Because patients having cardiac surgery are already known to have cardiac dysfunction, the purpose of an ECG is not to detect dysfunction, but rather to form a basis for comparison (i.e. to aid in the detection of intraoperative events) as well as to define the “ischemic threshold” (when used in the context of a stress test). ECG-based stress testing has lower sensitivity than TTE-based stress testing.
Pharmacologic Stress Testing
Dipyridamole-Isotope Scintigraphy (DIS)
Adenosine is a myocardial vasodilator. Dipyridamole blocks adenosine reuptake. Thus, dipyridamole maximally vasodilates coronary arteries, preferentially “stealing” blood away from stenotic vessels and simulating the effects of exercise or stress, an effect that can be visualized following injection of isotopes (ex. thallium, technetium scintigraphy). In order to approximate the amount of “susceptible” myocardium, two injections are required – an initial injection of dipyramidole + isotope (underfilled areas are either dead or susceptible), and a second injection of isotope alone (underfilled areas are dead, areas of myocardium which have re-filled as compared to the initial injection are “at risk”).
Dobutamine Stress Echocardiography (DSE)
Dobutamine Stress Echocardiography (DSE) differs from Dipyridamole-Isotope scintigraphy in that dobutamine actually causes the heart to work harder (as opposed to simulating it via coronary vasodilation) and uses TTE, not scintigraphy, to image the heart. DSE is as sensitive as Dipyridamole-Isotope scintigraphy and has the added advantage of assessing valvular competence and left ventricular function. Furthermore, DSE does not require a second set of imaging, and can thus be accomplished more quickly than Dipyridamole-Isotope scintigraphy.
Besides its obvious use in DSE (see above), echocardiography is useful in evaluation of left ventricular and valvular function. PA pressures can often be estimated from preoperative TTE (based on tricuspid regurgitation, if present).
The gold standard for CAD workup. Left-sided catheterization can be accomplished via both the radial and femoral arteries, although if the radial artery is a candidate graft (for CABG), the femoral (which generally leads to increased complications) may deserve additional consideration. While right-sided catheterization is often accomplished in the operating room (PA catheterization), a study of 200 patients undergoing L-heart catheterization for CAD workup showed that the addition of R-sided catheterization added 6 minutes of total time (90 seconds of fluoroscopy), but only changed management in 1.5% of cases [Hill JA et al. Am J Cardiol 65: 590, 1990]. Thus, routine right-sided catheterization cannot be recommended in the setting of a CAD workup.
Capable of assessing ejection fraction as well as segmental wall motion abnormalities (SWMAs). SWMAs are rated according to the following scale:
SWMA Rating System 0: normal 1: mild hypokinesis 2: moderate hypokinesis 3: severe hypokinesis 4: akinesis 5: dyskinesis (paradoxical or aneurysmal motion)
Rated based on LA opacification (1+ to 4+) and regurgitant fraction (mild to severe)
Angiographic MR (LA Opacification) Mild: (1+) contrast clears from LA in one beat (never complete opacification) Moderate: (2+) contrast clears from LA in several beats (never complete opacification) Moderate-Severe: (3+) complete opacification of the LA Severe: (4+) dense opacification with one beat, into pulmonary veins
Angiographic MR (Regurgitant Fraction) Mild: 20% RF Moderate: 20-40% RF Moderate-Severe: 40-60% RF Severe: > 60% RF
Normally the left main (1-2.5 cm) bifurcates into the left anterior descending (LAD) and the circumflex (CX) arteries. Rarely, it may split into three arteries, the LAD, CX, and ramus intermedius, or, alternatively, it may be absent (in which case the LAD and CX have their own ostia).
The LAD gives rise to diagonals (D) and septal perforators (S).
The CX (located in the AV groove) gives rise to the obtuse marginals (OM).
In 85-90% of patients (i.e. “right dominant”), the posterior descending artery (PDA) arises from the right coronary artery. In 10-15% (i.e. “left dominant”), it arises from the CX. Note that in 2/3 of patients, the apex is supplied by the LAD, but in 1/3 of patients it is supplied by the RCA.
By convention, stenosis is reported in terms of diameter. Thus, a 50% stenosis of the LAD refers to a 75% reduction in cross sectional area (and a 16-fold increase in resistance to flow). Collateralization begins to form when stenosis approaches 80%.
While catheterization is the gold standard for assessment of CAD, several caveats must be kept in mind. First, most patients undergoing catheterization have been fasted (and thus may exhibit decreased filling pressures. The exception is for patients with pre-existing renal disease, in whom hydration is accomplished overnight). Second, the patient’s medication regimen on the day of the procedure may be different than at baseline (ex. diuretics may be held). Third, these procedures usually involve some sedation, and potentially changes in PaO2 and PaCO2 [Kaplan JA, ed. Essentials of Cardiac Anesthesia. Saunders, 2008 p 32].