Threshold effects of respiratory muscle work on limb vascular resistance

Abstract

. Threshold effects of respiratory muscle work onlimb vascular resistance. Am J Physiol Heart Circ Physiol 282: H1732-H1738, 2002; 10.1152/ajpheart.00798.2001.-The purpose of this study was to determine whether the human diaphragm, like limb muscle, has a threshold of force output at which a metaboreflex is activated causing systemic vasoconstriction. We used Doppler ultrasound techniques to quantify leg blood flow (Q L) and utilized the changes in mouth twitch pressure (⌬P MT) in response to bilateral phrenic nerve stimulation to quantify the onset of diaphragm fatigue. Six healthy male subjects performed four randomly assigned trials of identical duration (8 Ϯ 2 min) and breathing pattern [20 breaths/min and time spent on inspiration during the duty cycle (time spent on inspiration/total time of one breathing cycle) was 0.4] during which they inspired primarily with the diaphragm. For trials 1-3, inspiratory resistance and effort was gradually increased [30, 40, and 50% maximal inspiratory pressure (MIP)], diaphragm fatigue did not occur, and Q L, limb vascular resistance (LVR), and mean arterial pressure remained unchanged from control (P Ͼ 0.05). The fourth trial utilized the same breathing pattern with 60% MIP and caused diaphragm fatigue, as shown by a 30 Ϯ 12% reduction in P MT with bilateral phrenic nerve stimulation. During the fatigue trial, Q L and LVR were unchanged from baseline at minute 1, but LVR rose 36% and Q L fell 25% at minute 2 and by 52% and 30%, respectively, during the final minutes of the trial. Both LVR and Q L returned to control within 30 s of recovery. In summary, voluntary increases in inspiratory muscle effort, in the absence of fatigue, had no effect on LVR and Q L, whereas fatiguing the diaphragm elicited time-dependent increases in LVR and decreases in Q L. We attribute the limb vasoconstriction to a metaboreflex originating in the diaphragm, which reaches its threshold for activation during fatiguing contractions. diaphragm fatigue; leg blood flow; metaboreflex threshold; Doppler ultrasound; bilateral phrenic nerve stimulation IT IS WELL ESTABLISHED THAT thinly myelinated group III and unmyelinated IV afferent nerve fibers innervate limb skeletal muscle. These afferents are stimulated by metabolic byproducts (23), mechanical deformation (29), temperature (18), and vascular distension during muscular contraction (4). Stimulation of these afferents during muscular contraction elicits a powerful pressor response where sympathetic vasoconstrictor outflow is augmented to both resting and exercising skeletal muscle. A threshold exists for activation of this metaboreflex as numerous human studies have demonstrated a link between the metabolic events occurring within a contracting muscle (forearm) and the cardiovascular adjustments to handgrip exercise (3, 21). The current concept is that inadequate blood flow (or O 2 delivery) to the contracting muscle leads to an increase in the concentration of metabolites and the stimulation of chemosensitive afferents (type IV). The resulting ...

Exercise-induced respiratory muscle fatigue: implications for performance

“…The gradual increase in muscle sympathetic nerve activity was accompanied by a significant decrease in limb vascular conductance and limb blood flow (108,109). The sympathetic and vascular responses that occur with highintensity dynamic contractions of the respiratory muscles are similar to those that occur with fatiguing contractions of the forearm musculature (108,120).…”

“…Respiratory muscle work had to be increased to near fatiguing levels for muscle sympathetic nerve activity to be increased (22,120), or for blood flow to be decreased in a resting (109) or an exercising limb (39,59,102,139). Exercise in hypoxia would also be expected to exacerbate the effects of respiratory muscle work on limb fatigue and exercise tolerance.…”

Electromyographic Pattern of Respiratory Muscles in Young People during Incremental Exercise

“…The total test time was 14 [5] minutes for the physically active and 11 [5] minutes for the sedentary group. There was no statistically significant difference between the total testing time of the two the groups, the value P was .22.…”

“…Ventilation occurs through the coordinated contraction of specific muscle groups, which in inspiration produces enough energy to distend the pulmonary structures [3,5]. In the healthy individual the ventilatory requirements during exercise, are easily achieved because the breathing muscles are anatomically suited for increasing ventilatory requirements and neural regulation of respiration is ideal [6].…”

Development of an Aquatic Exercise Training Protocol for the Asthmatic Population

“…Another cause for the reduction in exercise tolerance is respiratory fatigue, or specifically diaphragmatic fatigue (Sheel, Derchak, Pegelow, & Dempsey, 2002). Improvement in respiratory function is largely dependent upon diaphragm function.…”