March 13, 2012

Transient ischemic dilation in myocardial perfusion imaging

Measurement of ventricular size after stress compared to at rest an important diagnostic tool

Author: Tom Heston MD

People with known or suspected coronary artery disease frequently undergo myocardial perfusion imaging as part of the workup for disease and risk management. The stress versus rest gated SPECT myocardial perfusion scan is particularly useful because it evaluates overall physiologic function (exercise capacity), electrocardiographic changes with stress, left ventricular size, myocardial perfusion, and left ventricular ventricular function.

One useful measurement from gated SPECT imaging that helps provide an overall picture of cardiac health is the response of the left ventricle to stress. This can be quantified in two ways. The first is the "transient ischemic dilation" (TID) ratio. The TID ratio is the average left ventricular size after stress divided by the average left ventricular size at rest. When elevated to approximately 1.2 or greater, it increases the risk of advanced cardiovascular disease.

Because the "average" ventricular size is a computer generated measurement which is uncommonly used in other imaging procedures, an alternative to the TID ratio is the end systolic or end diastolic volume ratio, or a combination of both. Again, with these measurements, an elevated ratio raises the risk of advanced cardiovascular disease.

A recent manuscript in the Journal of Nuclear Cardiology attempted to quantify the upper limit of normal for end diastolic and end systolic volume ratios. They found the upper limit of normal for the end systolic volume ratio to be 1.46, the upper limit of normal for the end diastolic volume ratio to be 1.23, and the upper limit for the TID ratio to be 1.19 when performing single day rest followed by stress gated SPECT myocardial perfusion imaging. They looked at 259 patients. Patients with ratios above these limits were significantly more likely to have advanced cardiac disease.

These values are to be utilized with caution because the authors did not throw out the patients where end systolic volumes ratios are highly fragile, namely, in patients with a high ejection fraction and a small heart. In my personal database of over 3700 patients, the upper limit of normal for the end systolic volume (ESV) ratio is 1.34 and the upper limit for the end diastolic volume (EDV) ratio is 1.20. When throwing out all patients with an ejection fraction of 70 or greater, the upper limit for the ESV ratio falls to 1.27 but the upper limit for the end diastolic volume ratio remains almost the same at 1.19.

All stress vs rest volume ratios were significant predictors of advanced cardiac disease. Furthermore, they found that when there was at least a small perfusion defect in addition to an elevated stress vs rest ventricular volume ratio, the sensitivity of an elevated ratio went up while the specificity remained unchanged. The sensitivity of TID for the detection of advanced disease was about 65% to 70%.

Take-Home Messages from this study:
  • The study reaffirms that increased change in ventricular volume with stress is a marker of increased cardiovascular risk.
  • The TID ratio, end systolic volume ratio, and end diastolic volume ratio all are significant predictors of increased risk.
  • There may be some advantage to using the EDV and/or TID ratios, as they might be more resistant to variations in heart size compared to the ESV ratio.
  • The upper limit of normal for both the TID ratio and the EDV ratio is approximately 1.20.

Xu Y, Arsanjani R, Clond M et al. Transient ischemic dilation for coronary artery disease in quantitative analysis of same-day sestamibi myocardial perfusion SPECT. J Nucl Cardiol. 2012 Mar 8.

Tanaka H, Chikamori T, Hida S, Igarashi Y, Miyagi M, Ohtaki Y, Shiba C, Hirose K, Hatano T, Usui Y, Yamashina A. The diagnostic utility of the Heston Index in gated SPECT to detect multi-vessel coronary artery disease. J Cardiol. 2008 Feb;51(1):42-9.

NOTE: medicine is a rapidly changing field. Always consult with your physician regarding your personal health issues. Keep in mind the words from Mark Twain, updated for the Internet age: "Be careful of reading health information online, you might die of a misprint."