Parasternal Blocks: Superficial and Deep Approaches with Dr. Melody Anderson (Herman)

The desflurane vaporizer is a heated blender vaporizer, whereas vaporizers for sevoflurane, isoflurane and enflurane are variable bypass vaporizers. This means that a vaporizer for sevoflurane works by diverting a given amount of oxygen flow through the vaporizer in contact with liquid sevoflurane, where the oxygen becomes fully saturated with vapor before exiting the vaporizer. Therefore, the amount of sevoflurane exiting the vaporizer is subject to changes in atmospheric pressure and temperature. In contrast, the desflurane vaporizer works by heating liquid desflurane to 39°C, so that all of the liquid desflurane becomes vapor. No fresh gas flow flows through a desflurane sump. Rather, a certain volume of desflurane vapor is released into the circuit based on the total fresh gas flow and percentage set on the control dial. Therefore, the output of desflurane from the anesthesia circuit will be THE SAME whether at sea level or at altitude. The question of anesthetic depth requires a consideration of the partial pressure exerted by the anesthetic agent inside the patient's brain (Remember that the concept of "Minimum Alveolar Concentration" is given as a percentage, but this percentage assumes that one is at sea level). The partial pressure of an agent in the brain depends upon the atmospheric pressure. So, the partial pressure exerted by 6% desflurane at 760 mm Hg is .06 x 760 = 45.6 mmHg. The partial pressure exerted by 6% desflurane at 570 mmHg is .06 x 570 = 34.2 mmHg. Therefore, a patient anesthetized at high altitude would be expected to demonstrate a relatively lighter anesthetic depth compared to their sea-level counterpart.