The estimation of CFR using the method we have utilized is interesting due to its clinical and physiopathological value .
Despite dipyridamole is more suitable for the assessment of CFR [5, 6], dobutamine is one of pharmacologic agents most commonly used during stress echocardiography . However; the simultaneous estimation of CFR is not frequent in daily practice, with any of the above drugs; probably, this might be due to the lack of experience of echo-laboratories with this method, besides a relatively longer duration of the test that is not covered by health care programs.
Dobutamine exerts a complex effect on the coronary arteries: heart rate and myocardial contractility increase due to adrenergic stimulation, producing greater oxygen (O2) uptake and reactive hyperemia . The direct effect of dobutamine produces vasodilatation by direct effect on epicardial coronary arteries and microcirculation (stimulation of β1, β2 and α adrenergic receptors in the vascular wall) [18, 19].
Dobutamine hyperemia is equivalent to adenosine-induced vasodilatation in patients with ischemic response to dobutamine, but lower than that observed in non-ischemic patients. In normal patients, the effect of dobutamine on CFR is lower than that of adenosine according to studies using paired Doppler Flowires in the catheterization laboratory [20, 21].
In addition, dobutamine increases contractility and myocardial O2 consumption, and produces release of vasodilatory substances such as adenosine, which act on epicardial arteries and microvasculature. Finally, dobutamine, by increasing the heart rate, induces flow mediated epicardial vasodilation [22, 23].
The analysis of CFR has been studied in different clinical scenarios; however there is little information about coronary flow in each stage.
In a study performed at the Mayo Clinic by P. Pellikka et al., the feasibility of assessing the CFR was 97% from peak diastolic velocity. The authors concluded that CFR assessment during dobutamine stress echocardiography correlated well with wall thickening and detected ischemia early before development of wall motion abnormality, emphasizing the importance of determining CFR in this type of study .
We have not found any study analyzing and measuring the relation between heart rate and coronary flow reserve during this test.
In an excellent study, P. Meimoun et al. assessed CFR during dobutamine stress echocardiography and compared it to the CFR obtained with adenosine in the same group of patients and found a good correlation and concordance between the tests, in a wide range of LAD disease. Although a clear cut-off of heart rate was not established in that study, it was emphasized along the manuscript that a maximal achievable target heart rate was necessary during dobutamine infusion to obtain the good correlation and concordance between CFR-dobutamine and CFR-adenosine. Furthermore, a significant correlation was found in that study between CFR during dobutamine and change of rate-pressure product in patients with abnormal results .
In addition, the normal and abnormal values of CFR during dobutamine stress were clearly described in that study. A CFR < 2 with dobutamine stress echocardiography was found in all patients who had a positive test in the LAD territory (n = 8) and interestingly the same low CFR < 2 was obtained with adenosine in these patients .
Takeuchi et al.  studied 129 non selected patients. In patients without wall motion abnormalities, mean CFR was 2.76, similar to our findings. Coronary flow velocity was evaluated in each stage, yet changes in heart rate were not investigated.
It should be noted that the feasibility of analyzing CFR during dobutamine stress (around 90%) according to our previous experience and the results of previously mentioned studies [10, 11] is a little lower compared to CFR obtained using vasodilator agents (about 90-95%) . This might be explained by bias in patients selection as well as to the fact that, compared to dipyridamole, dobutamine increases heart rate, myocardial contractility and movements of the heart with displacement of the LAD; for these reasons, the estimation of CFR may be more difficult.
Finally, 97% patients achieved an adequate CFR with a delta HR of 50 bpm from the baseline stage. In the same sense, 95.6% of patients reached a normal CFR with 75% of their predicted maximum heart rate.
This study puts in evidence that it is technically recommended to obtain coronary artery flow in baseline conditions and after reaching the predicted heart rate (50 bpm greater than baseline HR or 75% of maximum predicted HR) without forgetting about wall motion response.