Chapter: Essential Anesthesia From Science to Practice : Applied physiology and pharmacology : Anesthesia and the lung
Pre-oxygenation/de-nitrogenation
Pre-oxygenation/de-nitrogenation
Generally,
tracheal intubation in adults is made easier with muscle relaxation, which
causes apnea. An apneic healthy adult will begin to desaturate (decreasing
oxyhemoglobin percentage, SpO2) within 2 minutes. Young and healthy
patients usually maintain close to 100% oxygen saturation while breathing room
air. Increasing the inspired oxygen concentration will raise the arterial
oxygen ten-sion, but add relatively little to the overall oxygen content of
blood, as explained above.
However,
to provide a reservoir of oxygen in the lungs, we apply a tight-fitting mask
and have the patient breathe 100% oxygen before inducing apnea. This reservoir
occupies the functional residual capacity (FRC) of the lung (the vol-ume
remaining after normal exhalation, see Fig. 10.6).
Notice that little of the total lung volume is actually exchanged with normal
tidal ventilation; therefore, several minutes of pre-oxygenation, or four to
five full vital capacity breaths, are required to maximize the oxygen depot in
the FRC.
Two requirements for effective denitrogenation: (i) we need to let the patient inhale pure oxygen, which means
the anesthesia machine must deliver at least a minute volume of oxygen to the
patient in order to prevent the rebreathing of exhaled nitrogen (the carbon
dioxide absorber takes care of the CO2). (ii) The patient must
ventilate his lungs long enough to wash out the nitrogen in the lungs. We
usually think of this process in terms of time constants, that is to say that
it will take about four time constants to approach near complete
denitrogenation of the lungs (there will still be nitro-gen in solution in the
body). One time constant is the time required to deliver a volume of gas equal
to the volume of the lungs. After one time constant, we will have replaced 63%
of the gas in the lungs (with additional time constants we get to 86.5%, 95.0%,
98.2% and 99%, respectively). In a healthy, non-obese adult, with adequate pre-oxygenation
(end-tidal oxygen concentration of >85%), this reser-voir will provide 5–8 minutes
of oxygen before the apneic patient’s blood begins to desaturate.
Some
factors can increase the rate of desaturation:
·
A smaller reservoir: decreased FRC as in obesity, pregnancy,
infancy.
·
Increased oxygen consumption: hyperthermia, obesity, pregnancy,
infancy.
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