Reflex effects of aerosolized histamine on phrenic nerve activity.

Studies were conducted in anesthetized, paralyzed dogs on the effect of aerosolized histamine on phrenic nerve activity. The paralyzed dogs were ventilated in phase with their recorded phrenic nerve activity at a constant inspiratory flow-rate, using a cycle-triggered ventilator. Phrenic nerve activity was measured before and during administration of aerosolized histamine while the inspiratory flow-rate and arterial blood gases were kept constant. In addition, before and after histamine, phrenic nerve activity was recorded for single bursts during which the ventilator was switched off. The effects of histamine on respiratory resistance were prevented by prior administration of isoproterenol and atropine. Although no changes occurred in respiratory resistance, histamine increased the instantaneous magnitude of phrenic nerve activity. The effect was evident early in the inspiratory period and was found even when the lungs were not inflated. Inflation of the lungs excited phrenic nerve activity; this effect increased after histamine. All of these actions of histamine were abolished by vagotomy. We conclude that histamine increased phrenic nerve activity during inspiration by a vagal reflex.

Dyspnea.

Dyspnea is the medical term for the patient's or subject's complaint of shortness of breath. It encompasses the respiratory discomfort experienced in many different diease states as well as the shortness of breath felt by a normal subject during or after strenuous exercise. Several parameters which have been shown to correlate with the onset or severity of dyspnea are described, including reduced vital capacity, the ratio of minute ventilation to vital capacity, reduced breathing reserve, the work of breathing, and the oxygen cost of breathing. Attempts at quantitation of dyspnea have usually consisted of measuring physiological parameters associated with the sensation, such as the "dyspneic index". The direct measurement of respiratory sensations using modern psycho-physical methods is at an early stage of development. Since the observation that the existence of dyspnea is often unrelated to any disturbance of arterial blood gas composition, it has been generally held that the mechanism of dyspnea is primarily neurophysiological. The neural pathways may conceptually be divided into those which transmit the "dyspnea message" from the respiratory apparatus to integrating centers in the brain, and those concerned with subsequently bringing the sensation to the level of consciousness. It seems likely that there is no single sensing mechanism and neural pathway which will be able to explain dyspnea in the diverse populations of patients and subjects who experience unpleasant respiratory sensations. Three theories concerning mechanisms of dyspnea are briefly described: "length-tension inappropriateness", vagal afferent activity especially from the J-receptors, and the recent concept of diaphragmatic fatigue. Some specific characteristics of the shortness of breath experienced in certain disease states are described, including chronic bronchitis and emphysema, bronchial asthma, pulmonary fibrosis and congestive heart disease.

Effects of hypercapnia and hypoxia on abdominal expiratory nerve activity.

We examined the effects of progressive hypercapnia and hypoxia on the efferent neural activity in a whole abdominal expiratory nerve (medial branch of the cranial iliohypogastric nerve (L1) in anesthetized, paralyzed dogs. To eliminate effects of phasic lung and chest-wall movements on expiratory activity, studies were performed in the absence of breathing movements. Progressive hyperoxic hypercapnia and isocapnic hypoxia were produced in the paralyzed animals by allowing 3-5 min of apnea to follow mechanical ventilation with 100% O2 or 35% O2 in N2, respectively; during hypoxia, isocapnia was maintained by intravenous infusion of tris(hydroxymethyl)aminomethane buffer at a predetermined rate. To quantify abdominal expiratory activity, mean abdominal nerve activity in a nerve burst was computed by integrating the abdominal neurogram and dividing by the duration of the nerve burst. Hypercapnia and hypoxia both increased mean abdominal nerve activity and decreased expiratory duration. In contrast to the ramplike phrenic neurogram, the abdominal neurogram consisted of three phases: an initial rising phase, a plateau phase in which abdominal nerve activity was approximately constant, and a terminal declining phase in which the activity returned to the base-line level. The height of this plateau phase and the rates of rise and decline of abdominal nerve activity all increased with increasing hypercapnia and hypoxia. We conclude that, with proprioceptive inputs constant, both hypercapnia and hypoxia are excitatory to abdominal expiratory neural activity.

Effects of hypercapnia and hypoxia on phrenic nerve activity and respiratory timing.

In the spontaneously breathing animal, respiratory responses to chemical stimuli are influenced by phasic proprioceptive inputs from the thorax. We have compared the effects of hypercapnia and hypoxia on the level and timing of phrenic nerve activity while these phasic afferent signals were absent. Progressive hyperoxic hypercapnia and isocapnic hypoxia were produced in anesthetized paralyzed dogs by allowing 3-5 min of apnea to follow mechanical ventilation with 100% O2 or 35% O2 in N2, respectively; during hypoxia, isocapnia was maintained by intravenous infusion of tris(hydroxymethyl)aminomethane buffer. The peak height (P) of nerve bursts, inspiratory time (TI), and expiratory time (TE) were measured from the phrenic neurogram. With the vagi intact or severed, hypoxia decreased TI, whereas hypercapnia did not; both stimuli decreased TE. At the same minute phrenic activity (P x frequency), P, TI, and TE were all less during hypoxia than during hypercapnia. The decreases in TI and TE with hypoxia were significantly less after carotid sinus denervation. The results indicate that the patterns of phrenic nerve activity in response to hypoxia and hypercapnia are different: hypoxia has a greater effect on respiratory timing, whereas hypercapnia has a greater effect on peak phrenic nerve activity. The effect of hypoxia on respiratory timing is largely mediated by the peripheral chemoreceptors.