RPFT Lung Volume Measurement & Diffusing Capacity 2 — Questions and Answers
Question 1: The single-breath diffusing capacity (DLCO) test primarily measures the transfer of which gas across the alveolar-capillary membrane?
- Oxygen
- Carbon dioxide
- Carbon monoxide (Correct answer)
- Helium
Correct answer: Carbon monoxide
DLCO uses carbon monoxide (CO) as a tracer because its high affinity for hemoglobin makes pulmonary capillary blood effectively an infinite sink, so diffusion is the rate-limiting step.
Question 2: During a single-breath DLCO maneuver, how long does the patient hold their breath at TLC after a rapid inhalation of test gas?
- 2 seconds
- 5 seconds
- 10 seconds (Correct answer)
- 20 seconds
Correct answer: 10 seconds
The standard breath-hold time is 10 seconds (range 8–12 s per ATS/ERS guidelines), which allows sufficient CO uptake for accurate calculation.
Question 3: Which of the following conditions is most likely to cause a DECREASED DLCO with NORMAL spirometry?
- Asthma
- Pulmonary arterial hypertension (Correct answer)
- Obesity
- Vocal cord dysfunction
Correct answer: Pulmonary arterial hypertension
Pulmonary arterial hypertension reduces pulmonary capillary blood volume and surface area for gas exchange, lowering DLCO despite preserved airflow.
Question 4: The DLCO result must be adjusted (DLCO/VA or KCO) when a patient cannot perform a full inhalation to TLC. What does VA represent in this equation?
- Vital capacity measured on the same maneuver
- Alveolar volume calculated from the helium dilution during the breath hold (Correct answer)
- Total lung capacity from a prior plethysmography session
- The anatomical dead space volume
Correct answer: Alveolar volume calculated from the helium dilution during the breath hold
VA is the effective alveolar volume derived from the dilution of the tracer gas (usually helium) inhaled during the same DLCO breath, reflecting the volume actually participating in gas exchange.
Question 5: A patient with significant anemia is found to have a low DLCO. What correction should be applied to interpret the result accurately?
- Correct for body surface area
- Correct DLCO for hemoglobin concentration (Correct answer)
- Apply the BTPS correction
- Adjust for barometric pressure only
Correct answer: Correct DLCO for hemoglobin concentration
Because hemoglobin carries CO, low hemoglobin reduces CO uptake independent of lung pathology; DLCO should be corrected to a standard Hb (e.g., 14.6 g/dL for men) to isolate the membrane component.
Question 6: Which of the following is an absolute contraindication to performing a DLCO test?
- A mild respiratory infection with stable vitals
- Carbon monoxide poisoning or recent exposure to high CO levels (Correct answer)
- Patient age over 70 years
- Mild claustrophobia
Correct answer: Carbon monoxide poisoning or recent exposure to high CO levels
If the patient has elevated carboxyhemoglobin from CO poisoning or heavy smoking, the test gas CO cannot be accurately measured, making the result invalid and potentially dangerous.
Question 7: According to ATS/ERS quality criteria, the minimum number of acceptable DLCO maneuvers required for a valid test session is:
- 1
- 2 (Correct answer)
- 3
- 5
Correct answer: 2
ATS/ERS guidelines require at least 2 acceptable maneuvers that agree within 2 mL CO/min/mmHg (10%) to report a mean DLCO.
RPFT Lung Volume Measurement & Diffusing Capacity 2
The single-breath diffusing capacity (DLCO) test primarily measures the transfer of which gas across the alveolar-capillary membrane?