Why Don’t You Feel Pain During Surgery? New Tool Helps Doctors Measure Pain Response

Pain control is a major concern for patients who need to have surgery. Nobody wants to wake up from surgery in pain, but too much pain medicine may cause difficulties in recovery.

Thanks to anesthesia, most surgical patients are unconscious and therefore unaware of any pain their bodies are experiencing. However, the body still senses the stimuli that would cause pain and responds to it involuntarily. This response is called nociception.

"Even when people are unconscious, pain signals still go to the brain," Daniel Sessler, MD, an anesthesiologist with the Cleveland Clinic, told Verywell. “We know people respond to painful signals even if they’re not conscious. Everyone is at risk of having pain during surgery. Even small operations cause plenty of pain.”

Your body may react to painful sensations by increasing your heart rate or blood pressure or by sweating. You may also move involuntarily. Since you cannot tell your surgical team you are in pain, anesthesia providers closely monitor for these signs during surgery. Because these reactions may be subtle, it’s possible for anesthesiologists to miss certain pain signals.

A new device is helping anesthesiologists more precisely identify and respond to pain during surgery. Developers at Medasense, a biotech company in Ramat Gan, Israel, created a finger probe to continuously monitor four different pain signals during surgery.

A New Way to Monitor Pain During Surgery

Medasense’s device, the PMD-200, is a nociception monitor that uses a finger probe with sensors to measure the body’s reaction to pain when a patient is under anesthesia.

The monitor analyzes a patient’s heart rate, heart rate variability (how slowly or rapidly their heart rate changes), and body temperature to come up with a nociception level index (NOL) reading.

The NOL index is a scale of 0-to-100 (no pain to extreme pain). The target NOL score for a patient under anesthesia is around 25.

In theory, a patient with a higher NOL reading needs more pain medication during surgery while a patient with a lower NOL has enough pain relief. A patient on the lower end of the scale may need less pain medication than the anesthesia provider would typically give.

While anesthesiologists already have a range of tools to keep an eye on a patient during surgery, no other technology that’s currently used in the U.S. gives them the ability to monitor the body’s response to pain as closely as the NOL index does.

To that end, the individualized information from the PMD-200 can help anesthesia providers carry out pain management for each patient. It’s currently authorized for use during operations to help personalize analgesia dosing to the patient’s actual requirement. It is not yet authorized to treat or monitor post-op pain.

Controlling Pain During Surgery Impacts Recovery, Too

Pain management during surgery is just the beginning. Not having enough pain medication during surgery can mean worse pain after surgery.

“Patients under anesthesia still have physiological pain. The body will sense the pain and respond to it with the ‘fight or flight’ response,” Galit Zuckerman Stark, CEO and founder of Medasense, told Verywell. “Once the patient becomes conscious again, they will feel the pain.”

Too much pain medication during surgery, however, can make it more difficult for someone to come out of anesthesia. This can cause oversedation, delirium, or confusion.

It’s not easy, but anesthesia providers need to find the sweet spot between too much and too little pain relief during surgery and maintain it.

“Pain control during surgery is a balance between anesthesia and analgesia. You need to know which lever to pull,” Padma Gulur, MD, an anesthesiologist with Duke University School of Medicine in Durham, North Carolina, told Verywell.

Because pain cases a stress response in the body, research suggests that reducing pain during surgery with the help of devices like the PMD-200 can improve recovery outcomes.

“By adjusting opioids to the NOL index during surgery, you give that patient the right dose for them, which may be more or less than an anesthesiologist routinely uses,” said Sessler, who serves as a consultant for Medasense. “When patients receive the right amount of pain medication for them as individuals, they should awaken quickly and without severe pain.”

Will NOL Monitoring During Surgery Become Standard?

There are some limitations to the PMD-200. Clinical trials have not consistently shown that NOL monitoring during surgery reduces short-term or long-term pain for patients. More research would be needed before NOL monitoring could become the standard of care.

“We believe that NOL monitoring can improve postoperative pain, but studies so far have been underwhelming,” Sessler said. “There also have not been many studies, and more are clearly needed.”

Some anesthesia experts have warned against treating patients based on a machine-provided number instead of their clinical condition—especially given the potential dangers of opioid overuse.

“When we start treating numbers, and we don’t necessarily get good outcomes from it, we have to ask, what is this adding?” Gulur said. “We have an opioid epidemic. We don’t want to use more opioids than necessary, especially if we do not see their benefits.”

With more research, though, it’s possible that NOL monitoring technology could be used outside of the operating room as well.

For example, patients in the intensive care unit (ICU) who need a breathing tube and ventilator are often given sedating medications to ease their discomfort.

“We saw NOL monitoring used in critical care during COVID,” Zuckerman Stark said. “Critical care patients are sedated just like surgery patients, but for days or weeks. We were concerned to see the amounts of opioids patients in the ICU were receiving over time.”

Eventually, NOL could even be useful outside of the hospital. While the device would look different, the same algorithms can be used to help non-speaking individuals indicate they are in pain. It could also be used to better respond to chronic pain.