A team of researchers from the Massachusetts Institute of Technology (MIT; Cambridge, MA) and Massachusetts General Hospital (MGH; Boston, MA) has developed an optical sensor that can be embedded into an epidural needle, helping anesthesiologists guide the needle to the correct location. The development helps address the complication of needles being inserted too far or placed in the wrong tissue.
Currently, anesthesiologists must guide a 4–6 in. needle through multiple layers of tissue to reach the epidural space surrounding the spinal cord. They know when the needle has reached the right spot based on how the tissue's resistance changes. However, some patients' tissues vary from the usual pattern, which can make it more difficult to determine whether the needle is in the right place. These complications can lead to reduced effectiveness of the pain-killing drug, or an excruciating post-procedure headache. In rare cases in which the needle goes too far or into a blood vessel, a stroke or spinal cord injury can occur.
To improve the accuracy of epidural needle placement, T. Anthony Anderson, an anesthesiologist at MGH and an assistant professor at Harvard Medical School, teamed up with researchers at MIT's Laser Biomedical Research Center, headed by Peter So, a professor of mechanical engineering and biological engineering.
So and MIT research scientist Jeon Woong Kang designed and tested several types of optical sensors that could be placed at the tip of an epidural needle and determined that the best is one that relies on Raman spectroscopy. This technique, which uses light to measure energy shifts in molecular vibrations, offers detailed information about the chemical composition of tissue. In this case, the researchers measured the concentrations of albumin, actin, collagen, triolein, and phosphatidylcholine to accurately identify different tissue layers.
This sensor provides immediate feedback telling the anesthesiologist which tissue the needle is in. As an epidural needle is inserted, it passes through five layers—skin, fat, supraspinous ligament, interspinous ligament, and ligamentum flavum—before reaching the epidural space, which is the target. Beyond that space lies the dura mater, a stiff membrane that surrounds the spinal cord and cerebrospinal fluid. The sensor, Kang says, continuously measures Raman spectroscopy signals to give the anesthesiologist the chemical composition of the tissue, identifying all tissue layers from skin to spinal cord.
The team found that Raman spectroscopy could distinguish each of the eight tissue layers around the epidural space with 100% accuracy. Two other techniques that they tested, fluorescence and reflectance spectroscopy, could distinguish some layers, but not all eight.
The researchers have tested the sensor in pig tissue and now plan to do further animal studies before testing it in human patients. They also plan to reduce the diameter of the sensor from 2 mm (which is too large to fit in the most commonly used epidural needles) to 0.5 mm. The researchers have started a company, Medisight Corp., to continue developing the technology, which they believe could also be applied to medical procedures, such as cancer biopsies or injecting drugs into the joints.
Full details of the work appear in the journal Anesthesiology; for more information, please visit http://dx.doi.org/10.1097/aln.0000000000001249.