Maintaining good cerebral perfusion pressure often times gets under-valued in the pre-hospital setting.
Studies have shown that even one incidence of a systolic pressure less than 90 can double the mortality rate in a head injured patient with increasing intra-cranial pressure. While watching the systolic pressure is acceptable, MAP is a more informed way to maintain adequate Cerebral Perfusion Pressure (CPP).
The value of the CPP is derived from the MAP-ICP. Thus, MAP is likely the better tool to use when trying to maintain an adequate blood pressure for brain perfusion.
We cannot know the ICP value in the field (unless we’re transferring someone with an ICP monitoring device). However, we can estimate it based on the clinical presentation of the patient. If head-injured patients begin to show clinical signs of ICP, we have to assume that it is greater than 15. The CPP goal for a head-injured/increasing ICP patient is 70 to 90. But, keep in mind that the mortality rate increases by about 20% for each 10mm drop.
In most other clinical situations, a MAP of 65 is acceptable, even desirable; however, with increasing ICP, such a value would not be sufficient. For example, let’s take a MAP of 65 and subtract an ICP of 15 (high end of the norm). That would leave us with a Cerebral Perfusion Pressure of only 50. This is well below the desired range of 70 to 90. If we suspect that our patient has increasing ICP, then that value is likely going to be above 15 already. So now we are looking at an even lower CPP.
This again increases mortality rate exponentially. It is a fine balancing act though, especially if we have an exsanguinated patient. We don’t want to wash out all their hemoglobin, but at the same time we’ll need to maintain the CPP.
Keeping patients breathing en route to the hospital
We also have to remember that a Cerebral Perfusion Pressure higher than 70 increases the risk of ARDS. This is all still a subject of much debate. Generally speaking, though, the Brain Trauma Foundation recommends maintaining optimal CPP as the focus in a TBI patient.
We can also help maintain CPP by utilizing ‘relative’ hyperventilation, in that our goal is an ETCO2 reading of 30-35 as opposed to the norm of 35 to 45. In the event that end-tidal monitoring is not possible, ventilating the patient on the higher end of 12-20 respirations per minute works as well. Ultimately, a little diesel therapy is what the patient really needs. Get them to definitive care as soon as possible. Maintaining good blood flow to the brain on the way, however, is a huge plus in helping them recover fully.