✓Many doctors involved in the critical care of head-injured patients understand intracranial pressure (ICP) as a number, characterizing the state of the brain pressure–volume relationships. However, the dynamics of ICP, its waveform, and secondarily derived indices portray useful information about brain homeostasis. There is circumstantial evidence that this information can be used to modify and optimize patients' treatment. Secondary variables, such as pulse amplitude and the magnitude of slow waves, index of compensatory reserve, and pressure–reactivity index (PRx), look promising in clinical practice. The optimal cerebral perfusion pressure (CPP) derived using the PRx is a new concept that may help to avoid excessive use of vasopressors in CPP-oriented therapy. However, the use of secondary ICP indices remains to be confirmed in clinical trials.
Marek Czosnyka, Peter Smielewski, Ivan Timofeev, Andrea Lavinio, Eric Guazzo, Peter Hutchinson, and John D. Pickard
Andrea Lavinio, Sally Harding, Floor Van Der Boogaard, Marek Czosnyka, Peter Smielewski, Hugh K. Richards, John D. Pickard, and Zofia H. Czosnyka
Exposing patients with ventricular shunts to magnetic fields and MR imaging procedures poses a significant risk of unintentional changes in shunt settings. Shunt valves can also generate considerable imaging artifacts. The purpose of this study was to determine the magnetic field safety and MR imaging compatibility of 5 adjustable models of hydrocephalus shunts.
The Codman Hakim (regular and with SiphonGuard), Miethke ProGAV, Medtronic Strata, Sophysa Sophy and Polaris programmable valves were tested in a low-intensity magnetic field, and then translational attraction (TA), magnetic torque (MT), and volume of artifacts on T1-weighted spin echo (SE) and gradient echo (GE) pulse sequences in a 3-T MR imaging unit were measured.
The ProGAV and Polaris valves were immune to unintentional reprogramming by magnetic fields up to 3 T. Other valves randomly changed settings, starting from the intensity of field: Sophy valve 24 mT, Strata valve 30 mT, and both Codman Hakim programmable valves from 42 mT. Shunt performances in the 3-T MR imaging unit are reported in the order of compatibility: 1) Codman Hakim regular, TA = 0.005 N, MT = 0.000 Nm, GE = 30 cm3, SE = 2 cm3; 2) Miethke ProGAV, TA = 0.001 N, MT = 1.4 × 10−3 Nm, GE = 231 cm3, SE = 13 cm3; 3) Codman Hakim with SiphonGuard, TA = 0.005 N, MT = 2.3 × 10−3 Nm, GE = 233 cm3, SE = 19 cm3; 4) Medtronic Strata, TA = 0.27 N, MT = 18.0 × 10−3 Nm, GE = 484 cm3, SE = 86 cm3; 5) Sophysa Sophy, TA = 0.82 N, MT = 38.9 × 10−3 Nm, GE = 758 cm3, SE = 72 cm3; and 6) Sophysa Polaris, TA = 0.80 N, MT = 39.6 × 10−3 Nm, GE = 954 cm3, SE = 100 cm3.
All valves, with the exception of the Polaris and ProGAV models, are prone to unintentional reprogramming when exposed to heterogeneous magnetic fields stronger than 40 mT. All tested valves can be considered safe for 3-T MR imaging. All valves generated a distortion of the MR image, especially the GE sequences.
Christian Zweifel, Andrea Lavinio, Luzius A. Steiner, Danila Radolovich, Peter Smielewski, Ivan Timofeev, Magdalena Hiler, Marcella Balestreri, Peter J. Kirkpatrick, John D. Pickard, Peter Hutchinson, and Marek Czosnyka
Cerebrovascular pressure reactivity is the ability of cerebral vessels to respond to changes in transmural pressure. A cerebrovascular pressure reactivity index (PRx) can be determined as the moving correlation coefficient between mean intracranial pressure (ICP) and mean arterial blood pressure.
The authors analyzed a database consisting of 398 patients with head injuries who underwent continuous monitoring of cerebrovascular pressure reactivity. In 298 patients, the PRx was compared with a transcranial Doppler ultrasonography assessment of cerebrovascular autoregulation (the mean index [Mx]), in 17 patients with the PET–assessed static rate of autoregulation, and in 22 patients with the cerebral metabolic rate for O2. Patient outcome was assessed 6 months after injury.
There was a positive and significant association between the PRx and Mx (R2 = 0.36, p < 0.001) and with the static rate of autoregulation (R2 = 0.31, p = 0.02). A PRx > 0.35 was associated with a high mortality rate (> 50%). The PRx showed significant deterioration in refractory intracranial hypertension, was correlated with outcome, and was able to differentiate patients with good outcome, moderate disability, severe disability, and death. The graph of PRx compared with cerebral perfusion pressure (CPP) indicated a U–shaped curve, suggesting that too low and too high CPP was associated with a disturbance in pressure reactivity. Such an optimal CPP was confirmed in individual cases and a greater difference between current and optimal CPP was associated with worse outcome (for patients who, on average, were treated below optimal CPP [R2 = 0.53, p < 0.001] and for patients whose mean CPP was above optimal CPP [R2 = −0.40, p < 0.05]). Following decompressive craniectomy, pressure reactivity initially worsened (median −0.03 [interquartile range −0.13 to 0.06] to 0.14 [interquartile range 0.12–0.22]; p < 0.01) and improved in the later postoperative course. After therapeutic hypothermia, in 17 (70.8%) of 24 patients in whom rewarming exceeded the brain temperature threshold of 37°C, ICP remained stable, but the average PRx increased to 0.32 (p < 0.0001), indicating significant derangement in cerebrovascular reactivity.
The PRx is a secondary index derived from changes in ICP and arterial blood pressure and can be used as a surrogate marker of cerebrovascular impairment. In view of an autoregulation–guided CPP therapy, a continuous determination of a PRx is feasible, but its value has to be evaluated in a prospective controlled trial.