365. If the alveolar to venous partial pressure difference of a volatile anesthetic (Pa-Pv) is positive (i.e., Pa > Pv) and the arterial to venous partial pressure difference (Pa-Pv) is negative (i.e., Pv > Pa) which of the following scenarios is most likely to be true? A. The vaporizer has been shut off at the end of the case B. Induction has just started C. Steady state has been achieved D. The volatile anesthetic has been turned down from steady state, but not off E. The vaporizer was shut off during emergence, then suddenly turned up because the patient moved before closure of the incision The delivery of anesthetic gases to a patient is a complex series of events that starts with the anesthesia machineand culminates with achievement of an anesthetic partial pressure in the brain (PBr).The partial pressure measured in the blood for any volatile is either rising (at first rapidly, then more slowly) or falling (rapidly at first then more slowly). The vessel-rich group reaches steady state in about 12 minutes (for any dialed level of volatile). The rest of the body, however, approaches, but virtually never reaches, equilibrium (e.g. the equilibrium half time for the fat group is 30 hours for sevoflurane). Hence, a true zero gradient is never achieved in the steady state. When the anesthetic is discontinued or reduced, there is a fall in the arterial partial pressure such that it is less than the venous partial pressure. In fact, when the venous partial pressure exceeds the arterial partial pressure it means the volatile has been reduced (or shut off) because the lungs are “cleansing” the blood as the volatile filled blood passes through them. The newly “cleansed” blood then finds it way to the left ventricle with a very low Pa for the volatile in question. The present example can only be explained if the volatile had just been turned off or down (lungs cleansing) then suddenly turned back up. In this brief “window” the alveolar partial pressure gradient would exceed the venous partial pressure because there is a net transfer of anesthetic into the blood exiting the lungs (pulmonary vein). Since this just happened (turned up), the body has not had sufficient time to reverse the gradient in the left sided arterial and venous system. Moments later, the left sided arterial volatile partial pressure will exceed the venous partial pressure and the patient will become “deeper”