CAD Flashcards
What determines myocardial oxygen demand?
inc if in inc HR, systolic pressure, volume (less of an effect) or contractility
How is myocardial oxygen supply regulated at rest?
- Goal = maintain flow
- If dec perfusion pressure then must also dec resistance
- R2 fluctuates to maintain constant Q despite varying perfusion pressures (P); R2 can only decrease so much so once perfusion pressure drops a lot, R2 vessels cannot dilate anymore to keep up
- Coronary vasodilator reserve = diff in resting and peak coronary flow w/ max R2 vasodilation
How is resting coronary blood flow preserved in presence of obstructive disease?
- R1 vessels are main sites of atherosclerosis which inc R1 resistance AND R3 may inc as ventricle stiffens
- SO… to maintain flow R2 resistance must decrease (vasodilation) even at rest
- At rest… auto-regulation maintains normal coronary flow in pt w/ CAD so cannot dx at rest
What happens to coronary supply in someone w/ CAD during exercise?
- During exercise… R2 vessels are already at max dilation —> flow can no longer match oxygen demand —> ischemia
- Made even worse as R3 increases w/ inc EDP and EDV
Why is the endocardium at greatest risk for ischemia? What are 2 ways it tries to compensate for this?
- Greatest inc in demand - greatest R3 resistance - more force on endocardium wall means more pressure which is one factor that inc myocardial oxygen demand
- More demand = needs more blood flow
- BUT has less time to get blood flow b/c most affected by systole - flow blocked to endocardium when ventricle contracts
- So must inc amount of flow in time it does have by…
- 1- Inc # vessels in endocardium
- 2- More R2 vasodilation in endocardium in normal heart; this means less reserve when CAD (already fully dilated normally)
% Stenosis as a Predictor of Flow (what would a better predictor be?)
- Percent stenosis is often used as a predictor of flow; claim that at rest you can maintain normal flow up to 80-90% stenosis then steep drop off in flow and during exercise you can maintain flow up to 50% stenosis then steep drop off in flow
- So 50% stenosis/obstruction used as cut-off; less than 50% is still okay and greater than 50% is significant lesion
- Problems w/ this…
- Study not reproducible
- How do you decide what original radius is? May be comparing it to an already mildly stenoses nearby vessel
- Length of lesion also affects flow (longer lesion = more severe drop off in flow)
- Better predictor = minimal luminal cross sectional area
3 Ways to Induce Stress in Stress Test
1- Exercise (incremental inc in speed and elevation every 3 min)
2- Vasodilators (adenosine) - vasodilation or coronary vessels will inc perfusion pressure
3- Inotropic agents (dobutamine) - inc HR and contractility; similar to exercise this will inc oxygen demand of the heart so body will compensate w/ inc flow
6 Predictors of Worse Outcomes on Stress ECG (which is #1 predictor?)
1- Greater ST depression magnitude
2- ST depression slope (down-slope worse than horizontal)
3- Chest pain during testing
4- Peak HR (if diminished - chronotropic incompetence) suggests higher mortality
5- Functional capacity of heart (BP, HR, how long pt can remain on treadmill w/ incremental inc) - #1 predictor of risk if abnormal function
6- HR recovery (HR should dec by 12 BPM+ in 1st min after stopping); poor recovery is linked to worse outcomes and varies directly w/ functional capacity
Sensitivity & Specificity of Stress ECG v. Stress SPECT
ECG
- Sensitivity - about 70%
- Specificity - about 75%
SPECT - inc sensitivity w/ same specificity
How does SPECT perfusion study add to prognostic power of stress testing?
- Stress test ECG can stratify patients into low-intermediate-high risk of adverse cardiac event
- Then if you go on to perform SPECT in intermediate group it further stratifies them based on # vascular territories w/ decreased perfusion
- If you go on to perform SPECT in high risk group but they have a normal SPECT this greatly reduces risk of a cardiac event/risk of mortality
How do SPECT perfusion studies work?
*short, vertical and long axis images of heart while exercising v at rest
- Look at perfusion at rest v. perfusion in exercise
- If dec perfusion at rest … infarction
- If only dec perfusion in exercise … exercise induced ischemia
What patients should receive a stress test?
- Use in intermediate risk patients (ex- abnormal angina)
- Bayes Theorem - those w/ low pre-test probability have low chance of disease even w/ pos result; those w/ high pre-test probability have high chance of disease even w/ neg test result; so those w/ intermediate pre-test probability have greatest benefit from the test in terms of differentiating whether they are at risk
- Risk based on inc age, gender (higher in men) and symptoms (mainly typical angina)
