A New Technique to Indicate Risk of SUDEP?

WASHINGTON, DC—Measuring transcutaneous carbon dioxide partial pressure (TCpCO_²) may identify increased seizure severity and duration, according to research described at the 67th Annual Meeting of the American Academy of Neurology. The technique also may indicate the risk for sudden unexpected death in epilepsy (SUDEP).

Drawing blood for arterial blood gas (ABG) volumes is the current gold standard for monitoring oxygen and carbon dioxide levels in peri- and postictal seizure stages. The technique, however, is invasive, painful, and insufficient to provide continuous monitoring over several minutes, said Derek Chong, MD, Director of EEG & Epilepsy Consult Services and Assistant Professor of Neurology at NYU Langone Medical Center in New York City.

In contrast, TCpCO_² measures carbon dioxide diffusion through the skin. “It correlates better with arterial carbon dioxide than end-tidal carbon dioxide and may be specifically better during seizures and for prolonged wear in the epilepsy monitoring unit,” said Dr. Chong. The transcutaneous system warms the capillary bed to better match the temperature of arterial blood. The system then measures the diffusion of carbon dioxide through the skin to the sensor.

TCpCO_² measurements previously had been used exclusively during stable conditions outside of a seizure episode, and the validity of measurement during a seizure remains to be evaluated. Dr. Chong presented data from the first study known to use TCpCO_² technology for seizure analysis.

Carbon Dioxide Was Associated With PGES
In a retrospective analysis using TCpCO_² recordings that captured 15 seizures in 10 adults, Dr. Chong and his colleagues found changes similar to those reported in prior studies using continuous EEG monitoring and ABG volumes. “The carbon dioxide—and not the oxygen—was associated with postictal generalized electrographic suppression [PGES] on multivariate analysis, and that was a pretty strong association,” he said.

Standard ABG measurements generally show that a decline in oxygen precipitates the clinical onset of a seizure about 40 seconds later, Dr. Chong explained. Oxygen saturation levels tend to decrease to hypoxic levels (ie, below 50%) and take some time to recover, while the pulse rate climbs to about 150 bpm, suggesting a high rate of oxygen demand.

The study participants’ TCpCO_²levels showed similar decreases in oxygen rates in PGES-related seizures, but with less significant changes in carbon dioxide levels. “We had seven generalized tonic–clonic seizures. Two were associated with PGES, and both of those had the highest changes in carbon dioxide, Dr. Chong said. These seizures were associated with an elevation of 20% or more. “The peak in the carbon dioxide [pressure] was about 8 [mmHg], and the peak tended to happen at least four minutes after the seizure or later,” he added.

Technique Requires Validation
Data were captured on eight other seizures. “Two of [the patients] also had associated increases in carbon dioxide but did not become hypoxemic, whereas others were hypoxemic but did not have a rise in carbon dioxide, so there’s some dissociation there,” Dr. Chong noted.

The stents, seizure duration, seizure type, and PGES were all directly correlated with the change in TCpCO_². On multivariate analysis, only PGES was associated with TCpCO_² elevations, and this result, Dr. Chong concluded, may denote greater severity and duration of seizure activity.

The next steps will be to validate the use of TCpCO_² and end-tidal carbon dioxide during seizures, said Dr. Chong. He and his colleagues plan to use these measurements in the design of a preoxygenation interventional study of PGES in SUDEP. The ultimate goal of future studies is to limit SUDEP, he concluded.

—Linda Peckel