Physiology and patho-physiology
The two most flow dependent substances which maintain CNS function and vitality are oxygen and glucose. Around 98-99% of metabolic activity in the CNS is fuelled by glucose.
Cerebral oxygen delivery relies on arterial inflow which comprises DO2 and MAP (see Assessment tutorial). In addition the intracranial pressure serves as a resistance to cerebral blood inflow so must be considered when assessing and treating. The cerebral perfusion pressure (CPP) is the difference between mean arterial blood pressure (MAP) and intra-cranial pressure (ICP). Anatomically the intra-cranial contents (brain and associated tissues, blood both venous and arterial and CSF) contribute significantly to ICP.
- CPP = MAP – ICP
- Normal CPP = MAP 80mmHg – ICP 10mmHg = 70mmHg
Intra-cranial pressure/volume relationship
As the CNS structures are located in a bony container (for protection) an increase in the volume of any of them will cause a rise in ICP. There comes a time when the rise in pressure for small rises in volume is very acute. This critical point is highlighted on Figure 1. The patient may arrive at this point relatively slowly with sub-acute or chronic conditions such as tumours or abscesses. However in acute processes such as haemorrhage or trauma-induced oedema progress to this stage can be alarmingly fast. Once the relationship is on the steep part of the curve intervention must occur immediately to reduce ICP (whilst maintaining MAP) as the brain will suffer ischaemic damage and the potential for tentorial and/or brain stem herniation is enormous. You will find out how to deal with this and potentially rescue the situation later.
Figure 1. ICP/volume curve.