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.