A Description of Collateral Ventilation in Lung Disease

Recently, on another blog, there was a question where someone was asking for a good description of Collateral Ventilation in relation to lung disease. The following is the answer I gave. I have repeated it here in the hope that someone else may benefit from it.

Let’s see if I can help explain this for you. It may be a bit long-winded, so please bear with me.
In the lungs of a normal, healthy person, there is one single route inwards and out again for each of the alveoli (alveoli are the tiny cavities at the end of the airways where the gas exchange with the blood happens). Think in terms of a bunch of grapes. Alveoli are the grapes, the airways are the stalks.
When the lungs get damaged, whether through disease or pollution or irritation, if the membrane suffers extensive damage, then some of that membrane is replaced, as part of the healing process, with scar tissue. If there are repeated episodes of disease, or sustained exposure to pollution or other causes of irritation, then the amount of scarring can become significant.
Scar tissue is not as elastic as the original membrane. It also does not permit gas exchange. As our lungs expand and contract, if the sites where scarring has occurred have become a significant size, then some tearing can happen. This tearing is minute, and is no cause for concern in terms of day to day wear and tear. It is the long term combined effect over years that is the problem. This is where we now jump to, several year down the line. Imagine that two adjacent alveoli have a lot of scarring, and another inflaming infection takes hold, and one particular breath is deeper and heavier than normal, and it causes a tear that goes through the tissue between the alveoli. You now have a hole that will not close. The tissue will heal, but leaves an enlarged cavity because that requires less stretching than the original formation. For the bunch of grapes, two grapes have been replaced by one damson fed by two stalks. After several more years, a number of damsons have formed, and some of those have merged into much larger plums. This is now advanced Emphysema, with some large cavities (called bullae), and we are at the point where medical intervention is required. One of the things that has to be considered to determine which operation is best to go for is how this damage presents itself.
The structure of the lungs is that they are divided into zones called lobes, three in the right, and two in the left. Thinking in terms of the bunches of grapes, consider that each lobe is home to one complete bunch whose only contact normally is through the main stem (the main airway). If the damage within the lungs is confined within the individual bunches, in other words, does not cross the boundaries between the lobes, then there is no colateral ventilation. Collateral ventilation occurs when the the tissue damage permits the passage of air between the lobes through holes between the lobes.
As the damage progresses, as the cavities form, so the internal support structure of the lungs gets reduced. This allows the lungs to become longer and they over inflate because of the loss of elasticity. They sit on the diaphragm, the bottom lobe of each lung gets compressed and generally cannot continue to work properly. The diaphragm now has to lift this extra weight with each breath taken. Through a day, that adds up to a lot of extra hard work. Taken with the loss of alveoli, and the presence of frequent infections etc, the lungs are now operating at maybe as low as 15% of their full capability when in good condition. The patient is permanently fatigued and needs lots of medication to keep the airways open. Often oxygen is needed. At this point, the consultant decides that surgical intervention is required. The first choice at the moment for most consultants is to use pulmonary valves. They are easy to fit, they allow the blocked off part of the lung to continue to pass CO2 and the lungs natural secretions out. Most importantly, they are reversible (if need be, they can be removed easily). The biggest deciding factor in choice for or against valves is whether or not there is colateral ventilation. If there is none, or is very minor, then valves could be considered. If there is colateral ventilation, then the usual decision is to go for lung reduction surgery.
As you can see, at the stage where there needs to be a choice made, the presence or not of colateral ventilation is important. Links to some videos that may help follow:

http://pulmonx.com/en/downloads/videos/zephyr-videos/zephyr-animation/

http://pulmonx.com/en/downloads/videos/chartis-videos/collateral-ventilation/

breathe easy, everyone.

John