What You Should Know About Using NIV Studies

Volume 15 - Issue 12 - December 2002
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Understanding The Value Of PPG

Photoplethesmography (PPG) transmits infrared light from an emitting diode into the tissue. The light is reflected back from the blood within the cutaneous microcirculation. The amount of reflected light varies with the blood content of the microcirculation. The phototransducer is taped to the end of the toe and a small digital blood pressure cuff is placed at the base of the digit (TBI). The recordings reflect flow through the subpapillary venous plexus and are most useful at documenting skin flow and differentiating between organic and vasospastic disease.

When PPG is inconclusive, you can use the laser Doppler to detect minute microcirculatory blood because it is based on reflected light from moving blood particles, not on pulsatile changes in blood filling the digit.¹⁰

NIV is a valuable tool in screening for peripheral arterial disease. The cost of NIV studies is substantially less in comparison to arteriography.

Author(s):

By Dana Giacalone, DPM, and Khurram Khan, DPM

The cuffs should be the same width or 25 percent larger than the width of the part they encircle. If the cuff is too narrow, the pressure will be falsely elevated. Using a Doppler probe, you would inflate the cuff so pedal sounds are no longer audible. Then slowly deflate the cuff and record the pressure in the cuff when you first hear the sound. This represents the pressure of the artery where the cuff is located.⁵
According to Bowker and Pfeifer, the pressure changes correlate directly with flow.⁵ So an index of 0.5 represents only 50 percent of the expected blood flow. A gradient of 40mmHg or greater between the ipsilateral or contralateral levels suggests an occlusion or highly stenotic segment. There is discrepancy with this value as Hoffman suggests 30mmHg and Pellerito suggests 20mmHg.^2,4 These values can be misleading if collateral flow is so extensive that a 20-30 mmHg drop does not occur or in a situation in which there are non-compressible vessels due to calcification.^1,4,5