An Experimental Paradigm for the Prediction of Post-Operative Pain (PPOP)

This video demonstrates three pain testing protocols used in an experimental paradigm for the prediction of postoperative pain. The first model involves women rating the pain of punctuate mechanical pressure applied to the forearm. The second model involves a dynamic test with women rating the pain of a heat stimulus applied to the skin of the forearm, followed by women rating the pain of the identical heat stimulus while they simultaneously have their opposite hand submerged in a hot water bath.

In the third model, the area of dynamic hyperalgesia for punctuate mechanical stimuli and tactile pain threshold is measured 48 hours after surgery taken together. These three clinical pain testing protocols may allow the identification of patients at risk for developing chronic pain. Hi, I'm Ruth Landa from the Department of Anesthesiology and Pain Medicine at the University of Washington Medical Center in Seattle.

Today we will show you three procedures that allow to evaluate the pain modulatory system in surgical patients. The first procedure measures the extent of mechanical temporal summation, which indicates the degree to which the excitatory pain pathway is activated or central sensitization. The second procedure measures the efficiency or robustness of the diffuse noxious inhibitory control pathway or nic, and this test is called the NIC paradigm.

This pain modulating pathway is the neurophysiological basis for the well-known phenomenon of pain, inhibits pain in remote areas of the body. Finally, the third procedure measures hyperalgesia surrounding an incision in a dynamic manner and that allows to evaluate central sensitization. We are currently using these three procedures to evaluate pain modulation during pregnancy and are hoping to validate these tests as ways to identify women's individual risk to develop chronic pain after cesarean delivery.

So let's get started. To begin the MTS assessment, the researcher first applies a 180 gram von fray filament on the woman's forearm once or twice to familiarize her with the pin prick sensation. For this test, I'll use this plastic fiber to touch your forearm like this.

Then I'll ask you to rate the intensity of the pin prick pain on a zero to 100 pain scale with zero being no pain and 100 being the worst pain of hable. Then the Von Frey filament is applied 11 times within a one centimeter diameter circle, and the woman is asked to rate the pinprick pain of the first and 11th touches on a zero to 100 verbal numerical pain scale And read it now.Five. The denic assessment is conducted in a private exam room where the woman is seated in a comfortable position with a pillow placed in her lap.

She's told that small talk and conversation will not take place during the test in order to obtain consistent results. She's also asked to concentrate as best she can on the instructions that are described to her. First, the 11 point verbal numerical pain scale is explained.

For this test, we'll be using a zero to 10 pain scale with zero being no pain and 10 being the worst pain imaginable. And then the test is briefly explained. This test involves the heat, this heat thermo placed on your dominant forearm, and at the end of the test, you will have your left hand in the water bath for a full minute.

The heat ther mode and the water bath do not reach temperatures that will burn your skin and you can stop the test at any time you like. The heat ther mode is carefully placed on the inside part of the woman's voler forearm and secured with a Velcro strap. Now, relax your arm without disturbing the position of the thermo.

The first part of the test involves familiarizing the woman with two distinct heat sensations and getting her used to rating the painful sensation upon request. The thermo increases in decreases at eight degrees Celsius per second from 32 to 43 degrees Celsius, and then again from 32 to 44 degrees Celsius. Each target temperature first 43 degrees Celsius and then 44 degrees Celsius remains for seven seconds.

The inter stimulus interval is set to two seconds. The subject is asked to rate the pain on an 11 point verbal numerical pain scale. Six seconds after the thermo reaches the target temperature For the first part of the test, you will feel two heat sensations each lasting seven seconds.

When I say rate it now, please rate the pain you feel of each sensation on a zero to 10 scale with zero being no pain and 10 being the worst pain imaginable. The thermo is activated And rate it. Now three.

The thermo returns to the baseline temperature of 32 degrees Celsius, and after six seconds at 44 degrees Celsius, the researcher asks the woman to rate the pain again And rate it now three. The next part of the test involves applying three consecutive temperatures to determine the temperature, the woman reports with a pain intensity of six on the zero to 10 scale. This is done by first heating the thermo to 45, 46 and 47 degrees Celsius in random order.

Okay, now we're going to do the exact same thing, but you'll feel three heat sensations. Instead of two, The thermo is activated and after six seconds at the first temperature, the researcher asks the woman to rate the pain. Rate it now, This is repeated for the other two temperatures And rate it now six and rate it now seven.

Depending on the responses of the woman for each temperature, the range of the three temperatures will be increased or decreased by one degree Celsius. The thermo is not able to reach temperatures above 48 degrees Celsius. This process is repeated until the subject rates a six for a particular temperature and ready.

Now six. Next the pain six temperature is confirmed. Now you'll feel just one heat sensation.

The thermo is activated and after six seconds at the pain six temperature, the researcher asks the woman to rate the pain And rate it now six. Now the thermo is removed from the woman's forearm for at least five minutes. This is a good time for the subject to complete questionnaires and paperwork.

After at least five minutes, the thermo is placed in the exact same position on the woman's forearm. The next part of the test involves the thermo heating up to the pain six temperature for 30 seconds, the woman is asked to rate the pain at 0, 10, 20, and 30 seconds during the test stimulus. Now you'll feel a heat sensation for 30 seconds.

Please rate the pain when I say rate it now and rate it now four and rate it now six and rate it now six and rate it now six. The thermo is then removed from the woman's forearm for at least five minutes during which time more questionnaires and paperwork may be completed. After at least five minutes, the thermo is once again placed in the exact same position on the woman's forearm.

For the last part of the test, the denic pathway will be tested with the hot water bath conditioning stimulus. The water in the water bath is kept at a constant 46.5 degrees Celsius. The woman will submerge her non-dominant hand up to the wrist in the water for a full minute, and during the last 30 seconds of the minute, the thermo heats up to the same pain.

Six, temperature and rate your left hand.Eight. The thermo is activated once the subject places her hand in the water bath, but the thermo remains at the baseline 32 degrees Celsius for 30 seconds while the non-dominant hand is submerged in the water bath. After 10 seconds, the researcher points to the woman's non-dominant hand and asks her to rate the pain.

Again, this is repeated two more times at 20 seconds and 30 seconds. And rate your left hand eight and rate your left hand seven. Now the thermo heats to the pain six temperature, and the researcher asks her to rate the thermo arm at 40, 50 and 60 seconds.

Next, I'll ask you to rate your thermo arm and rate your red arm four. And rate your right arm four. And rate your right arm Three.

Thank you. We're finished. You can take your left hand outta the water bath.

The assessment of the area of secondary hyperalgesia around the incision is performed 48 hours after surgery. For this procedure, the woman should be lying flat on a bed stimulation with a 180 gram von fray. Filament is started from the outside of the hyperalgesic area where no pain sensation is experienced and moved toward the incision until the patient reports a distinct change in pain Perception.

Ow.Ow.The point where a painful sore or sharp feeling appears is marked and the distance to the incision is measured. If no change in sensation appears stimulation is stopped, 0.5 centimeters from the incision. The area of secondary hyperalgesia is determined by testing along radial lines separated by 2.5 centimeters.

Around the incision, the observations are then translated onto graph paper. The pain threshold for mechanical static punctuate stimuli is assessed with an electronic von Fray device. The woman is instructed to close her eyes during the procedure.

Various pressures are applied in ascending order, one centimeter above the abdominal incision for approximately one second, and the applications are separated by at least 30 seconds. Do you wanna speed there? Okay.Mechanical temporal summation is calculated by subtracting the pain rating after the first touch from the pain rating after the 11th touch.

To calculate a DN score, the average of the last three pain ratings of the conditioned test stimulus is subtracted from the average of the last three pain ratings of the unconditioned test stimulus. The area of secondary hyperalgesia is calculated as the sum of the distances from each point of secondary hyperalgesia to the incision divided by the length of the incision. The tactile pain threshold is defined as the smallest force grams per millimeter squared that is perceived as painful.

Three determinations are made for each assessment, and a mean is calculated to date. We have performed MTS and DN preoperatively in 75 women, 38 in Brazil, and 37 in Seattle. A graphic representation of pre cesarean denic score is shown here.

Efficient endogenous analgesia is represented by a positive DX score, zero to positive five. While lack of endogenous analgesia is represented by a negative DN score zero to negative five. Finally, the correlation between preoperative DN score over MTS and 48 hour wound hyperalgesia is shown.

In this graph, We have just shown you two procedures that test the excitatory and inhibitory pain pathways of the pain modulatory system, and two short tests to measure the sensitivity around an incisional wound in the obstetric patient. The first procedure tested the excitatory pain pathway through mechanical temporal summation. The second tested the diffuse noxious inhibitory control pathway using heat pain as the test stimulus and a hot water bath as the conditioning stimulus.

The third procedure performed 48 hours after surgery, assessed central sensitization by first evaluating secondary mechanical wound hyperalgesia, and then evaluating the tactile pain threshold. Taken together the three clinical pain testing protocols may allow to identify patients at risk for chronic pain. So that's it.

Thanks for watching and good luck with your experiments.