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))
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
What is the typical chart notation for ABG values?
- 28-year-old man brought to the EC unconscious
- He has a past history of suicidal ideation
- ABG shows pH 7.1, PCO2=24, PO2=80, HCO3=12, BUN=17, Na=145, K=4.9, CI=111 Glucose=168, Creatinine=2.3, Serum osmolarity=332.
1. What is the acid base disturbance?
2. What is the anion gap in this case? What are causes of anion gap metabolic acidosis?
3. How can the electrolytes/osmolality help with the diagnosis? How might the osmolality identify ingested agents as cause of acid-base disturbance?
- Primary metabolic acidosis (low pH, low pCO2, low HCO3)
- AG = Na - (Cl + HCO3) = 145 - (111 + 12) = 22 (high)
- Causes: AMUDPILES
- Calculate osmolarity: 2Na + glucose/18 + BUN/2.8
- Osmolar gap = measured serum osmolarity - calculated osmolarity
- gap = 332 - 305 = 27
- Normal < 10
- Gap indicates that he might have ingested something
- 25-year-old man with DM Type 1 is admitted to the ICU with DKA
- He felt ill for one day and held his insulin because of poor oral intake. He is lethargic on examination.
- Arterial blood gases show pH=7.25, PCO2= 28, PaO2=85.
- Serum electrolytes show Na=134, K=3.7, Cl=110, HCO3=8, BUN=22 and Creatinine=1.2
1. What is the acid base disturbance?
2. What is the anion gap in this case?
3. Is the compensation as expected? If not, what does it mean?
4. What is the likely explanation of this acid-base disturbance?
5. What is delta-delta? What does this signify
- Primary metabolic acidosis (low pH, low PCO2, low HCO3)
- AG = Na - (Cl + HCO3) = 134 - (110 + 8) = 16 (high)
- eCO2 = 1.5 (HCO3) + 8 +/-2 = 18 - 22
- pCO2 is higher than expected (28 > 22), thus respiratory acidosis
- Caused by diabetic ketoacidosis with hypoventilation or airflow obstruction (new 2 things, one explanation for each primary condition)
- ΔAG = 6, ΔHCO3 = 16; thus Δ-Δ mismatch!
- Fall in HCO3 > increase in AG
- Thus, there is an additional metabolic acidosis (non high AG)
What is osmolar gap?
- How do you calculate osmolarity?
- What is a normal gap value?
Osmolarity = 2Na + glucose/18 + BUN/2.8
Osmolar gap = measured serum osmolarity - calculated osmolarity
Normal gap
- This patient had a smoking history of 74 pack years and was still smoking.
- He complained of progressive breathlessness and wheezing on mild exertion.
- He had a family history of pulmonary disease.
1. How would you interpret this test?
2. Can you make a statement as to the patient’s underlying lung disease?
3. Does the reduced DLCO suggest anything?
- FEV1/FVC = 53% (decreased)
- FEV1 is decreased (31%)
- FVC is also decreased (45%)
Thus, this is obstructive lung disease
- TLC is 9.37 (142%); high. Thus, he is hyperinflated
- RV/TLC is also increased; trapping air
- He is just severe obstruction (no restriction unless lung volume is < 80%); this is probably emphysema
- Also has very decreased DLCO
- How do you classify the flow-volume curve? (obstructive/restrictive/mixed)
- What types of disease process may be in your differential?
- What does the DLCO imply?
- Spirometry suggests restrictive disorder; need to look at lung volumes
- Do have decreased volumes
- How would you interpret the results?
- What type of disease processes can cause this type of PFTs?
Mixed restrictive and obstructed
- Restrictive from reduced TLC
- Obstruction from low flows
- What is your interpretation of this test?
It is normal; this is not a lung problem
Young male after ATV accident
- A 36 y/o man is evaluated for dyspnea on exertion, hoarseness and difficulty swallowing for 8 months.
- He was involved in ATV accident one year ago and had multiple intubations.
- CXR is normal and PFT is shown below.
- How would you interpret the spirometry?
- How are the lung volumes?
- Please comment on the flow volume loop
Flat curves; fixed obstruction
