Cardiac Stress Testing
- Rationale: noninvasive means to diagnosis high-grade epicardial stenoses (typically >50%)
- Does not detect subclinical CAD, ruptured plaque, or viable myocardium
- Physiologic, not anatomic test – does not actually image coronaries
- Modalities:
- Exercise ECG (ETT)
- Physiology: patient exercises which monitored continuously via ECG, and repolarization changes/symptoms are noted
- Advantages: widely available, cheap, provides information on functional status
- Disadvantages: poor performance (sensitivity 68%, specificity 77%), cannot be used with certain baseline ECG abnormalities (LBBB, preexcitation, baseline ST depression, LVH)
- Based on this, ETT should only be used to gauge a patient’s exercise capacity, and if one is truly interested in ischemia, an imaging modality should be ordered
- Radionuclide myocardial perfusion imaging (single photon emission computed tomography)
- Physiology: radiolabeled isotopes (e.g. thallium, technetium-labeled compounds) are injected, and when uptaken by cardiac myocytes, are indicative of perfusion (delivery of tracer), and viability (uptake)
- Image at rest → absence of uptake typically indicates infarcted myocardium
- Image under stress → absence of uptake indicates either ischemic or infarcted myocardium
- Mismatches in uptake are ischemic regions
- In addition to mismatched perfusion, transient ischemic dilation (marker of multivessel disease) and post-stress LVEF are prognostic factors
- Advantages: performance (sensitivity 88%, specificity 77%), can measure LV size and EF, vasodilator MPI can be used in setting of existing LBBB
- Disadvantages: radiation exposure, measures only relative blood flow between regions (can miss “balanced ischemia”), artifacts from breast/diaphragmatic tissue
- Tracers:
- Thallium: redistribution properties allow for viability assessment, but given long half-life can only use lower doses, which compromises image quality, and lower energy increases artifact
- Technetium: does not redistribute, thus allowing for gated imaging and LVEF estimation, but limited ability to evaluate viability
- Method of stress:
- If possible, exercise patient, as it provides additional prognostic information:
- Exercise capacity
- Exercise BP: drop in BP suggests severe 3-vessel disease
- Chronotropic competence
- Post-HR recovery: impaired vagal tone is poor prognostic sign
- Ventricular ectopy
- Pharmacologic:
- Vasodilator to cause relative increase in flow to non-diseased coronary arteries (adenosine, dipyridamole): caution in reactive airway disease
- Dobutamine: not typically used in MPI, as performance is inferior, but may be indicated in patients with severe COPD or asthma
- Stress echo
- Physiology: regional ischemia provokes by stress is evidenced by wall motion abnormalities
- Advantages: performance similar to MPI (sensitivity 76%, specificity 88%), provides structural information, cheaper and faster than MPI, no radiation, dobutamine stress can be used in setting of existing LBBB
- Disadvantages: subjective interpretation
- Method of stress:
- As above, exercise if possible
- Pharmacologic
- Dobutamine: increases HR/contractility and myocardial oxygen demand, thus inducing wall motion abnormalities in ischemic regions
- Vasodilators: not used, as sensitivity is inferior
- Impact of medications:
- Given their anti-ischemic effects, beta blockers, calcium channel blockers, and nitrates can result in false negative results and should be held for 4-5 half lives
- Beta blockers and calcium channel blockers can also prevent peak HR from being reached in exercise tests
- Caffeine can interfere with pharmacologic MPI (adenosine receptor antagonist)
(Christopher Woo MD, 4/19/11)