I bring you a weekly bite-sized chunk of the science behind helicopter human factors and CRM in practice, simplifying the complex and distilling a helicopter related study into a summary of less than 500 words.
TITLE:
Tolerance to head-up tilt and suspension with elevated legs.
WHAT?
Nearly 30 year-old study examining how the body tolerates prolonged upright positioning and whether supporting and elevating the legs during suspension can reduce the risk of pre-syncope. Syncope is a temporary loss of consciousness caused by insufficient blood flow to the brain, usually followed by rapid, spontaneous recovery. In helicopter hoisting there is a risk of cardiovascular collapse when casualties are hoisted vertically, especially following water immersion due to circulatory changes that reduce blood pressure and cerebral blood flow.
WHERE?
Denmark, by the Copenhagen Muscle Research Center and the Royal Danish Air Force.
WHEN?
The study was published in Aviation, Space, and Environmental Medicine in 1998.
WHY?
The study wanted to establish the extent to which lifting casualties vertically during helicopter rescue, particularly people recovered from water or boats, could provoke fainting or even cardiovascular collapse. They therefore investigated participants’ normal tolerance to upright positioning and whether maintaining venous return by elevating the legs could reduce this risk.
HOW?
79 healthy volunteers underwent passive 50° head-up tilt for up to one hour. A separate group of nine were suspended for an hour using two padded strops (one around the chest and another behind the knees) keeping the torso upright but elevating and supporting the legs. Participants remained motionless while blood pressure, heart rate and indicators of central blood volume and oxygenation were monitored. Testing stopped if pre-syncopal symptoms developed.
FINDINGS:
69 of 79 (87%) subjects developed pre-syncopal symptoms during head-up tilt, half of them within 27 minutes and 20% within 10 minutes.
By contrast, only one of nine (11%) experienced pre-syncope during suspension with elevated legs. Physiological measurements indicated that leg elevation helped maintain central blood volume and venous return.
The study also references a fatal case involving an apparently healthy soldier left suspended vertically by a chest strop, illustrating the potential consequences of prolonged unsupported suspension.
SO WHAT?
For helicopter hoisting, the study was early experimental evidence that casualty orientation matters. A conscious casualty who appears stable before extraction may become pre-syncopal during vertical lifting because gravity shifts blood away from the central circulation. The risk may be amplified following cold-water immersion, while a chest strop can further compromise venous return.
Supporting and elevating the legs substantially improved tolerance in this experiment. The authors therefore considered a more horizontal position preferable and proposed a two-strop arrangement supporting the chest and knees when practicable.
It is now well understood in modern hoist operations that how a casualty is suspended is a physiological as well as a technical consideration. Hoist procedures, equipment design and training should consider body orientation, leg support and the duration of unsupported vertical suspension.
REFERENCE:
Madsen, P., Svendsen, L. B., Jørgensen, L. G., Matzen, S., Jansen, E., & Secher, N. H. (1998). Tolerance to head-up tilt and suspension with elevated legs. Aviation, Space, and Environmental Medicine, 69(8), 781–784
