Dynamics of Respiration during Rapid Rate Mechanical Ventilation in Anesthetized and Paralyzed Rabbits

Abstract

METHODSpressure-flow relationships and expressed as changes in compliance, resistance, and inertance (1-7). Analyses using this approach have customarily relied on two assumptions: first, that the pressure-flow relationships in the respiratory system are linear, and second, that the pressure waveform applied to the respiratory system is sinusoidal. However, the validity of these assumptions as applied to the mechanically ventilated infant may be questioned: the first because of the nonlinear pressureflow behavior of the endotracheal tube (8), and the second because a longer expiratory than inspiratory time is necessary to avoid gas trapping in the lungs as ventilatory rate is increased.In this study, we analyzed the effects of rapid rate mechanical ventilation [as commonly applied to infants with respiratory failure (9)] on the pressure-flow behavior of the respiratory system and its components: endotracheal tube, lungs, and chest wall. We examined how these components interact to affect tidal volume and its distribution at rapid ventilatory rates in anesthetized and paralyzed rabbits. Unlike previous studies (1-3, 6, 7), our analysis takes into account both the nonlinearities of the pressure-flow relationship and the differences between inspiration and expiration.Preparation. Ten New Zealand White rabbits weighing 2.50-3.41 kg (2.94 ± 0.30 kg, mean ± SD) were anesthetized with halothane and placed in the supine position. The marginal vein of the ear was cannulated for fluid and drug administration, and a tracheostomy was performed below the first tracheal cartilage. Through the tracheostomy, we introduced an endotracheal tube and a catheter for pressure measurement, which were enclosed by a stainless steel tube of 0.55 cm of external diameter and 2 cm of length (Fig. I). The endotracheal tube (Portex, Inc., Wilmington, MA) had an internal diameter of 0.3 cm and was 16.5 cm long. The catheter for pressure measurement was made of a length of PE-160 tubing and had four side holes within I cm of its occluded end. Gas leaks were prevented by placing a tie around the trachea and by filling the space between the endotracheal tube, the catheter, and the inner wall of the stainless steel tube with silicon rubber. The stainless steel tube prevented the tie around the trachea from compressing the endotracheal tube and the catheter, thereby preserving their dynamic response. In addition, the stainless steel tube maintained the relative positions of the endotracheal tube, the tracheal catheter, and the trachea. Given the distance between the lateral holes of the tracheal catheter and the tip of the endotracheal tube (3 cm), the tracheal diameter of the rabbits (average = 0.55 cm), and the Reynolds number of the tracheal flow, the error in the measurement of tracheal pressure caused by the Bernoulli effect would not have exceeded 3%, even at peak flows (10).A double lumen polyethylene catheter (0.22 cm of external diameter) was inserted in the right pleural space through a small 750 ABSTRACT. Infants with respiratory failure...

High-Frequency Ventilation

“…[25][26][27][28][29][30] The first such in vitro study measured delivered tidal and minute volumes as ventilator rates increased progressively from 20 to 150 breaths/min. The technique seems simple and easy.…”