41. PFT’s Explained For You



  • Short Explanation
  • Long Explanation
  • Another Long Explanation

Short Explanation

I’m no expert on it, but I am told by my CF doctor that three numbers are looked at in CF:

FVC: Forced Vital Capacity. This is the total amount of air you can blow out forcefully.

FEV1: Forced Expiratory Volume in 1 second. This is the amount of air you can blow out in one second. This is the number, I am told, that is most relevant. When it falls below 30%, it’s time to start looking into a tx.

FEF 25%-50%: I don’t know what this stands for, but it is a measure of small airway capacity. I understand this is usually quite low in CF patients, even if the other two are high. (ie: mine is about 14%, but my FVC is about 72%). I guess this is where the oxygen gets to the blood. The living-related tx people told me that, the reason a living tx patient feels so good, even though he doesn’t have a complete set of lungs, is because CF patients are so used to going around with low FEF 25%-50%, so that, even in the worst case, a living-related tx’er ends up w/ 40%.

I understand that anywhere from 75%-125% is a normal deviation for any of these measures.

As far as the reliability of PFT’s goes, I’m told that they are only considered reliable within 15% because they are effort dependent and easily influenced by other factors. In an effort to correct this discrepancy (because a 10% drop in volume can portend bad things!!) I make sure I do EXACTLY the same regimen at the same time intervals before my appt’s.

Long Explanation

To address a few questions off the top:

Accuracy: the accuracy of these tests are dependent on a few factors – the effort of the person taking the test is a biggie (that’s why we make you do it three times- to make sure it is reproducible), the equipment (is it checked regularly and has calibrations done i.e. comparing known volumes to the result from the machine), and the operator(do they use the equipment properly?, do they coach you well?). Labs should have the operators of the equipment do “calibrations” (for lack of a better word) and check their accuracy on the equipment.

Barometric pressure can play a role too – new machines correct for it themselves.

Yes, these tests can be inaccurate- but hopefully, if all the above things are in line – then they will give some valuable info, particularly a trend. By comparing present tests to previous – that can help.

It is helpful if you can have a similar routine prior to having tests – that makes the trending that much more accurate. Everyone’s lung function has variations over the course of the day – so time of day is important too.

There are standards for all of this put out by the American Thoracic society (ATS) that dictate how the tests should be done, what makes them accurate, the specifications of the equipment etc. Ask your friendly PFT person if you want to know more. The standards actually dictate that three attempts must be within 5% of each other to be considered accurate – obviously, sometimes this is not possible – so we use the best result.

More General info:

When the MD’s say percentages – it is of predicted normal for your height, sex, age, race. Normal is considered about 75-80% to 120% of the predicted value for you. Studies have been done with thousands of people to develop these predicted values.

OK – it’s time to play connect the dots – or use your imagination! Does the shape below look familiar? (it’s not exact – but the best I can do with my senile computer – it is also the shape for healthy lungs)

It is what’s called a Flow- Volume loop. Volume is the X-axis and Flow is the Y-axis.

This can be spit out of a lot of machines when you do Spirometry (big breath all the way in – now blow it out , blow blow blow blow, all the way — bring back bad memories????)

the top is peak flow (a nice sharp point is good!) . . . . . FEF25 . . y . . . .FEF50 a . . x . . over the line is you breathing out I . . start of breath out is on left s . .FEF 75 . . . . x axis . . . . the little circle is normal breathing —————-.—-.——————————————- . . . . . . under the line is you breathing in . . start of breath in is on the right . . . .

A lot of the values in question are derived from this diagram.

FVC – forced vital capacity is the distance along the x axis , the amount of air you can blow out forcefully and completely (i.e. until you feel there is none left) from a full breath in. (there is actually air left in your lungs – this is called residual volume -this value goes up in CF as there is air trapping behind the mucus)

FEV1 – Forced expiratory volume in 1 second – they look at one second along the x axis and figure out the volume. Normally someone should be able to get around 80% of the FVC out in 1 second.

FEV1/FVC this ratio is a VERY broad indicator of obstruction in the airways (particularly the larger airways) it does not often show it in the small airways

FEF25-50 Forced expiratory Flow 25 – 50% (25-75 is common too) it doesn’t have to do with oxygen – but the flow rates (the speed of the air leaving your lungs) beyond the initial spike in the diagram. The percentages tell you at which time the flow measurement is taken. (at 25% of the full breath out, 50% etc.)

You can also have averages of these.

With FLOW– the peak flow tells you about your large airways (bronchi- the ones with smooth muscle , that’s why this is useful with asthma), As you get towards the 50,75% FEF’s it’s telling you about your middle to smaller airways and how the flow is doing down there.

PEF(R) Peak expiratory Flow (rate) the speed of the breath at the beginning of a breath out.

PIF – same as above but for inspiration (breath in).

MVV -Maximum Voluntary Ventilation (usually over a minute) – how much volume you can move in 1 min going at top speed and with BIG breaths.

TLC – total lung capacity – usually measured by the “body box” or by dilution. I suspect the box would work better with CF – because dilution might miss some air stuck behind some mucus.

VC – Vital capacity – is that thing from a deep breath in to the residual volume (mentioned above) difference between TLC and RV.

RV/TLC – tells you how much air is staying in your lungs after you breathe out as far as you can – indicates how much “gas trapping” there is.

In CF – RV would go up with more air being trapped – so the RV/TLC ratio would go up.

FRC – Functional residual capacity – it’s like the RV plus a little bit. instead of breathing all of the way out – this measure is only taken from the end of a regular breath out.

Man, I wish I could draw this!!!!!!! Imagine( or draw) a nice sine wave as your regular breaths in and out – then go higher with the wave on a big breath in and then lower as you breathe out further than usual. Then try and plot TLC, VC, FRC Tidal volume (normal breaths in and out), VC. It might help a bit.

So that’s volumes and Flows in a nutshell. (I think – I hope not too much jargon either)

Other things they can measure:

Diffusion capacity – this is the oxygen thing – they take a TINY bit of Carbon monoxide and let you breathe it in a mixture of oxygen etc. They measure what’s left to see how much is and how fast it is absorbed into your blood.

Airway Resistance Raw

I’m not certain what the sRaw and Vtg(Raw) are – but the Raw cmH2O/L/s is the unit it is measured in. cmH20 is a unit of pressure -exactly what it says- think of a column of water – and depending on how high (-in centimeters) it is – the pressure is higher at the bottom. So- Resistance is the pressure it takes to move a liter of gas in a second through the airways.

Resistance will increase as airways get narrower – from mucus or fibrosis etc.

Another Long Explanation

with kind permission of the author: Len Moriyama, RPFT, RRT, RCP Respiratory Technician for Alpha1 Registry California Pacific Medical Center, San Francisco

“PFTs”, or pulmonary function testing, is simply the evaluation of lung function. PFTs are used to determine:

  • the presence of lung disease or abnormality of lung function
  • the extent of abnormalities
  • the extent of impairment caused by abnormal lung function
  • the progression of the disease
  • the nature of the physiologic disturbance
  • a course of therapy and treatment, as indicated

As you may have experienced, seeing your PFT results can be quite overwhelming. It is my hope that this simplification of what the abbreviations (i.e. FVC) mean, as well as the observed values, will help you to understand your own test results. Obviously, not all the measured values on your test have as much significance as others, therefore, I will try to define and discuss only those which are most commonly measured and have the most significance.

PFT definitions

Normal: for comparative purposes, observed values that fall at 80% or greater than the predicted values.

Predicted Values: these are average numbers based upon one’s age, height, sex, and at some institutions, race.


  1. Pulmonary mechanics
    1. FVC – Forced Vital Capacity is the maximum volume of gas that can be forcefully and rapidly expired after a maximal inspiration. This maneuver may also be called a “Flow-Volume Loop”, which is the same maneuver that is graphically displayed comparing volume and flow rate. significance: May be normal or reduced in emphysema because of loss of support for small airways.
    2. FEV1 – Forced Expiratory Volume after 1 second is the volume of gas expired after one second from the beginning of the FVC maneuver. significance: Usually decreased in obstructive airways due to mucus secretion, bronchospasm, inflammation, or loss of elastic support of the airways themselves, as in emphysema.
    3. FEV1/FVC – The ratio of FEV1 to FVC expressed in a percentage. significance: usually decreased in obstructive airways and is independent of the relative values of FVC and FEV1.
    4. FEF 25-75% – Forced Expiratory Flow from 25 to 75%, is the average rate of flow during the middle half of an FVC maneuver based upon a segment of the FVC that included the flow from medium-sized and small airways. Also known as “mid flow rates”. significance: usually decreased in obstructive airways due to mucus secretion, bronchospasm, inflammation, or loss of elastic support of the airways (i.e. emphysema).
    5. FEFMAX or PEFR – Forced Expiratory Flow at Maximum effort or Peak Expiratory Flow Rate, is the maximum flow rate attained during the FVC maneuver. significance: Generally, FEFMAX is a good index of the patient’s effort and relative strength in forceful exhalation. Also, may have relative meaning in evaluation of the effectiveness of a patient’s cough.
    6. Raw – Airway resistance is the “resistance” created by the airways and the friction against the walls of the airways caused by the flow of air through these passageways. This test is performed in the so-called “body box” or Plethysmograph, where one performs “panting” maneuvers. significance: Raw is increased in emphysema because of narrowing and collapse in some of the larger airways, as well as more distal bronchioles. “Normal” would be a value which is less than the predicted value.
  2. Lung volumes
    “Air-trapping” – a term used to describe the state where one actually maintains a large amount of air in one’s chest, even at the end of both normal and maximal exhalation.

    1. SVC – Slow Vital Capacity is the volume of gas measured on a slow, complete expiration after or before a maximal inspiration, without forced or rapid effort. significance: often the SVC is significantly larger than FVC, which indicates to some degree the existence of air-trapping. Generally, with obstructive lungs, SVC is within normal limits. It may be less than normal due to the collapse of airways due to lack of support.
    2. FRC or TGV – Functional Residual Capacity, or Thoracic Gas Volume, is the volume of gas remaining in the lungs after exhalation of a normal breath. This test is performed in several different ways. Perhaps the most accurate is by the “body box”; other less accurate ways (especially in light of poorly communicating airways) include the Nitrogen Washout method, Helium Dilution, and Single-Breath Nitrogen Washout. significance: relative to the predicted values, larger values of FRC or TGV indicate the degree of air-trapping (i.e. emphysema).
    3. RV – Residual Volume is the volume of gas remaining in the lungs after complete exhalation. Value is obtained by any of the above methods mentioned for FRC. significance: Like FRC, RV is compared to the predicted values to determine the degree of air-trapping.
    4. TLC – Total Lung Capacity is the volume contained in the lungs at the end of a maximal inspiration. This value is obtained from determination of SVC and FRC, in methods previously mentioned. significance: because of air-trapping associated with emphysema, the TLC is usually larger than predicted.
    5. Diffusing capacity
      DLCO – Diffusing Capacity measures the transfer of a diffusion-limited gas (Carbon Monoxide, CO 0.3%) across the alveolocapillary membrane. CO combines with hemoglobin approximately 210 times more readily than oxygen does. In the presence of normal amounts of hemoglobin and normal ventilatory function, the primary limiting factor of diffusion of CO is the status of the alveolocapillary membrane. significance: DLCO is usually decreased in emphysema because of the decrease in the total surface area, loss of capillary bed, increased distance from terminal bronchiole to alveolocapillary membrane, and the mismatching of ventilation to blood flow.
  3. Other measurements
    1. ABG – Arterial Blood Gasses are samples drawn from an artery (usually the radial near one’s wrist), which enables us to measure the following values:PH – the measurement of your arterial blood’s acid-base balance. This “balance” is controlled by several factors, but primarily metabolism and ventilation. If there is an imbalance with one or the other or both, there will be a definite effect on the PH. The approximate “normal” range of PH is 7.335 to 7.45, with 7.4 being the mean. Values less than 7.4 are considered to be more “acidic”, while those greater than 7.4 are more basic or “alkaline”. 2.PaCO2 – Partial Pressure of Carbon Dioxide in arterial blood. This value is an indicator of how effectively your lungs are able to rid themselves of a by-product of metabolism, CO2. The “normal” range for PaCO2 is 35 to 45 mm Hg. Elevated values greater than 40-45 mainly indicate that the lungs are not able to rid themselves of the CO2 (“CO2 retainer”). This may be evident in emphysema, where much of the lungs are unable to effectively exchange gasses. 3.PaO2 – Partial Pressure of Oxygen in arterial blood. This is a measure of the actual amount of oxygen there is in your arterial blood. The “normal” for PaO2 is generally greater than 75-80 mm Hg, relative to your age. For the most part, it should be greater than 55-60 mm Hg, otherwise supplemental oxygen would be indicated. (Note of caution: More is not always better. For those of you on supplemental oxygen, please contact your physician before you significantly increase your O2 flow).
    2. PULSE OXIMETRY or SaO2 – This is a non-invasive measure of one’s oxygen saturation; that is, the amount of oxygen saturated in one’s hemoglobin in terms of a percentage. This is not as accurate as the values obtained from an ABG and should only be used as a gauge of one’s oxygenation. Normal ranges are between 95-100%. Supplemental oxygen is not generally instituted unless SaO2 is less than 88-90% at rest.


Laboratory Evaluation of Pulmonary Function, William F. Miller, Robert Scacci, Lee Roy Gast, J.P. Lippincott Company, Philadelphia, PA, 1987.

Manual of Pulmonary Function Testing, Fifth Edition, Gregg L. Ruppel; Mosby Year Book, USA, 1991.

Respiratory Physiology – the Essentials, John B. West; Williams and Wilkins Company, Baltimore, MD, 1987.


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