Pulmonary mechano-receptors strongly affect the breathing pattern in surfactant-depleted animals

Objective: We hypothesized that pulmonary mechano-receptors influence the breathing pattern also in a lung condition similar to RDS in neonates. Vagal cooling experiments suggest that the deep slow breathing observed after vagotomy is not only a result of loss of pulmonary stretch receptor feedback, but also a result of blocked input from C-fibers (unmyelinated fibers). If these effects on breathing change after surfactant depletion/replacement is not known. To study the influence of vagal afferents on breathing pattern, gas exchange, and lung mechanics in young cats with normal, surfactant-depleted and surfactant-treated lungs. Methods: 12 young cats (weight 2.1 +- 0.2kg) (mean +- SD) were anaesthetised with chloralose, intubated intratracheally, and studied during spontaneous breathing on CPAP before and after lavage and after surfactant replacement. Respiratory rate (RR), tidal volume (Vt), inspiratory time (Ti), expiratory time (Te), arterial blood gases, and oesophageal pressure were measured. To block afferent nerve activity, vagal nerves were cooled with Peltier elements to + 7 C (myelinated afferents) and + 0 C (all afferents). Statistics: One way RM ANOVA and Bonferroni t-tests. Results: Cooling the vagal nerves from + 38°C, to + 7°C and then to + 0°C resulted in an overall decrease in RR of 87% from 11/min at 37°C, 51% from 26/min, and 51% from 24/min in healthy, surfactant-depleted, and surfactant-treated lungs respectively. It further resulted in an increase in Vt (140% from 15ml/kg, 225% from 8ml/kg, 156% from 9ml/kg respectively), Ti (126%, 158%, 164% respectively), and Te (96%, 169%, 205% respectively) (RM ANOVA: p<0.001 for all four variables) without any changes in arterial blood gases. Interbreath variability increased in Vt, Te and airflow after surfactant depletion (p<0.047). Conclusions: Deep slow breathing occurred during vagal cooling in all animals and was more pronounced in surfactant-depleted and surfactant-treated animals, suggesting that pulmonary mechano-receptors strongly affect the breathing pattern in lung conditions encountered in newborn infants.