How to think about stress-EKG testing

Cardiac investigation is getting all digital and full of imaging, and so it’s getting expensive. It’s nice to have EKG stress test equipment in your clinic, for lower-cost investigation.  It’s quite reliable, used right, and we sell lots of packages to happy clinicians all the time. 

The guidelines do say to choose your patients carefully.  The ideal candidate for a stress EKG test is an adult with suspected coronary artery disease, or someone with known CAD whose clinical status is changing.  Clinicians also use them for lower-risk stable angina patients free of active ischemic or heart failure symptoms, and for risk stratification of patients prior to discharge, or for pre-operative evaluation of selected patients sometimes. For all stress-EKG patients, baseline EKG should be close to normal. 

Clinicians aren’t doing stress tests in patients with pacemakers, or complete left bundle branch block, or greater than 1 mm ST depression or pre-excitation pattern on the EKG.  Other contraindications include acute coronary syndrome in the last 5 days, poorly controlled heart failure, severe symptomatic aortic stenosis, acute endo-, myo-, or pericarditis, hemodynamically significant and uncontrolled arrhythmias, pulmonary embolism/infarction, and severe peripheral vascular disease. 

There are risks even for patients who qualify, though they’re slim.  You’ll need to brief your staff on medications that should ideally be stopped prior to the test, like AV nodal blocking agents such as beta-blockers to reduce blunting the heart rate response, digoxin also to reduce artefactual effects on the EKG, and nitrates which may increase false negative rates.

When you do the test, you normally do a standard 12-lead EKG at baseline. The Mason-Likar modification in which the extremity electrodes are moved to the torso can be used to reduce motion artifact.  Remember that this may cause rightward axis shift, increased R wave amplitude and loss of Q waves in the inferior leads, and development of new Q waves in aVL.

It’s not for us to tell you how to interpret your findings and make diagnoses, but we can tell you what the guidelines these days say about the kind of reliability you can expect from your stress EKG equipment. 

The exercise test alone has a lower sensitivity and specificity than imaging, but it has a higher negative predictive value.  This is why it’s especially valuable in low risk patients.  Be advised, though, that it has lower predictive value in elderly people and women when used by itself.  In fact, in asymptomatic women, low exercise capacity, slow heart rate recovery and inability to reach target heart rate have been found to be more important predictors of outcome than exercise induced EKG changes.  Obviously, in patients with prior myocardial infarction or coronary interventions, adding imaging gets you better sensitivity to ischemia.

That exercise-EKG testing also requires a normal baseline EKG may be a problem for some patients.  We’ve found papers that advise that intrinsic electrical conduction abnormalities can compromise usefulness of the data – this is with things like left ventricular hypertrophy, left bundle branch block, or medications like digoxin.

We’re advised that using stress EKG to rule out CAD, whether with imaging or without, should be done with caution, because patients’ sensitivities may have been over-estimated due to referral bias.  We’ve also read that patients with negative stress tests can actually have underlying one or two-vessel CAD, clinically dormant.

But -- using angiography as the gold standard for detection of CAD probably over-estimates the actual prevalence of the clinically significant syndrome.  Adjusting for that bias, the estimated sensitivities may fall significantly, effectively reducing the predictive value of ANY stress test, with or without imaging. 

This goes for any of the stress-EKG equipment models you might buy from us.  We’ve got an enormous range for you to choose from, in any case.  While you’re browsing, have a look at the Quinton Q4500.  It’s a tidy, one-cart package, extremely flexible, with lots of good filtration for things like artifact and baseline wander.  Reconditioned, it’s a genuine value.  Pair it up cost-effectively with the Quinton ST55 Treadmill, another first-class pre-owned find, designed expressly to work with Quinton machines.  

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