Do not give up hope
I am sorry that your Dad is having so many problems, but as I read this, I thought–What a fighter he is!!!! He/his body is trying so hard.
People are amazing in how beautifully bodies balance and work even after so many decades of life’s challenges. However, once one problem hits, it is harder to balance the rest and you can start seeing more of this type of health oscillation where it becomes so difficult to get back to “stable” and the balance is tenuous. When someone has a bad infection, the body tries to fight it and there is an intense inflammatory response. this can make inflammatory diseases (like GBS/CIDP) worse. The body sends out as many white blood cells as possible to fight the infection and the lack of reserves then makes the WBC low and the body more susceptible for a while. Third spacing is when fluid leaks outside of the blood vessels and this frequently happens with sepsis (even when the body acts like there is a bad infection, but no positive cultures for infection happen). The leaking is into the extremities (edema =swelling and puffiness) or into the lungs (as seen by the need for a chest tube to drain this “extra” fluid) or anywhere else or diffusely. All this extra fluid can also overload the heart and cause congestive heart failure. Whenever someone drops their blood pressure very low, doctors give fluids or “pressors” (medicines to recover the blood pressure–think of it as medicines to cause the blood vessel walls to press together to keep up the blood pressure) to bring it back up because the body needs the blood pressure to be high enough to make blood go especially to the very important brain. Kidneys are also very sensitive to low blood pressure and if the kidneys stop working as well, it is harder to get rid of the extra fluid after the body begins to stabilize. This fluid builds up in the lungs (which is dangerous), annoys the heart, swells the legs (which predisposed to clotting), etc. It is a “domino effect” where one thing leads to another that leads to another. It gets harder and harder for a body to re-balance it all. As the others have said, often the autonomic system that controls blood pressure and heart rate/function does not work normally in GBS/CIDP. This also makes it more likely to have greater swings from side to side and more challenges to re-stabilize. As well, it is harder to treat an infection if you do not know what it is and not treating well enough (because of this lack of knowing exactly what or how much is needed because one does not know the exact cause of the infection–bacteria type or place) can lead to relapse of the infection or to development of a second “resistant” infection. From what you have said about this being an abrupt onset after he was doing better, it sounds like it had to be treated as sepsis as it was. It also sounds like they and he are fighting hard to rebalance and get over this. Please expect that it will be slow. When there is a lot of serious backsteps in progress to recovery, often the approach is to try a slow and steady path rather than a quick one because it is far better not to have another significant step back. Nature (as a word for the awesome design of God) is the greatest healer. As a person who works in healthcare, I am so awed by this and know that in people as strong as your Dad, people in healthcare just are helping hands in this process. It may be flustrating that progress is slow or that there is no magic repair, but do not give up hope.
It is late and I may not be clear, but the main points again are that your Dad is trying hard, that although it is really hard to rebalance he has on his very positive side both that he was previously healthy and that he had rebalanced after the previous setbacks (he has resilience). Our thoughts are with you.
With Hope for cure of these diseases.