Dehydrated patients performed worse on an arithmetic test compared to rehydrated patients. (Image source: Thinkstock)
Dehydrated patients performed worse on an arithmetic test compared to rehydrated patients. (Image source: Thinkstock)

Distraction is known to decrease pain threshold.  Would dehydration also affect pain experience?

Dr. Yuichi Ogino, Department of Anesthesiology, Gunma University Graduate School of Medicine, Maebashi, Japan, and colleagues from the Department of Nursing, Faculty of Health and Welfare, Kawasaki University of Medical Welfare, Kurashiki, Japan, and the Department of Anesthesiology, Saiseikai-Hita Hospital, Hita, Japan, studied pain thresholds and cortical activation in response to evoked pain stimulus using fMRI and an executive function task (calculation test) in five healthy patients who on one day performed 40 minutes of aerobic exercise in addition to a 12-hour fast without any oral rehydration (dehydration day) and on another day consumed up to 2000 mL from the night before the test day until the time they started exercise and up to 1000 mL during exercise (rehydration day).  The results of their study were published in the June 2014 issue of Anesthesia & Analgesia in the article titled “Dehydration Enhances Pain-Evoked Activation in the Human Brain Compared with Rehydration.”

Pain was generated using the cold pressor test whereby frozen ice packs were applied to the subject’s medial forearm.  The pain threshold during dehydration was 13.4 ± 3.6 seconds vs.16.2 ± 3.4 seconds during rehydration.  The change in pulse rate after exercise was higher after dehydration, and, of course, dehydrated subjects were thirstier.  Arithmetic performance was lower after dehydration.  Neuroimaging studies showed that regions such as the anterior cingulate cortex, bilateral insula, thalamus, and cerebellum were activated more in dehydrated patients compared to rehydrated patients in terms of peak and cluster.

Is dehydration bad?  We know that fluid restriction can reduce hospital stay and the incidence of complications for certain procedures such as abdominal surgery. However, clearly the key is balance – finding fluid management strategies that provide the benefits of fluid restriction, without increasing the risks associated with dehydration.

This study had a modest sample size, and used an experimental pain model (frozen ice packs) that differed from postoperative surgical pain. However, the findings are provocative and should be considered hypothesis generating. Future studies looking at the benefits of fluid restriction should consider pain scores one of the outcomes assessed in helping to find optimal fluid management.