Abstract
BackgroundSpreading depolarization (SD) and the initial, still reversible phase of neuronal cytotoxic edema in the cerebral gray matter are two modalities of the same process. SD may thus serve as a real-time mechanistic biomarker for impending parenchyma damage in patients during neurocritical care. Using subdural platinum/iridium (Pt/Ir) electrodes, SD is observed as a large negative direct current (DC) shift. Besides SD, there are other causes of DC shifts that are not to be confused with SD. Here, we systematically analyzed DC artifacts in ventilated patients by observing changes in the fraction of inspired oxygen. For the same change in blood oxygenation, we found that negative and positive DC shifts can simultaneously occur at adjacent Pt/Ir electrodes.MethodsNurses and intensivists typically increase blood oxygenation by increasing the fraction of inspired oxygen at the ventilator before performing manipulations on the patient. We retrospectively identified 20 such episodes in six patients via tissue partial pressure of oxygen (ptiO2) measurements with an intracortical O2 sensor and analyzed the associated DC shifts. In vitro, we compared Pt/Ir with silver/silver chloride (Ag/AgCl) to assess DC responses to changes in pO2, pH, or 5-min square voltage pulses and investigated the effect of electrode polarization on pO2-induced DC artifacts.ResultsHyperoxygenation episodes started from a ptiO2 of 37 (30–40) mmHg (median and interquartile range) reaching 71 (50–97) mmHg. During a total of 20 episodes on each of six subdural Pt/Ir electrodes in six patients, we observed 95 predominantly negative responses in six patients, 25 predominantly positive responses in four patients, and no brain activity changes. Adjacent electrodes could show positive and negative responses simultaneously. In vitro, Pt/Ir in contrast with Ag/AgCl responded to changes in either pO2 or pH with large DC shifts. In response to square voltage pulses, Pt/Ir falsely showed smaller DC shifts than Ag/AgCl, with the worst performance under anoxia. In response to pO2 increase, Pt/Ir showed DC positivity when positively polarized and DC negativity when negatively polarized.ConclusionsThe magnitude of pO2-induced subdural DC shifts by approximately 6 mV was similar to that of SDs, but they did not show a sequential onset at adjacent recording sites, could be either predominantly negative or positive in contrast with the always negative DC shifts of SD, and were not accompanied by brain activity depression. Opposing polarities of pO2-induced DC artifacts may result from differences in baseline electrode polarization or subdural ptiO2 inhomogeneities relative to subdermal ptiO2 at the quasi-reference.
Highlights
Spreading depolarization (SD) and the initial, still reversible phase of neuronal cytotoxic edema in the cerebral gray matter are two modalities of the same process [1]
We found that the change in polarity of the pO2-induced direct current (DC) artifacts in our in vitro experiments could be produced by reversing the polarization of a Systemic-hyperoxia-induced DC artifact
If neurointensivists wish to use Pt electrodes to predict and detect ischemic conditions in the brain by monitoring SDs, the awareness of pO2 and pH interference with Pt electrodes is important because changes in pO2 and pH can produce large DC artifacts that should not be confused with SDs
Summary
Spreading depolarization (SD) and the initial, still reversible phase of neuronal cytotoxic edema in the cerebral gray matter are two modalities of the same process [1]. Using subdural platinum/iridium (Pt/Ir) electrodes, SD is observed as a large negative direct current (DC) shift. We systematically analyzed DC artifacts in ventilated patients by observing changes in the fraction of inspired oxygen. For the same change in blood oxygenation, we found that negative and positive DC shifts can simultaneously occur at adjacent Pt/Ir electrodes. Conclusions: The magnitude of pO2-induced subdural DC shifts by approximately 6 mV was similar to that of SDs, but they did not show a sequential onset at adjacent recording sites, could be either predominantly negative or positive in contrast with the always negative DC shifts of SD, and were not accompanied by brain activity depression
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