CAD Flashcards
(12 cards)
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)
