Vicente A. Diaz, MD (New Haven, CT), Robert B. Schonberger, MA (New Haven, CT), Kwang Suh, MA, PhD (Cleveland, OH), Rafat Ansari, PhD (Cleveland, OH), David G. Silverman, MD (New Haven, CT), Ron A. Adelman, MD, MPH (New Haven, CT)

PURPOSE:

Do choroidal vascular beds exhibit the surge in parasympathetic activity previously seen in forehead vascular beds in response to intravenous phenylephrine? How does this effect choroidal blood flow?

METHODS:

Six healthy subjects were recruited to participate; subjects did not have any chronic medical conditions and were instructed to abstain from nicotine or caffeine for 24 hours prior to the study. Baseline choroidal LDF readings were taken while subjects breathed in response to a metronome (12 breaths/min). Then phenylephrine infusion was started (0.4-0.8 mcg/kg/min). BP, EKG, respirations, and cutaneous LDF readings of forehead and index finger were monitored throughout the study. Additional choroidal LDF readings were taken when blood pressure increased by 10%, indicating a systemic response. Spectral-domain
analysis was then used to assess power of high frequency (HF) oscillations.

RESULTS:

There was a significant increase in high frequency oscillations in the choroidal microvascular beds in response to phenylephrine infusion (p=0.05). Also, finger blood flow decreased significantly from baseline (p=.04), while choroidal microvascular beds maintained or increased flow (p=NS). A compensatory baroreceptor response was indicated by a decline in
HR in all 6 subjects (p=0.03). Many studies over the past 40 years have proven that high frequency oscillations represent pure parasympathetic activity. Using the Oculix device, through our collaboration with NASA, we show that choroidal blood vessels exhibit a surge of parasympathetic activity in response to intravenous phenylephrine, a systemic
vasoconstrictor. We believe that autonomic autoregulation has evolved to maintain perfusion to essential organs during a vasoconstrictive or ischemic challenge.

CONCLUSION:

There is a surge in HF oscillations, known to be a marker of the parasympathetic system, in response to phenylephrine induced vasoconstriction. These HF oscillations allow choroidal vessels to maintain blood flow, while the finger showed a significant decrease in flow. Future studies will be conducted to determine the role of this response in diseases such as AMD, DM and HTN.