Thin-fiber phrenic afferents mediate the ventilatory response to diaphragmatic ischemia.

We assessed the role of groups III and IV phrenic afferents in the ventilatory response to diaphragmatic ischemia in mechanically ventilated, chloralose-anaesthetized dogs using the in-situ isolated and innervated left hemidiaphragm preparation. The inspiratory motor drive to the right (Rt Edi) and left (Lt Edi) diaphragms, parasternal (Eps), and alae nasi (Ean) muscles was measured from the peak integrated EMG activities. When left diaphragmatic ischemia was produced in the control group (n = 6) by occluding the left phrenic artery for 20 min, LtEdi increased to 158%, RtEdi to 160%, Eps to 150% and Ean to 135% of baseline values. Left diaphragmatic tension, however, remained unchanged during the ischemia period. In the capsaicin-treated group (n = 6), we injected repeated doses of capsaicin, a selective stimulant of groups III and IV afferents, into the left phrenic artery to eliminate inputs from these afferents. Repeated injections of capsaicin are known to induce prolonged periods of afferent dysfunction. The first two injections of capsaicin (1 mg each) produced transient activation of the inspiratory muscles and higher breathing frequencies. Subsequent injections, however, failed to elicit any ventilatory changes. When diaphragmatic ischemia was induced after the last injection of capsaicin, no changes in the Right Edi, Eps and Ean were observed, whereas Left Edi and left diaphragmatic tension declined significantly. We conclude that increased inspiratory motor drive during selective diaphragmatic ischemia is mediated through the activation of groups III and IV phrenic afferents.