Reliability of the Blood Flow Velocity Pulsatility Index for Assessment of Intracranial and Cerebral Perfusion Pressures in Head-Injured Patients

Aim: It is generally accepted that a reliable noninvasive tool for screening for evidence of increased intracranial pressure (ICP) and lowered cerebral perfusion pressure (CPP) would be highly valued in clinical practice. It has been postulated that Gosling pulsatility index (PI) assessed with transcranial Doppler (TCD) has a diagnostic value for noninvasive estimation of ICP and CPP. Our aim was to revisit this hypothesis using a database of digitally stored signals from a cohort of head-injured patients.

Methods: We analyzed prospectively collected data of patients admitted to the Cambridge Neuroscience critical care unit who had continuous recordings of ABP, ICP, and cerebral blood flow velocities (FV) using TCD. PI was calculated (Fvsys-Fvdia/Fvmean) over each recording session. Statistical analysis was performed using Spearman rank correlation, Receiver-Operator-Characteristics (ROC) methods and modeling of a nonlinear PI-ICP/CPP graph.

Results: Total of 762 recorded daily sessions from 290 patients were analyzed with a total recording time of 499.9 hours. The correlation between PI and ICP was 0.31 (p<0.P0561) and for PI and CPP -0.41 (p<0.P0561). The 95% prediction interval of ICP values by a given PI from the mean is more than 15 mm Hg and for CPP more than 25 mm Hg. The diagnostic value of PI to assess ICP, area under curve (AUC) ranges from 0.62 (ICP>15 mm Hg) to 0.74 (ICP>35 mm Hg). For CPP, the AUC ranges from 0.68 (CCP<70 mm Hg) to 0.81 (CCP<50 mm Hg). Distributions of the probability of elevated ICP/lowered CPP depending on PI are shown. No association with of PI and outcome was found.

Conclusion: The diagnostic value of TCD FV pulsatility index for direct noninvasive assessment of ICP and CPP is limited, especially in the ICP and CPP ranges where they start to become critical. However, its measurement can be readily and repeatedly performed, it can still potentially be used as an additional screening tool in case of uncertainty, whether a patient requires invasive ICP monitoring or not. But just in the case, when PI shows very low or very high values.

Acknowledgments We thank all people, including nursing staff, who participated in the head injury monitoring program between 1991 and 2009. This work was supported by the Swiss National Science Foundation (PBBSP3-125550 to CZ and PASSMP3-124262 to EC), Bern, Switzerland.