The standard explanation I’ve seen for the danger of introducing air into the human circulatory system is that “it blocks blood flow.” Either this is wrong or something’s missing, because an air bubble in a blood vessel should move along just like the blood in the blood vessel does; the air doesn’t have any better grip on the walls than blood does. The blood behind the bubble is pushed forward by the heart, and that blood pushes the air bubble forward, and the air pushes the blood in front of it forward. I can certainly imagine that if you get a “bubble” big enough so that you’ve got air flowing through large swaths of your brain you might have a very bad day, but the hazard always seems to be ascribed to much smaller quantities of air than that.
I am aware that centrifugal pumps (e.g. an automotive coolant pump) and positive-displacement pumps with really awful seals need to be primed before they are able to properly move the working fluid along. The heart appears to be a positive-displacement pump, so the only way air could present a hazard is if the heart valves are extremely leaky, making it difficult to build up pressure when filled with a low-density fluid (air). In a normal human being (i.e. someone who is not a candidate for valve replacement), are the heart valves really that leaky?
The air bubble can compress under pressure of the heart beating and thus not transmit the same pressure forward. Blood, being mostly liquid, is essentially non-compressible and thus transmits more readily maintains circulatory pressure.
Do you remember the image linked to in my post (#8) in this old thread on the same subject? Even a 60 cc bubble wasn’t too much for that 82-year-old guy. Wonderful pictures.
Thank you! Yes, I was on my phone, which I blame for both the typo and the not being able to search for that thread. I was hoping someone would find it.
On review, I probl’y shoulda just kept quiet, as it’s been way too long a day and I can’t brain anymore.
I can certainly imagine that if you get a “bubble” big enough so that you’ve got air flowing through large swaths of your brain you might have a very bad day, but the hazard always seems to be ascribed to much smaller quantities of air than that. *
How big of a bubble is needed to block a capillary? I don’t know, but it seems like it might be an important consideration since air in an artery will eventually be pushed into a capillary. Air in a vein will eventually be pushed into the heart and then the lungs, where all the vessels become capillaries again. So I’m thinking little bubbles might ride along for a while, but eventually things are going to seriously squeeze down to 5 or 10 microns.
Typical blood pressure is around 100 millimeters of mercury, or 1.9 psi. That’s measured relative to ambient atmospheric pressure, typically about 14 psi. Assuming the worst case, blood pressure at the vena cava is exactly 14.7 psi, and at the aorta, the pressure is 15.9 psi. Under these conditions, an air bubble passing through the heart would be reduced to about 92% of its initial volume. In order for the heart to stop pumping blood (because it’s just squeezing the same bubble of air over and over again), the heart’s chambers would have to change volume by only that much; if they contracted much more than that, they would squeeze at least some of the air bubble out to the aorta and bring at least some fresh blood in on the next intake event.
So there’s a relevant question for the nearest doctor: by what percentage to the heart’s chambers change in volume during its cycle? Note that this is not just the volume expelled during each heart beat: we need to know what that volume is in comparison to the volume of each heart chamber.
So who’s got the numbers?
Air lock is associated with centrifugal pumps that generate barely enough head to keep the working fluid moving, and can’t generate enough head to lift the working fluid up to the highest point in the piping (where the air is) without having a pressure assist from a comparably tall column of fluid at the pump’s inlet. So for air lock to be a problem, it seems to me that the heart would have to be incapable of pumping blood up to the head without a “suction assist” from the veins, which implies that under normal circumstances there would be a vacuum condition in the veins of the head. I don’t believe this is the case, but if I’m wrong, I’m listening.
For the left ventricle, ejection fraction is usually 50-65%. For air embolism, we are more concerned with the right ventricle; I don’t know the figure for this, but I think it is similar (perhaps a bit lower).
There have been rare reports of large air embolism rising to the head and causing strikes by impairing venous drainage in cerebral veins and venous sinuses.
As noted before, even a small air embolism that gets to the sytemic arterial side (such as through an atrial or ventricular septal defect or through an arteriovenous shunt in the lungs) can cause strikes and heart attacks (and infarctions in other more tolerant organs). I don’t like to see people get *too *casual about air in the venous line.
But mostly a little air in a vein is no big deal. When I was a kid, I was very impressed by this undetectable killing method, used by television and movie murderers. (I think this method was considered in ‘Murder on the NIle’ and a variant was attempted on the TV show, “UFO”.) Now I just smirk.
Cerebral blood vessels are able to change the flow of blood through them by altering their diameters in a process called autoregulation…
Perhaps the air embolism interferes wth auto-regulation, the system is quite complex…
The air may even kill cells in/near the arteries and veins, by starving them of sugar,food,or simply oxygen.
