Blood Velocity Variation in Right Renal and Superior Mesenteric Arteries During Cardio-pulmonary Bypass

The cardiopulmonary by-pass (CPB) technique, used in cardiac surgery to obtain a bloodless operating field and an immobile heart, determines important effects on the blood vessel wall, especially when a continuous and non-continuous blood flow is used. In fact, a reduction in NO production by the endothelium, an increase in systemic vascular resistance and an increased risk of cerebral and renal hypoperfusion have been observed and can result in potential organ damage.

Acute kidney injury (AKI) after heart surgery is a major cause of mortality and morbidity. Its incidence varies according to different definitions, but can reach 30%. In some series, 1-5% of patients require renal replacement therapy (RRT) in the postoperative period presenting a mortality that can reach 50-70%. However, even more limited increases in serum creatinine (sCr) are associated with worsening prognosis and the risk of chronic kidney disease (CKD). The pathophysiology of AKI in cardiac surgery is complex and still partly unknown. It is believed that one of the main causative factors is hypoperfusion and renal hypoxia, in particular of the medullary region; this would result in a vasoconstriction of the afferent arterioles to the glomerulus and a reduction in filtration. Risk factors associated with the increased incidence of AKI include bleeding, use of the aortic pump, excessive cardiopulmonary bypass duration, excessive haemodilution, insufficient pump flow, or insufficient blood pressure. Hypothermia, which also has a protective effect against hypoperfusion and tissue hypoxia, could induce AKI by increasing renal vascular resistance and favoring medullary hypoxia during subsequent rewarming.

In addition to AKI, another complication of cardiac surgery, rarer but associated with a higher mortality, is acute mesenteric ischemia; the most frequent type is non-occlusive mesenteric ischemia (NOMI) which seems to have as a predisposing cause a reduction or maldistribution of splanchnic blood flow and the use of vasoconstrictors.

Recently a technique has been described that allows to measure the blood velocity in the right renal artery and in the superior mesenteric artery using the transesophageal echocardiogram (TEE); this technique allows to view these arteries and measure the speed of the blood with good precision because the insonation angle (ie the angle formed by the ultrasound flow and the direction of the blood vessel) is adequate. In cardiac surgery, this methodology allows you to monitor blood velocity in the right renal artery and superior mesenteric artery during surgery. Some authors have used it to conduct pilot studies in which the blood velocity values in the renal arteries during cardiac surgery were used to calculate the pulsatility and resistivity indices, as predictors of the risk of postoperative AKI. The calculation of these indices, however, requires the use of a pulsatile blood flow to generate a periodic variation of the blood velocity, and they are not evaluable during CPB since the current practice in almost all centers is to use a continuous blood flow. At present, therefore, despite the fact that TEE is routinely used for monitoring renal perfusion during cardiac surgery, the blood velocity in the renal and mesenteric arteries has been little studied during CPB and has never been evaluated during CPB with continuous flow; in particular, the possible variation in blood velocity measured during CPB compared to the baseline values measured before extracorporeal circulation and its correlation with the onset of postoperative renal failure is not known.