The Least Work of Breathing: Applying principles of Adaptive Support Ventilation

Adaptive Support Ventilation (ASV) is an advanced closed-loop mode of ventilation only available on Hamilton Medical ventilators (G5, Galileo, C2) in the United States.


ASV is considered an “Optimal” mode of ventilation, in which the ventilator automatically adjusts multiple set-points (pressure, tidal volume, frequency, & I: E ratio). Furthermore, the ventilator applies a mathematical model (Otis equation) to find the least work of breathing.


Providing minimal work of breathing is a primary goal for the mechanically ventilated patient. A load that is too high will eventually lead to ventilatory muscle fatigue and may lead to lung injury and/or liberation failure. Every practitioner should attempt to minimize the work of breathing (WOB) by continuously assessing the patient ventilator interaction and adjusting settings appropriately.


If available the practitioner could employ ASV, however what if you don’t have a Hamilton ventilator or unfamiliar with ASV?



One can apply the principles of the Otis equation and set an optimal respiratory rate to provide the least WOB.


The Otis equation

1. The Otis equation is not easy as one might think; I tried to program it into my TI-83 programmable calculator & failed miserably with logic errors.

1. One can use the “Auto Otis” [1] a programmable spread sheet to find the optimal breathing pattern, and use these for your tidal volume and frequency settings. This is a very helpful tool in setting the ventilator based on pulmonary mechanics. However, one always has to open up the spread sheet to utilize it, not ideal in the clinical setting.
2. After reviewing the equation & the spread sheet, I concluded that the Otis equation’s main calculation revolves around the measured expiratory time constant. So one can calculate the expiratory time constant and set the respiratory rate on the following table I created.

Note- some ventilators automatically calculate the time constant, however one can calculate the time constant with the following equation:

X= RAW*Cdyn

X = Time Constant
RAW = Airway resistance
Cdyn = Dynamic compliance





Image 1: Optimal Respiratory Rate Table. 





Viewing the table the operator matches the calculated/measured time constant (on the right side) then sets the ventilator frequency to the corresponding rate on the left side of the table.

Example: I have a calculated time constant of 0.7 second; this falls in between 0.75-0.65 second, so my set frequency should be 13 breaths/ minute.

RELATED LINKS

New auto-weaning ventilator might make pulmonologist obsolete

1. Lotti, G. & Brunner, J. (2002). Auto Otis: Calculator of Minimum Work of Breathing & Best Ventilatory Pattern. Pavia, Italy & Rhazuns, Switzerland.