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. Thanks for watching, you can help support
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