Monthly Archives: February 2014

Space blankets are worthless

Well, that’s not entirely true. They just aren’t good at being blankets. They fell out of favor quickly in hospital use, but survivalists still advocate for their use in wilderness settings. There’s at least one ultramarathon that makes participants carry one at all times. Why do they do this?

Space blankets came into vogue during the 60s and 70s, as a response to the “space race”. Anything that had to do with interstellar travel was popular, and out of that technology boom came the thin plastic coated with metal on one side.

They work quite well in space, since they’re small, weigh very little, and reflect radiated heat. Well, they work well on equipment, and only for keeping it cool. Heat from the sun is effectively reflected by the blankets. The problem is, humans don’t radiate much. Most heat loss is by convection, and the space blanket does little for this outside of being a wind break.

An even funnier consideration is referenced in the paper as well. The blankets only reflect heat on the size that has been metallized. Since this is usually only one side, and often clear plastic is used, you have a 50/50 shot of putting it on backwards if you’re not careful, and not reflecting any heat back to you. All of the negatives of looking silly without any of the benefits.

Now that the usefulness of the metal has been show to be invalid, let’s talk to the other putative benefits of space blankets. They can be made into shelters, rain catchers, water-repellent devices such as ponchos and boot liners, and all sorts of first aid items, from wraps to slings to wound dressings. Truth be told, a space blanket would probably work in that situation. A trash bag of any decent thickness would work just as well, and cost a whole lot less. You could get them in fancy colors if you wanted to use them as a signal in snow, and they come in multiple sizes.

Sure, the reflective side of the blanket could serve as a signal, but you’ve already got a mirror on you. (You DO have a signal mirror, don’t you?) If you’ve got one in your pack, don’t throw it out just yet though. You could always use it in a pinch, as long as it hasn’t become stuck to itself by prolonged storage. You might want to check before the next hike though.

So if your friend finishes a road race and is worried about getting cool too fast, give them a jacket. And if you’re in an emergency department and the patient is cold, give them a warm blanket.

Hypothermia and the use of space blankets: a literature review.
http://www.ncbi.nlm.nih.gov/pubmed/9325662

Preventing paresthesias when hiking

Appalachian trailParesthesthias commonly occur in long distance hiking, such as thru-hiking the Appalachian Trail (seen to the left) or the Pacific Coast Trail. They occur in both upper and lower extremities, and can certainly make hiking less fun. Of course, there can be serious causes to paresthesias, so care needs to be taken to not dismiss cerebrovascular accidents, toxidromes, and complications of diabetes. Once those are ruled out, the paresthesia is likely related to repetitive trauma or compression of nerves.

Shoulder straps from backpacks can cause ulnar paresthesias, and most commonly present as numbness or tingling of the ring and little fingers. Hip straps can cause numbness of the outer thigh, usually called meralgia paresthetica. It is also known as Bernhardt-Roth syndrome. Tarsal tunnel syndrome, or posterior tibial neuralgia, can be caused by footwear that compresses the area behind the medial malleous (known perhaps unsurprisingly as the tarsal tunnel). This usually causes numbness or pain of the sole, but can include the toes. If just the toes themselves are affected, then it is known as digitalgia paresthetica, and is commonly attributed to type of shoe padding.

Most of these are well known to long distance hikers, but they can happen to short distance hikers who are out of shape, as well as military recruits. Once they present, the first suggestion is to alter whatever piece of gear is the offending agent. This can mean adjusting or padding the straps of one’s pack, or changing footwear. This may stop it from getting worse, but may not makes symptoms go away.

The question is, can we prevent this? The neuropathy can last for months and be fairly debilitating. This article wasn’t trying to answer just that question, but was instead trying to determine prevalence and predictors of all injuries and illness in long distance hikers. They did this by anonymously surveying thru-hikers of the PCT and AT near the northern terminus of each trail. The responses were included if they had hiked at least 500mi in the last season, and were older than 18. Of the 143 surveys they received, only 15 were excluded.

Paresthesias were common, with 48% prevalence. The survey did allow for localization, but hikers were like most survey takers, and declined to input free text. Since the surveys did have sections asking about footwear and pack weight, the authors had enough of a sample size to analyze these factors.

Unsurprisingly, pack weight had a statistically significant affect on prevalence of paresthias, with packs between 10-20lbs having a ~30% rate of paresthesias, 21-30lbs at ~50%, and >31lbs having a nearly 70% prevalence. Obviously the take home message is to carry less with you if you want to prevent numbness or tingling. 

What is interesting is footwear. While never reaching significance, there was an obvious trend toward minimalist footwear as well. Hiking boots were worst, with ~70% paresthesias, then it drops sharply for low top hiking boots at ~40%. Running shoes were a little lower, and sandals were at the bottom with <30% prevalence of paresthesias. Maybe those guys with Chacos were doing it right all along.

So is ultralight the way to go? From this study, it seems to be a logical and healthy alternative to what many are doing.

Of note, there was also a 50% rate of wilderness-associated diarrhea, in line with earlier studies. I guess they should have washed their hands more.

The Impact of Footwear and Packweight on Injury and Illness Among Long-Distance Hikers
http://www.ncbi.nlm.nih.gov/pubmed/19737037