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University of Wisconsin-Madison, Department of Population Health Sciences, John Rankin Laboratory of Pulmonary Medicine, Madison, Wisconsin 53705
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 (QL) and utilized the
changes in mouth twitch pressure (
PMT) 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 ± 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 QL, limb vascular resistance (LVR), and mean arterial pressure remained unchanged from control (P > 0.05). The fourth trial
utilized the same breathing pattern with 60% MIP and caused diaphragm
fatigue, as shown by a 30 ± 12% reduction in PMT
with bilateral phrenic nerve stimulation. During the fatigue trial,
QL and LVR were unchanged from baseline at minute
1, but LVR rose 36% and QL fell 25% at minute
2 and by 52% and 30%, respectively, during the final minutes of
the trial. Both LVR and QL 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
QL, whereas fatiguing the diaphragm elicited time-dependent
increases in LVR and decreases in QL. 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
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