Vasoconstriction with phenylephrine increases cardiac output in preload dependent patients

Area of Science:

• Anesthesiology
• Cardiovascular Physiology
• Pharmacology

Background:

• General anesthesia reduces sympathetic outflow, impairing vasomotor control and cardiac preload.
• Phenylephrine (PE), a pure α1-agonist, is used for intraoperative hypotension but its effect on cardiac output via venous recruitment is debated.
• Human studies on PE's impact on preload-dependent circulation using indicator dilution techniques are lacking.

Purpose of the Study:

• To investigate the effect of phenylephrine infusion on restoring preload and augmenting stroke volume and cardiac output in preload-dependent patients under general anesthesia.
• To evaluate the controversial potential of phenylephrine for increasing cardiac stroke volume and output through venous recruitment.

Main Methods:

• 20 patients undergoing general anesthesia for gastrointestinal surgery were monitored using arterial catheter and LiDCO unity monitor.
• Patients were placed in head-up tilt to induce preload responsiveness (SVV > 12%).
• Phenylephrine infusion was administered to restore preload (SVV < 12%), with cardiac output measured by indicator dilution at baseline, during head-up tilt, and after PE infusion.

Main Results:

• Head-up tilt increased stroke volume variation (SVV) to 19% and decreased cardiac index (CI) and stroke volume index (SVI).
• Phenylephrine infusion reduced SVV to 6%, significantly increasing CI and SVI.
• All observed hemodynamic changes during phenylephrine infusion were statistically significant (p < 0.001).

Conclusions:

• Phenylephrine infusion in preload-dependent patients under general anesthesia increases venous return.
• This effectively abolishes preload dependency, as indicated by SVV normalization.
• Phenylephrine administration augments cardiac stroke volume and cardiac output, as measured by indicator-dilution technique.