Positive end-expiratory pressure (PEEP) is commonly used during mechanical ventilation in several clinical settings. Such a change in mechanical environment will affect the mechanosensory input from pulmonary slowly adapting receptors (SARs), altering cardiopulmonary function. However, we know little about how SARs behave during prolonged, graded PEEP application.
We systematically examined the SAR response to PEEP application in anesthetized, open-chest and mechanically ventilated rabbits. We recorded single-unit activities from 18 SARs in the cervical vagus nerve and examined their response to increase of PEEP from 4 to 10 cm water for 20 minutes.
The mean activity of the units increased immediately from 35.7±4.2 to 80.5±7.3 imp/s at the fifth breath after PEEP application (n=14, p<0.001) and then gradually returned to 56.5±5.7 imp/s at the end of 20 minute of PEEP application (p<0.001). Similarly, the unit activities at the peak and valley of cyclic airway pressure increased from 84.2±5.2 (peak) and 8.5±3.7 (valley) to 137.1±11.4 and 47.8±6.2 imp/s, respectively (p<0.001). The activities gradually adapted to 118.5±9.3 (p<0.05) and 22.1±4.9 (p<0.005) imp/s after 20 minutes. The rest four units ceased firing at 34.7 seconds (ranging 10-56 seconds) after their activity reached peak and the cyclic airway pressure maintained high during PEEP application. These inactivated units could be reactivated by lowering PEEP and inactivated again by returning PEEP to the high level. Activation and inactivation occurred abruptly, i.e., the unit activity oscillated between high frequencies and 0, suggesting that over-excitation inactivates the sensory units.
Sustained high PEEP stimulates SARs and the SAR activity habituates through multiple mechanisms including receptor inactivation by over-excitation.
Understanding of the sensory inputs in the regulation of cardiopulmonary function during PEEP application will assist in developing better strategies of mechanical ventilation.
J.J. Guardiola, None.