Evidence based medicine and Black Swans

Evidence based medicine tells us that
a) beta blockers are useful for those with atrial fibrillation (1),
b) that beta blockers are useful for those in heart failure with systolic dysfunction (2).

Can we extrapolate and say that beta blockers will therefore be useful for those suffering from atrial fibrillation and heart failure with systolic dysfunction? Will we prescribe beta blockers for those with atrial fibrillation and heart failure based on evidence? I am sure most of us will answer “yes” to both questions.

Unfortunately this view turns out to be incorrect as new evidence from a meta-analysis of beta blockers in patients with atrial fibrillation and heart failure with systolic dysfunction shows. The authors of this analysis, published in JACC Heart Failure in February 2013, say: The main finding of the present meta-analysis indicates that the effect of beta-blockers in patients with Heart Failure (HF) and Atrial Fibrillation (AF) is significantly different from the effect of these drugs in patients with HF and sinus rhythm. Indeed, beta-blockers were not found to have a favourable effect on HF hospitalizations or mortality in 1,677 AF patients who had been enrolled in placebo-controlled, randomized studies (3).

Unexpected results or events are what the author and Dean’s Professor in the Sciences of Uncertainty at the University of Massachusetts, Nasim Nicholas Taleb, calls Black Swans. The term arises from the once-held conviction that all swans are white because no one had ever seen a black swan for hundreds of years. A Black Swan, by definition, is a rare event that cannot be predicted by existing knowledge but which, when it occurs, can have tremendous impact on our lives or radically change our thinking. We make ourselves open to Black Swans when we allow our knowledge to blind us by making us believe that we know everything. Black Swans remind us that we must never be too confident that we know everything simply because we know a lot of stuff.

Evidence based medicine is a great repository of knowledge. When a form of treatment is proposed, we often ask the question: Is it evidence based? The assumption is that, if it is evidence based, we can be sure of the kind of effect the proposed treatment will have. Black Swans tell us that evidence based medicine should be used for predicting results only after being aware of variations in clinical situations between the past and the present. There is a subtle randomness about reality, and variations between individuals, that should make us aware of possible errors in extrapolating data from the past.

Intellectual humility is a mark of wisdom because it makes doctors aware that there are many things they do not know. Only through wisdom can we avoid being struck by Black Swans.

References: 
1. Rodney H. Falk. Atrial Fibrillation. N Engl J Med 2001; 344:1067-1078 
2. Wilson S. Colucci. Use of beta blockers in heart failure due to systolic dysfunction. Uptodate.
3. Rienstra M, Damman K, Mulder BA, et al. Beta-Blockers and Outcome in Heart Failure and Atrial Fibrillation: A Meta-Analysis. JCHF. 2013;1(1):21-28.