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

Pulmonary stretch receptor activity during partial liquid ventilation with different pressure waveforms

BACKGROUND

The aim of the present study was to investigate pulmonary stretch receptor activity (PSR) under different peak inspiratory pressures (PIPs) and inspiratory pressure waveforms during partial liquid (PLV) and gas ventilation (GV).

METHODS

PSR instantaneous impulse frequency (PSRf_imp) was recorded from single fibers in the vagal nerve during PLV and GV in young cats. PIPs were set at 1.2/1.8/2.2/2.7 kPa, and square and sinusoidal pressure waveforms were applied.

RESULTS

PSRf_imp at the start of inspiration increased with increasing PIPs, and was steeper and higher with square than with sinusoidal waveforms (p < 0.05). Total number of impulses, peak and mean PSRf_imp were lower during PLV than GV at the lowest and highest PIPs (p < 0.025). Time to peak PSRf_imp was shorter with square than with sinusoidal waveforms at all pressures and ventilations (p < 0.005). Irrespective of waveform, lower PIPs yielded lower ventilation during PLV.

CONCLUSION

As assessed by PSRf_imp, increased PIPs do not expose the lungs to more stretching during PLV than during GV, with only minor differences between square and sinusoidal waveforms.

Bronchopulmonary C-fibers modulate the breathing pattern in surfactant-depleted juvenile cats

The aim of this study was to investigate the influence of nonmyelinated C-fibers on the breathing pattern by cooling the vagal nerves to temperatures at which myelinated nerve transmission from pulmonary stretch receptors is blocked (+7 degrees C) and further at which nonmyelinated fiber input is blocked (0 degrees C), in anaesthetized spontaneously breathing juvenile cats with normal (L(N)), surfactant-depleted (L(D)) and surfactant-treated (L(T)) lungs. In L(N), vagal cooling from +7 to 0 degrees C decreased respiratory frequency (f(R); -8%; p < 0.01), and increased tidal volume (V(T); +40%; p < 0.01). In the presence of shallow fast breathing in L(D), f(R) decreased (+38 to +7 degrees C: -26%; p < 0.015 and +7 to 0 degrees C: -24%; p < 0.001) and V(T) increased (+37%; p < 0.049 and +88%; p < 0.016). In L(T), f(R) decreased (+7 to 0 degrees C: -21%; p < 0.001), whereas V(T) remained the same at 0 degrees C (+12%; NS). These findings show for the first time that the activity of bronchopulmonary C-fibers have a prominent role in modulating the breathing pattern in juvenile cats with surfactant-depleted lungs.

Maintained inspiratory activity during proportional assist ventilation in surfactant-depleted cats early after surfactant instillation: phrenic nerve and pulmonary stretch receptor activity

BACKGROUND

Inspiratory activity is a prerequisite for successful application of patient triggered ventilation such as proportional assist ventilation (PAV). It has recently been reported that surfactant instillation increases the activity of slowly adapting pulmonary stretch receptors (PSRs) followed by a shorter inspiratory time (Sindelar et al, J Appl Physiol, 2005 [Epub ahead of print]). Changes in lung mechanics, as observed in preterm infants with respiratory distress syndrome and after surfactant treatment, might therefore influence the inspiratory activity when applying PAV early after surfactant treatment.

OBJECTIVE

To investigate the regulation of breathing and ventilatory response in surfactant-depleted young cats during PAV and during continuous positive airway pressure (CPAP) early after surfactant instillation in relation to phrenic nerve activity (PNA) and the activity of PSRs.

METHODS

Seven anesthetized, endotracheally intubated young cats were exposed to periods of CPAP and PAV with the same end-expiratory pressure (0.2-0.5 kPa) before and after lung lavage and after surfactant instillation. PAV was set to compensate for 75% of the lung elastic recoil.

RESULTS

Tidal volume and respiratory rate were higher with lower PaCO2 and higher PaO2 during PAV than during CPAP both before and after surfactant instillation (p < 0.05; both conditions). As an indicator of breathing effort, esophageal deflection pressure and PNA were lower during PAV than during CPAP in both conditions (p < 0.02). Peak PSR activity was higher and occurred earlier during PAV than during CPAP (p < 0.01), and correlated linearly with PNA duration in all conditions studied (p < 0.001). The inspiratory time decreased as tidal volume increased when CPAP was changed to PAV, with the highest correlation observed after surfactant instillation (r = -0.769). No apneic periods could be observed.

CONCLUSION

PSR activity and the control of breathing are maintained during PAV in surfactant-depleted cats early after surfactant instillation, with a higher ventilatory response and a lower breathing effort than during CPAP.