Thursday, March 1, 2012

The Minimal SSI Strategy

Stress Index (Si) Displayed on the SERVO-i Ventilator. 

In 2009 Brunner & Wysocki proposed that there is an optimal breathing pattern to minimize stress & strain during mechanical ventilation [1]. Stress and strain are primary causes of ventilator induced lung injury (VILI), so it would be imperative to provide a breathing pattern which decreases the chances of VILI. 

The researchers computed various breathing patterns and end expiratory lung (EELV) volumes to model their effects in regards to stress & strain. The results showed that there was an optimal breathing pattern in which provided the least amount of stress & strain (a.k.a. minimal stress strain index, mssi). 

After reviewing this strategy I have been trying to apply it, which was not easy.

The strain component is easy to apply, this is using a breathing pattern that ensures the minimal tidal possible (I use 4ml/kg/IDBW for a target tidal volume, and titrate the set rate to meet the minute ventilation demand). This has led to ventilator settings where set tidal volume is less than the ARDS Net strategy & higher rates (this corresponds to the researchers computed model). 

As for determining the best EELV, this is more difficult; I originally set my PEEP based on low-flow P/V curve maneuvers

Now with the SERVO-i’s new available software one can actually find the optimal EELV based on the “Stress Index” (Si). The ventilator will actually now calculate this value which is displayed & trended allowing for easy titration of PEEP. 

The Si based on CT scan evidence of optimal EELV [2]. A Si of 0.9 to 1.1 indicates ideal lung recruitment.

A Si of > 1.1 indicates over-distention & a Si 0f < 0.9 indicates under recruitment, both contributing to VILI.

With this new tool one can easily utilize the  mssi & obtain a optimal breath pattern to prevent VILI.

[1].  Brunner, J. & Wysocki, M. (2009). Is there an Optimal Breath Pattern to Minimize Stress & Strain duringMechanical Ventilation? Intensive Care Medicine. 35: 1479-1483.