Sensing Brain Pressure without Surgery
One of the most important things to monitor in patients who’ve sustained a severe blow to the head or a serious hemorrhage is pressure in the brain. This can reveal an increase in the brain’s volume, thanks to bleeding, swelling, or other factors, which can compress and damage brain tissue and starve the organ of blood. Increases in pressure have also been implicated in other, less critical neurological problems, such as migraines and repeated concussions. But current methods for monitoring intracranial pressure are highly invasive—a neurosurgeon drills a hole in the skull and inserts a catheter, which carries a risk of infection.
Thomas Heldt, a research scientist at the Research Laboratory of Electronics at MIT, and collaborators Faisal Kashif and George Verghese, also at MIT, hope to change that with a new, noninvasive method for monitoring intracranial pressure. While the technology is still in its early stages of development, initial studies on data from comatose patients show that it is about as accurate as intracranial monitoring with a catheter and more accurate than other, less invasive options, which involve inserting a catheter into the tissue layers between the inner skull and the brain. Heldt presented the research at the Next-Generation Medical Electronic Systems workshop at MIT earlier this month.
“If we had a way of determining pressure in the field, even a simple heuristic, like whether pressure is greater than 20 mmHg (millimeters of mercury—the standard measure at which physicians intervene), it would be hugely helpful,” says Rajiv Gupta, director of the Ultra-High-Resolution Volume CT Lab at Massachusetts General Hospital, in Boston. “Triage is based on that.” Gupta was not involved in the research.
To assess pressure noninvasively, Heldt’s team started by creating a simple circuit model of pressure in the brain using knowledge of brain anatomy and how blood and cerebrospinal fluid flow through the organ. They then developed an algorithm to calculate intracranial pressure for a given level of arterial blood pressure and cerebral blood flow. Arterial blood pressure can be measured either with a catheter inserted into the wrist, or indirectly with a finger cuff, a device similar to an arm blood-pressure cuff but which provides continuous readings of blood pressure. A noninvasive ultrasound technique known as transcranial Doppler can detect velocity of cranial blood flow, which is directly related to the flow itself.
Researchers validated the approach using previously collected data from 45 comatose patients. The estimate matched the gold standard measure with a deviation of about eight to nine mmHg. Other methods for measuring pressure, such as catheters inserted into the space between the skull and brain tissue, vary by 10 mmHg from reading to reading in the same brain.
Heldt says the goal is to achieve accuracy within four to five mmHg, which will enable physicians to distinguish between a safe pressure—a healthy person’s intracranial pressure ranges from about seven to 15 mmHg—and one that requires intervention. When pressure rises to between 20 to 25 mmHg, physicians try to bring it down to a safer range, either through steps as simple as making the patient sit up, or as severe as taking away a piece of the skull to relieve pressure.
Researchers are about to begin a new test of the technology with collaborators at Beth Israel Deaconess Medical Center in Boston using data collected in real time from intensive care unit (ICU) patients. They hope that better-quality data will improve the accuracy of the measure. (The previous data set was collected more than a decade ago, with older equipment.) They also hope to show that a noninvasive method of collecting arterial pressure will work as well as intra-arterial monitoring.
While the researchers are initially focused on validating the technology in ICU patients, where they can compare the measure to intracranial catheters, they say the biggest potential for the tool is in examining patients with mild traumatic brain injury, recurrent migraine, and certain vestibular disorders.
The cumulative effect of mild brain injury is of great concern to both athletes and the military, given growing evidence that repetitive damage can have serious long-term effects. “For mild traumatic brain injury, we don’t know what intracranial pressure does,” says Heldt. Recent research in rats has shown that exposure to a blast, which generates a pressure wave, triggers an increase in intracranial pressure; the bigger the blast, the bigger the increase in pressure. Eventually, the researchers plan to develop miniaturized devices that could be deployed on the battlefield or the sports field.
Heldt adds that his team isn’t the first to try to assess intracranial pressure based on arterial and cerebral blood flow. But previous efforts used data mining or machine learning approaches to create the algorithm. Such approaches require a database of previous measures. If a new patient is substantially different from those in the database, the algorithm fails. By incorporating simple physiological knowledge of the brain, his team could create a model that doesn’t require any previous knowledge of the patient or anyone else.
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