Is FEV1/FVC ratio higher in younger people or older people?
How are normal values for FEV1/FVC ratio determined?
The results are compared to known "normals" based on age, gender, race, and height and given as a % predicted. A normal range is within 80-120% of % predicted.
What is the hallmark of obstructive lung disease in pulmonary function tests?
Reduced FEV1/FVC ratio. A ratio under 0.7 is considered to be obstructive lung disease
How do you diagnose restrictive disease with spirometry?
You can't. You have to measure lung volumes to diagnose restrictive disease. In restrictive disease, the FEV1/FVC ratio is preserved or can even be elevated.
Explain the expiration and inspiration curves on the flow-volume loop
A normal expiration curve rapidly reaches peak-expiratory flow rate during the first 1/3 of expiration. During the second 2/3 of expiration, expiration is "effort independent" which means that it is not increased by increased effort. The decline is linear because of the elastic recoil of the lungs and resistance of airways.
The inspiration curve is a fairly symmetric half-loop.
For flow-volume loops, how do the expiratory loops for obstructive and restrictive diseases compare to a normal loop?
Both obstructive and restrictive diseases will have a lower peak expiratory flow rate. The obstructive loop is at higher lung volumes (left shifted) and will have "coving" which is a non-linear drop off of airflow due to the smaller airways being obstructed.
The restrictive loop is at lower lung volumes. The shape is generally the same as the normal loop but shifted to the right and a lower peak. Comparing to the normal loop, the restrictive loop has higher airflow at the same volume. This is called "supranormal airflow".
What does variable extrathoracic obstruction look like on a flow-volume loop?
The expiration curve is normal because the trachea has a positive relative pressure during expiration which holds the obstruction open. The inspiratory curve will be flattened because the trachea has a negative relative pressure during inspiration which draws the obstruction narrowing the airway.
What does variable intrathoracic obstruction look like on a flow-volume loop?
During expiration, the relative positive pressure of the lungs will cause a narrowing in the airway due to the intrathoracic obstruction. This will cause a flattening of the expiratory curve.
The inspiratory curve stays normal. During inspiration, there is relative negative pressure of the lungs which will cause an opening of the airway at the point of intrathoracic obstruction.
What does fixed obstruction look like on a flow-volume loop?
Both inspiratory and expiratory curves are flat.
When interpreting flow-volume loops, what is the memory aid for determining where the obstruction is?
Draw an arrow from the flat end of the loop towards the rounded/pointy end to remind yourself of where the obstruction is. The downward arrow is to remind you that the obstruction is variable intrathoracic and the upward arrow is to remind you that the obstruction is variable extrathoracic.
What is the helium/nitrogen dilution method? What is an issue with this method?
This is a method for measuring lung volume. You have the patient breathe in an inert gas and hold it for 10 seconds(helium or nitrogen or anything that won't be readily absorbed into the blood stream). You have them blow it back and you can measure the volume of gas.
The problem with this method is that it requires uniform diffusion of gas for accuracy. It is less accurate in obstructive lung diseases due to air trapping. In obstructive lung diseases, this method will underestimate the lung volume.
What is the most accurate way to measure lung volumes?
Plethysmography (the body box).
The patient sits in a body plethysmograph and breathes against a shutter that opens and closes with the patients breathing to determine lung volumes. It does this with Boyle's Law (P1V1 = P2V2). This method is most accurate as it does not require diffusion of gas so it can accurately measure patients with air trapping such as asthma and emphysema.
What is thoracic gas volume (TGV)?
TGV (thoracic gas volume) is the same as FRC (functional residual capacity)
What lung volume value(s) indicate hyperinflation?
When TLC or FRC is greater than 120% predicted.
For RV, it has to be greater than 140% of predicted.
If RV is greater than 140% predicted but TLC and FRC are normal, there is air trapping.
What does the patient have if RV is greater than 140% predicted but TLC and FRC are normal?
What lung volume value(s) indicate that a patient has lung restriction?
TLC or FRC is less than 80% predicted
What is DLCO?
DLCO is the Diffusion Capacity of the Lung for Carbon Monoxide (CO)
This test involves measuring the partial pressure difference between inspired and expired carbon monoxide. It relies on the strong affinity and large absorption capacity of erythrocytes for carbon monoxide and thus demonstrates gas uptake by the capillaries that are less dependent on cardiac output.
What 4 factors influence DLCO?
1. Surface area
2. Membrane thickness
3. Diffusion gradient of gas
4. Presence of hemoglobin
How does emphysema decrease DLCO?
The destruction of alveolar units
How does interstitial lung disease or pulmonary fibrosis decrease DLCO?
The membrane thickness is greatly increased so the diffusion capacity is reduced
How does alveolar filling (pulmonary edema or pneumonia) decrease DLCO?
Alveoli are filled by water or pus which increases diffusion distance and decreases surface area.
How does decreased pulmonary blood flow (pulmonary vascular disease) decrease DLCO?
Increased resistance and reduced flow in the pulmonary vascular beds reduces the amount of hemoglobin that goes through per unit time which reduces the amount of CO that can be taken up
What pathology can increase DLCO?
Alveolar hemorrhage causes an increase in DLCO because hemoglobin are spilled into the alveoli which suck up all the CO
What 4 things increase DLCO? (greater than 120% expected)
1. Polycythemia (disease state in which the proportion of blood volume that is occupied by red blood cells increases)
2. Interstitial edema
3. Asthma (reason is unknown)
4. Alveolar hemorrhage
What 5 things decrease DLCO? (less than 80% expected)
2. Pulmonary vascular disease
3. Interstitial lung disease
5. Pulmonary edema or pneumonia
In what cases do you need to correct for lung volume when measuring DLCO?
This is done when there are chest wall/pleural diseases (e.g. pleural effusion, neuromuscular weakness, obesity, etc.) or resection of the lung (surgical removal of cancer, etc). In these cases, their DLCO may be low but there may not be an intrinsic problem with the alveoli. Correcting for lung volume will help find out of the low DLCO is from an alveolar diffusion problem.
How do you measure compliance in patients?
This isn't done often but you can have a patient breathe in until TLC and the patient will blow out while stopping at different intervals. A catheter is used to measure pressures at the different intervals and a pressure volume (compliance) curve is drawn from the data collected.
Compare the pressure-volume curves in these diseases to a normal pressure-volume curve:
3. Obesity, decreased strength, pleural disease, chest wall abnormalities
4. pulmonary fibrosis.
What are bronchodilator challenges for and what indicates a positive finding?
Bronchodilator challenge is to determine whether an obstruction is reversible (e.g. asthma). This is done by giving a bronchodilator (typically albuterol) and to see if the patient has an increase of at least 12% in their FEV1 and or FVC and at least a 200 cc absolute increase in volume.
What is the methacholine challenge and what is a positive finding?
Methacholine is a bronchoconstrictor. You start by administering a low dose of Methacholine and you slowly increase to see if the patient has a decrease of at least 20% in their FEV1 and/or FVC when given less than or equal to 8mg/ml of methacholine. If you give enough methacholine, even healthy persons will have greater than 20% decrease in FEV1 and/or FVC but patients with asthma will have this change at low doses of methacholine.
What is the exercise induced bronchoconstriction test?
You have a patient exercise while inhaling cold air and if they have at least a 20% decrease in their FEV1 and/or FVC, the test is positive. All people have some amount of bronchoconstriction during this kind of activity but patients with asthma will have a change of 20% or more.