and HCO3 moving)
- If respiratory disorder, is it acute or chronic
- If a metabolic acidosis, is it AG or non-AG
—- If AG acidosis, what is the dAG and dHCO3
- Is compensation adequate
- What clinical problems might cause the problem
What is the compensation in metabolic acidosis?
Respiratory system increases ventilation to lower PCO2 (and correct pH balance)
Expected new pCO2 = 1.5 (HCO3) + 8 +/- 2
What is the compensation for metabolic alkalosis?
Respiratory system decreases ventilation to elevate PCO2 (and correct pH balance)
Expected new pCO2 = 0.7 (HCO3) + 20?
((Decreased ventilation to elevate PCO2
1 mEq ↑ HCO3 -> 0.7 mmHg ↑ PCO2))