Do preceding vasoconstrictions influence the 'inspiratory gasp test'?

Diabetic or other neuropathies can affect the autonomic innervations of vessels and heart. Using non-invasive and inexpensive autonomic tests, one can detect autonomic neuropathy. One of these tests is the inspiratory gasp response (IGR). It appears as a short reduction of skin blood volume (SBV) following deep inspiration (DI). Unfortunately, because of a high degree of variability of the study results, the clinical merits of this test have been questioned. However, there remains vagueness about underlying mechanisms or pitfalls. The aim of the study was to identify preceding vasoconstrictor episodes as putative pitfalls in assessing IGR. With regard to the hypothesis that a preceding vasoconstrictor episode may affect the amplitude of the following one, we investigated SBV in fingertips of 25 healthy test subjects (14 females, 11 males; mean age 24 years), using the photoplethysmographic technique. After adaptation to environmental conditions the subjects were instructed to perform five DI in various time lags (90, 30, 20 and 180 s). We found that the shorter the time interval to a preceding IGR, the lower the amplitude of the following IGR. After an interval of 180 s, the amplitude increased again. We conclude that this newly observed phenomenon might explain the contradictory results of the IGR test found in some earlier studies. We suppose that reduced IGR may depend on alterations in peripheral norepinephrinergic transmission processes (e.g. emptying of norepinephrine reservoirs). In clinical studies it should be ruled out that a diagnostically assessed IGR is influenced by a preceding vasoconstrictor episode.