Airway anesthesia during positive and negative inspiratory pressure breathing in man

Role of airway receptors in the reflex responses of human inspiratory muscles to airway occlusion

1. This study was designed to investigate the role of airway receptors in the responses of a range of inspiratory muscles to airway occlusion. The occlusion had a rapid onset (< 10 ms), lasted 250 ms and produced only a slight impediment to inspiration. 2. Based on analysis of single trials and averaged rectified electromyographic responses (EMG) in six subjects, there was a major inhibition (IR) with an onset at 34 +/- 2 ms and a trough at 65 +/- 2 ms, and an excitation (ER) with a peak at 105 +/- 2 ms. These two responses are reflex given that voluntary reaction times to a tap on the chest wall occurred at latencies longer than the peak of ER. 3. The responses to airway occlusion did not appear in limb muscles which contracted phasically with inspiration. 4. Anaesthesia of the surface receptors of the upper airway did not attenuate the responses to occlusion. Because this procedure does not eliminate the inputs from muscle and deep laryngeal pressure receptors, two subjects were tested when intubated with a cuffed endotracheal tube so that the occlusion was delivered only to structures below the level of the trachea. Responses to airway occlusion were preserved when all upper airway receptors were 'bypassed'. 5. Responses to airway occlusion also remained after prolonged inhalation of nebulized lidocaine (lignocaine) sufficient to block the cough reflex. 6. The receptors mediating the responses to airway occlusion are therefore likely to reside in inspiratory muscles acting on the chest wall. If so, the short-latency inhibition contrasts with the excitatory stretch reflex responses observed in limb muscles.

Frequency of mechanical ventilation and respiratory activity after double lung transplantation

We investigated the contribution of pulmonary afferent nerve fibers to the control of inspiratory activity in awake humans. Eight double lung transplant outpatients and eight normal subjects were hyperventilated with a mechanical ventilator. Respiratory frequency was increased until no respiratory activity was detectable. Then, by either adding CO2 in the inspired gas or decreasing respiratory frequency, end-tidal PCO2 (PETCO2) was increased until inspiratory activity (i.e. change in inspiratory airway pressure peak and/or time profile) was detected. In normal subjects, PETCO2 threshold for inspiratory muscle recruitment was significantly lower when frequency was decreased than when CO2 was added (31.3 +/- 6.8 Torr vs. 38.2 +/- 8.1 Torr respectively, P < 0.005). This was not the case in the double lung transplant group (31.5 +/- 6.5 Torr vs. 32.9 +/- 5.8 Torr). These findings suggest that pulmonary afferent nerves have an inhibitory effect on inspiratory activity in humans.

Respiratory response to positive and negative inspiratory pressure in humans

To investigate the effect of positive or negative inspiratory pressure on respiration, eight subjects breathed, either without or with added external dead space (VD, 600 ml), through either added inspiratory laminar flow resistances (RES; peak inspiratory airway pressure, Pinsp, down to -9 cmH2O) or with inspiratory pressure support (IPS; Pinsp up to +10 cmH2O). IPS, triggered by the subject's inspiratory effort, provided positive airway pressure throughout inspiration, but allowed for attainment of the subject's own respiratory pattern. The following main results were obtained with IPS or RES relative to the control (no IPS, no RES): (1) with VD, IPS led to small, but significant, increases in tidal volume (VT), respiratory frequency (fR) and ventilation (VE), with no changes in inspiratory time (TI) or duty cycle (TI/TT). Mean inspiratory flow (VT/TI) increased, and mouth occlusion pressure 0.1 sec after onset of inspiration (P0.1) decreased significantly with IPS. The changes during RES were essentially in the opposite direction; (2) without VD, similar, but smaller effects were observed, and only the changes in VT/TI and P0.1 during IPS were significant; (3) highly significant decreases were observed during IPS in end-tidal PCO2 (PETCO2); on the average from 39.6 to 29.2 Torr without VD, and from 45.7 to 39.3 Torr with VD breathing. A small, but significant decrease in PETCO2 occurred also during RES with VD. We conclude that while resistive loading is nearly completely compensated with but small changes in PETCO2, inspiratory pressure support leads to marked hyperventilation, which is not effectively counteracted by central timing commands.