Maintenance of end-expiratory recruitment with increased respiratory rate after saline-lavage lung injury

Abstract

Cyclical recruitment of atelectasis with each breath is thought to contribute to ventilator-associated lung injury. Extrinsic positive end-expiratory pressure (PEEPe) can maintain alveolar recruitment at end exhalation, but PEEPe depresses cardiac output and increases overdistension. Short exhalation times can also maintain end-expiratory recruitment, but if the mechanism of this recruitment is generation of intrinsic PEEP (PEEPi), there would be little advantage compared with PEEPe. In seven New Zealand White rabbits, we compared recruitment from increased respiratory rate (RR) to recruitment from increased PEEPe after saline lavage. Rabbits were ventilated in pressure control mode with a fraction of inspired O(2) (Fi(O(2))) of 1.0, inspiratory-to-expiratory ratio of 2:1, and plateau pressure of 28 cmH(2)O, and either 1) high RR (24) and low PEEPe (3.5) or 2) low RR (7) and high PEEPe (14). We assessed cyclical lung recruitment with a fast arterial Po(2) probe, and we assessed average recruitment with blood gas data. We measured PEEPi, cardiac output, and mixed venous saturation at each ventilator setting. Recruitment achieved by increased RR and short exhalation time was nearly equivalent to recruitment achieved by increased PEEPe. The short exhalation time at increased RR, however, did not generate PEEPi. Cardiac output was increased on average 13% in the high RR group compared with the high PEEPe group (P < 0.001), and mixed venous saturation was consistently greater in the high RR group (P < 0.001). Prevention of end-expiratory derecruitment without increased end-expiratory pressure suggests that another mechanism, distinct from intrinsic PEEP, plays a role in the dynamic behavior of atelectasis.

A Numerical Model of the Respiratory Modulation of Pulmonary Shunt and PaO2 Oscillations for Acute Lung Injury

“…In comparison, our simulations show a much smoother transitions of PaO 2 after the onset of inspiration (expiration), without reaching a ''plateau'' before the onset of the following expiration (inspiration), in agreement with the findings of Baumgardner et al 3 The model of lung mechanics adopted in this study was not capable of generating intrinsic PEEP (except for very high respiratory rates). However, this did not prevent a satisfactory simulation of the experimental results and is in accordance with the findings of Syring et al 17 of a lack of intrinsic PEEP generation in a rabbit model very similar to that considered for our study, supporting their final suggestions that ''another mechanism, distinct from intrinsic PEEP, plays a role in the dynamic behavior of atelectasis. ''…”

Influence of Inspiration to Expiration Ratio on Cyclic Recruitment and Derecruitment of Atelectasis in a Saline Lavage Model of Acute Respiratory Distress Syndrome*

“…Syring et al, for example, reported maximal Pao 2 oscillations in lavaged rabbits at an I:E ratio of 2:1, suggesting that in that model derecruitment was slightly faster than recruitment. The general principle that shortening exhalation times can prevent end-expiratory collapse without an increase in intrinsic PEEP, however, was also demonstrated in the study by Syringe et al (34), in that study by increasing RR. Other potential mechanisms of VILI that could accompany IRV, such as higher static stretch and higher mean airway pressures, possibly resulting in increased inflammation (35)(36)(37)(38), were not addressed in our study.…”

The effect of increased apparatus dead space and tidal volumes on carbon dioxide elimination and oxygen saturations in a low-flow anesthesia system

“…1, Table 3). Table 2 Comparison of tidal volumes (V T s), ventilator peak pressure (P peak ), ventilator plateau pressure (P plateau ), and inspired carbon dioxide concentration (FICO 2 ) between the dead space (DS) and non dead space (NDS) groups 15 …”