Quantification and interpretation of nitric oxide-dependent cutaneous vasodilation during local heating

Human cutaneous microdialysis approaches for assessing nitric oxide (NO)-dependent blood flow include local heating (LH) of the skin until a plateau is reached, followed by infusion of a NO synthase inhibitor such as <i>N</i>^G-nitro-l-arginine methyl ester (l-NAME); however, varied methods of quantifying and expressing NO-dependent vasodilation can obfuscate data interpretation and reproducibility. We retrospectively assessed NO-dependent vasodilation during LH to 39°C or 42°C, calculated as the <i>1</i>) absolute contribution of the NO-dependent component (along with baseline and the non-NO-dependent component) to the total cutaneous vascular conductance (CVC) response to LH, normalized to maximal CVC (%CVC_max); <i>2</i>) difference in %CVC_max (Δ%CVC_max) between the LH plateau and post-NO synthase inhibition (l-NAME plateau; Δ%CVC_max = LH plateau - l-NAME plateau); <i>3</i>) percentage of the LH plateau attributable to Δ%CVC_max (%plateau = Δ%CVC_max/LH plateau × 100); and <i>4</i>) %plateau when correcting for baseline. The LH plateaus during 39°C and 42°C were 48 ± 17%CVC_max (9 ± 5% baseline; 2 ± 4% non-NO dependent; 36 ± 15% NO dependent) and 88 ± 10%CVC_max (15 ± 8% baseline; 9 ± 10% non-NO dependent; 64 ± 13% NO dependent), respectively. The absolute contributions of the non-NO-dependent and NO-dependent components of the response (<i>P</i> < 0.0001) and the Δ%CVC_max (66 ± 14 vs. 38 ± 15%) were greater during 42°C compared with 39°C (all <i>P</i> ≤ 0.02); however, there were no differences between the two protocols in %plateau (75 ± 13 vs. 80 ± 10%; <i>P</i> = 0.57) or %plateau_BL (88 ± 14 vs. 95 ± 8%; <i>P</i> = 0.31). For both protocols, the values were greater for %plateau_BL versus Δ%CVC_max and %plateau (<i>P</i> ≤ 0.0001), and for %plateau versus Δ%CVC_max (<i>P</i> ≤ 0.05). Quantification of NO-dependent skin vasodilation responses to LH is dependent upon the mathematical approach and verbal description, which can meaningfully impact data interpretation and reproducibility.<b>NEW & NOTEWORTHY</b> Local heating protocols are commonly used in conjunction with intradermal microdialysis for assessing nitric oxide (NO)-dependent microvascular function in humans, but various methods used to quantify and describe NO-dependent vasodilation may impact data interpretation. We compared four approaches for quantifying NO-dependent cutaneous vasodilation during local heating at 39°C and 42°C. We identify discrepancies in calculated NO-dependent dilation responses that are dependent upon the mathematical approach and meaningfully impact data interpretation and reproducibility.