The splenic switch off (SSO) sign is routinely used in clinical practice to assess adequacy of adenosine stress during perfusion CMR studies with absence of SSO suggested as evidence of suboptimal stress. Myocardial perfusion mapping is a novel tool that provides pixelwise maps of myocardial blood flow (MBF), allowing for measurement of increase in MBF in response to adenosine at a regional level.
To assess the performance of peak stress MBF (SMBFmax, the myocardial segment with the maximum stress MBF value) measured using perfusion maps to assess for maximal stress compared to the SSO sign.
In total, 216 subjects were recruited to the study. Twenty-two healthy controls underwent stress perfusion CMR in order to assess normal increase in MBF in response to adenosine and identify threshold values for SMBFmax. A "coronary physiology cohort" of 37 patients underwent stress perfusion CMR and invasive coronary physiology assessment on the same day to compare CMR perfusion parameters to reference standard invasive measures of hyperaemia (defined as at least two out of three of: 1) ventricularisation of the distal pressure waveform, 2) disappearance of the dicrotic notch on the distal waveform, 3) separation of mean aortic and distal pressures) . Finally, a "clinical cohort" of 157 patients were studied to compare perfusion mapping parameters to SSO.
From the healthy volunteer scans, SMBFmax >1.43ml/g/min was derived as the threshold value for hyperaemia (defined as 1.96 standard deviations below the sample mean of segments with lowest stress MBF values). This threshold was then tested in the coronary physiology cohort: 100% of patients with invasive evidence of hyperaemia demonstrated mapping values greater than the predefined threshold of SMBFmax >1.43ml/g/min, despite absence of SSO in 19% of cases (Panel A). When the threshold was applied to 157 patients who underwent adenosine stress CMR, 93% of cases had hyperaemia defined by at least one segment with SMBFmax >1.43ml/g/min compared to only 71% using SSO (Panel B). Overall there was no difference in stress perfusion parameters in those with or without SSO (SMBFmax: 2.58±0.89ml/g/min SSO vs 2.54±1.04ml/g/min no SSO, p=0.84; myocardial perfusion reserve: 2.69±1.06 SSO vs 2.87±1.14 no SSO, p=0.35). Increase in heart rate >16bpm was able to predict stress response (SMBFmax >1.43ml/g/min) with sensitivity 63% and specificity 91% (AUC 0.87, p<0.001) and performed better than the SSO sign (AUC 0.62, p<0.001 for comparison of methods). We developed a diagnostic algorithm based on perfusion maps to assess stress response (Panel D).
Conclusion Adenosine-induced increase in MBF measured using perfusion mapping can be used assess hyperaemic response during stress CMR studies identify studies with sub-maximal stress.