We thank Dr. Eisenkraft for taking the time to write regarding our recent article1 and describe to us a detailed alternative scheme by which expiratory limb ventilation can be provided. The suggestion is valid and not the one that we thought of in this emergency. We were unaware of the Kummar et al. description, which does not explain how the Bain circuit was pressurized on his Aisys machine (GE Healthcare, Madison, WI). Dr. Eisenkraft’s alternative demands mental preparation for such emergencies, just as we taught our option in previous simulations, and would require that the clinician recall the alternate common gas outlet circuitry immediately within a crisis situation. Although we admire his technically accurate methods of scavenging the volatile agent, we believe that such connections would not be available or clinically necessary in a brief emergency situation. From a technical perspective, we would like to raise three issues with his alternative.
Room air entrainment: what Dr. Eisenkraft describes is analogous to a Mapleson D (Bain) circuit, with the fresh anesthetic gas traveling down the inspiratory limb from the alternate common gas outlet to the Y-piece, but it differs on the distal expiratory limb end because the exhaled gas from the patient is scavenged to atmosphere upon reaching the self-inflating manual ventilation device (SIMVD) valve, and it does not mix within the SIMVD reservoir. When the reservoir is released after a manual inspiratory squeeze, it will entrain room air. When it is subsequently squeezed, the SIMVD reservoir would deliver room air retrograde to the patient, thereby diluting the exhaled and fresh anesthetic gas and oxygen mixture. Our alternative to connect the SIMVD to oxygen would theoretically deliver a higher concentration of oxygen, but it would not provide anesthetic gas.
High fresh gas flow: Dr. Eisenkraft’s alternative is superior to ours in delivering anesthetic agent but would deliver enriched oxygen and maintain desired anesthetic concentration only if high fresh gas flow is provided via the alternate common gas outlet. We calculate a minimum fresh gas flow requirement of 18 l/min to prevent dilution by the SIMVD room air, given the example of 600 ml VT delivered over 2 s. If oxygen was connected to his SIMVD, it would further enrich the oxygen concentration, but it would dilute the anesthetic agent. We agree that this connection is not necessary if his fresh gas flow is high enough.
Rebreathing carbon dioxide: analogous to the Bain system, and at high fresh gas flow, Dr. Eisenkraft’s alternative ought to cause less rebreathing of carbon dioxide than our to-and-fro ventilation method.
In summary, we applaud this alternative suggestion as long as the clinician uses high fresh gas flow and desires the continuity of volatile anesthetic, but in the emergency situation we describe, we feel more secure in delivering higher oxygen concentrations from our SIMVD reservoir connected to auxiliary oxygen and do not see the need for scavenging arrangements. Our technique can be used on ANY anesthesia machine (without the alternate common gas outlet) and may require less technological understanding.