Low-flow

Low-flow anesthesia for the early morning

Warning! Under Construction!!

Future plans for this page:
uptake curves for anesthetics
interactive simulation
links
costs
theory of low-flow/ closed circuit anesthesia
history
hardware (anesthesia machine) info
simulators

How to do low flow anesthesia

Requirements Quantifiable leak in anesthesia machine and circut of less than 100ml/min
Calibrated oxygen monitor
Agent analyzer
Ascending bellows ventilator

Priorities

Assure adequate oxygen concentration
Watch O2 monitor
Increase O2 flows if concentration drops

If you are delivering nitrous oxide an accurate oxygen monitor is critical. Nitrous uptake is such that you may be at FiN2O of 70% with nitrous flows of 250 ml/min and O2 flows of 300 ml/min. This is because the patient's consumption of oxygen is a significant fraction of the delivered O2, and stays relatively constant, whereas the uptake of N2O decreases dramatically (and predictably) with time. The safest step to take if the FiO2 is low or drops is to increase the oxygen flow (WARNING: increasing any flows also increases the delivery of volatile agent to the circut. Therefore you risk overdosing the patient unless you decrease your vaporizer setting or carefully watch your agent analyzer.

Assure adequate alveolar ventilation

Watch bellows
Watch spirometer
Watch patient

You will have to alter your ventilator settings as you change gas flows.

Assure adequate anesthetic levels

Watch patient
Watch agent analyzer

You will see a large disparity between the concentration set on your vaporizer and the inspired concentration measured by the gas analyzer. This is expected.
WARNING: Loading with high concentrations of agents (overpressure) can lead to an overdose. Watch your patient and your agent analyzer carefully.
Low flow techniques require that denitrogenation and initial bolus of agent be given with relatively high flows (2-4 l/m).
Note: in order to change depth of anesthesia rapidly, the flow rate may need to be increased.

Disclaimer: This page is intended for informational purposes only.

A brief History of low flow anesthesia

Low flow anesthesia could not come into being until circle systems for anesthesia were developed. Hence, until CO2 absorbers were available, rebreathing was something to minimize, rather than maximize. Low flow was used with cyclopropane to limit the atmospheric leak of a highly explosive gas.

Links:

University of Michigan Department of Anesthesiology
Gasnet
ASA
Ohmeda
North American Drager
APSF (Anesthesia Patient Safety Foundation)
Association for Low Flow Anaesthesia
UMMC Machine Check-out
for more information, contact:

jszocik@umich.edu