Bronchitis means inflammation of the bronchial tubes in the lung, and it’s said to be chronic
when it causes a productive cough—which means produces mucus—for at least 3 months
each year for 2 or more years.
Chronic bronchitis is actually lumped under the umbrella of chronic obstructive pulmonary
disease (or COPD), along with emphysema.
These two are different in that chronic bronchitis is defined by clinical features, like a productive
cough, whereas emphysema is defined by structural changes—specifically enlargement of the
air spaces.
That being said, they often coexist, probably because they share the same major risk factor
— smoking.
Other risk factors for chronic bronchitis include exposure to air pollutants like sulfur
and nitrogen dioxide, exposure to dust and silica, as well as genetic factors like having
a family history of chronic bronchitis.
With COPD, the airways become obstructed, and the lungs don’t empty properly, and
that leaves air trapped inside the lungs.
For that reason, the maximum amount of air people with COPD can breath out in a single
breath, known as the FVC, or forced vital capacity, is lower.
This reduction is especially noticeable in the first second of air breathed out in a
single breath, called FEV1—forced expiratory volume (in one second), which typically is
reduced even more than the FVC.
A useful metric therefore is the FEV1 to FVC ratio, which, since the FEV1 goes down even
more than FVC, causes the FEV1 to FVC ratio to go down as well.
Alright so say normally your FVC is 5 L, and your FEV1 is 4 L, your FEV1 to FVC ratio would
end up being 80%.
Now, someone with COPD’s FVC might be 4 L instead, which is lower than normal, but
the volume of air that he or she can expire in the first second is only 2 L, so not only
are both these values lower, but their ratio is lower as well—and this is a hallmark
of COPD.
All that had to do with air breathed out right?
Conversely, for air going in, the TLC, or total lung capacity, which is the maximum
volume of air that can be taken in or inspired into the lungs, is actually often often higher
because of the air trapping.
Alright, so chronic bronchitis is a type of COPD that’s diagnosed based on clinical
symptoms, specifically coughing up a lot of mucus.
But why does this happen?
Well, first off, in the lungs, the walls of normal airways have a couple layers to think
about.
Lining the lumen of the airways you’ve got the epithelium, composed of ciliated pseudostratified
columnar epithelial cells, which are named that because these epithelial cells have hair-like
projections called cilia, their nuclei don’t align so it looks like they’re more than
one layer even though they’re not, hence, pseudostratified, and because the cells are
mostly tall and narrow - or columnar in shape.
This layer also contains the occasional Goblet cell which makes some of the mucus that lines
the airway.
Going deeper past that layer you’ve got the basement membrane and loose connective
tissue, called the lamina propria—which together with the epithelium makes up the
mucosa.
Beyond the mucosa, there is smooth muscle followed by more connective tissue, and together,
these two layers make up the submucosa and this is where the bronchial mucinous glands
live.
These are the glands that secrete the majority of the mucus into the lumen of the bronchi,
helping to catch and filter out particles and pathogens.
Finally, in the bronchi, but not the bronchioles, there is also a layer of cartilage below the
submucosa which stiffens the bronchus and helps to keep it open.
Alright so people who smoke expose their airways to all sorts of irritants and chemicals.
Whatever the irritants are, their effect is to stimulate hypertrophy and hyperplasia of
the mucinous glands in the main bronchi, as well as the goblet cells in the smaller airways
- the bronchioles, which increases mucus production in both locations.
Since the bronchioles are smaller, even a slight increase in mucus can lead to airway
obstruction, so this contributes to the majority of the air trapping.
To make matters worse, though, smoking makes the cilia short and less mobile, making it
harder to move mucus up and out of bronchioles towards the back of the throat to get swallowed.
As a result of having too much mucus and poorly functioning cilia, people with chronic bronchitis
end up relying on coughing to get rid of their mucus plugs.
One measurement, typically done post-mortem, is called the Reid index, which is the ratio
of the thickness of the bronchial mucinous glands, relative to the total thickness of
the airway - from the epithelium to the cartilage.
Normally, this ratio should be less than 40%, but it can be over 40% for people with chronic
bronchitis, because of the hyperplasia and hypertrophy of the glands.
Even though an increased Reid index goes along with chronic bronchitis, the diagnosis is
still done clinically and this measurement is not usually used diagnostically.
All this mucus in the lungs causes people with chronic bronchitis to wheeze
due to narrowing of the passageway available for air to move in and out, these people also
have crackles or rales caused by the popping open of small airways.
People with chronic bronchitis also often present with hypoxemia, low oxygen in the
blood, and hypercapnia, increased carbon dioxide in the blood.
This is because the mucus plugs in the airways block air flow, right?
Which causes the partial pressure of CO2 to increase in the lungs.
Increased PCO2 means that the partial pressure of O2 in the lungs goes down, and a lower
PO2 means less oxygen gets to the blood, causing hypoxemia.
This trapped CO2 in the lungs also makes it harder for CO2 to get out of the bloodstream,
which also explains the hypercapnia.
The increased CO2 levels in the blood can get so bad that some people develop cyanosis,
which is a blue discoloration of the skin, and this is why patients with chronic bronchitis
are sometimes referred to as blue bloaters.
This is compared to the term pink puffers which describes patients with emphysema.
Alright so in addition to those things, in the the areas with decreased gas exchange,
blood vessels undergo vasoconstriction in an attempt to shunt blood to an area with
better exchange; which, if it’s localized to one area of the lungs, that would work
pretty well, but when a large proportion of the lungs aren’t exchanging oxygen effectively,
a large proportion of blood vessels start to clamp down.
And this has the effect of increasing pulmonary vascular resistance, and to maintain pulmonary
blood flow the body responds by developing pulmonary hypertension.
Over time, this increases the work needed by the right side of the heart to pump blood
to the lungs, and eventually the right side enlarges, leading to right-sided heart failure,
a process called cor pulmonale.
And finally another consequence of mucus plugging in chronic bronchitis is that people can develop
lung infections behind the mucus blockages in the airway, and these infections can worsen
the pulmonary and cardiac symptoms.
Treatment of chronic bronchitis largely involves reducing risk factors, like for example stopping
smoking, but also managing associated illnesses.
Supplemental oxygen, as well as certain medications like bronchodilators, inhaled steroids, and
antibiotics to control secondary infections might also be used.
Okay, to recap, chronic bronchitis is a type of chronic obstructive pulmonary disease or
COPD, where exposure to chemicals and irritants—like with smoking—stimulates increased mucus
production in the airways, which causes a productive cough that lasts for at least 3
months each year for at least 2 years.
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