To the Editor
I write in response to the letter from Frances E. Noe and William E. Farrant in the Winter, 1995-96, APSF Newsletter titled “Is Low Flow Really Safe?”
The effect of fresh gas flow rate on delivered tidal volume and minute ventilation is understandable and predictable. Unlike ICU ventilators, fresh gas flow contributes to minute ventilation during mechanical ventilation with a standard anesthesia system ventilator. When the pop-off valve in the ventilator is closed during a mechanical inspiration, fresh gas flow entering the system is no different than gas being introduced from the ventilator bellows (Miller (ed), Anesthesia 3rd Edition, pg. 197).
The magnitude of this effect can be easily approximated. With an I:E ratio set at 1:2, an increase in fresh gas flow of 3 liters-min/1 will result in approximately a 1 liter-min/1 increase in minute ventilation. A corollary of this effect is that changing I:E ratio will also alter minute ventilation when fresh gas flow rates are high. Note that the tidal volume numbers written on the ventilator bellows casing, and the “set tidal volume” were created for some arbitrary fresh gas flow rate, presumably 5 liters-min/1. I have verified these approximations many times by changing flow rates during a general anesthetic with mechanical ventilation and measuring the resultant change on the spirometer to demonstrate this effect to anesthesiology residents.
Certainly, it is easiest to use a system which displays measured exhaled tidal volume and minute ventilation. However, understanding the effects of fresh gas flow rates on minute ventilation should allow the practitioner to safely use lower or low flow techniques and compensate accordingly. Adequacy of ventilation should always be assured in other ways as well, such as observation of chest excursion and end-tidal CO2 monitoring. I encourage Dr. Noe and Mr. Farrant to reconsider their use of low flow techniques.
John R. Feiner, M.D. Assistant Professor Department of Anesthesia University of California San Francisco, CA