Re: The parachute analogy
As Christmas approaches and The BMJ is putting its midwinter edition to bed we reflect on 13 years of needless confusion around the parachute analogy.  When is it helpful to apply it and when is it not? For example, it applies to tension pneumothorax where there is a high and imminent likelihood of death. Prompt insertion of a needle or tube alleviates the problem with an audible hiss and an immediate, clinically evident beneficial effect which is mechanistically explicable and, if before and after radiology is available, the effect can be irrefutably confirmed. Our own concern about the parachute analogy is because it is brought up in rebuttal of the need for the PulMiCC randomised controlled trial of lung metastasectomy.
There has now been a randomised trial of parachutes. Researchers dropped rag dolls with and without parachutes in random order from a height of 20.83 metres and then checked to see whether the various air or water filled balloons within them were ruptured. Under their controlled test conditions the dolls with parachutes had fewer ‘organ’ injuries and fractures. In spite of this evidence, it is unlikely that commercial airlines would consider it worthwhile to put parachutes under the seats of passengers or crew. One reason might be that the risk of death from this cause is so low that the intervention cannot be justified but more likely is that the efficacy of parachutes under specific circumstances does not translate into general effectiveness in modern civil aviation. General awareness of the analogy is unsurprising: Smith and Pell have been cited 829 times (make that 830 now) and some of those readers have been seriously misled.
Analogies can lead to useful discussions, but need to be taken in context when making decisions on patient care.
On a radiological “now you see, it now you don’t” basis metastasectomy works but unlike dropping out of a plane, lung metastases are not a cause of imminent death. In fact, unlike the fall, it is not the metastasis that kills you. The careful selection of a small minority (≤3%) of all the possible candidates to be offered elective metastasectomy makes it utterly unlike dropping out of a plane and the analogy is inapplicable. In modern multimodality cancer care, repeated interventions are interspersed in the years of a patient’s cancer journey. Under these circumstances, discerning the signal from the noise is not as easy as seeing why people dropping out of planes for recreation are well advised to use a parachute.
1 Smith GC, Pell JP: Parachute use to prevent death and major trauma related to gravitational challenge: systematic review of randomised controlled trials. BMJ 2003;327:1459-1461.
2 Wallace A: Managing in flight emergencies. BMJ 1995;311:374-376.
3 Czorlich P, Burkhardt T, Buhk JH, Matschke J, Dreimann M, Schmidt NO, Eicker SO: Does usage of a parachute in contrast to free fall prevent major trauma?: a prospective randomised-controlled trial in rag dolls. Eur Spine J 2016;25:1349-1354.
4 Treasure T, Mineo T, Ambrogi V, Fiorentino F: Survival is higher after repeat lung metastasectomy than after a first metastasectomy: Too good to be true? J Thorac Cardiovasc Surg 2015;149:1249-1252.
5 Jawed I, Wilkerson J, Prasad V, Duffy AG, Fojo T: Colorectal Cancer Survival Gains and Novel Treatment Regimens: A Systematic Review and Analysis. JAMA Oncol 2015;1:787-795.
6 Glasziou P, Chalmers I, Rawlins M, McCulloch P: When are randomised trials unnecessary? Picking signal from noise. BMJ 2007;334:349-351.
Competing interests: No competing interests