The term insufflation refers to a breathing system that is not in contact with the patient but instead blows oxygen and the anesthesia gases across a patient’s face. There are 2 common methods where insufflation is used in modern practice. The first is in pediatric patients who will not allow the placement of an intravenous line or the application of an oxygen face mask to their face. In this case the oxygen mask is held near the patient’s face and when they breath in, they inhale the oxygen and the anesthesia gases that are supplied by the mask. The other example of insufflation is during cataract surgery and procedures performed typically under conscious sedation or monitored anesthesia care (MAC) when the patient’s face is fully covered with the surgical drapes. Frequently oxygen tubing is then placed under the drapes to blow oxygen at high flows near the patient’s face to minimize the rebreathing of carbon dioxide. The disadvantage of insufflation is positive pressure ventilation cannot be done and the concentration of oxygen delivered is variable even at high flow rates.

In modern medicine, open-drop anesthesia systems are no longer in use, but since they were one of the earliest methods for providing anesthesia, they deserve a brief description. These were the primary methods for administering ether or chloroform anesthesia to patients. They consisted of a face mask with a gauze-covered opening where a highly volatile anesthetic was then dripped onto the gauze. When the patient inhaled, the flow of air through the gauze caused the liquid volatile anesthetic to become a gas. The depth of anesthesia was controlled by the amount of volatile anesthetic liquid applied to the gauze. The major disadvantage of this system is the patient had to be breathing spontaneously and there was no way to monitor the concentration of volatile anesthetic the patient was inspiring.

The development of the Mapleson circuits attempted to overcome the major shortcomings of the insufflation and open-drop systems. Compared to previous breathing systems, the Mapleson circuits had the following advantages:

Since there are no unidirectional valves or CO₂ absorbers, the CO₂ is eliminated by fresh gas flows that flush the CO₂ into the atmosphere or scavenger via the pop-off valve. The Mapleson circuits are classified based on the location of the following parts as seen in ◘ Fig. 34.1:

Although the Mapleson circuits all have the same components, the comparison of the circuits and the amount of CO₂ rebreathing that occurs can be complex. Rebreathing that occurs for each circuit is affected by the position of the pop-off valve in relationship to the fresh gas input and the reservoir tubing and bag. Each type of Mapleson system has different levels of fresh gas flows required to prevent rebreathing. In addition, each class of circuit responds differently to spontaneous versus controlled ventilation in relation to the fresh gas flows required to prevent rebreathing.

Mapleson A (see ◘ Fig. 34.1) is considered the most efficient circuit for spontaneous ventilation but is not recommended for controlled ventilation due to the need for unpredictably high fresh gas flows required to prevent rebreathing. By changing the position of the pop-off valve and the fresh gas inlet the system becomes a Mapleson D (see ◘ Fig. 34.1), which is significantly more efficient for controlled ventilation. This is because the fresh gas flushes the exhaled gases away from the patient and toward the pop-off valve. The Bain circuit is a modified Mapleson D system that incorporates the fresh gas inlet tubing inside the larger corrugated breathing tube. This decreases the size of the system and allows warming of the inspired gases by the exhaled gases. A major disadvantage of Bain circuit is if the fresh gas inlet tubing is disconnected or kinked, this will lead to significant rebreathing of exhaled gases.

The circle system was designed to overcome some of the limitations of the Mapleson circuits. Through the rebreathing of exhaled gases humidity and heat are maintained. Since the exhaled gases are rebreathed, this leads to a conservation of volatile anesthetics, oxygen, and air. These advantages have resulted in the circle system being the ventilation system used in all modern anesthesia machines. While the oxygen, nitrogen, and the volatile anesthetics are rebreathed, CO₂ absorbers chemically neutralize the CO₂ so there is no rebreathing of CO₂. However, to achieve these results the complexity of the breathing system was substantially increased. As shown in ◘ Fig. 34.2 the necessary components of a circle system include: