Preoxygenation: A valuable safety measure in veterinary anesthesia

Preoxygenation is one of the easiest and least expensive ways to greatly improve the safety of your patients during the induction phase of anesthesia.

Preoxygenation for 3-5 minutes prior to the administration of induction drugs helps to replace nitrogen in the lungs with oxygen, which greatly reduces the risk of hypoxemia that can result from the respiratory depressive effect of most induction drugs. This is especially true for those of us living and working at higher altitudes.

When we talk about hypoxemia, we are really looking at PaO2, which is the partial pressure of oxygen dissolved in arterial blood. PaO2 is also related to the percent saturation of hemoglobin (SaO2) by the oxygen-hemoglobin dissociation curve:

Note the sigmoid shape of this curve, especially the sharp decrease in PaO2 below ~ 60 mmHg and ~ 90% SaO2. This region of the curve indicates a severe, potentially life-threatening hypoxemia. The farther away your patient is away from this steep drop-off point (i.e., farther to the right on the curve), the lower the likelihood that she will become severely hypoxemic.

In normal, healthy patients, PaO2 should be no more than ~ 10% less than the alveolar oxygen, PAO2, described by the alveolar oxygen equation:

PAO2 = [(Pbar – PH2O) x FIO2 ] – (PaCO2 / 0.8)

where

PAO2 = partial pressure of oxygen in the alveoli

Pbar = barometric pressure (~760 mmHg at sea level and ~ 620 mmHg in the part of Colorado that I’m from)

PH2O = water vapor pressure = 47 mmHg

FIO2 = inspired oxygen concentration (21%, or 0.21, breathing room air)

PaCO2 = partial pressure of CO2 dissolved in arterial blood = ~ 40 mmHg in normal awake patients

Now, let’s plug in some numbers to prove how beneficial preoxygenation really is!

If you have a patient breathing room air and no induction drugs have been administered, at sea level: PAO2 = [(760-47) x 0.21] – (40/0.8) = 99.7 mmHg. If everything is normal, the PaO2 should be no lower than 10% less, or ~ 89.7 mmHg. Substituting 620 as the barometric pressure, in my part of Colorado the PAO2 = ~ 70.3 mmHg and PaCO2 = 63.3… already entering that steep drop off!

Once you administer induction drugs, you get an increase in PaCO2 due to hypoventilation. If your patient is still breathing room air when this happens, then at sea level: PAO2 = [(760-47) x 0.21] – (50/0.8) = 87.2 mmHg and PaO2 = 78.5. In Colorado, PAO2 = 57.8 mmHg and PaO2 = 52 mmHg. Very, very dangerous!

If you administer oxygen via an open mask, you increase the FIO2 from 21% to ~ 40%. At sea level, PAO2 = [(760-47) x 0.4] – (50/0.8) = 222.7 mmHg and PaO2 = 200.43 mmHg. Even in Colorado, PAO2 = 166.7 mmHg and PaO2 = 150.03 mmHg.

Starting your patient off far to the right on the oxygen-hemoglobin dissociation curve buys her precious time against life-threatening hypoxemia until an airway can be secured and 100% oxygen administered. This is especially important in the event that intubation is prolonged and/or the patient has significant underlying disease that already compromises her ability to adequately deliver oxygen to her tissues. We never want our patients to experience hypoxemia and the resulting cyanosis that is evident in this beagle.

Most sick and/or adequately sedated patients will tolerate an open mask with very little restraint, and hopefully this brief discussion of respiratory physiology has convinced you to incorporate the important safety measure of preoxygenation into your induction procedure!