Prospective Study on Noninvasive Assessment of Intracranial Pressure in Traumatic Brain-Injured Patients: Comparison of Four Methods

Abstract

Elevation of intracranial pressure (ICP) may occur in many diseases, and therefore the ability to measure it noninvasively would be useful. Flow velocity signals from transcranial Doppler (TCD) have been used to estimate ICP; however, the relative accuracy of these methods is unclear. This study aimed to compare four previously described TCD-based methods with directly measured ICP in a prospective cohort of traumatic brain-injured patients. Noninvasive ICP (nICP) was obtained using the following methods: 1) a mathematical "black-box" model based on interaction between TCD and arterial blood pressure (nICP_BB); 2) based on diastolic flow velocity (nICP_FVd); 3) based on critical closing pressure (nICP_CrCP); and 4) based on TCD-derived pulsatility index (nICP_PI). In time domain, for recordings including spontaneous changes in ICP greater than 7 mm Hg, nICP_PI showed the best correlation with measured ICP (R = 0.61). Considering every TCD recording as an independent event, nICP_BB generally showed to be the best estimator of measured ICP (R = 0.39; p < 0.05; 95% confidence interval [CI] = 9.94 mm Hg; area under the curve [AUC] = 0.66; p < 0.05). For nICP_FVd, although it presented similar correlation coefficient to nICP_BB and marginally better AUC (0.70; p < 0.05), it demonstrated a greater 95% CI for prediction of ICP (14.62 mm Hg). nICP_CrCP presented a moderate correlation coefficient (R = 0.35; p < 0.05) and similar 95% CI to nICP_BB (9.19 mm Hg), but failed to distinguish between normal and raised ICP (AUC = 0.64; p > 0.05). nICP_PI was not related to measured ICP using any of the above statistical indicators. We also introduced a new estimator (nICP_Av) based on the average of three methods (nICP_BB, nICP_FVd, and nICP_CrCP), which overall presented improved statistical indicators (R = 0.47; p < 0.05; 95% CI = 9.17 mm Hg; AUC = 0.73; p < 0.05). nICP_PI appeared to reflect changes in ICP in time most accurately. nICP_BB was the best estimator for ICP "as a number." nICP_Av demonstrated to improve the accuracy of measured ICP estimation.

Keywords: noninvasive ICP monitoring; transcranial Doppler; traumatic brain injury.

FIG. 1.

Schematic representation of the black-box model for nICP estimation. A known transfer function between ABP and FV alongside modification (transcranial Doppler ultrasonography) characteristics is used as a means to continuously find a relationship between ABP and nICP transformations (unknown transfer function). ABP, arterial blood pressure; FV, flow velocity; nICP, noninvasive assessment of intracranial pressure.

FIG. 2.

Systolic and diastolic flow velocities behaviour during a drop of cerebral perfusion pressure. Low FVd component in this case indicates inadequate CPP (resulting from plateau increase of intracranial pressure). CPP, cerebral perfusion pressure; FVs, systolic flow velocity; FVd, diastolic flow velocity.

FIG. 3.

Representation of the CrCP interaction with ICP and WT in a situation of intracranial hypertension (plateau increase of ICP). ABP, arterial blood pressure; CrCP, critical closing pressure; ICP, intracranial pressure; WT, vascular wall tension.

FIG. 4.

Representation of the flow velocity components (FVs, FVd, and FVm) used for pulsatility index calculation. CBF, cerebral blood flow; FVs, systolic flow velocity; FVd, diastolic flow velocity; FVm, mean flow velocity.

FIG. 5.

Example of recording of nICP with four investigated methods in a case when ICP changed considerably. (A) ICP; (B) nICP_BB; (C) nICP_FVd; (D) nICP_CrCP; (E) nICP_PI. ICP, intracranial pressure; nICP, noninvasive assessment of ICP; BB, black box; FVd, diastolic flow velocity; CrCP, critical closing pressure; PI, pulsatility index.

FIG. 6.

Linear regressions and Pearson's correlation coefficients with p values for nICP_BB (A), nICP_FVd (B), nICP_CrCP (C), and nICP_PI (D). ICP, intracranial pressure; nICP, noninvasive assessment of ICP; BB, black box; FVd, diastolic flow velocity; CrCP, critical closing pressure; PI, pulsatility index.

FIG. 7.

Bland-Altman and error histograms plots showing bias and 95% CI for prediction of ICP for nICP_BB (A), nICP_FVd (B), nICP_CrCP (C), and nICP_PI (D). ICP, intracranial pressure; nICP, noninvasive assessment of ICP; BB, black box; FVd, diastolic flow velocity; CrCP, critical closing pressure; PI, pulsatility index; CI, confidence interval.

FIG. 8.

Receiver operating characteristic curve showing AUC for averaged nICP estimator (nICP_Av). AUC, area under the curve; nICP, noninvasive assessment of intracranial pressure.

FIG. 9.

Example of transcranial Doppler ultrasonography recording in which it is possible to observe considerable presence of noise (time series) and depleted signal resolution (zoom in). CBF, cerebral blood flow.