Picture this.
A 60 year old male with a prior history of CHF and COPD is
brought to your ED by ambulance in respiratory distress. On exam, he looks sick
and is wheezing. While waiting for a
portable x-ray, you give him sublingual nitrates, frusemide, salbutamol
(albuterol), steroids and morphine as you really have no idea what the heck is
going on. BiPAP is getting set up. But what if you had a tool at the bedside
that could give the diagnosis in seconds?
Bring on lung ultrasound. Answer in seconds? Well, maybe.
This systematic review and meta-analysis tried to determine
the sensitivity and specificity of ultrasound using B-lines in diagnosing acute cardiogenic pulmonary edema (ACPE) in
patients presenting to the ED with acute shortness of breath.
The authors conducted an impressive study with an excellent search of the literature,
quality assessment and analysis of the data. This was really well done.
This study design is good for generating statistical power
and getting summary estimates. However the Achilles heel of any meta-analysis
is publication bias and the
potential for the garbage-in and
garbage-out phenomenon. No matter how much one performs a beautiful study,
one is always limited by the quality of the original data. Put another way, two third graders do
not make a sixth grader.
Skipping right to the results- they found seven studies to
combine. After some serious number crunching they gave summary estimates of the
sensitivity to be 94% (95% CI
81-98%), specificity 92% (95% CI
84-96%), positive likelihood ratio 12.4
and negative likelihood ratio was 0.06.
These are impressive numbers! But unfortunately, there are some
big limitations.
Ultrasound does not
have “fixed” test characteristics. Studies of ultrasound are usually done by a group of experts in motivated centres. But in real life, the sensitivity and the specificity
change with each person that picks up the probe as experience is variable. So it is a bit silly
to report results to a couple of decimal places when it is really going to change substantially according to operator experience. In addition, I would imagine there
would be some spectrum bias- i.e. the test works better in patients that are
sicker.
This meta-analysis included only two studies conducted in Emergency Departments. Both were small and
heterogeneous. What if there were other unpublished small studies that were
negative? The results could be quite different.
In the end, we really
don’t know the answer. But I think it probably works ok with experienced
point-of-care sonographers in with respiratory distress.
More research on this application of ED ultrasound would be
fantastic but unfortunately will suffer from many of the limitations noted
above. (I didn't even mention issues with the gold standard). It may be that we will remain stuck with limited good quality evidence.
Therefore we will rely on experience, common sense and consensus. Eminence
based medicine anyone? And while you're there pass me the ultrasound machine.
Covering:
Deeb, M, Barbic Skye, Featherstone R, et al. Point-of-care
Ultrasonography for the Diagnosis of Acute Pulmonary Edema in Patients
Presenting with Acute Dyspnea: A Systematic Review and Meta-Analyisis. Acad
Emerg Med 2014;21:844:852
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