9/10- Cases 1: ABGs/Pulmonary Function Flashcards
(34 cards)
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))
What is the compensation for acute respiratory acidosis?
pCO2 Δ10 mmHg -> pH Δ 0.08
ΔpH = 0.08 x (PaCO2 - 40)/10
pCO2 ↑1mmHg -> HCO3 ↑ 0.1 mEq
What is the compensation for chronic respiratory acidosis?
pCO2 Δ10 mmHg -> pH Δ 0.03
ΔpH = 0.03 x (PaCO2 - 40)/10 pCO2
↑1mmHg -> HCO3 ↑ 0.4 mEq
What is the compensation for acute respiratory alkalosis?
pCO2 Δ10 mmHg -> pH Δ 0.08
ΔpCO2 = 0.08 x (pH - 7.4)/10
pCO2 ↑1mmHg -> HCO3 ↑ 0.2 mEq
What is the compensation for chronic respiratory alkalosis?
pCO2 Δ10 mmHg -> pH Δ 0.03
ΔpCO2 = 0.03 x (pH - 7.4)/10
pCO2 ↑1mmHg -> HCO3 ↑ 0.4 mEq
- 65-year-old man with past history of COPD, presented with shortness of breath following a viral upper respiratory tract infection
- His blood gases on room air showed: pH=7.28, PCO2=56, PO2=50
- Primary condition?
- Acute or chronic?
- A-a gradient?
- What could the cause be? What does A-a gradient imply?
- Primarily = respiratory acidosis (pH is low and pCO2 is high)
- Acute (determined by compensation equations):
- change in pH = 0.08 x (PaCO2 - 40)/10
- thus 7.4 - 7.28 = 0.12 ~ 0.12
- PAO2 = 80
- A-a gradient = 30 (high)
- Abnormally increased A-a difference implies abnormal lung function (i.e. chronic disease or acute disease like pneumonia)
- In this case, one would want to exclude COPD exacerbation, asthma exacerbation, work up for pneumonia…
What can cause respiratory acidosis?
CNS depression
- Narcotic OD
- Neurological disorders
Chest wall disorders
- Amyotropic lateral sclerosis
- Guillan-Barre syndrome
Obstructive lung diseases
- COPD
- Asthma
What are common causes of hypoxemia? Examples?
Think about pathway of air:
- Decreased inspired O2 (high altitude)
- VQ mismatch (pulmonary embolism)*
- Shunt (intra-cardiac R to L shunt)*
- Hypoventilation (sedatives)
- Diffusion abnormality (interstitial lung disease)*
- *Have high A-a gradient*
- A 20-year-old man, IV drug abuser was brought in the ER with altered mental status.
- His blood gas analysis showed: pH=7.08, PC02=80, P02=45.
1. What is the primary acid base disturbance? (Acute or chronic)
2. What is the A-a difference? Is this normal?
3. What is the most likely diagnosis?
- Primary respiratory acidosis (low pH, high pCO2)
- Acute (based on compensation equation):
ΔpH = 0.08 x (PaCO2 - 40)/10
- 7.4 - 7.08 = 0.32 = 0.08 x (80-40)/10
- (for chronic, pH would have been 7.28)
- PAO2 = 50
- A-a gradient = 5 (normal)
- Most likely due to hypoventilation (since the only hypoxemic causes with normal A-a gradient are hypoventilation and decreased PiO2 from altitude or low FiO2)
- 70-year-old man has a 5 year history of intermittent watery diarrhea
- In addition he has had prostate surgery, has symptoms of prostatic obstruction, and NIDDM (non-insulin dependent diabetes) for 5 years
- pH=7.32, PCO2=33, PO2=87, HCO3=18, Na=133, K=2.7, CI=105.
1. What is the acid-base disturbance?
2. What is the anion gap in this case?
3. Is the compensation as expected?
4. What is the likely explanation of this acid-base disturbance?
- Primary metabolic acidosis (low pH, low pCO2, low HCO3)
- AG = Na - (Cl + HCO3)
- 133 - (105 + 18)
- AG = 10
- eCO2 = 1.5 (HCO3) + 8 +/-2
- eCO2 = 1.5 (18) + 8 +/-2
- eCO2 = 33 to 37
- Compensated
- Cause = diarrhea
