cardiac stress testing

see also:

• Mx of possible acute coronary syndrome without ST elevation (NSTEACS)
• cardiac stress ECG testing

• the sensitivity of the various modalities of stress testing is generally low, identifying less than half of those who will go on to have a further adverse cardiac event
• the problem arises from the fact that, unlike as is the case with stable angina where there is a chronic limitation to coronary blood flow, patients with ACS tend to have a transient, dynamic thrombotic occlusion of a coronary vessel, hence if performed, these tests should ideally be done at the time of the ACS, PRIOR to discharge.
• traditional 12 lead ECG stress testing has largely been displaced by other investigations given its poor sensitivity of about 30% and specificity (particularly in women), with a NPV of around 86%
• patients with a high pre-test probability of coronary artery disease with risk of a major adverse cardiac event (MACE) within 1 year of > 10% should go straight to coronary angiography

• see diagnostic tests - sens, spec, PPV, NPV, etc
• the positive predictive value is generally low (<10% for cardiac death and < 20% for cardiac death or AMI), and thus, they should only be performed on patients with a reasonable pre-test probability - ie. not on 30yr olds who are worried they might have heart disease but do not have clinical features to suggest cardiac disease.

• probably should be considered for invasive investigation rather than stress testing as even a negative test result will not lower the probability of ACS sufficiently to warrant avoiding an invasive test.
• clinical risk markers are perhaps more reliable than stress testing in determining who should have invasive investigation such as coronary angiography.

• if pre-test probability of MACE in the next year is > 2-3% then these patients are most likely to benefit from “stress testing” given that LR+ of these tests are around 4-5.

• these patients probably should be counseled before embarking on stress testing as they are much more likely to have a false positive result than a true positive result and thus may end up having unnecessary and risky invasive testing.
• for instance if sensitivity is 50% and specificity is 90% and the pre-test probability is only 5%, then of 1000 patients tested, 125 will have a positive result, but of these, 95 will have a false positive result.
• for patients with “non-ischaemic sounding chest pain”, over 90% of positive ECG stress tests will be false positives! ¹⁾
• patients with normal ECG's, troponins, no high risk factors such as PH IHD/diabetes and pain is not typical angina pain, probably have a probably of MACE of 0.5% in the next 1 year, thus for an investigation to be useful in this population to put them over the 10% MACE threshold for coronary angiography referral, the test's LR+ must be at least 20 and none of our current tests have a LR+ anywhere near that (most are around 4-5).
  • thus for every 1000 patients in this group, 5 will be at risk of MACE but 25 will be “positive” of which 20 are false positive and be unnecessarily referred for invasive coronary angiography

• in women with an interpretable ECG, exercise echocardiography was more sensitive than exercise ECG testing (86% versus 61%), and significantly more specific (79% versus 70%)²⁾. These figures give a LR+ of 4 and an LR- of 0.18
• more likely to give false negative result in women than in men as women more likely to have single vessel disease.

• The addition of any form of radionuclide myocardial perfusion imaging to stress ECG testing greatly increases diagnostic accuracy compared to stress ECG alone in both sexes. This may be particularly true in women.
• overall sensitivity of 93%, specificity of 78%, and predictive accuracy of 88% for ≥50% stenosis on angiography for women. These give a LR+ of 4.3 and an LR- of 0.09.
• more likely to give false negative result in women than in men as women more likely to have single vessel disease.
• expensive (~$A1000) and has the highest radiation dose (~ 600 CXR's)

• an emerging investigation
• much lower radiation than stress MIBI when using the latest 320 slice CT scanners
• studies using 64 slice MDCT looking for stenoses > 50% gives sensitivity 98% (88-100%) and specificity 88% (59-100%), and LR+ of 8, and NPV of 95-100% but these studies do not look at MACE which is more clinically relevant
• ~ half of the intermediate risk chest pain patients will have C/I
• negative test effectively rules out obstructive coronary disease (but remember half of AMI's occur in non-obstructive coronary arteries!)
• positive test is rather non-specific and requires further testing or invasive coronary angiography
  • if low pre-test probability, most will be false positives
• presence of severe coronary calcification interferes with interpretation of test and reduces specificity to around 50%
• not useful for segments < 1.5mm diameter

• PH coronary artery disease
• metformin
• inability to slow heart rate adequately to below 70 beats / min
• inability to breath hold for 5 secs
• renail impairment
• thyroid disease
• irregular rhythms (eg. AF, frequent ectopics)
• contrast allergy
• pregnancy, breast feeding
• young women

• spatial resolution 0.35mm-1.0mm (dual source 64 slice = 0.6mm; 320 slice = 0.5mm; cw catheter angiography 0.1-0.2mm)
• temporal resolution 35-200msec (dual source 64 slice = 83msec; 320 slice = 175msec; cw catheter angiography 5-10msec)
• 320 slice can capture the whole heart in one rotation within 350msec as it covers 16cm instead of only 4cm per rotation as with 64 slice.
• the Monash 320 slice scanner can accommodate patients up to 295kg
• lifetime risk of cancer per 10mSv = 1 in 2000 although new Canadian study³⁾ suggests 3% risk of cancer within 5 years per 10mSv in AMI patients with males having a 1.5x hazard risk compared to females of same age and dose.
• radiation dose of 320 slice scanner = 4mSv BUT avoid in young females as breast irradiation⁴⁾
  • catheter angiogram = 3-6mSv
  • CXR = 0.02mSv
  • Tc Sestamibi rest-stress scan = 11-14mSv
  • thallium stress test = 22-43mSv
  • calcium scoring with MDCT = 1-2mSv
  • 64 slice cardiac CT = 7-21mSv ⇒ presumed lifetime risk of cancer per CT = 1 in 284 (40yr old female), 1 in 1007 (40 yr old male) but this may need to be revised given the Canadian study.
• 320 slice CT protocol can give Calcium Score, myocardial perfusion and function, as well as hi resolution coronary angiography
• Monash 320 slice CTCA chest pain Mx algorithm:⁵⁾
  • negative initial troponin + risk stratification giving TIMI score 0-4 +absence of CTCA exclusion factors
  • CTCA performed:
    • no CAD nor plaque ⇒ discharge home
    • minor CAD (<50%) ⇒ rpt troponin at 6hrs, if negative then discharge
    • mod. CAD (50-70%) or abnormal LV function ⇒ rpt troponin at 6hrs, if negative then discharge home with early outpatient stress echo.
    • significant CAD (>70%) or rpt troponin abnormal ⇒ admit for catheter angiography
  • results of study of 111 patients having CTCA (mean age 58yrs, 62% male):
    • 31% discharged home after single troponin and CTCA
    • 48% discharged home after CTCA and repeat troponin
    • 21% admitted for catheter angiography of which 17% were negative thus false positive CTCA rate of 17%
    • in other words only 4% of this group had significant CAD but:
      • ALL were exposed to the contrast and radiation risks of CTCA
      • 21% were exposed to the significant risks of coronary angiography, although this aspect is probably reasonable, given that 19% who had coronary angiography had significant disease (hence CTCA in this staudy has a LR+ of about 5) and this would appear to be above the test threshold for coronary angiography of around 10% 1yr cardiac event rates.