Vasoactive Response of the Thoracic Aorta to Phenylephrine Infusion During Cardiac Surgery

The human aorta is commonly viewed as a passive conduit, but its potential vasoactive behavior could affect its Windkessel function and thereby its response to hypertension treatment involving vasoactive drugs. This study aims to investigate the in vivo vasoactive response of the thoracic aorta to phenylephrine during cardiac surgery. Patients undergoing elective cardiac surgery receiving phenylephrine for intraoperative hypotension were included. Transesophageal echocardiography was used to record the descending thoracic aorta before and after phenylephrine administration, while simultaneously recording pressure. The following parameters were calculated while correcting for pressure by exponential curve fitting: luminal area, distensibility, local pulse wave velocity (PWV) and the pressure-independent stiffness index (γ). Autonomic nervous system activity was assessed via heart rate variability analysis. Of the 70 recruited patients, 29 patients with 60 paired measurements were included. Phenylephrine administration significantly increased blood pressure (+22.3±13.3/+11.1±8.3 mmHg, p<0.001) and thereby increasing uncorrected luminal area (systolic: +0.21±0.21 cm 2 , p<0.001) and PWV (+1.4±1.3 m/s, p<0.001) and decreasing distensibility. Interestingly, these effects persisted after correcting for blood pressure, although smaller in mean magnitude (isobaric luminal area: +0.06±0.16 cm 2 , p=0.007; PWV: +0.4±1.1 m/s, p=0.004; γ: +0.90±2.26, p=0.003). A decrease in sympathetic tone is suggested by elevated R-R interval variability (+0.24±0.69 ms, p=0.019) and decreased heart rate (-3±3 min⁻¹, p<0.001). The thoracic aorta showed a pressure-independent vasodilative response to phenylephrine, thereby challenging its traditional role as passive conduit. This paradoxical response may be explained from the observed decrease in sympathetic tone. Responses showed strong patient-to-patient variability, highlighting the need to measure effects patient-specifically.