One of the issues seen all too often with new Paramedics is missing that all too obvious in hind sight call when a patient presents with marked bradycardia and altered LOC. The first, and correct, differential diagnosis that pops in your head is symptomatic bradycardia. Better start a line and get an amp of atropine on board. But, this only correct if the bradycardia is the cause of the altered LOC and not a symptom of something else. What do we know of that could possibly cause these two signs? Check one more vital sign and the answer should be staring you in the stethoscope.
Remember it is called cushing’s triad for a reason. Not a very good reason as it turns out as there are in fact 4 associated signs caused by what is called cushing’s reflex. These 4 signs being 1) Bradycardia, 2)Hypertension, 3)Change in respiratory pattern (usually Cheyne-Stokes), 4) Hyperthermia.
If you have a basic understanding of why these signs occur it makes diagnosis much easier.
Remember that this condition can be caused by any event that causes the pressure in the cranial vault to increase (ICP). That is anything from trauma to a tumor.
With the increase in ICP the brain itself has only one way out of an intact cranial vault, the foramen magnum, and if you have ever looked at a skull you know that huge human brain is not coming out through that dinky little hole. Au contraire mon ami, it will come out, it just won’t work again once it gets free.
Now that we know what is happening let’s take a look at what is going on behind the scenes that is causing these externally measurable signs that we will later document with confidence as cushing’s triad.
Firstly as the ICP increases it gets harder for the systolic blood pressure to overcome the added resistance inside the brain. The oxygen sensors in the brain stem start saying “hey! not getting quite enough blood and oxygen up here”
In response the heart pumps a little harder (not faster) This brings the pressure up to the point where the brain is a bit happier with its supply of blood and tasty oxygen. This exchange will repeat itself continuously until the baroreceptors in the aortic arch notice that the blood pressure is getting a bit on the high side. If you think of these two sets of sensors as different branches of the body’s government you then realize that there is no way they are ever going to communicate with each other. One result of this lack of communication is that the aortic arch baroreceptors issue an order to slow the heart rate to reduce the blood pressure. As soon as this happens the brain stem starts yelling for more blood again so the BP rises in response. Now the aortic arch guys are turning the heart rate down even more in response to this latest increase in BP……and so it goes until the ICP starts to push the still intact brain through that dinky little hole, the foramen magnum. This is when the third and fourth signs start to appear. Housed in the bottom part of your brain is the breathing center and the thermoregulatory center and they do not like being squeezed like ripe tomatoes . It is not just the pressure of these centers being squeezed that cause the irregular breathing and hyperthermia , there is a whole ischemia thing going on now because of the squeezing, but for our purposes it is sufficient to accept that the physiologic changes are occurring secondary to the squeezing of the brain stem.
Paramedic Review 