Flashcards in Blood Gases and function tests and ventilation Deck (50)
What is important to remember about assessing alveolar ventilation?
Cannot assess it just by looking at a patient
What is the pCO2 equation?
PCO2 = 0.863 x (CO2 production/alveolar ventilation)
What are the physiologic processes that can cause hypoventilation?
Reduced total ventilation
Normal total but increased dead space ventilation
What can cause alveolar hyperventilation?
Increased central respiratory drive - from pain or anxiety, compensation for metabolic acidosis
What is a compensated respiratory acidosis?
Near normal ph with high bicarbonate and pCO2
What determines if the lungs are doing a good job getting oxygen from atmosphere to blood?
The A-a gradient
Difference between oxygen in alveoli and oxygen in arterial blood
What is an elevated A-a gradient?
Anything over 15 mm Hg
Increases with age
How can high A-a gradient due to diffusion block be recognized?
Only reduces arterial blood ox during things like exercise, not at rest
Happens in fibrotic lung diseases
How can high A-a gradient due to v:q mismatch be recognized?
Reduced ventilation but normal perfusion
Most common cause
How can high A-a gradient due to a shunt be recognized?
pO2 not increased even if patient breaths 100% oxygen
R to L shunt suspected if paO2 <95% while patient breathing 100% oxygen
What is suspected if paO2 is not affected but SaO2 is decreased?
Carbon monoxide poisoning
When can SpO2 differ from SaO2?
When SaO2 <85% - ignore SpO2
Can be normal in CO poisoning
No info about alveolar ventilation or acid base balance
Inaccurate in patient with poor perfusion of extremities (shock)
How can cyanide poisoning be recognized?
Adequately oxygenated blood is reaching the tissue but tissue cannot utilize it
Consequences of tissue hypoxia but all stats are normal
What are the five causes of tissue hypoxia?
Reduced paO2 with normal pAO2
Reduced CaO2 with normal paO2 (anemia, CO poisoning)
Reduced tissue perfusion (hypotension, arterial occlusion)
Reduced tissue oxygen utilization (cyanide poisoning)
How is the A-a gradient calculated?
A-a = (150 - 1.25(paCO2)) - pAO2
How can bicarbonate be determined to assess renal compensation ?
Henderson hasselbach equation
PH = 6.1 + log(HCO3-/0.03paCO2)
What are the five common indications for PFTs?
Assessment of patient with pulm complaints
Determine pattern of respiratory impairment
Occupational or environmental reasons
What are four categories of information obtained from PFTs?
Lung volumes and capacities
Maximal inspiratory pressure and maximum expiratory pressure
What is total lung capacity?
Total volume of gas within lungs after maximal inspiration
What is residual volume?
Volume of gas remaining in lungs after maximal expiration
What is vital capacity?
Volume of gas expired when inspiring all the way to TLC and exhaling down to RV
Largest volume of air that can be moved into or out of the lungs
Made up of IRV, TV, ERV
What is functional residual capacity?
Volume of gas in lungs at resting state (during tidal breathing)
How does spirometry work?
Deep inspiratory breath to TLC then forced exhalation to RV
Measure how fast and how long
What values can you obtain from spirometry and which can't you?
Can get FEV1 and FVC
Cannot get RV, TLC, or FRC
Does provide inspiratory capacity
What is normal FEV1/FVC?
What are possible methods of measuring lung volumes?
Body plethysmography (patient sits in box and performs maneuvers, uses boyles law)
Dilutional lung volumes using helium dilution or nitrogen washout - severe COPD may cause falsely low value, use other method
These measure FRC
How can different lung volumes be calculated if FRC is obtained?
FRC - ERV = RV
IC + FRC = TLC
VC + RV = TLC
TLC is important for diagnosing restrictive lung diseases
What are the gold guidelines for defining airflow OBSTRUCTION?
FEV1/FVC less than 70%
Other than a fixed ratio, what value can be used to determine if obstruction is present?
Lower limits of normal
Obstructive if ratio below the 5th percentile of predicted