The overall goal of this electrophysiological measurement is to objectively asses nociception in newborn infants. This method can help answer key questions in the neonatal field such as how non-verbal patients perceive pain and how it differs in response to different conditions. The main advantage of this technique is that it can be used in neonates born at term or late pre-term starting at 34 weeks of gestational age.
The implications of this technique extends toward investigation of pain because it is the first objective measurement of nociception in non-verbal patients. Generally, individuals new to this method will struggle because experience with EEG is required. We first had the idea for this method when we observed after delivery different pain responses in neonates born spontaneously as compared to those born by elective ceasarean section.
But also at the neonatal intensive care unit, babies are exposed to so many potentially painful procedures. You don't know how much they perceive even when they are under analgesic drugs. Now we can objectively measure their noxious response.
Before starting, ensure the baby is quiet and settled. The baby can be asleep but sucking motion should be absent during the recording because of movement artifacts. Measure the neonate's head circumference with a measuring tape to define the size of the EEG cap.
Then identify the active electrode position by marking the middle point between nasion and inion and the middle point between left and right pre-auricular point with a skin marker pencil. A schematic of the electrode positions is shown here. Identify the positions for the ground electrode on the right forehead and for the reference electrode over the left mastoid.
Clean the three electrode sites using a cotton swab soaked in skin disinfectant. Then gently scrub the electrode sites with EEG prepping paste to lower the impedance and place the EEG cap with the electrodes attached on the neonate's head. Next, use a syringe with a short plastic needle to inject conductive EEG gel into the electrodes to optimize the contact between the electrodes and the scalp.
Then to ensure the impedance is below 50 kilo ohms, connect an impedance meter to the electrodes and set it below 50 kilo ohms. The light will turn green if the impedance is below 50 kilo ohms. After this, connect the electrodes to the amplifier.
Position a camera to record the neonate's facial expressions. Then connect the flat-tip probe to the contact trigger device which is fixed to the EEG recording device. Begin by choosing a new work file.
Set the sampling rate to 2, 000 Hertz. Set the low cutoff filter to a frequency of one Hertz. Set the high cutoff filter to 70 Hertz and the notch filter to 50 Hertz.
Select a study name to store the data. Start the EEG and the video recording. Whilst the neonate's right hand is held in a horizontal position, record background EEG activity.
Annotate the EEG recording manually to record periods where no stimuli are applied and the infant is resting. Once the background EEG is stable, administer the required amount of flat-tip probe stimuli to the neonate's right hand. Be careful to use the flat-tip probe perpendicularly to the neonate's hand so the tip does not bend and the correct force is applied.
When the flat-tip probe reaches the nominal force on the skin, a trigger signal is generated by the contact trigger device. This signal is sent to the computer tagging the EEG recording with the trigger mark. After the stimuli are administered, stop all the recordings and document the experimental setting details.
After taking off the electrodes at the infant's head, clean the scalp with a wet towel to remove any residuals. A flat-tip probe stimulus of 32 millinewtons was applied to the right hand of one patient. The shaded region shows the time window of interest which occurs 200 to 500 milliseconds after the stimulus.
After a single stimulus, the noxious-evoked response is visible at approximately 300 milliseconds post stimulus onset. The principle peak at 250 milliseconds is indicated with an arrow. This trace demonstrates the average response of 50 stimuli applied with a force of 32 millinewtons to the same patient.
Note that the noxious-evoked potential is clearer if more stimuli are applied. Woody filtering can be used to adjust for slight variability in the response latency. The predefined template shown in red is projected onto the EEG to calculate the magnitude of the noxious-evoked response.
Once mastered, this technique can be done in 15 minutes if it is performed properly. While attempting this procedure, it's important to remember to wait to apply the stimuli until the infant is calm. After watching this video, you should have a good understanding of how to objectively measure nociception at bedside.
