Download - Teaching Evidence-Based Medicine
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TeachingEvidence-Based Medicine
Gary S Gronseth, MD, FAANProfessor of NeurologyUniversity of Kansas
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To Teach EBM…• Explicitly Reason
– Exclude the unreasonable– Distinguish opinion from principles
• Rate Evidence on a Hierarchy– Understand two sources of error– Love the 2 x 2 table
• Emphasize Evidence never enough– Apply to your patient– Incorporate patient values
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A case…• A 58 year-old right-handed man suddenly
developed problems speaking, right lower facial weakness and right hand clumsiness. His symptoms slowly resolved over a week.
• He had a history of controlled HTN and no other risk factors.
• Head MRI: small left frontal infarct.• EKG: sinus rhythm. • MRA: no cranial artery stenosis. • Echocardiogram: PFO
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The Physician’s Dilemma
To Close or Not to Close
Even if the answer is unknown, a decision must be made!
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Clinical Reasoning
Close PFO?
“Where I trained”
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To Teach EBM…• Explicitly Reason
– Exclude the unreasonable
Clinical Reasoning
Close PFO?
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Deceitful
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“Closure of PFO in patients with cryptogenic ischemic stroke is
the standard of care in the community.”
“The consequences of a second stroke are potentially devastating. PFO closure
is mandatory.”
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Fallacious• Irrelevant• Rhetoric• Psychological appeal• Emotion-Driven• Persuasion
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Patient
Intervention
Co-intervention
Outcome
Determining relevance:Define the question
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For patients with cryptogenicstroke and PFO
does PFO closurevs no PFO closure
reduce the risk of the next stroke
Determining relevance:Define the question
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Popular Appeal
“Closure of PFO in patients with cryptogenic ischemic stroke is the standard of care in the community.”
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Begging the Question
“The consequences of a second stroke are
potentially devastating. PFO closure is
mandatory.”
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Irrelevant Outcomes
I’ll be sued.
I’ll be reimbursed
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Deceitful
Fallacious
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To Teach EBM…• Explicitly Reason
– Exclude the unreasonable
Deceit
Close PFO?
Fallacy
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Deceitful
Fallacious
Reasoned
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Reasoned
•Relevant•Logical appeal•Data-Driven •Truth
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To Teach EBM…• Explicitly Reason
– Exclude the unreasonable– Distinguish opinion from
principles
Principles
Close PFO?
Evidence
Judgment
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Principles
Decision
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Deductive InferenceFrom Principles
• The left side of the brain controls the right side of the body
• My patient can’t control the right side of his body
• My patient has a problem with the left side of his brain
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Principles
Use a Parachute?
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Principles
Close PFO?
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PFO
• Fibrous adhesions fail to seal the atrial septum after birth
• Persistence of a potential shunt between the right and left atria of the heart
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PFO might allow paradoxical embolism
• Small emboli normally filtered by lung without clinically important consequence
• In patients with PFO, emboli can travel to the brain and cause ischemic stroke
• Closing the PFO will prevent future strokes
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Principles
Close PFO?
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Reasoned•Relevant•Reason•Logical appeal•Data-Driven •Truth
Deduction(Principles)
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Principles
Close PFO?
Evidence
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• Bob had a stroke and PFO and wasn’t treated with closure, he had another stroke
• Jane has a stroke and PFO. We should treat her with closure to prevent another stroke.
• John had a stroke and PFO and was treated with closure, he didn’t have another stroke.
• Sue had a stroke and PFO and was treated with closure, she didn’t have another stroke.
Analogy and Inductive Inference
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Evidence
What happened to
patients?
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Principles
Close PFO?
Evidence
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Reasoned•Relevant•Reason•Logical appeal•Data-Driven •Truth
Induction (Evidence)
Deduction(Principles)
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Principles
Close PFO?
Evidence
Judgment
Best Guess
Opinion
Hypothesis
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Reasoned•Relevant•Reason•Logical appeal•Data-Driven •Truth
Intuition(Judgment)
Induction (Evidence)
Deduction(Principles)
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Distinguishing Opinion from Principles• Is there equipoise?
– Do reasonable people disagree?– Would an IRB approve a trial?– Is there an ongoing trial?
• Evidence separates judgment from principles
Principles
Close PFO?
Evidence
Judgment
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Theory
Scientific Method
Experiment
Hypothesis
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To Teach EBM…• Explicitly Reason
– Exclude the unreasonable– Distinguish opinion from
principles
Principles
Close PFO?
Evidence
Judgment
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To Teach EBM…• Explicitly Reason
– Exclude the unreasonable– Distinguish opinion
from principles• Rate Evidence on a Hierarchy
Strong
Weak
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• Jane has a stroke and PFO. We should treat her with closure to prevent another stroke.
• John had a stroke and PFO and was treated with closure, he didn’t have another stroke.
Strong
Weak
• Sue had a stroke and PFO and was treated with closure, she didn’t have another stroke.
• Bob had a stroke and PFO and wasn’t treated with closure, he had another stroke
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Inferences from Evidence…
Are not valid or invalid
Are never certain
Strong
Weak
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• Jane has a stroke and PFO. We should treat her with closure to prevent another stroke.
• John had a stroke and PFO and was treated with closure, he didn’t have another stroke.
Strong
Weak
• Sue had a stroke and PFO and was treated with closure, she didn’t have another stroke.
• Bob had a stroke and PFO and wasn’t treated with closure, he had another stroke
Informally recalled cases
Why is this a weak inference?
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Inferences from informally recalled cases can mislead
• Too few cases• Selective recall: remember those
– That are more recent– With extreme results– That support our pre-conceptions
Experts are not immune to these limitations
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To Teach EBM…• Explicitly Reason
– Exclude the unreasonable– Distinguish opinion from principles
• Rate Evidence on a Hierarchy– Understand two sources of error
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• Often too few cases
• Selective recall: remember those– That are more recent– With extreme results– That support our pre-
conceptions
Two Sources of Error
Systematic
Bias
Random
Chance
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Find More Cases
Retrospective Observational Sudy 2002 to 2010
Of all Stroke and PFO Cases: 319
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Rats…I’m going to have to
start counting these cases
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To Teach EBM…• Explicitly Reason
– Exclude the unreasonable– Distinguish opinion from principles
• Rate Evidence on a Hierarchy– Understand two sources of error– Love the 2 x 2 table
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Relationships between variables
PFO Closure and Stroke
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2 X 2 TableOutcome
Treatment No stroke
Stroke All
Closure 167
No Closure 152
Total 239 80 319
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Expected if No RelationshipOutcome
Treatment No stroke
Stroke All
Closure 125 42 167
No Closure 114 38 152
Total 239 80 319
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Expected if no RealtionshipOutcome
Treatment No stroke
Stroke All
Closure 75% 25% 100%
No Closure 75% 25% 100%
Total 75% 25% 100%
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“Actual”Outcome
Treatment No stroke
Stroke All
Closure 150 17 167
No Closure 89 63 152
Total 239 80 319
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“Actual”Outcome
Treatment No stroke
Stroke All
Closure 90% 10% 100%
No Closure 59% 41% 100%
Total 75% 25% 100%
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2 X 2 TableOutcome
Treatment No stroke
Stroke All
Closure a b 167
No Closure c d 152
Total 239 80 319
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Relative Risk stroke =b/(a+b)
d/(c+d)
Risk difference stroke = b/(a+b) - d/(c+d)
Measures of AssociationOutcome
Treatment No stroke
Stroke All
Closure a b 167
No Closure c d 152
Total 239 80 319
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Measure of AssociationRelative Risk Stroke Outcome
Outcome
Treatment No stroke
Stroke
Closure 90% 10%
No Closure 59% 41% RR Stroke10/41 = 0.24
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Cryptogenic stroke patients receiving Closure were 0.24 times less likely to have stroke.Therefore, I should offer my patients with
stroke and PFO Closure.
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To Teach EBM…• Explicitly Reason
– Exclude the unreasonable– Distinguish opinion from
principles• Rate Evidence on a
Hierarchy– Understand two sources of
error– Love the 2 x 2 table
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Random (Sampling) Error--Incorrect result from bad luck
Equally likely to be too high or too lowStatistical power/precision
--Measured by:
P-values (p < 0.001)
Confidence intervals RR 0.24: (95% confidence intervals 0.15 to 0.40)
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Systematic Error
Incorrect results from poor study design or execution
More likely to be too high or too low
Risk of Bias Measured:
Semi-quantitatively
Class of Evidence
0.5 0.75 1 1.25 1.50.25
TruthMeasured
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Our Study
+Cl
-Cl
Stroke No Stroke
Patients not receiving Closure were more often older, diabetic and hypertensive
Sometimes had to “guess” the outcome from the record.
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Major Sources of Bias
+Cl
-Cl
Poor Good
Confounding
Misclassification
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Lower Risk of BiasThe Randomized Masked Trial
+Cl
-Cl
Poor Good
R
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Randomized Masked Trial
SingleCase Report
What is the risk of Bias?
Strong
Weak
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Find the best evidenceSearch online databasesMEDLINE
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There is insufficient evidence to
support or refute the benefit or
lack of harm of PFO closure.
Conclusion
Strong
Weak
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Despite the weak evidence, a
decision must be made.
Decide
Strong
Weak
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To Teach EBM…• Explicitly Reason
– Exclude the unreasonable– Distinguish opinion from
principles• Rate Evidence on a Hierarchy
– Understand two sources of error– Love the 2 x 2 table
• Emphasize Evidence never enough– Apply to your patient– Incorporate patient values Principles
Close PFO?
Evidence
Judgment
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Induction is never certain
• Often the evidence is weak• Even when strong, the Evidence never
perfectly applies to your patient
• Explicitly consider how well the evidence applies to your patient
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Incorporating patient values
Benefits Risks
Uncertainty
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Know what is not Known
• If you fail to acknowledge the uncertainty and tell the patient we know that the PFO should or should not be closed…
• You have failed to distinguish opinion from principles. Principles
Close PFO
Evidence
Judgment
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To Teach EBM…• Explicitly Reason
– Exclude the unreasonable– Distinguish opinion from
principles• Rate Evidence on a Hierarchy
– Understand two sources of error– Love the 2 x 2 table
• Emphasize Evidence never enough– Apply to your patient– Incorporate patient values Principles
Decision
Evidence
Judgment