Ventilatory effects of stimulation of phrenic afferents

Diaphragm stimulation elicits phrenic afferent-induced neuromuscular plasticity

We hypothesized that activation of phrenic afferents induces diaphragm motor plasticity. In anesthetized and spontaneously breathing rats we delivered 40 Hz, low threshold (twitch and 1.5X twitch threshold), inspiratory-triggered stimulation to the left hemidiaphragm for 30 min to activate ipsilateral phrenic afferents. Diaphragm amplitude ipsilateral and contralateral to stimulation were increased for 60 min following both currents compared to time controls not receiving stimulation. Diaphragm stimulation was repeated in laminectomy controls or following a unilateral C3-C6 dorsal rhizotomy to eliminate phrenic afferent volleys. Laminectomy controls expressed neuromuscular plasticity post-stimulation. In contrast, ipsilateral and contralateral diaphragm amplitude following dorsal rhizotomy was lower than laminectomy controls and no different than time controls, suggesting diaphragm motor plasticity was not induced post-rhizotomy. Our results indicate that diaphragm stimulation induces a novel form of plasticity in the phrenic motor system which requires phrenic afferent activation. Respiratory motor plasticity elicited by diaphragm stimulation may have value as a therapeutic strategy to improve diaphragm output in neuromuscular conditions.

Phrenic afferent activation modulates cardiorespiratory output in the adult rat

Phrenic afferents project to brainstem areas responsible for cardiorespiratory control and the mid-cervical spinal cord containing the phrenic motor nucleus. Our purpose was to quantify the impact of small- and large-diameter phrenic afferent activation on phrenic motor output. Anesthetized and ventilated rats received unilateral phrenic nerve stimulation while contralateral phrenic motor output and blood pressure were recorded. Twelve currents of 40-Hz inspiratory-triggered stimulation were delivered (20 s on, 5 min off) to establish current response curves. Stimulation pulse width was varied to preferentially activate large-diameter phrenic afferents (narrow pulse width) and recruit small-diameter fibers (wide pulse width). Contralateral phrenic amplitude was elevated immediately poststimulation at currents above 35 µA for wide and 70 µA for narrow pulse stimulation when compared with animals not receiving stimulation (time controls). Wide pulse width stimulation also increased phrenic burst frequency at currents ≥35 µA, caused a transient decrease in mean arterial blood pressure at currents ≥50 µA, and resulted in a small change in heart rate at 300 µA. Unilateral dorsal rhizotomy attenuated stimulation-induced cardiorespiratory responses indicating that phrenic afferent activation is required. Additional analyses compared phrenic motor amplitude with output before stimulation and showed that episodic activation of phrenic afferents with narrow pulse stimulation can induce short-term plasticity. We conclude that the activation of phrenic afferents 1) enhances contralateral phrenic motor amplitude when large-diameter afferents are activated, and 2) when small-diameter fibers are recruited, the amplitude response is associated with changes in burst frequency and cardiovascular parameters.NEW & NOTEWORTHY Acute, inspiratory-triggered stimulation of phrenic afferents increases contralateral phrenic motor amplitude in adult rats. When small-diameter afferents are recruited, the amplitude response is accompanied by an increase in phrenic burst frequency, a transient decrease in mean arterial blood pressure, and a slight increase in heart rate. Repeated episodes of large-diameter phrenic afferent activation may also be capable of inducing short-term plasticity.

Inhibition of central activation of the diaphragm: a mechanism of weaning failure

During a T-tube trial following disconnection of mechanical ventilation, patients failing the trial do not develop contractile diaphragmatic fatigue despite increases in inspiratory pressure output. Studies in volunteers, patients, and animals raise the possibility of spinal and supraspinal reflex mechanisms that inhibit central-neural output under loaded conditions. We hypothesized that diaphragmatic recruitment is submaximal at the end of a failed weaning trial despite concurrent respiratory distress. Tidal transdiaphragmatic pressure (ΔP_di) and electrical activity (ΔEA_di) were recorded with esophago-gastric catheters during a T-tube trial in 20 critically ill patients. During the T-tube trial, ∆EA_di was greater in weaning failure patients than in weaning success patients (P = 0.049). Despite increases in ΔP_di, from 18.1 ± 2.5 to 25.9 ± 3.7 cm H₂O (P < 0.001), rate of transdiaphragmatic pressure development (from 22.6 ± 3.1 to 37.8 ± 6.7 cm H₂O/s; P < 0.0004), and concurrent respiratory distress, ∆EA_di at the end of a failed T-tube trial was half of maximum, signifying inhibition of central neural output to the diaphragm. The increase in ΔP_di in the weaning failure group, while ∆EA_di remained constant, indicates unexpected improvement in diaphragmatic neuromuscular coupling (from 46.7 ± 6.5 to 57.8 ± 8.4 cm H₂O/%; P = 0.006). Redistribution of neural output to the respiratory muscles characterized by a progressive increase in rib cage and accessory muscle contribution to tidal breathing and expiratory muscle recruitment contributed to enhanced coupling. In conclusion, diaphragmatic recruitment is submaximal at the end of a failed weaning trial despite concurrent respiratory distress. This finding signifies that reflex inhibition of central neural output to the diaphragm contributes to weaning failure.

NEW & NOTEWORTHY Research into pathophysiology of failure to wean from mechanical ventilation has excluded several factors, including contractile fatigue, but the precise mechanism remains unknown. We recorded transdiaphragmatic pressure and diaphragmatic electrical activity in patients undergoing a T-tube trial. Diaphragmatic recruitment was submaximal at the end of a failed trial despite concurrent respiratory distress, signifying that inhibition of central neural output to the diaphragm is an important mechanism of weaning failure.

Respiratory resetting elicited by single pulse spinal stimulation

Intraspinal microstimulation (ISMS) can effectively activate spinal motor circuits, but the impact on the endogenous respiratory pattern has not been systematically evaluated. Here we delivered ISMS in spontaneously breathing adult rats while simultaneously recording diaphragm and external intercostal electromyography activity. ISMS pulses were delivered from C2-T1 along two rostrocaudal tracts located 0.5 or 1 mm lateral to midline. A tungsten electrode was incrementally advanced from the dorsal spinal surface and 300μs biphasic pulses (10-90 μA) were delivered at depth increments of 600 μm. Dorsal ISMS often produced fractionated inspiratory bursting or caused early termination of the inspiratory effort. Conversely, ventral stimulation had no discernable impact on respiratory resetting. We conclude that ISMS targeting the ventral spinal cord is unlikely to directly alter the respiratory rhythm. Dorsal ISMS, however, may terminate the inspiratory burst through activation of spinobulbar pathways. We suggest that respiratory patterns should be included as an outcome variable in preclinical studies of ISMS.

Anatomy and physiology of phrenic afferent neurons

Large-diameter myelinated phrenic afferents discharge in phase with diaphragm contraction, and smaller diameter fibers discharge across the respiratory cycle. In this article, we review the phrenic afferent literature and highlight areas in need of further study. We conclude that 1) activation of both myelinated and nonmyelinated phrenic sensory afferents can influence respiratory motor output on a breath-by-breath basis; 2) the relative impact of phrenic afferents substantially increases with diaphragm work and fatigue; 3) activation of phrenic afferents has a powerful impact on sympathetic motor outflow, and 4) phrenic afferents contribute to diaphragm somatosensation and the conscious perception of breathing. Much remains to be learned regarding the spinal and supraspinal distribution and synaptic contacts of myelinated and nonmyelinated phrenic afferents. Similarly, very little is known regarding the potential role of phrenic afferent neurons in triggering or modulating expression of respiratory neuroplasticity.

Tidal volume and diaphragm muscle activity in rats with cervical spinal cord injury

[Purpose] The purpose of this study was to make an experimental model of cervical spinal cord injury (CSCI) using Wistar rats, in order to analyze the influence of CSCI on the respiratory function. [Subjects] Thirty-two male 12-week-old Wistar rats were used. [Methods] The CSCI was made at the levels from C3 to C7, and we performed pneumotachography and electromyography (EMG) on the diaphragm. Computed tomography was used to determine the level of spinal cord damage. [Results] After the operation, the tidal volume of the rats with a C3 level injury decreased to approximately 22.3% of its pre-injury value. In addition, in the same rats, the diaphragmatic electromyogram activity decreased remarkably. Compared with before CSCI, the tidal volume decreased to 78.6% of its pre-injury value in CSCI at the C5 level, and it decreased to 94.1% of its pre-injury value in CSCI at the C7 level. [Conclusion] In the rats that sustained a CSCI in this study, the group of respiratory muscles that receive innervation from the thoracic spinal cord was paralyzed. Therefore, the EMG signal of the diaphragm increased. These results demonstrate that there is a relationship between respiratory function and the level of CSCI.

Cardiovascular effects of the respiratory muscle metaboreflexes in dogs: rest and exercise

In awake dogs, lactic acid was injected into the phrenic and deep circumflex iliac arteries to elicit the diaphragm and abdominal muscle metaboreflexes, respectively. At rest, injections into the phrenic or deep circumflex iliac arteries significantly increased mean arterial blood pressure 21 +/- 7% and reduced cardiac output 6 +/- 2% and blood flow to the hindlimbs 20 +/- 9%. Simultaneously, total systemic, hindlimb, and abdominal expiratory muscle vascular conductances were reduced. These cardiovascular responses were not accompanied by significant changes in the amplitude or timing of the diaphragm electromyogram. During treadmill exercise that increased cardiac output, hindlimb blood flow, and vascular conductance 159 +/- 106, 276 +/- 309, and 299 +/- 90% above resting values, lactic acid injected into the phrenic or deep circumflex iliac arteries also elicited pressor responses and reduced hindlimb blood flow and vascular conductance. Adrenergic receptor blockade at rest eliminated the cardiovascular effects of the respiratory muscle metaboreflex. We conclude that the cardiovascular effects of respiratory muscle metaboreflex activation are similar to those previously reported for limb muscles. When activated via metabolite production, the respiratory muscle metaboreflex may contribute to the increased sympathetic tone and redistribution of blood flow during exercise.

Phrenic long-term facilitation requires spinal serotonin receptor activation and protein synthesis

Respiratory long-term facilitation (LTF) is a form of serotonin-dependent plasticity induced by intermittent hypoxia. LTF is manifested as a long-lasting increase in respiratory amplitude (and frequency) after the hypoxic episodes have ended. We tested the hypotheses that LTF of phrenic amplitude requires spinal serotonin receptor activation and spinal protein synthesis. A broad-spectrum serotonin receptor antagonist (methysergide) or protein synthesis inhibitors (emetine or cycloheximide) were injected intrathecally in the cervical spinal cord of anesthetized rats. Control rats, injected with vehicle (artificial CSF), exhibited an augmented phrenic burst amplitude after three 5 min episodes of hypoxia (78 +/- 15% above baseline, 60 min after hypoxia; p < 0.05), indicating LTF. Pretreatment with methysergide, emetine, or cycloheximide attenuated or abolished phrenic LTF (20 +/- 4, 0.2 +/- 11, and 20 +/- 2%, respectively; all p > 0.05). With protein synthesis inhibitors, phrenic LTF differed from control by 15 min after intermittent hypoxia. As an internal control against unintended drug distribution, we measured respiratory LTF in hypoglossal (XII) motor output. At 60 min after intermittent hypoxia, all treatment groups exhibited similar XII LTF (artificial CSF, 44 +/- 10%; methysergide, 40 +/- 5%; emetine, 35 +/- 9%; and cycloheximide, 57 +/- 29%; all p < 0.05), suggesting that drugs were restricted at effective doses to the spinal cord. We conclude that phrenic LTF requires spinal serotonin receptor activation and protein synthesis. Serotonin receptors on phrenic motoneuron dendrites may induce new protein synthesis, thereby giving rise to phrenic LTF.

Powerful respiratory stimulation by thin muscle afferents

The ventilatory responses to electrical stimulation of phrenic afferents were examined in spontaneously breathing dogs at different levels of sodium pentobarbital anesthesia. High intensity stimulation (activation of all the afferents, including thin fibers) increased ventilation (V(E)). The increase in V(E) was comparable to that of breathing 10% CO2 and was inversely related to anesthesia level. Under light anesthesia, V(E) increased to 282+/-36% of the control value when the phrenic nerve was stimulated at 130 times the twitch threshold (n = 15; P < 0.01). The increase in V(E) was due to increases in breathing rate (193+/-19%) and tidal volume (V(T)) (143+/-8%). On the other hand, inspiratory time (T(I)) decreased. Thus, average airflow rate (V(T)/T(I)) increased to 204+/-23%. After administration of 20 and 40% of the initial dose of pentobarbital, V(E) response was attenuated to 157+/-21 and 121+/-4%, respectively. We conclude that thin muscle afferents are capable of eliciting pronounced ventilatory stimulation. The small responses observed earlier were likely due to depth of anesthesia.

Respiratory muscle fatigue

Ventilatory failure may accompany a variety of pulmonary and neuromuscular diseases. There has been much controversy about whether this failure is due to respiratory muscle fatigue at peripheral sites or a failure of drive at sites within the central nervous system. The chapter reviews this topic.

Phrenic afferent stimulation by bradykinin and the distribution of the inspiratory motor drive

Activation of thin-fiber (groups III and IV) afferents from the diaphragm using capsaicin or ischemia increases the respiratory muscle activity. To assess whether bradykinin causes similar effects, we injected boluses of bradykinin into the phrenic artery of in situ, isolated and innervated left hemi-diaphragm preparations in 8 alpha-chloralose anesthetized, vagotomized, mechanically ventilated dogs. Inspiratory motor drive during spontaneous breathing attempts was assessed from the integrated EMG activity of several inspiratory muscles. Fifty micrograms of bradykinin increased peak integrated EMG activities of alae nasi to 110%, genioglossus to 189%, left diaphragm to 115% (P < 0.05) and parasternal to 109% (P < 0.01) of baseline activity 60 sec after the injection. Inspiratory time decreased by 10% (P < 0.01). The mean arterial blood pressure increased by about 10 mmHg. Responses were similar with 10, 25 and 100 micrograms of bradykinin. After left phrenicotomy, bradykinin did not affect inspiratory muscle EMG or respiratory timing. In conclusion, thin-fiber phrenic afferent activation by bradykinin exerts an excitatory but disproportionate influence on the inspiratory motor drive.

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.

Ventilatory effects of the interaction between phrenic and limb muscle afferents

We studied the effects on ventilation and ventilatory muscle activation of stimulation of the central ends of the left phrenic and gastrocnemius nerves separately and concurrently in 10 spontaneously breathing, alpha-chloralose anaesthetized dogs. The nerves were stimulated for 1 min, at a frequency of 40 Hz and pulse duration of 1 ms. The phrenic nerve was stimulated at 20 and 40 times twitch threshold (TT). During these stimulation periods ventilation increased by 39% and 79% of control values, respectively. The gastrocnemius nerve was stimulated at 20 times TT. This produced a 90% increase in ventilation. Stimulation of either nerve resulted in increases in the activity of the right diaphragm, parasternal intercostal and alae nasi muscles comparable in magnitude to the increase in tidal volume. The activities of the genioglossus and transversus abdominis muscle increased to a much greater extent than did the other muscles under all conditions. In contrast, triangularis sterni activity remained unchanged during stimulation of either nerve. The phrenic nerve was then stimulated at 40 times TT for 1 min with superimposed gastrocnemius nerve stimulation (20 times TT) during the last 30 s. Ventilation had risen by 66% after 30 s of phrenic nerve stimulation. With the addition of gastrocnemius nerve stimulation, ventilation rose by a further 84% for a total increase of 150% of the control value. Mathematical summation of the responses to individual nerve stimulation at these intensities predicted a 156% increase in ventilation. Similar degrees of summation were found with respect to respiratory muscle activation. We conclude that the interaction between phrenic and limb muscle (gastrocnemius) afferent is additive with respect to their effects on ventilation.

Phrenic afferents and ventilatory control

It has been understood since the late 1800s that the diaphragm has significant sensory innervation. The role of phrenic afferents in the control of breathing, however, has been obscure. The phrenic nerve has been shown to contain a full array of afferent fibers. However, proprioceptive (group 1 fibers) afferents are few compared to postural muscles or the intercostals. The diaphragm, unlike the inspiratory intercostal muscles, has a small complement of spindle afferents and not all of these spindles are supplied with fusorial fibers. Reduced spindle afferents under gamma control help to explain previous studies of the diaphragm that have failed to reveal autogenic facilitation, that is, a reflex-mediated increase in drive during inspiratory loading. Nevertheless, some clinical studies have revealed increased activation of the diaphragm when its length is reduced. Group 1 fibers, which are predominantly tendon organ afferents in the diaphragm, have been shown to have a phasic inhibitory function. A reduction in this inhibition brought about by a reduction in diaphragmatic length during lung inflation may explain the increased diaphragmatic activation reported in clinical studies. Phrenic afferents have been shown to have multiple spinal and supraspinal projections. Recent studies have explored the ventilatory effects of thin fiber afferents (group III and IV fibers) in the phrenic nerve. Stimulation of these afferents has been shown both to inhibit and excite ventilation. These afferents arise from polymodal receptors that respond to both mechanical and chemical stimulation. Activation of these receptors may occur in a variety of conditions and the ventilatory response may be determined by the specific receptor activated.

Localization of substance P immunoreactivity in phrenic primary afferent neurons

Retrograde tracing with a fluorescent dye (Fast Blue) combined with immunohistochemistry was used to localize the putative neurotransmitter, substance P, in phrenic primary afferent neurons. Fast Blue was injected into the diaphragm and was found to label phrenic primary afferent neurons in sections from the fifth and sixth cervical dorsal root ganglia. The same sections were then treated with antiserum to substance P. A total of 11.4% of labelled phrenic primary afferent neurons contained substance P immunoreactivity. The diameters of the neurons ranged between 17 to 45 microns with a mean size of 29.7 +/- 0.7 microns (N = 81). The results suggest that substance P could be involved in mediating the transmission of sensory information from the diaphragm to the CNS.