Spirometry

Volumes            and                Capacities

Pulmonary function tests like spirometry are quick and easy to perform and also cheap.  Most spirometers are simply a gauge that measures the volume of air that you exhale.  Exhale.  I can’t really measure the volume that you inhale. However, I can certainly have you blow up a balloon or a bellows. Then, I can measure the air that collects in it.  In a biology lab, you would probably use an old-fashioned spirometer. It inflates a bellows similar to an accordion. The bellows lays flat when not inflated and stretches out when inflated.  Kind of like an accordion or the bellows that you would use to fan a fire. 

I can directly measure volumes.  Not all volumes, but enough of them that I can do a few calculations to get the rest.  Capacities are calculated.  These are volumes that can’t be measured directly, but can be calculated from a few key measurements. 

Volumes

If you are sitting down and not exercising, just breathe a few breaths using quiet breathingNormal, passive breathing at rest..  Recall that this type of breathing uses only the diaphragm and external intercostalsElevator Between ribs; lifts rib cage during breathing in. to inhale. No muscles are used to exhale.  The volume that you are moving in and out is roughly the same with each breath. This is known as your tidal volumeThe amount of air inhaled or exhaled in a normal breath., or TV. 

OK, now breathe normally and on your next breath out, breathe out all the air that you can.  You can even bend over, collapsing your lungs even more.  The volume of air that you just exhaled is the Expiratory Reserve Volume or ERV.  Think about this name Expiratory, exhale, Reserve, above normal, Volume. 

OK, now breathe normally and on your next breath in, breathe in all the air that you can.  This extra air is called the IRV or the Inspiratory Reserve Volume.  Both of these reserve volumes can be used if you need to use them.  For example, exercise immediately asks you to increase the depth and rate of breathing.  To increase the depth of breathing, you would move more air with each breath. You would also use these reserve volumes.

There is one more volume, but it can’t be measured on a living person.  This is the residual volume(RV) – The amount of air remaining in the lungs after maximal exhalation. or the volume of your lung that does not inflate with each breath.  Remember that not all of your alveoliMicroscopic air sacs in the lungs where gas exchange occurs between air and blood. inflate upon inhalation.  The residual volume is an estimate of that volume and is usually provided on a standardized chart.  Basically, you have to look up the residual volume.

Capacities

We have now discussed all the air movementA fundamental property of life involving motion of the body or its parts. that we could measure or look up.  Those were all called volumes, because we can measure them.  You breathe in and out this relatively normal tidal volume. If you need to employ more lung tissue, you can do so by using those reserve volumes. 

Now, let’s calculate some of the capacities.  To do this, we only need to employ addition and subtraction.  First to calculate is the total lung capacity(TLC) – The total volume of air the lungs can hold. or TLC.  This capacity is the total amount of air that could fill your lungs if every single alveolus inflated upon inhalation.  To find TLC, we just have to add together the tidal volume, the reserve volumes and the residual volume.  TLC = TV + ERV + IRV + RV

The vital capacity or VC can be referred to as forced vital capacity(FVC) – The maximum amount of air a person can forcibly exhale after a deep inhalation. or FVC.  This is because we measure FVC is usually measured via a forcible exhale.  Vital capacity can be calculated by adding together all the volumes that represent air movement in one huge breath.  VC = TV + ERV + IRV. 

The last capacity important to us is the Functional Reserve Capacity or FRC.  This is the ERV, what you can breathe out, and the residual volume, the unused space. So FRC = ERV + RV.  This calculation is important in people with COPD.  It is usually small because they are not breathing out all the way.

FEV1 and FVC

All the capacities and volumes in the previous slides are nice. However, they are not what is really used if you go for your first round of pulmonary function tests. If you are asked to breathe through a spirometer, one volume and one rate will be measured.  Take in a really deepAway from the surface of the body. breath and blow it out as fast as you can.  Now do that by blowing out your breath through one of those red cocktail straws. These straws are meant for mixing, not drinking.  Ahhhhh, you might be able to blow out the same volume of air both times. However, you will do it at different rates. Beathing normally, you could probably exhale a pretty big volume of air in the first second of your exhale.  Through the cocktail straw, you’d blow out considerably less air in that first second. 

Measuring what you can exhale in the first second can help us measure restrictive diseases such as bronchitisInflammation of the bronchi, leading to mucus buildup, coughing, and breathing difficulties. and asthmaA chronic condition characterized by airway inflammation, narrowing, and excessive mucus production,.  These disease affect the conducting system, not the lungs.  As in the patient in our case study, he is young, healthy, and an athlete. There is nothing wrong with his lungs.  But, he has asthma.  So, if his asthma is active, he can expel the same volume of air. However, it will take him a long time to do it.  This leads us to the measurement that is most useful in asthma: the FEV or the forced expiratory volume.  We may choose to measure this volume in the first second of exhale.  In that case, we’d call it the FEV1.  If I wanted to measure the amount in 2 seconds, it’d be the FEV2, and so on. 

For a patient with asthma, their FVC might not be different when the condition is active. It might also remain the same when it’s not active.  The FVC won’t change because their lungs are not affected by asthma.  The amount of air they can expel in the first second is significantly reduced. This is especially true for a patient with asthma. 

OK, so if I just measure your FEV1, my FEV1, and our case study patient’s FEV1, I can compare them. No. If you want to use FEV1, you have to express it as a proportion of the FVC. 

Example here to determine why it is that you cannot just compare Fe V1 across patients. Let’s say that we have three people one weighing 100 lbs one weighing 200 lbs and one weighing 300 lbs. The person weighing 100 lbs loses 10 lbs or 10% of their weight. The person weighing 300 lbs also loses 10 lbs which is only 3% of their body weight. So both of my patients here lost 10 lbs. However, in terms of the proportion of body weight, they lost 10% and 3%. This difference is very significant. The FEV1 has to be put into a percentage to make a comparison.  We do this by dividing FEV1 by the FVC.  This gives us a proportion we can work with.

Diseases

Ya know what, I really hate these two terms classifying pulmonary diseases.  I just feel like it is the opposite.  Let me explain.

Restrictive diseases are ones that restrict the rate at which you can inhale or exhale, forcibly or not.  Anything that would be like blowing through a straw is a restrictive disease.

Obstructive diseases are ones that obstruct the ability of the lung to use total lung capacity.  Diseases that damage or fill in the lung’s surface area fall into this category. Diseases that affect lung complianceThe ease with which the lungs expand and contract during breathing. are also included.

Am I wrong?  Does the term obstructive feel more like, “Your 3-year-old swallowed a toy and has an airway obstruction.”  The term restrictive feels more like this. “Your chest is restricted from reaching full capacity. There is a 10 pound cat on your chest.” 

Despite my inability to keep the term straight, they both produce very different curves on this graph.  Remember, they key is that we have to compare FEV1/FVC.  Comparing the FVCs won’t help, and comparing the FEV1s kinda helps.  We have to calculate FEV1/FVC to make a true comparison across individuals. This also applies across an untreated patient and a treated patient.

Exercise Induced Asthma

As with our patient in the case study, his FEV1/FVC is very low. It is significantly worse compared to what he can do with that inhaler.  We can calculate this ratio and compare it for the spirometry data before and after his albuterol inhaler.  Now, his trachea and bronchiThe large airways that branch from the trachea into the lungs, dividing into smaller bronchioles. are wide open and he achieves FEV1 much more quickly AND it is also higher.  He can blow out more air, more quickly after the albuterol treatment.