Sunday, April 3, 2011

Adaptive Support Ventilation: the “Pareto Principle” of Mechanical Ventilation.

One reader asked me “what do you think about ASV” (Adaptive Support Ventilation) in which I replied back “I think it is the Pareto Principle or Pareto Efficiency of mechanical ventilation”.

So what is the Pareto Principle?

The Pareto Principle is simply the observation that “most things in life are not distributed evenly” [1]. An example would be that 20% of the input creates 80% of the results (aka. the 80/20 rule).

So how does this apply to mechanical ventilation?

Well with ASV the operator only sets three main parameters:

1. High Pressure Limit

2. Ideal Body Weight

3. Percent Minute Ventilation (Desired minute ventilation)

Additionally, one sets FiO2, and can set pressurization rate, end-time sensitivity, and tube resistance compensation for spontaneous breaths.

The ventilator then uses a mathematical model (Otis equation) and threshold limits to set the target tidal volume, inspiratory time, and respiratory frequency. The ventilator will also adjust all these settings as the patients lung mechanics change.

The mode can be a CMV, IMV, or CSV mode based on the patients spontaneous activity.

The mode safety thresholds prevent tachypnea, auto-PEEP, and excessive dead space ventilation, high-pressure & maintain minimum minute ventilation.

Many skeptics believe that their settings are superior to the machines; there are various peer reviewed journal articles that disprove this belief.

I myself evaluated this in 22 patients, comparing physician settings to ASV calculated settings. In regards to set frequency & tidal volume, there were no statistical differences between the machines and the physician choices. Additionally, when calculating the work of breathing in joules/second from the chosen settings there was no statistical differences between the physician and the machine choices (p value 0.003, paired two sample t-test).

Although ASV is an advanced automated mode there still needs to be practitioner engagement. The operator needs to be familiar with the mode & with traditional pressure control & pressure support ventilation. ASV requires little operator input and provides substantial benefits.

New auto-weaning ventilator might make pulmonologist obsolete

1. Pareto Principle