Passive expiratory flow-volume curve is not an accurate method to measure the respiratory time constant in rabbits

Neonatal Resuscitation With T-Piece Systems: Risk of Inadvertent PEEP Related to Mechanical Properties

Background: Resuscitation of infants using T-piece resuscitators (TPR) allow positive pressure ventilation with positive end-expiratory pressure (PEEP). The adjustable PEEP valve adds resistance to expiration and could contribute to inadvertent PEEP. The study indirectly investigated risk of inadvertent peep by determining expiratory time constants. The aim was to measure system expiratory time constants for a TPR device in a passive mechanical model with infant lung properties. Methods: We used adiabatic bottles to generate four levels of compliance (0.5-3.4 mL/cm H₂O). Expiratory time constants were recorded for combinations of fresh gas flow (8, 10, 15 L/min), PEEP (5, 8, 10 cm H₂O), airway resistance (50, 200 cm H₂O/L/sec and none), endotracheal tube (none, size 2.5, 3.0, 3.5) with a peak inflation pressure of 15 cm H₂O above PEEP. Results: Low compliances resulted in time constants below 0.17 s contrasting to higher compliances where the expiratory time constants were 0.25-0.81 s. Time constants increased with increased resistance, lower fresh gas flows, higher set PEEP levels and with an added airway resistance or endotracheal tube. Conclusions: The risk of inadvertent PEEP increases with a shorter time for expiration in combination with a higher compliance or resistance. The TPR resistance can be reduced by increasing the fresh gas flow or reducing PEEP. The expiratory time constants indicate that this may be clinically important. The risk of inadvertent PEEP would be highest in intubated term infants with highly compliant lungs. These results are useful for interpreting clinical events and recordings.

Bronchodilation induced by muscular contraction in spontaneously breathing rabbits: neural or mechanical?

The respective contribution of mechanical and neural mechanisms to the bronchodilation occurring during exercise is not fully identified in spontaneously breathing animals. The airway response to electrically induced muscular contractions (MC) was studied after vagal cold block in 9 spontaneously breathing rabbits. The forced oscillation respiratory system resistance (Rrs) was measured at vagal nerve temperatures 37°C, 8°C and 4°C. Rrs was found to decrease significantly during MC in all conditions. The occasional occurrence of a deep breath was responsible for a sudden decrease in Rrs. However, when the deep breath was absent - after vagal cooling and in some experiments at 37°C - the bronchodilation was frequently dissociated from the change in breathing pattern, most likely illustrating a neural mechanism. Altogether, while some bronchodilation may be ascribed to the mechanical stretching of the airways, Rrs decreasing with little change in breathing pattern is likely related to a reflex effect, possibly a sympathetic-borne mechanism.

Airway response to induced muscular contraction in spontaneously breathing rabbits

The airways are thought to dilate during exercise in humans but the time course and mechanisms of the response are not fully described. The aim of the study was to document changes in airway calibre during electrically induced muscular contractions (MC) in spontaneously breathing rabbits. Experiments were also performed after induced bronchoconstriction to assess the effect of change in breathing pattern on airway calibre during MC. Respiratory resistance (R(RS)) was measured in 12 rabbits using the forced oscillation technique at 20 Hz before, during and after 30s MC in control conditions and after methacholine induced bronchoconstriction (Mch). MC was associated with significant decrease in R(RS) both at control and Mch. The MC induced increase in V(E) occurred with significant albeit small change in mean tidal volume (V(T)) at Mch but not control. An augmented breath (AB) occurred in 29/35 MCs and was usually associated with an abrupt drop in R(RS). The decrease in R(RS) induced by AB was significantly larger at Mch compared with control. Passively inflating the lung after MC induced significantly larger decrease in R(RS) than AB during MC. The data indicate bronchodilation by MC in spontaneously breathing rabbits. The mechanisms appear to include AB dependent airway wall stretching as well as removal of cholinergic input to the airway smooth muscle.