Cooling Study 3 of 4

Results

A total of 88 experimental trials were conducted on the twelve subjects.  Two subjects experienced relapses and dropped out of the study without completing a set of paired trials.  Twenty six data sets that met the criteria for paired trials (identical work loads and a maximum of seven days separating the trials) were collected from the remaining ten subjects.  Cooling treatment increased performance. 

When the individual subject’s mean data were analyzed according to subject and treatment, treatment extended exercise duration by an average of 35% (42.8 ± 16.4 with cooling compared to 31.7 ± 9.8 minutes without cooling, mean ± standard deviation, n=10, p<0.003, paired test) and ranged from 8% to 65%.  

When the exercise duration data were analyzed according treatment only (n=26), mean exercise duration with treatment was 43.6 ± 17.09 min (mean ± SD) compared to 32.8 ± 10.9 without treatment: a mean increase of 33% (p< 5.0·10-6, two tailed paired t  6 test).  The magnitude of the treatment effect was correlated with exercise duration during the control trials (Figure 1).  Ninety percent of the variance in the improvements with the treatment could be accounted for by an exponential function fitted to the data (y = 12.505e0.0356x with y and x being exercise duration with treatment and without treatment, respectively). 

There were no discernable patterns in heart rates (Figure 2).  Cardiac drift - a rise in heart rate during sustained fixed load exercise - was observed in all trials.  Initial heart rate, final heart rate, and rate of cardiac drift (change in heart rate/time) varied among the subjects and trials. 

There was no significant difference between treatment groups in initial heart rates [87 ± 7 beats per minute (bpm) with cooling compared to vs.  88 ± 10 bpm without cooling, mean ± standard deviation, n = 10, p_ 0.56, paired t-test].  Treatment significantly affected maximum heart rates (123 ± 18 bpm with cooling compared to 118 ± 20 bpm without cooling, mean ± standard deviation, n = 10, p _ 0.03, paired t-test).  The higher maximum heart rates observed during the cooling treatment trials were likely related to increased exercise durations with the cooling treatment.  However, the rates of cardiac drift were proportional to neither work loads nor exercise duration times.  Treatment did not affect the cardiac drift in a consistent manner: in some cases treatment increased cardiac drift; in others, treatment decreased cardiac drift; while, in others, treatment had no effect heart rate patterns. 

Anecdotally, most subjects reported feeling better during the cooling trials.  Several subjects reported that instead of experiencing the usual progressive fatigue when exercising, their symptoms occurred in waves when they used the heat extraction device.  Another subject reported experiencing an unusual set of symptoms when treated.  Instead of feeling “cloudy” during exercise, he felt a tingling in his legs. 

Discussion

Under the experimental conditions described in this report, the use of a device that facilitates removal of heat from the circulating blood provided a performance benefit to individuals with MS who had a history of transient worsening of symptoms associated with conditions that would impose a heat load on their bodies.  In this study, the dependent variable was physical performance while the independent variable was the application of a cooling technique. 

Methodological limitations in the study design may have weakened the association between the independent and dependent variables and should be considered when evaluating the merit of the data presented in this report.  We are aware of three methodological limitations in the study design: 1) the lack of a placebo control, 2) the researchers were not blinded to the treatments, and 3) the absence of a temperature measure or a subjective assessment of thermal comfort. 

To facilitate heat transfer into or out of a body, it is necessary to apply a heat source or sink to a surface of the body.  The sensations of temperature result from the differential activation of warm and cold cutaneous temperature sensors that respond to changes in local skin temperature and, thus, individuals can perceive the presence of a thermal source or sink when it is applied to the body surface.  Blocking the afferent input from the cutaneous thermosensors in local skin regions would be a way to eliminate the sensory input, but use of a nerve block to control for a placebo effect seemed excessive for this preliminary study. 

We are unaware of a practical means to blind subjects to treatment when a treatment entails applying a thermal stimulus to the skin surface.  In these studies the researchers as well as the subjects were aware of the treatments being applied during a given trial.

A common confound in exercise performance trials is researcher bias.  In trials on healthy subjects with subjective stop criteria, peak performance can be influenced by external motivational factors such as differences in the researcher’s confidence to push the subject.  Researcher bias, while always a potential confound, likely had little influence on exercise duration in these trials on individuals with MS because the primary stop criterion for exercise was an exacerbation of physical symptoms rather than being related to motivational factors.  Researcher bias could have been eliminated from these trials by physically isolating the subjects from the researcher. 

However, since these subjects were physically compromised and exercising on a treadmill often to near the point of physical collapse, it was deemed more prudent to have the researchers directly observe the subjects during the trials and be immediately available to assist the subjects at the termination of exercise.  Neither a real measure of body temperature nor a subjective assessment of perceived temperature was monitored during these trials.  The effects of temperature on individuals with MS are well known: increases in temperatures often exacerbate symptoms while decreases in temperatures can improve them. 

However, the magnitude of thermal stimulus necessary to elicit a symptom exacerbation in MS patients can be minor and may be insufficient to be noted as a change in a measured deep body temperature.  In previous studies substantial improvements in fatigue, muscle strength and standing balance were observed when individuals with MS were actively cooled using cooling garments, but there was not an associated effect of lowering tympanic temperatures. 

Since the effects of cooling have been demonstrated without an associated effect on core temperatures and placement of a core temperature probe can be stressful and unpleasant for the subjects, core temperatures were not measured in this study.  The cause of the physical performance improvements could be multi-factorial and thus, without a direct measure of core temperature, we can only speculate that that the treatment used in these studies provided a cooling benefit which resulted in the performance improvement.