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.