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Restrictive lung disease: Is smoking a cause outside the box?

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It is an accepted fact that cigarette smoking causes chronic obstructive lung disease (COPD).  But it is not a considered cause of restrictive lung disease. In fact, conventional wisdom is to not acknowledge smoking as a cause of restrictive lung disease. There are reasons to rethink this belief though. One reason is frequent findings on pulmonary function tests of smokers.  Another is a newly acknowledged link between smoking and interstitial lung disease

What is Restrictive Lung Disease?  

Restrictive lung disease (RLD) is a state in which the lungs do not fully expand or stretch during the entry of air. As a consequence, it restricts how deep a breath a person can take. The disturbance is one of lung compliance, not blockage in airways. The problem can be within the lungs themselves or outside of the lungs. When the problem is within the lungs there is inflammation and/or scarring of the lung interstitium. The resulting stiffness in turn leads to a decrease in the volume of air that can enter the lungs while breathing in. Problems outside of the lungs that restrict the ability to breathe in can involve the pleura, chest wall, or the nerves and muscles as a unit, which are involved in breathing.

Restrictive Lung Disease and Lung Volumes

The clearest understanding of the effect of restrictive lung disease is in view of how it impacts inspiratory capacity and total lung capacity. Inspiratory capacity (IC) is the maximum volume of air that one can breathe into the lungs after a normal expirationTotal lung capacity (TLC) is the maximum volume of air the lungs will hold.  It is the IC plus the amount of air in the lungs after a full exhalation. Both capacities are less than normal in the presence of restrictive lung disease.

A rough indicator of the IC is the vital capacity (VC). The VC is the maximum amount of air one can expel from the lungs after taking as deep a breath as possible. The most common means of detecting restrictive lung disease is a reduced forced vital capacity (FVC) measured during spirometry. The FEV1 might also be less than restrictive lung diseasenormal with restrictive lung disease because it is a part of the FVC. If the latter is the case though, the FEV1/FVC ratio must be 70% or greater.  The FEV1/FVC ratio is FEV1 divided by FVC, and then multiplied by 100.

COPD and Lung Volumes

In contrast to restrictive lung disease COPD has its greatest effect on exhalation.  The reason is the tendency of airways in the lungs to collapse with the expulsion of air in the case of COPD.  This results in a decrease in the amount of air that one can expel in 1 second (FEV1) as well as a decreased FEV1/FVC ratio. In terms of air flow volume, COPD is an FEV1 of less than 80% of predicted along with and FEV1/FVC of less than 70%. The FEV1/FVC criterion is based on the fact that blockage of airways leads to a greater decrease in the amount of air expelled in the first second than the total amount of air expelled.

Smoking as a Cause of Restrictive Lung Disease Rationale

Inductive reasoning is a basis for surmising a link between cigarettes smoking and restrictive lung disease. After all, why would the toxins in tobacco cause inflammation in airways and damage alveoli yet not cause damage and scarring in adjacent interstitial tissue?  After all, there is proof that smoking causes damage to various tissues throughout the body.   

More along the lines of deductive reasoning, smoking might be a cause of RLD because of the restrictive findings in many smokers who undergo a pulmonary function test.  The main finding is an FVC of less than 80% of predicted. One might argue that obesity is the cause of the decreased FVC.  But many of these patients are not obese and don’t have any other cause of the restrictive defect revealed by the test.  Many smokers meet the criteria for COPD based on the FEV1 and FEV1/FVC ratio.  But when they also have an FVC of less than 80% of predicted that denotes mixed obstructive and restrictive disease.

A low DLCO is another PFT finding that lends credence to the notion that smoking is a cause of RLD.  The reason is a low reading means there is a problem with gas exchange in the lungs.  In the absence of pulmonary hypertension, the most common reason for impaired gas exchange in the lungs is interstitial lung disease (ILD), a forerunner and companion of restrictive lung disease.

Even more compelling of a link between smoking and RLD, is the increase in the prevalence of (ILD) in smokers; the reason being ILD is a forerunner and companion of RDL. This is relatively new information which is the result of the recent trend of screening smokers for lung cancer with low-dose CT scans.  The scans have demonstrated different forms and degrees of ILD in those patients. The disease pattern is fibrotic (shows scar tissue in the lungs).  In other cases the pattern is one of other signs of ILD without scarring.  The nonfibrotic forms of ILD tend to improve and not progress to fibrosis with smoking cessation. The fibrotic forms have a much worse prognosis though regardless of a change in smoking.

It will require much research to make it official.  But it should come as no surprise if the future proves that tobacco is not just a common cause of restrictive lung disease outside the box.  But it is also a cause inside the box………of cigarettes. If so, it will be another among a host of reasons to stop or not start smoking.

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