Background: Vasoactive drugs are routinely used clinically to alter mean arterial blood pressure (MAP) and cardiac output (CO) and to maintain organ perfusion. However, the effect of such drugs on microvascular visceral blood flow (MiBF) is not fully understood. We aimed to track changes in renal MiBF, using the well-validated technique of Contrast Enhanced Ultrasound (CEUS), across a range of MAP and CO generated via the vasoactive drugs, phenylephrine and ephedrine. Methods: Baseline cardiovascular measurements were recorded, with renal MiBF determined via CEUS as renal microvascular transit time (RTT). Phenylephrine was then administered, via a standardized protocol, to increase MAP and CO, with repeat CEUS. Following return to baseline, the above was repeated using ephedrine. CEUS time-intensity curves were constructed and renal MiBF calculated.
Results: In 11 male volunteers (median age 32), phenylephrine increased MAP (98.7 vs 110.8 mmHg, p<0.001), but not CO (4211 vs. 4089 ml.min-1, p=0.42), while ephedrine increased CO (4110 vs 6097, p<0.001) and MAP (95.6 vs 100.9, p=0.02). Phenylephrine reduced time to organ perfusion (TTOP) (22.3 vs 18.4 secs, p=0.009), but not RTT (14 vs 13.2 secs, p=0.46). Ephedrine decreased TTOP (21.1 vs 14.7 secs, p=0.003), and RTT (3.5 vs 9.6 secs, p=0.007). Change in CO predicted change in TTOP (r2=0.26, p=0.02), but not RTT (r2=0.04, p=0.43). Change in MAP did not predict change in TTOP (r2=0.02, p=0.58), or RTT (r2<0.001, p=0.89). Conclusion: Changes in MAP and CO fail to predict renal MiBF. Key points Question: Do macrovascular changes predict renal microvascular perfusion. Findings: Changes in cardiac output and mean arterial blood pressure fail to predict renal microvascular perfusion. Meaning: Clinical measurement of macrovascular indices may not correlate with microvascular indices.