Spontaneous breathing affects the spatial ventilation and perfusion distribution during mechanical ventilatory support

Uppsala University, Uppsala, Uppsala, Sweden
Critical Care Medicine (Impact Factor: 6.31). 06/2005; 33(5):1090-5. DOI: 10.1097/01.CCM.0000163226.34868.0A
Source: PubMed


In acute respiratory failure, gas exchange improves with spontaneous breathing during airway pressure release ventilation (APRV). The mechanisms for this improvement are not fully clear. We have shown that APRV with spontaneous breathing reopens nonaerated lung tissue in dorsal juxtadiaphragmatic regions. We hypothesized that spontaneous breathing during APRV may redistribute ventilation and perfusion toward these reopened regions.
Prospective, randomized, controlled study.
Animal research laboratory
Twenty controlled mechanically ventilated pigs.
Lung injury was induced by injection of oleic acid into the central circulation; thereafter, pigs were randomized to APRV with or without spontaneous breathing. To induce spontaneous breathing during APRV with spontaneous breathing, the mechanical respiratory rate was decreased by 50% in this group.
We measured respiratory mechanics, hemodynamics, gas exchange including the multiple inert gas elimination technique, and the spatial ventilation and perfusion distribution using single photon emission tomography. At similar minute ventilation and airway pressures, shunt remained stable during APRV with spontaneous breathing, whereas it increased during APRV without spontaneous breathing during the 2-hr study period (p = .006). Single photon emission tomography showed more ventilation (p < .001) and pulmonary blood (p < .025) flow in dorsal, juxtadiaphragmatic lung regions when spontaneous breathing was present.
The beneficial effects of spontaneous breathing on intrapulmonary shunt and oxygenation are explained both by increased ventilation of aerated dependent lung tissue and by opening up nonaerated tissue so that ventilation is distributed to a larger share of the lung. Redistribution of perfusion is possibly secondary to the altered ventilation. The overall effect is a more efficient use of available lung tissue for gas exchange.

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    • "An animal study found less depression of CO and oxygen delivery (DO2) with PCV + SB compared with PCV at similar transpulmonary pressures [40]. In our study, the CO during PCV + SB and PCV – SB was comparable to previously published studies [12,16]., and the more pronounced increase in the PCV – SB group does not lead to a significant increase in DO2. This suggests that the higher CO was required to maintain an adequate DO2. "
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    • "Our results are supported by recent studies that demonstrate that superimposed spontaneous breathing during airway pressure release ventilation redistributes tidal ventilation toward dependent lung regions just near the diaphragm [24]. This conclusion was derived using single photon emission tomography in the pig model. "
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    • "The net result is an improvement in oxygenation with potentially decreased VALI in these dependent lung regions. These beneficial effects have been confirmed in animal models of ALI/ARDS with spontaneous breathing during APRV [42] [43] [44]. In humans, the oxygenation effects of spontaneous breathing during APRV may accrue over time (ie, 24 hours) because of slow, progressive recruitment of these lung units [45]. "
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