Non-invasive vascular impedance measures demonstrate ocular vasoconstriction during isometric exercise

Area of Science:

• Ophthalmology
• Cardiovascular Physiology
• Biomedical Engineering

Background:

• Vascular network impedance calculation is crucial for circulation studies.
• Ocular vascular studies have historically used simpler resistance measures, unsuitable for pulsatile flow.
• Impedance indices offer a better assessment of pulsatile opposition to blood flow and vasculopathy.

Purpose of the Study:

• To demonstrate a novel, non-invasive method for determining ocular vascular impedance.
• To assess ocular vascular impedance under the physiological stress of sustained isometric exercise.

Main Methods:

• Non-invasive measurement of ocular blood flow and carotid arterial blood pressure waveforms.
• Calculation of ocular blood flow using the Langham-Silver method via intraocular pressure fluctuations.
• Determination of carotid blood pressure waveforms using a SphygmoCor tonometer.
• Harmonic analysis (Fast Fourier transform) of simultaneous waveforms under resting and exercise conditions.
• Calculation of the first four impedance values as ratios of pressure to flow harmonics.

Main Results:

• 12 healthy volunteers (6 male, 6 female, mean age 27) participated.
• Isometric exercise significantly increased mean carotid blood pressure and heart rate.
• Significant changes in ocular vascular impedance were observed during exercise: the first impedance value increased (p=0.01), and the third impedance value decreased (p=0.01).

Conclusions:

• A practical, non-invasive method for calculating ocular impedance moduli was established.
• Observed impedance changes during exercise align with known vascular vasoconstriction responses.
• Ocular impedance moduli show promise for investigating eye conditions involving vascular obstruction.