PFTs -- What, Why, When, How

Pulmonary function tests (PFTs) are the most common way to measure and track the extent that someone has lost respiratory function. In the US, in ALS, PFTs are most often done to justify Medicare or other insurance coverage for BiPAP and possibly other equipment such as a suction machine, cough assist device and/or oscillating vest. PFTs may also help establish eligibility for clinical trials.

However, PFTs don't show how much longer someone will live, or what settings should be used for BiPAP. The speedometer in your car does not show whether it is safe to drive at that speed given traffic and weather conditions. And it does not show whether you will need to slam on your brakes in the next five minutes, or how good your tires are.

PFTs are best at predicting two things: when things are relatively good (normal values generally confirm at least a year of life remaining with ALS) and when things are going south. The fine points of how fast the journey will be, cannot be answered by PFTs.

One reason for this is that often people end up using different muscles to help them breathe when weakness begins, so tests may look unrealistically good for a while, and then when the "substitute" muscles become weak as well, a dramatic drop may occur.

Also, many PALS begin breathing more slowly and/or shallowly to compensate for their weakened breathing, so even though they are moving less air, they are not building up too much CO2. Opioid drugs like codeine and morphine can also make breathing shallower and/or irregular, and can cause “central apnea,” where your brain does not send all of its normal signals to the muscles that control breathing, telling them to help you take a breath.

So even with multiple PFT measures that are captured during the tests, the day-to-day picture about how well oxygen is getting into tissues and CO2 is being exhaled, is not always clear. PALS who gradually become less mobile may not exert themselves as much, so shortness of breath may not be obvious. Sometimes, PFT results may show no respiratory dysfunction, even though muscles are working overtime and need help.

Therefore, to truly understand respiratory impairment, no single number is “the best” for all. However, in the US, only two can be used to get insurance to pay for your BiPAP. In addition, if a PALS shows the signs of respiratory impairment listed above, they probably could benefit from a BiPAP, no matter what the PFTs say, which is reflected in the European guidelines but not in the US ones.

Preparing for Testing
On testing day, you may be asked to skip certain inhalers or medications. However, bring your inhaler if you have one; you may be asked to use it during the test. Wear loose clothing and avoid big meals before testing.
◦ Take medications as usual unless you are told otherwise. If they will be due during the test, take them before.
◦ Do not smoke for at least six hours prior to testing. [Don't smoke at all in ALS!]
◦ Try not to use any inhalers on test day.

Common PFT Measures
The most useful measures in assessing respiratory function in ALS are FVC, SVC, MIP, SNIF/SNIP, MEP, and CPF/PCF, each outlined below. Other results will often be listed on the report; however, these are typically more important in lung diseases such as COPD and asthma, so they may be normal in ALS.

You will sit for the FVC (Forced Vital Capacity) test and also be asked to lie down for “supine” (on your back) measurement. FVC uses more muscle groups than other inspiratory tests, making it a good barometer of breathing overall as a way to see how well some muscles that are weak are being substituted with stronger ones.

Supine measurement is often lower but more important since you lie down when you sleep, and that is usually when breathing is hardest. If you use a power chair, you might be tilted back in it for a supine measurement.

Do not confuse "SVC" (slow vital capacity) with supine FVC measurement, even though you might be tempted to abbreviate supine FVC as SVC. It has no abbreviation.

Supine FVC is typically lower than upright FVC in restrictive lung diseases such as ALS (up to 15%). So especially if upright FVC does not support reimbursement for BiPAP, ask that supine FVC be measured as well.

With a clip on your nose to keep air from coming out of it, you place your lips tightly around a plastic mouthpiece. Then you take in as big and deep a breath as possible and then blow out as hard and fast (for FVC) or as steadily and long (SVC) as you can. Often, testing will be repeated at least three times.

However, FVC requires the facial muscle strength and motor control to form and hold a tight seal around the mouthpiece. Also, it can be low for other reasons than ALS, such as another lung problem, smoking (if you do, quit!!) or obesity.

PALS with severe bulbar disease have difficulties with FVC because the upper airway may collapse when they breathe in, and they may start coughing during exhalation.

In such cases, the tech may use their hands to create a tight seal for the PALS. Or, if that is not possible, the SNIP may be considered a more accurate measure.

The technician may ask you to use an inhaler to help open your airways and then repeat the test to see if your breathing improves. The testing takes only a few minutes.

SVC (slow vital capacity) is like the FVC, but you are asked to take a regular deep breath instead of a fast one. SVC is often considered a better indicator of progression than the FVC, but US insurers do not accept it as evidence for reimbursing BiPAP. SVC is easier to perform than FVC, especially in PALS with significant upper motor neuron disease or with severe respiratory dysfunction.

To test MIP (maximum inspiratory pressure), you take a rapid breath in through your mouth. The force you generate is measured with a probe. This helps assess sternocleidomastoid (neck) muscle strength. You may get up to three tries.

PALS with facial weakness or upper motor neuron damage may find it hard to create and hold a seal with their lips and/or to breathe in quickly. The technician or therapist may manually help you maintain the seal. In the US, a value of <60 cm can be used to justify BiPAP reimbursement.

SNIF (Slow Nasal Inspiratory Force) is sometimes called SNIP (Slow Nasal Inspiratory Pressure>). It’s like a MIP, except you breathe in using the nose instead of the mouth, so it is a better test than the MIP for those with facial weakness. The diaphragm generates most of the force generated during this test. SNIF/SNIP requires effort, good central motor control, and appears to be subject to a learning effect (you get better the more times you do the test).

You sniff, usually with one nostril blocked, and pressure generated by the other one is measured with a probe. This helps assess diaphragm strength. You may get up to 10 tries.

European guidelines call for BiPAP use when SNIP < 40 cm H2O, or in some cases when it's <60 and breathing is harder lying down. US payors do not use SNIP, however, only FVC and/or MIP.