Tschakovsky, Michael E.

The introduction of duplex Doppler ultrasound almost half a century ago signified a revolutionary advance in the ability to assess limb blood flow in humans. It is now widely used to assess blood flow under a variety of experimental conditions to study skeletal muscle resistance vessel function. Despite its pervasive adoption, there is substantial variability between studies in relation to experimental protocols, procedures for data analysis, and interpretation of findings. This guideline results from a collegial discussion among physiologists and pharmacologists, with the goal of providing general as well as specific recommendations regarding the conduct of human studies involving Doppler ultrasound-based measures of resistance vessel function in skeletal muscle. Indeed, the focus is on methods used to assess resistance vessel function and not upstream conduit artery function (i.e., macrovasculature), which has been expertly reviewed elsewhere. In particular, we address topics related to experimental design, data collection, and signal processing as well as review common procedures used to assess resistance vessel function, including postocclusive reactive hyperemia, passive limb movement, acute single limb exercise, and pharmacological interventions. Listen to this article’s corresponding podcast at https://ajpheart.podbean.com/e/guidelines-for-doppler-ultrasound-and-resistance-vessel-function/ .

Phenylephrine is generally regarded as a “pure” α1-agonist. However, after treatment of the forearm with the α-adrenergic-blocking drug phentolamine, brachial artery infusion of phenylephrine can cause transient forearm vasodilation. To determine whether this response was β-receptor mediated, phenylephrine, phentolamine, and propranolol were infused into the brachial arteries of six healthy volunteers. Forearm vascular conductance (FVC) was also calculated and expressed as arbitrary units (units). Infusion of phenylephrine by itself (0.5 μg · dl forearm volume−1 · min−1) caused a sustained decrease ( P < 0.05) in FVC from 3.5 ± 0.7 to 0.9 ± 0.2 units ( P < 0.05). Infusion of the α-blocker phentolamine increased ( P < 0.05) baseline FVC to 5.7 ± 1.3 units. Subsequent infusion of phenylephrine after α-blockade caused FVC to increase ( P < 0.05) for ∼1 min from 5.7 ± 1.3 to a peak of 13.1 ± 1.8 units. Propranolol had no effect on baseline flow, and subsequent phenylephrine infusion after α- and β-blockade caused a small, but significant, sustained decrease in FVC from 5.1 ± 1.0 to 3.6 ± 0.8 units. There were no systemic effects from the infusions, and saline infusion at the same rate (1–2 ml/min) had no forearm vasoconstrictor or dilator effects. These data indicate that in humans phenylephrine can exert transient β2-vasodilator activity when its predominant α-constrictor effects are blocked.

We tested the hypothesis that ACh or nitric oxide (NO) might be involved in the vasodilation that accompanies a single contraction of the forearm. Eight adults (3 women and 5 men) completed single 1-s-duration contractions of the forearm to raise and lower a weight equivalent to ∼20% maximal voluntary contraction through a distance of 5 cm. In a second protocol, each subject had a cuff, placed completely about the forearm, inflated to 120 mmHg for a 1-s period, then released as a simulation of the mechanical effect of muscle contraction. Three conditions were studied, always in this order: 1) control, with intra-arterial infusion of saline; 2) after muscarinic blockade with atropine; and 3) after NO synthase inhibition with N G-monomethyl-l-arginine (l-NMMA) plus atropine. Forearm blood flow (FBF), measured by combined pulsed and echo Doppler ultrasound, was reduced at rest with l-NMMA-atropine compared with the other two conditions. After the single contraction, there were no effects of atropine, butl-NMMA reduced the peak FBF and the total postcontraction hyperemia. After the single cuff inflation, atropine had no effects, whereasl-NMMA caused changes similar to those seen after contraction, reducing the peak FBF and the total hyperemia. The observation thatl-NMMA reduced FBF in response to both cuff inflation and a brief contraction indicates that NO from the vascular endothelium might modulate the basal level of vascular tone and the mechanical component of the hyperemia with exercise. It is unlikely that ACh and NO from the endothelium are involved in the dilator response to a single muscle contraction.

Despite being the subject of investigation for well over 100 years, the nature of exercising muscle blood flow control remains, in many respects, poorly understood. In this review we focus on the potential role of nitric oxide in vasodilation of muscle resistance vessels during a bout of exercise. Its contribution is explored in the context of whether it contributes to steady-state exercise hyperemia, the dynamic adjustment of muscle blood flow to exercise, or the modulation of sympathetic vasoconstriction in exercising muscle. It appears that the obligatory role of nitric oxide in all three of these categories is modest at best. The elucidation of the integrated nature of exercise hyperemia control in terms of synergy and redundancy of mechanism interaction remains in its infancy, and much more remains to be learned about the role of nitric oxide in this type of integrated control.