Exercise stress test ( Also called treadmill test ) is an important investigation not only in patients with suspected CAD but also in established CAD . In the former group , it helps us to exclude CAD in patients with chest pain and in the later group , it helps us to assess functional capacity , risk stratification and to detect any additional ( New or residual ) ischemia.
Stress test being a physiological test , has a huge advantage of assessing the adequacy of myocardial blood flow without even knowing the coronary anatomy , while Coronary angiogram (CAG) has a zero physiological value* in spite of excellent assessment of the coronary anatomy !
It is an irony , in the assessment of angina we are expected to assess the physiological adequacy of myocardial blood flow , we have kept coronary angiogram as a gold standard over and above the much neglected physiological stress test.
Of course, the limitation of stress test is that , it has only 75% specificity( to rule out CAD ) and about 80% sensitivity (To detect CAD ) .In simple terms stress test is likely to miss 20% times to miss a CAD in patients with CAD and 25% of times falsely diagnose CAD in patients without CAD.
In the above statistics , coronary angiogram was considered gold standard . The problem with this data is that , CAG is not the real gold standard ,but it was nominated as a gold standard . We now know normal coronary angiogram is not equivalent to normal coronary arteries and vice versa.
While both test have limitations , it is logical to believe CAG has an edge over stress test since it visualises the anatomy. But , once an obstruction is demonstrated by CAG, stress test scores over in assessing the physiological impact of the lesion.
Is a 70% LAD lesion significant or not ?
Stress test will give vital information to answer this question.If this patient performs 10-12Met exercise without symptoms it means , the obstruction is not impeding the flow even during stress. He may do well with medical therapy.
What does a positive stress *mean for the patient and for the physician ?
(* A false positive EST in LVH, anemia, baseline ST shifts are included in discussion )
- A positive stress test with or without angina at low workload <5 METS indicates very significant obstructive CAD either in left main , or proximal LAD/LCX. They should get immediate CAG.
- A positive stress test at load 5-10METS is again significant and patients should get early CAG
- A positive stress test with angina at good work load >10-12 mets would indicate insignificant or minimally obstructive CAD.
- A positive stress test at the peak of exercise at good work load > 10-12METS without angina could indicate a false positive or very minimal CAD.
For the physician , the proper way of interpretation should be , the fact that a person performs 10-12 METS indicate the myoacardial blood flow would be more than adequate in most life situations. Knowing the coronary anatomy serves no purpose here, as no revascularisation will be attempted even if he is going to have a significant CAD ( Which again , is also highly unlikely ) .He should be managed with appropriate lifestyle (Diet, activity, relaxation ) anti anginal drugs, aspirin , good lipid control and plaque stabilisation with statins .
Can a patient with critical left main or proximal LAD perform >10METS in exercise stress test ?
No , large clinical experience (Also refered to Class C evidence by ACC/AHA!) indicate no patient with critical left main or equivalent disease can perform 10 METS excercise
While , EST may be less hyped investigation, but it is the only noninvasive test , ( that too , simple and cheap ) that can rule out * a significant left main or equivalent almost 100% correctly .
Now that, the results of COURAGE and BARI 2D have clearly indicated medical therapy is best form of management in chronic CAD , ( except in severe obstructive CAD in vital locations) a positive EST at > 10-12Mets , has absolutely no indication* to for doing a CAG.
*Some would advocate a policy of doing a CAG as a baseline investigation in all patients with positive EST to know the coronary anatomy and will not proceed onto revascularisation if there is insignificant lesions.
Further , real life experience has taught us , routine CAG in these patients
- Increases patient anxiety as he is given a report with a diagram of obstructed heart vessels
- Leads to multiple cardiac consultations
- Divergence of opinions
- Finally end up in the likely hood of a inappropriate revascularisation for a insignificant distal CAD.
Every patient, who has positive stress test , ( Please note , it could even be true positive ) need not undergo CAG . Most interventional cardiologists could feel otherwise , but one should also remember , There is one more role for the interventional cardiologist ie , to intervene when inappropriate interventions are done to their patients.