Is Low Flow Really Safe?

Frances E. Noe, M.D.; William E. Farrant, B.S.

To the Editor:

It disturbs us that your bulletin and other publications are stating with no qualifications that total gas flows as low as 1 to 1 1/2 liters during anesthesia are “safe.”

We have been checking the performance of several brands of mechanical ventilators used for clinical anesthesia with a lung simulator as “patient.” Our work (Scientific Exhibit, ASA, 1994) showed that at a set volume of 500 ml TY, most ventilators delivered an expired volume of 320-360 ml. The ideal volume for total gas flow for ventilator function appears to be 3 liters; above that level, delivered volume as measured from expiration is higher than set volume and below that level, delivered volume is less than set volume.

Those who have facilities for measured expired volume of ventilation can compensate for these changes, but many anesthesia units do not have this particular equipment.

One of our current projects will investigate gas exchange and pulmonary distribution of gas at low rates of total gas flow.

Frances E. Noe, M.D., and William E. Farrant, B.S. University of North Carolina Chapel Hill, NC