The main finding of our study was that concomitant 6-min low-grade exercise supplementation with ≥35 W, through the use of the bicycle protocol, increased heart-to-background count ratios such as H/L and H/below the H ratios in the patients who underwent ATP stress MPI. The other findings of this study were that in the case of the H/below the H ratio, the artifact was independently associated with the LVEDV after exercise in addition to the increment of RPP.
Exercise stress is well known to increase heart-to-background count ratios as compared with drug stress alone and to reduce hepatic tracer uptake relative to the heart. Side effects are common during standard adenosine stress testing. However, few significant side effects attributable to the ATP infusion in combination with exercise were observed, and this procedure was well tolerated in this study. It is conceivable that exercise increases sympathetic nerve activity, which improves atrioventricular conduction. Therefore, noncardiac side effects such as hypotension, and major arrhythmias such as atrioventricular block are significantly reduced after exercise stress . Exercise also suppresses the vasodilation of adenosine in diaphragmatic regions and the heart-to-background count ratio is higher in the exercise groups in correlation with the exercise level achieved . Samady et al. found that a 6-min adenosine infusion with concomitant low-level treadmill exercise reduced unfavorable side effects, enhanced image quality, and may have resulted in greater detection of ischemia . Enhancement of image quality was also observed when the adenosine infusion study was performed as compared with the bicycle-exercise protocol in patients treated with beta-blockade . However, the opposite results were reported by Jamil et al., who showed no change in defect size or severity following exercise, compared with the standard 6-min adenosine infusion .
A clear increase in H/L ratio is an important finding because an increased H/L ratio has been shown to result in fewer artifacts in the inferior and inferoseptal regions of the heart [1, 2]. In our study, we included only patients whose SSSs were <3 to evaluate precisely the extent of background level. When combined with ATP plus short-duration exercise stress, mild stress was adequate to obtain a significant quality image. Some differences were found between the results in the two regions regarding the quantification of the target-to-background ratio. The H/below the H ratio represents a comparison of the image quality of the heart with those of organs adjacent to the heart, such as the diaphragm, intestine, and other digestive tissues, and was inversely dependent on LVEDV in association with the increment of RPP. Enlargement of the left ventricle may reduce the sharpness of the SPECT image because of the increase in the dispersion of 99mTc tetrofosmin in case of H/below the H ratio. By contrast, male sex, irrespective of left ventricular size, was an independent factor for H/L or H/below the H ratio. Sex-related differences in artifacts may result from the grade of fat tissue and complicated body structures, including the lung, spine, and chest wall. A pertinent question that should be answered in the future is how a higher image quality can be obtained in patients with enlarged left ventricles when the use of a new type of stress method is becoming widespread in myocardial perfusion SPECT.
A considerable interpatient variability exists in the heart-to-background count ratios, but some extent of the exercise level achieved greater heart-to-background count ratio, which may depend on the extent of the RPP. A short-duration exercise stress of ≥35 W may enhance sympathetic activity abruptly and adequately, thus leading to the desirable results. Improvement of the heart-to-background count ratio was present at the lower exercise levels, which most patients could be expected to achieve. Six-min ATP infusion with a moderately low-grade exercise protocol during the same period is preferable because it can be incorporated into the standard drug stress regimen with no time loss.
Depending on the protocol design, the type of tracer, moment of tracer injection, and type and duration of exercise may vary [14-18] and elicit different results. Whether these issues have any effects on the accuracy of the perfusion study are currently unclear. We could not use SPECT-computed tomography scanner to reduce artifacts. As we randomly assigned patients into four groups but enrolled only those with SSSs of <3, the number of patients differed among the groups. Under this condition, the defect severity and sensitivity to coronary artery disease in each group could not be evaluated.