The Evolutionary Onion

©2021 Jeffrey E. Isaaac, PA-C

An earlier newsletter discussed one of our general concepts in wilderness medicine called "Ideal to Real". It generated a lot comment and interest, so we'll try another.

The Evolutionary Onion is an analogy created by Dr. Peter Goth to explain how central nervous system (brain) function reflects the quality of perfusion and oxygenation, that is; ability of the circulatory and respiratory systems to perfuse vital organs with oxygenated blood. It is a simple tool that can help you detect a serious medical problem early enough to do something about it.

To review your seventh grade science…the function of the respiratory system is to supply outside air to the alveoli of the lungs where it comes into close proximity to the circulating blood. This allows oxygen from the air to diffuse into the blood stream and excess carbon dioxide to diffuse from the blood plasma into the air. Adequate oxygenation requires adequate respiration.

The function of the circulatory system is to force blood through the capillary beds in all body tissues, including the lung where oxygenation occurs. This is called "perfusion", and it serves to bring blood into close proximity to individual cells to allow oxygen, nutrients, and waste to diffuse between the cells and the blood stream. Considerable pressure is required to overcome the natural resistance to flow. We routinely measure this perfusion pressure as arterial "blood pressure".

All living tissue must be continuously supplied with oxygenated blood to maintain metabolism and function. Anything that interferes with this is a serious problem. The preservation of perfusion and oxygenation is the major goal of emergency medical care.

Since nervous system tissue is exquisitely sensitive to oxygen deprivation, the brain will exhibit the earliest signs of a developing problem with perfusion and oxygenation. The severity of the symptoms relates well to the severity of the problem. We measure these effects by evaluating level of consciousness and mental status.

Picture the brain as a sort of onion with increasingly complex layers of function from inside out. The basic automatic functions such as vasoconstriction, heart rate, and the control of consciousness extend from the deeper, more primitive layers in the brain stem. The outer layers of the cerebral cortex control higher functions such as personality, judgment, and problem solving.

These outer layers are also the first to be affected when problems develop with perfusion and oxygenation. The earliest vital sign changes seen are the beginning stages of peeling the onion. We call this "altered mental status".

Early mental status changes can be very subtle. A crewmember that is normally cooperative and pleasant may become grumpy and irritable. A normally grumpy and irritable crewmember may become pleasant and cooperative. In spite of the tremendous temptation to leave him that way, this change in mental status must be investigated.

As a problem progresses, patients may remain conscious and alert but become anxious, uncooperative, or respond in ways that don't fit the situation. They may act intoxicated, belligerent, or very confused.

More extreme injury affects the deeper layers of the brain causing a decrease in level of consciousness. When the onion has peeled this far, the situation has become desperate. The progression can also be reversed if the underlying problems with perfusion and oxygenation are corrected.

So, when you’re trying to decide if your foredeck ape is just cold or truly hypothermic, check his brain function. Not acting normally? He’s in trouble!

Is your mate with the bad cough in serious condition? If she’s anxious, irritable, and distracted when she’s usually right on top of her navigation, you’d better take a closer look at her illness.

Does your child with the fever of 103F need to go to the hospital? If he’s running around the deck, swinging from the rig, and destroying delicate electronics just like he always does, he’s probably fine for now.

Certainly mental status is not the only indicator. We look at all six vital signs and the patient exam when making an assessment. But knowing just how sensitive an indicator the brain can be gives you an early warning system that’s easy to monitor from a distance. In the wilderness or offshore setting, consciousness and mental status offer the most reliable field measurement of perfusion and oxygenation.