Survey of cerebral perfusion pressure measurement: location of the arterial transducer in the patient managed at 30° elevation

Introduction

In 2003 the Brain Trauma Foundation (BTF) published an update notice to its guidelines on the management of severe head injury in adults, focusing on cerebral perfusion pressure (CPP), this being the physiologic variable that drives cerebral blood flow. Guidance suggests that CPP should be maintained at a minimum of 60 mmHg.

Clinical practice in the United Kingdom may involve nursing the severely head-injured patient at an angle of 30°, although there is no consensus as to whether patients should be nursed flat or with their head up to 30°.

The location of the arterial transducer will influence the measured MAP and hence the CPP (CPP = MAP - ICP). This survey was conducted to establish current practice in measuring MAP, and to ascertain when nursing a patient at 30° whether the arterial transducer was referenced to the level of the external auditory meatus or the left atrium.

Methods

A telephone survey was conducted of the 28 UK neuroscience ICUs. The nurse in charge was identified and asked, 'When measuring CPP on neurosurgical patients nursed at 30°, is the arterial transducer placed at the level of the head or the heart?'

Results

The results suggest differing practice within the United Kingdom. Twenty units positioned the transducer at the level of the left atrium while six units used the level of the external auditory meatus. One unit sited the transducer at the wrist (no extension line used), and in one unit the practice varied depending upon which consultant was on duty. Variation in practice was evident between dedicated neurosurgical as well as general ICUs.

Discussion

At 30° above the horizontal, the hydrostatic effect may reduce MAP in the head by up to 10 mmHg. The associated fall in ICP has been shown to be around 3 mmHg. The measured CPP would therefore be 7 mmHg lower if the transducer is at the level of the head as compared with the heart. This discrepancy is of fundamental importance as McGraw's model directly relates outcome to the measured CPP. The BTF recommends a minimum target CPP of 60 mmHg but makes no allowance for posture. In the clinical setting when nursing a patient at 30° with the arterial transducer at the level of the heart, a target CPP of 60 mmHg may result in CPP within the great vessels of the head below the BTF recommendation.

Without a common approach it is difficult to discuss the ideal target CPP. Location of the arterial transducer needs to be clarified as it may result in an outcome difference dependent upon practice, and multicentre trials require a standardised approach. Until this is addressed authors should state the level at which the arterial transducer is referenced in the measurement of CPP.