Indices to quantify changes in intracranial and cerebral perfusion pressure by assessing agreement between hourly and semi-continuous recordings

Objectives: Little published data exists on whether nurse-recorded end-hour values of intracranial pressure (ICP) and cerebral perfusion pressure (CPP) are representative of continuous monitoring during the hour. There is also no standard method of quantifying the observed perturbations in cerebral hemodynamics. This study compared the level of agreement between end-hour values and computer downloaded observations of ICP and CPP at 15-min intervals. We also developed the intracranial hypertension index and the cerebral hypoperfusion index to quantify perturbations in cerebral hemodynamics. Each of these indices relates the number of abnormal observations to the total number of observations taken. Methods: Prospective, non-interventional study. Results: The bias and precision between the two methods for ICP and CPP were -0.002+/-2.6 mmHg and -1.1+/-6.2 mmHg, respectively. A strong correlation existed between the hourly mean calculated from the 15-min and the end-hour values for both ICP (r(2)=0.95, p<0.0001) and CPP (r(2)=0.78, p<0.001). The intracranial hypertension index was 40% from the 15-min measurements and 41% from the hourly observations (p=NS). The cerebral hypoperfusion indices were 13.4% and 13.1% with the 15-min and end-hour values, respectively (p=NS). Conclusions: The end-hour values of ICP and CPP are as accurate as more frequent measurements during the hour and are adequate for purposes of epidemiological research and medico-legal audit. The intracranial hypertension and cerebral hypoperfusion indices may be useful in describing cerebral hemodynamics for future interventional studies and for assessing quality in the delivery of neuro-critical care.