Phrenic Electroneurogram Research Articles

Diaphragm impairment in patients admitted for severe COVID-19.

Among patients affected by the virus COVID-19, physicians have observed ventilation disorders. It is relevant to assess neurological involvement, including the role of diaphragmatic function. Its possible impairment could be related to the systemic inflammatory response and disease progression that both typify COVID-19 infection. We distinguished two groups (severe group (SG) and mild group (MG)) according to the severity of respiratory symptomatology. We performed neurophysiological and sonography studies to evaluate the diaphragmatic function. Regarding the sonography variables, we identified statistically significant differences in the right mean diaphragmatic thickness along with the expiration, showing 1.56 mm (SEM: 0.11) in the SG vs 1.92 mm (SEM: 0.19) in the MG (p = 0.042). The contractibility of both hemidiaphragms was 15% lower in the severe group, though this difference is not statistically significant. In our examination of the neurophysiological variables, in the amplitude responses, we observed a greater difference between responses from both phrenic nerves as follows: the raw differences in amplitude were 0.40 μV (SEM: 0.14) in the SG vs 0.35 μV (SEM: 0.19) in the MG and the percentage difference was 25.92% (SEM: 7.22) in the SG vs 16.28% (SEM: 4.38%) in the MG. Although diaphragmatic dysfunction is difficult to detect, our combined functional and morphological approach with phrenic electroneurograms and chest ultrasounds could improve diagnostic sensitivity. We suggest that diaphragmatic dysfunction could play a relevant role in respiratory disturbance in hospitalised patients with severe COVID-19.

Assessment of the Use of Multi-Channel Organic Electrodes to Record ENG on Small Nerves: Application to Phrenic Nerve Burst Detection.

Effective closed-loop neuromodulation relies on the acquisition of appropriate physiological control variables and the delivery of an appropriate stimulation signal. In particular, electroneurogram (ENG) data acquired from a set of electrodes applied at the surface of the nerve may be used as a potential control variable in this field. Improved electrode technologies and data processing methods are clearly needed in this context. In this work, we evaluated a new electrode technology based on multichannel organic electrodes (OE) and applied a signal processing chain in order to detect respiratory-related bursts from the phrenic nerve. Phrenic ENG (pENG) were acquired from nine Long Evans rats in situ preparations. For each preparation, a 16-channel OE was applied around the phrenic nerve’s surface and a suction electrode was applied to the cut end of the same nerve. The former electrode provided input multivariate pENG signals while the latter electrode provided the gold standard for data analysis. Correlations between OE signals and that from the gold standard were estimated. Signal to noise ratio (SNR) and ROC curves were built to quantify phrenic bursts detection performance. Correlation score showed the ability of the OE to record high-quality pENG. Our methods allowed good phrenic bursts detection. However, we failed to demonstrate a spatial selectivity from the multiple pENG recorded with our OE matrix. Altogether, our results suggest that highly flexible and biocompatible multi-channel electrode may represent an interesting alternative to metallic cuff electrodes to perform nerve bursts detection and/or closed-loop neuromodulation.

Ascorbyl palmitate augments hypoxic respiratory response in the cat.

The redox signaling is germane for the hypoxia-sensing mechanisms at the carotid body. This raises the strong possibility that agents possess reducing and antioxidant attributes, such as ascorbate, could influence the hypoxic respiratory response. However, water solubility of ascorbate makes its effectiveness at membrane-associated target sites dubious. In this study, we sought to determine the effect of ascorbyl-6-palmitate (AP), a lipid-soluble derivative of ascorbate which penetrates biomembranes, on hypoxic respiration in the anesthetized, paralyzed and ventilated cat. AP was given by gavage: 600 mg/kg daily for 6 days before the beginning of the acute experiment. Respiration was then assessed from the phrenic electroneurogram, from which peak phrenic amplitude, a surrogate of tidal component, respiratory frequency, and their product, the minute phrenic output, were quantified. The response to normocapnic hypoxia, 7% O(2) in N(2), in the AP-treated cats was compared with that in controls. We found that AP augmented hypoxic respiration, delayed the appearance of hypoxic depression and decreased it, although the stimulatory/depressant character was preserved. The results suggest that the ascorbate moiety of AP interacts with the hypoxia-sensing mechanisms. Ascorbate may affect hypoxic respiration at multiple stages of chemotransduction pathways, which are subject to continuing uncertainties. The study highlights the augmentative effect of AP, a redox modulator, on hypoxic respiration, which may have a therapeutic potential.

Ascorbyl Palmitate Augments Hypoxic Respiratory Response in the Cat

The redox signaling is germane for the hypoxia-sensing mechanisms at the carotid body. This raises the strong possibility that agents possess reducing and antioxidant attributes, such as ascorbate, could influence the hypoxic respiratory response. However, water solubility of ascorbate makes its effectiveness at membrane-associated target sites dubious. In this study, we sought to determine the effect of ascorbyl-6-palmitate (AP), a lipid-soluble derivative of ascorbate which penetrates biomembranes, on hypoxic respiration in the anesthetized, paralyzed and ventilated cat. AP was given by gavage: 600 mg/kg daily for 6 days before the beginning of the acute experiment. Respiration was then assessed from the phrenic electroneurogram, from which peak phrenic amplitude, a surrogate of tidal component, respiratory frequency, and their product, the minute phrenic output, were quantified. The response to normocapnic hypoxia, 7% O₂ in N₂, in the AP-treated cats was compared with that in controls. We found that AP augmented hypoxic respiration, delayed the appearance of hypoxic depression and decreased it, although the stimulatory/depressant character was preserved. The results suggest that the ascorbate moiety of AP interacts with the hypoxia-sensing mechanisms. Ascorbate may affect hypoxic respiration at multiple stages of chemotransduction pathways, which are subject to continuing uncertainties. The study highlights the augmentative effect of AP, a redox modulator, on hypoxic respiration, which may have a therapeutic potential.

Augmentation of hypoxic respiration after brief hyperoxia in the anesthetized cat: Putative function of GABAA neurotransmission

In this study, we attempted to determine the role of GABA neurotransmission in augmentation of hypoxic respiration by antecedent hyperoxic breathing. The experiments were performed in anesthetized, paralyzed and vagotomized cats divided into control and bicuculline (a GABA(A) receptor blocker)-injected groups. The experimental protocol consisted of exposing the animals to successive hypoxic-hyperoxic-hypoxic conditions. Respiration was assessed using phrenic electroneurograms, from which the peak phrenic height, a surrogate of the tidal volume component, and respiratory rate were obtained, and their product, the respiratory minute output, was calculated. We found that prior hyperoxic ventilation increased the subsequent respiratory response to hypoxia by an average of 23.5%, compared with the preoxygen response. This increase was driven by volume respiration. The biphasic character of the hypoxic respiratory response, consisting of stimulatory and depressant phases, was sustained. Bicuculline abolished the augmentative effect on hypoxic respiration of prior hyperoxia, which suggests that oxygenation induces GABA(A)-mediated hyperexcitability of respiratory neurons, possibly by the liberation of reactive oxygen species. We concluded that GABA neurotransmission is pertinent to the effect of hyperoxia on hypoxic respiratory reactivity.

Augmentation of Hypoxic Respiration after Brief Hyperoxia in the Anesthetized Cat: Putative Function of GABAA Neurotransmission

In this study, we attempted to determine the role of GABA neurotransmission in augmentation of hypoxic respiration by antecedent hyperoxic breathing. The experiments were performed in anesthetized, paralyzed and vagotomized cats divided into control and bicuculline (a GABA_A receptor blocker)-injected groups. The experimental protocol consisted of exposing the animals to successive hypoxic-hyperoxic-hypoxic conditions. Respiration was assessed using phrenic electroneurograms, from which the peak phrenic height, a surrogate of the tidal volume component, and respiratory rate were obtained, and their product, the respiratory minute output, was calculated. We found that prior hyperoxic ventilation increased the subsequent respiratory response to hypoxia by an average of 23.5%, compared with the preoxygen response. This increase was driven by volume respiration. The biphasic character of the hypoxic respiratory response, consisting of stimulatory and depressant phases, was sustained. Bicuculline abolished the augmentative effect on hypoxic respiration of prior hyperoxia, which suggests that oxygenation induces GABA_A-mediated hyperexcitability of respiratory neurons, possibly by the liberation of reactive oxygen species. We concluded that GABA neurotransmission is pertinent to the effect of hyperoxia on hypoxic respiratory reactivity.

The role of raphe and tractus solitarius neuronal structures in the modulation of respiratory pattern in rabbits.

We studied the effects of MNR stimulation on phrenic (Phr) electroneurogram and external intercostal muscles (EI) electromiogram in spontaneously breathing rabbits. Additionally, experiments were performed before and after lignocaine blockade of nucleus tractus solitarii (NTS) to determine whether the information from MNR is transmitted via NTS neurones. The completeness of the blockade of NTS region was checked by studying the Hering-Breuer reflex. MNR was stimulated at the level 2-7 mm rostral to the obex. Stimulation at the rostral part of this region produced inhibition of phasic inspiratory activity, whereas stimulation in the caudal part elicited tonic activity throughout the respiratory cycle. These effects were more pronounced on EI than Phr. Responses to MNR stimulation were attenuated after lignocaine blockade, suggesting that the neurons located in NTS take part in the transmission of the modulatory information from the MNR to respiratory motoneurones.

Neurochemical control of tissue resistance in piglets.

Lung resistance may be influenced by chemoreceptor activity and modulated by inspiratory neural output; however, it is unknown whether the contractile elements of lung tissue participate in these changes during early development. In anesthetized paralyzed open-chest piglets, we measured phrenic electroneurogram, lung resistance (RL), and tissue resistance utilizing alveolar capsules to partition the hypercapnic and hypoxic responses of RL into tissue (Rti) and airway resistance (Raw) components. Inhalation of 7% CO2 significantly increased RL (7.4 +/- 0.5 to 11.3 +/- 0.6 cmH2O.l-1.s), Rti (5.2 +/- 0.5 to 6.9 +/- 0.5 cmH2O.l-1.s), and Raw (2.2 +/- 0.2 to 4.4 +/- 0.4 cmH2O.l-1.s). Inhalation of 12% O2 caused more modest increases in RL, Rti, and Raw. Oscillations in tracheal and alveolar pressures appeared in synchrony with phrenic activity in response to both chemoreceptor stimuli. Cholinergic blockade eliminated these oscillations and significantly reduced the hypercapnic and hypoxic responses of RL, Rti, and Raw. These data demonstrate for the first time that hypercapnia and hypoxia elicit a cholinergically mediated increase in Rti which, just like the airway component of RL, is modulated by inspiratory neural output and is present during early development. Such coordination in neural function throughout the respiratory system may serve to optimize gas exchange during early postnatal life.

Carotid bodies and ventilatory response to hypoxia in aminophylline‐treated piglets

Peripheral chemoreceptors may be immature in neonatal animals, exhibiting maturational changes in the perinatal period. Even though methylxanthines are respiratory stimulants, many premature neonates do not respond to them. Thus, we hypothesized that carotid body activity is necessary for aminophylline to reverse hypoxia-induced respiratory depression. We exposed 16 anesthetized newborn piglets (age 2-7 days) to hypoxia (inhalation of 12% oxygen) for 5 min. Aminophylline (15 mg/kg iv) was administered either prior to (11 piglets) or following (5 piglets) carotid body denervation (CBD). Before CBD, hypoxia elicited transient initial increases in tidal volume (from 79 +/- 4 to 99 +/- 1% of maximum, mean +/- SE), minute ventilation (from 64 +/- 5 to 93 +/- 4%), and peak phrenic electroneurogram (from 63 +/- 8 to 91 +/- 6%, all P < 0.05). This was followed by a decrease in tidal volume, minute ventilation and phrenic electroneurogram (all P < 0.05). Prior to CBD, aminophylline pretreatment prevented the decrease in all the measures of respiratory output during late hypoxia. After CBD, hypoxia induced an initial and sustained depression of ventilation (tidal volume from 100 to 33 +/- 14%; frequency from 94 +/- 4 to 42 +/- 17%; minute ventilation from 100 to 32 +/- 14%, all P < 0.05) and phrenic electroneurogram (peak phrenic from 100 to 47 +/- 18%; minute phrenic from 85 +/- 6 to 55 +/- 21%, both P < 0.05). Administration of aminophylline after CBD did not prevent the profound respiratory depression elicited by hypoxia in the chemodenervated piglets.(ABSTRACT TRUNCATED AT 250 WORDS)

Delayed poststimulus decrease of phrenic motoneuron output produced by phrenic nerve afferent stimulation.

The immediate effects of phrenic afferent nerve activation on ventilation have been shown to be both excitatory and inhibitory. Long-lasting inhibitory effects on respiratory motoneuron output have been reported after stimulation of afferent nerves from limb muscles. However, whether respiratory muscle afferent nerves can produce this effect is unknown. We therefore hypothesized that activation of phrenic afferent nerves may produce a prolonged decrease of respiratory motoneuron output. Six alpha-chloralose-anesthetized dogs were studied after vagotomy and bilateral carotid sinus nerve section. The dogs were paralyzed, and end-tidal CO2 was controlled by mechanical ventilation. The proximal end of the cut thoracic phrenic nerve was electrically stimulated for 1 min at intensities that produced activation of thin-fiber afferents. The contralateral efferent phrenic integrated electroneurogram (ENG) was recorded. During stimulation, phrenic ENG activity increased. ENG activity was recorded during recovery and reached a peak decrease compared with control of 19 +/- 11% (SD) 9.0 +/- 6 min after stimulation and returned to control after 30 min. A qualitatively similar response was seen after stimulation of the gastrocnemius nerve. We conclude that activation of thin-fiber afferents in the phrenic nerve can produce a delayed and prolonged decrease of respiratory motoneuron output similar to that of limb muscle afferent nerves.

The effects of changes in left ventricular pressure on respiratory activity in anaesthetized dogs.

This study was undertaken to determine whether moderate increases in left ventricular systolic pressure in anaesthetized dogs result in changes in respiratory activity. Left ventricular systolic pressure was changed using a preparation in which the applied pressure did not distend the aortic arch and in which left atrial and carotid sinus pressures were held constant. The systemic circulation was perfused at constant flow and respiratory activity was assessed from the phrenic electroneurogram. Increases in aortic root and left ventricular systolic pressures resulted in a decrease in arterial perfusion pressure, no significant change in heart rate and a decrease in phrenic nerve activity. These responses differed from those to changes in carotid sinus pressure, which were decreases in perfusion pressure and heart rate but no significant steady-state change in phrenic activity. Responses to changes in ventricular pressure were obtained to each small step between 16 and 28 kPa (120 and 210 mmHg). Following cervical vagotomy, changes in ventricular pressure resulted in neither respiratory nor vascular responses. These results indicate that left ventricular receptors may play a role in the control of respiration.

The absence of cardiovascular and respiratory responses to changes in right ventricular pressure in anaesthetized dogs.

1. This study was undertaken to determine whether physiological changes in pressure localized to the right ventricle result in reflex cardiovascular or respiratory responses. 2. Right ventricular systolic pressure was changed using a preparation in which right atrial and carotid sinus pressures were held constant. The pulmonary and hence the systemic circulation were perfused at constant flow. Vascular resistance and respiratory activity were assessed from the systemic arterial pressure and the phrenic electroneurogram. 3. Changes in right ventricular systolic pressure did not result in any consistent changes in heart rate, systemic arterial blood pressure or phrenic nerve activity. 4. Expected responses occurred to changes in the stimuli to carotid baroreceptors and chemoreceptors, distension of pulmonary arterial baroreceptors, and injections of veratridine into the left ventricle and pulmonary circulation. This suggests that the absence of responses to right ventricular distension was unlikely to have been due to damage to nervous pathways. 5. These results indicate that it is unlikely that there are reflexes arising from the right ventricle which have a major role in cardiovascular or respiratory control.

Cooling the intermediate area of the ventral medullary surface affects tracheal responses to hypoxia

The intermediate area of the ventral medullary surface (VMS) influences changes in airway tone caused by hypercapnia and intrapulmonary irritant receptor activation. These studies evaluated the effects of cooling the intermediate area of the VMS on the reflex hypoxic responses of the trachealis smooth muscle and of the phrenic nerve. Anesthetized, paralyzed cats were hyperventilated with 100% oxygen to produce phrenic neural apnea. Tracheal tone was measured indirectly by evaluating pressure changes in an innervated tracheal segment and the phrenic electroneurogram was determined from the central end of a cut cervical root. Switching the inspired gas to 12% O 2 increased tracheal pressure of 11 of 12 cats but caused phrenic activity to reappear in only 6 of the animals. Ventilation with 6% O 2 significantly increased tracheal constriction prior to phrenic activity. After intravenous administration of atropine methyl nitrate tracheal responses to hypoxia were abolished but phrenic neural responses were unaltered. Neither the tracheal pressure not the phasic phrenic electroneurogram responded to hypoxia after cutting the carotid sinus nerves. When the intermediate area of the VMS was cooled to 20°C prior to ventilation with the hypoxic gases, both tracheal and phrenic responses were significantly diminished. While the cats were hyperventilated with 6% O 2, cooling of the intermediate area significantly diminished tracheal pressure and phrenic nerve activity and both returned to the same levels after rewarming. Cooling of the intermediate area blunted tracheal and phrenic responses to carotid body stimulation by NaCN. However, the appearance of tracheal constriction prior to the onset of phasic phrenic activity may suggest that increased trachealis tone may occur independent of cyclical respiratory activity.

Effects of almitrine on genioglossal and diaphragmatic electromyograms.

We previously demonstrated dose-dependent increases in both hypoglossal and phrenic electroneurograms after almitrine in anesthetized, paralyzed, and vagotomized cats. We have now investigated the effect of this peripheral chemoreceptor stimulant on diaphragmatic and genioglossal (GG, an upper airway-maintaining muscle) electromyograms in five unanesthetized, chronically instrumented, spontaneously breathing adult cats during slow-wave sleep. In 12 studies almitrine doses of 1.0-6.0 mg/kg increased inspired minute ventilation (VI), frequency (f), and tidal volume (VT) and decreased expiratory time (TE). However, almitrine doses as high as 6.0 mg/kg failed to augment phasic inspiratory GG activity. To determine why almitrine induced phasic inspiratory upper airway activity in anesthetized, vagotomized cats but not in sleeping cats, additional studies were performed. In four dose-response studies in three pentobarbital-anesthetized cats, almitrine, 1.0-6.0 mg/kg, did not produce phasic inspiratory GG activity. Almitrine did induce phasic inspiratory GG activity in two of three studies in three vagotomized, tracheostomized, alpha-chloralose-urethan-anesthetized cats. These results suggest that almitrine would not be useful in obstructive sleep apnea, yet because almitrine markedly increased VI, f, and VT and decreased TE in unanesthetized sleeping cats the drug may be effective in patients who lack normal central neural respiratory drive, such as the preterm infant.

1857 EFFECT OF ALMITRINE ON HYPOGLOSSAL AND PHRENIC ELECTRONEUROGRAMS

Almitrine increases breathing by stimulating peripheral chemoreceptors. Previous studies suggest clinical usefulness in adults with COPD but few data are available to decide whether almitrine would be helpful in diseases involving pharyngeal airway obstruction such as apnea of prematurity or obstructive sleep apnea. We investigated the effect of intravenous almitrine on hypoglossal (HG, an upper airway nerve), and phrenic (PHR) neural activity in eight chloralose-urethane anesthetized, paralyzed, vagotomized, ventilated cats. Recordings were made of raw and integrated HG and PHR electroneurograms (ENGs), PACO2, PaO2, arterial blood pressure and rectal temperature. We found that: 1) in a dose-response study (N=3 cats) at doses of 0.1 - 4.0 mg/kg, almitrine doses as low as 0.1 mg/kg increased both HG and PHR ENG activity, with a maximum effect at 1.0 mg/kg; 2) holding PaCO2 at 40 mmHg, almitrine markedly increased HG and PHR ENG activity at all PaO2 values from 35-175 mmHg (N=5 cats); 3) holding PaO2 above 150 mmHg, almitrine increased HG and PHR ENG activity at all PaCO2 values from 30-70 mmHg (N=4 cats); 4) in a ventilatory parameter timing study almitrine increased VT/Ti and decreased Ti/Ttot at normoxia and eucapnea (N=6 cats). If the finding that almitrine increases upper airway-maintaining activity can be confirmed in unanesthetized sleeping animals, almitrine may be useful in obstructive sleep apnea and apnea of prematurity.

Time course of phrenic activity and respiratory pressures during airway occlusion in cats.

The morphology of integrated ("moving time average") phrenic electroneurograms (EPHR) and of tracheal (Ptr) and transdiaphragmatic (Pdi) pressure waves during occluded inspirations was studied in eight anesthetized cats breathing air and various hypercapnic and hypoxic mixtures. The shape of the rising part of EPR-, Ptr-, and Pdi-time profiles varied between animals (from convex to concave), but in each animal it remained virtually unchanged by hypoxia and hypercapnia. The shape of the Ptr and Pdi occlusion waves reflected the shape of EPHR. The relationship of EPHR to Pdi and Ptr did not change with chemical drive. It is concluded that central inspiratory activity (CIA) (as reflected by EPHR and its mechanical transforms Pdi and Ptr) increases in amplitude with stimulation of breathing but that the profile of CIA remains essentially unchanged. However, substantial differences in the time course development of phrenic activity, Pdi, and Ptr exist between cats. The fixed interrelationships among EPHR, Pdi, and Ptr indicate a proportional increase in activity among all inspiratory muscles with increased chemical drive.

Phrenic activity, respiratory pressures, and volume changes in cats.

In eight anesthetized cats we measured the integrated ("moving time average") phrenic activity [using phrenic electroneurogram (EPHR)] and the active transdiaphragmatic pressure [Pdi(mus)] during room air breathing, hypoxia, and hypercapnia. The relationship between Pdi(mus) and EPHR was unaffected by either hypoxic or hypercapnic stimulation of breathing, suggesting that in spontaneously breathing cats the pressure losses are negligible. In all cats, however, there was a substantial volume-related decrease in Pdi(mus), indicating that with increasing lung volume the effectiveness of the diaphragm as a pressure generator decreases. In addition, we have developed a model that allows prediction of the time course changes in lung volume for different morphology of inspiratory driving pressure. This model explains many of the features of control of breathing found experimentally in our cats.

Pulmonary arterial distension and activity in phrenic nerve of anesthetized dogs

In chloralose-anesthetized dogs a constant-flow, right atrium-to-left pulmonary artery, right-heart bypass was created. The right lung root was completely occluded and an isolated pouch of the main pulmonary arteries was perfused with venous blood at controlled nonpulsatile pressures between 10 and 90 Torr. An increase in pulmonary arterial pouch pressure caused an increase in systemic arterial pressure and in respiratory drive. In animals with afferent nerves intact, the increase in respiratory drive was indicated by an increase in the amplitude of the integrated phrenic electroneurogram. In animals with the left vagus nerve cut immediately above the left lung root, the increase in respiratory drive was evident by a shortening of the time of expiration. All responses were abolished by bilateral section of the cervical vagosympathetic trunks. It is concluded that in the preparation described pulmonary arterial distension causes a reflex increase in systemic vascular resistance and in respiratory drive, the afferent path of the reflex being in the vagus nerves.

Arterial blood gases during raised intracranial pressure in anesthetized cats under controlled ventilation.

✓ In anesthetized paralyzed cats ventilated with air, blood gases were analyzed repeatedly before and during episodes of raised intracranial pressure (ICP). The ICP was raised by infusion via the lumbar subarachnoid or intraventricular route, and increases were maintained for at least 30 minutes. A minor degree of hypoxemia commonly developed, but was always associated with hypercapnia; normoxia was restored by increasing the ventilation sufficiently to restore normocapnia. Relative under-ventilation is thus liable to develop if the minute volume is maintained constant when ICP is raised, probably because of increased metabolic rate which may be associated with a rise in temperature; there is no evidence to implicate more obscure causes of hypoxemia in this circumstance. Pulmonary hemorrhage and edema were found post mortem in nine of 20 animals, but only two of these had developed greater hypoxemia than could be accounted for by under-ventilation. Phrenic electroneurograms were recorded, and respiratory activity was shown to persist during prolonged periods of cerebral ischemia.

Respiratory modulation of barareceptor and chemoreceptor reflexes affecting heart rate and cardiac vagal efferent nerve activity.

1. Brief stimuli were delivered to the carotid chemoreceptors or baroreceptors in dogs anaesthetized with chloralose. Chemoreceptor stimulation was achieved by rapid retrograde injection of 0.2-0.5 ml. CO2 equilibrated saline through a cannula in the external carotid artery. Baroreceptor stimulation was achieved by forceful retrograde injection of 2-5 ml. air-equilibrated saline into the external carotid artery after first clamping the common carotid artery. 2. prompt decreases in heart rate were elicited by brief sudden chemoreceptor or baroreceptor stimuli when these were delivered during the expiratory phase of respiration. The stimuli did not modify the control heart rate pattern when delivered in the inspiratory phase of respiration. This respiratory modulation of reflex effectiveness persisted when the animals were completely paralysed and the phase of the respiratory cycle was monitored through a phrenic electroneurogram. 3. single cardiac vagal efferent nerve fibres were dissected from the cut central end of the right cervical vagus nerve. They were classified as cardiac efferents by their cardiac and respiratory rhythmicity, and by their increased activity in response to stimulation of a carotid sinus nerve or to mechanical elevation of the systemic arterial pressure. These efferent fibres increased their activity in response to brief chemoreceptor or baroreceptor stimuli delivered in expiration, but did not respond to stimuli delivered in inspiration. This respiratory modulation of both reflexes persisted after bilateral cervical vagotomy.