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Surfactant replacement partially restores the activity of pulmonary stretch receptors in surfactant-depleted cats

¹Department of Women's and Children's Health and Department of Physiology and Medical Biophysics, Uppsala University, Uppsala, Sweden; and ²Division of Neonatology, Department of Obstetrics and Gynecology, Klinikum Grosshadern, Ludwig Maximilian University, Munich, Germany

Submitted 7 April 2005 ; accepted in final form 29 September 2005

Single units of slowly adapting pulmonary stretch receptors (PSRs) were investigated in anesthetized cats during spontaneous breathing on continuous positive airway pressure (2–5 cmH₂O), before and after lung lavage and then after instillation of surfactant to determine the PSR response to surfactant replacement. PSRs were classified as high threshold (HT) and low threshold (LT), and their instantaneous impulse frequency (f_imp) was related to transpulmonary pressure (Ptp) and tidal volume (VT). Both the total number of impulses and maximal f_imp of HT and LT PSRs decreased after lung lavage (55 and 45%, respectively) in the presence of increased Ptp and decreased VT. While Ptp decreased markedly and VT remained unchanged after surfactant instillation, all except one PSR responded with increased total number of impulses and maximal f_imp (42 and 26%, respectively). Some HT PSRs ceased to discharge after lung lavage but recovered after surfactant instillation. The end-expiratory activity of LT PSRs increased or was regained after surfactant instillation. After instillation of surfactant, respiratory rate increased further with a shorter inspiratory time, resulting in a lower inspiratory-to-expiratory time ratio. Arterial pH decreased (7.31 ± 0.04 vs. 7.22 ± 0.06) and PCO₂ increased (5.5 ± 0.7 vs. 7.2 ± 1.3 kPa) after lung lavage, but they were the same after as before instillation of surfactant (pH = 7.21 ± 0.08 and PCO₂ = 7.6 ± 1.4 kPa) during spontaneous breathing. In conclusion, surfactant instillation increased lung compliance, which, in turn, increased the activity of both HT and LT PSRs. A further increase in respiratory rate due to a shorter inspiratory time after surfactant instillation suggests that the partially restored PSR activity after surfactant instillation affected the breathing pattern.

slowly adapting pulmonary stretch receptor; lung mechanics; breathing pattern; surfactant