Is there an optimal breath pattern to minimize stress and strain during mechanical ventilation?

Inappropriate selection of tidal volume and rate on mechanical ventilators in patients with reduced lung volume may cause lung damage. In spite of this rather recent insight, the optimal breath pattern and the relative importance of elevating end-expiratory lung volume (EELV) are still debated. A recent hypothesis is that lung injury is caused by excessive stress and strain. This paper elaborates on that hypothesis and proposes a new approach to optimizing the breath pattern. An index to quantify the impact of positive pressure ventilation on the lungs is defined (Stress-Strain Index, SSI) and calculated as a function of the breath pattern (tidal volume V (t) and respiratory rate f) for five different levels of EELV. The breath pattern at which SSI is minimal (mSSI strategy) was compared with three other strategies: the "6 ml/kg V (t)," minimal work of breathing and minimal force to breathe, for the different EELV levels. In the mathematical analysis, SSI was mainly determined by EELV and was much higher with low EELV. For each EELV level, a distinct minimum of SSI was found, defined by a particular V (t) - f combination. The mSSI strategy yielded lower V (t) and higher f (0.252 l and 39 b/min) as compared to the "6 ml/kg V (t)" strategy (0.420 l and 17 b/min). The EELV is the main determinant of the SSI. For a given EELV, the SSI can be minimized by an optimal V (t) - f combination.