Renal blood flow in critically ill cardiac patients: effect of dobutamine on total flow and selective redistribution of the cardiac index

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Baseline regional distribution of cardiac output (CO) depends on the physiological determinants of the end-organs. In congestive heart failure (CHF), this distribution is modified in favour of the coronary, brain and kidney circulations, depending on the main regulatory mechanisms of the vasomotor system, including the sympathetic nervous system, the vascular endothelium and the local non-endothelial mechanisms.

In this study, we measured the renal blood flow (RBF) and CO in 20 critically ill cardiac patients (17 males, mean age 49±10 years), utilizing both techniques of color coded duplex and echocardiography. All patients had CHF due either to dilated cardiomyopathy (12 patients) or ischemic heart disease (8 patients). A control group of 14 males (mean age 41±8.6 years) was also included. Following clinical evaluation, including the history and 12-lead ECG, all patients and control group were subjected to duplex assessment of RBF (ml/min/m²), in post-absorptive state by measuring the internal diameters of both renal arteries at their aortic origins, and then estimation of the renal artery pulsed flow wave to measure the time average velocity, (the average of the velocity spectrum in one second) and then multiplied by 60 to express the flow in one min. The CO was measured echocardiographically through the pulsed wave Doppler trans-aortic flow. The whole procedure was repeated, first under low dose(5 μg/kg/min), and then under high dose (20 μg/kg/min) dobutamine, with 24 h apart. The regional renal percentage (RRP) was calculated and expressed as percentage of the cardiac index.

Compared to the control group, patients, at baseline, showed statistically significant lower CO, RBF and RRP (mean =16.7±4.3% vs 19.3±4.1%, P<0.003). On low dose dobutamine, they showed a significantly higher CO and RBF with an insignificant increase in RRP (mean =17.3±2.3%) due to renal vasodilatation. On the other hand, reaching the higher dose of dobutamine, CO showed a further statistically significant increase, whereas RBF and RRP decreased significantly (mean = 14.2±3.4%, P<0.02), due to renal vasoconstriction.

Conclusion

The effects of different doses of dobutamine on the CO were not paralleled by similar effects on the RBF and the RRP. In patients with CHF, the adjustment of the proper dose of dobutamine should follow the regional renal blood flow rather than the absolute values of the CO.