In an experimental study with healthy volunteers, we are going to test the effect of breathing pattern manipulation on esophageal pain. Esophageal pain will be induced by electrical stimulation.
First, and for each subject, with threshold determination we will find the stimulus intensity (electrical current in mA) to be used during the intervention. The current method is to find the stimulus that can induce a pain rated by the subject as 6 in an 11-point NRS with 2 anchors; 0 = no pain, 10 = most intense pain imaginable. Then, we use this stimulus during the intervention and see how pain perception is influenced by the intervention. 15 stimuli will be applied during each intervention and subjects will rate the pain immediately after each stimulus using the NRS (from 0 to 10). In this way, all subjects are receiving same pain intensity in terms of the score, 6.
But why 6? because we assume that 6 is a "moderate" pain. However, although many subjects may consider 6 as a moderate pain, for some (not few) 6 is a "severe" pain and this is our concern. So, although all subjects will receive same pain intensity by score, it's not the same if we ask them to label it.
The question is: which one is more important? to have same pain intensity among subjects by same score or by same label?
I would recommend having the pain stimulus consistent at 6 across all participants, but adding perception of pain as a covariate
According to McCaffery, 0 is defined as “no pain at all”, 1 - 3 as “mild pain”, 4 - 6 as “moderate pain”, 7 – 9 as “severe pain”, and 10 as “the worst imaginable pain”.
McCaffery M. Using the 0-to-10 pain rating scale. Am J Nurs. 2001;101(10):81-2.
moderate VAS : 5-7. Please regard this as a subjective science. VAS of 8 in unable to get out of bed and/or perform daily activities. VAS of 9 ; consideration should be made for an urgent appointment with your primary doctor and pain management physician. VAS of 10 requires hospitalization.
Diagnostic and/or Imaging studies should be considered in patients' with VAS 8-10. Moderate pain maybe treated with NSAIDs and Physical Therapy . opiate analgesics and nerve blocks or possible surgery may be required for severe pain patients.
Again, I cannot stress this enough; Acknowledgment and severity of pain is subjective. Treat the pathology causing the pain and not just the pain itself.
'Pain measure and cut-offs - no worse than mild pain as a simple, universal outcome' by Moore published in Anaesthesia in 2013 contains a graph that compares NRS and cathegorical pain rating. As you can see, there is a spread of NRS for moderate from 4 - 6.
As to your question, you are doing simultanious observations in the same patient. As I understand you are interested in the difference in NRS. A patient will compare initial pain stimulus to the new stimulus and give you a new score comparing both. The outcome of interest will be delta NRS. The exact individual perception is therefore less relevant if you choose the moderate range between 4 and 6.
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