In the preceding two communications we described a model for the relation between respiratory neural and mechanical outputs. In the present report we test the accuracy of the model in predicting volume and flow from occlusion pressure wave forms, and vice versa. We performed single-breath airway occlusions in 21 unconscious subjects and determined the time course of occlusion pressure. We also measured the passive properties of the respiratory system. The time course of volume and flow was predicted from the occlusion pressure wave forms, and the results were compared with the spontaneous breaths immediately preceding occlusion. Inspiratory duration, shape and amplitude of occlusion-pressure wave forms, and the passive properties of the respiratory system varied widely among subjects. There was good agreement between predicted and observed values in all cases. Except for some prolongation of inspiration (Hering-Breuer reflex), diaphragmatic activity did not change during occlusion. Since occlusion pressure is proportional to inspiratory activity, we conclude that the model described provides a good approximation of the relation between inspiratory activity and spirometric output.
Vagal influence on inspiratory motor output was assessed in 20 normal subjects and in 12 patients with respiratory disorders under enflurane anethesia using the method of airway occlusion. The change in inspiratory duration during occlusion (delta TI) was measured from mechanical parameters (respiratory flow and tracheal pressure). In eight of the subjects, however, the effect of occlusion and augmentation of tidal volume was further evaluated from diaphragmatic electromyogram. In normal subjects delta TI (mechanical) averaged 0.15 s (range -0.1 to +0.77 s) and correlated with the duration of inspiration during occlusion. Electromyographic observations indicated that the change in neural TI exceeds the change in mechanical TI by approximately 0.2 s and that augmentation of tidal volume shortens TI with no apparent volume threshold. There was a tendency for vagal influence to be higher with restrictive lung disease and lower with obstructive airway disease. These observations indicate that a majority of humans display a significant vagal influence on TI in the spontaneous tidal volume range under anesthesia.
We reported earlier on the changes in excitability of central respiratory switching mechanisms in the course of a brief inspiratory-inhibiting vagal stimulus (J. Appl. Physiol. 50: 1183-1192, 1981). To further define the dynamics of central processing of such input we studied the changes in the excitability of timing mechanisms in the immediate (less than 1.0 s) and late (1-20 s) periods after stimulus removal. We also examined the changes in respiratory timing in the course of protracted (greater than 20 s) stimulation. Studies were done using pentobarbital-anesthetized cats. For studies involving long-term stimulation or late off responses, cats were paralyzed, vagotomized, carotid denervated, and artificially ventilated. We found that the inspiratory inhibitory influence of a brief stimulus continues, in a declining fashion, for 0.3-10 s after removal of the stimulus. This was followed by a paradoxical response, inspirations were prolonged and expirations were shortened, which was maximal 1-2 s after stimulus removal and which declined gradually over a period of 6-16 s. There was progressive decline in inspiratory-shortening expiratory-prolonging influence in the course of sustained stimuli. These results indicate substantial adaptation in the course of even brief stimuli and provide an explanation for inspiratory-expiratory duration and expiratory-inspiratory duration linkages.
When an electrical stimulus is applied to the cervical vagus during inspiration it results in earlier termination of that phase. We investigated the temporal behavior of the effect of such a stimulus in the interval between its application and inspiratory termination to gain insight into the central processing of these signals. In pentobarbital-anesthetized cats we determined the threshold for inspiratory termination by delivering stimulus trains of constant duration to one vagus nerve at different times after inspiratory onset. The threshold frequency was obtained when the other nerve was unstimulated and when it was stimulated using trains of constant frequency (conditioning stimulus) beginning soon after inspiratory onset. The difference in threshold between the two test indicated the effectiveness of the conditioning stimulus in reducing threshold for inspiratory termination. The effectiveness of the conditioning stimulus increased progressively, reached a peak value at approximately 0.8 s, and progressively declined thereafter. This behavior indicates substantial integrative and accommodative processing of inspiratory terminating vagal inputs.
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