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OPEN DROP ANESTHESIA
Although open-drop anesthesia is not used in mod-ern medicine, its historic significance warrants a brief description here. A highly volatile anesthetic— historically, ether or chloroform—was dripped onto a gauze-covered mask (Schimmelbusch mask) applied to the patient’s face. As the patient inhales, air passes through the gauze, vaporizing the liquid agent, and carrying high concentrations of anes-thetic to the patient. The vaporization lowers mask temperature, resulting in moisture condensation and a drop in anesthetic vapor pressure (vapor pres-sure is proportional to temperature).
A modern derivative of open-drop anesthe-sia utilizes draw-over vaporizers that depend on the patient’s inspiratory efforts to draw ambient air through a vaporization chamber. This tech-nique may be used in locations or situations in which compressed medical gases are unavailable (eg, battlefields).
Draw-over devices have nonrebreathing circuits that use ambient air as the carrier gas, although supplemental oxygen can be used, if available. The devices can be fitted with connections and equip-ment that allow intermittent positive-pressure ven-tilation (IPPV) and passive scavenging, as well as
continuous positive airway pressure (CPAP) and positive end-expiratory pressure (PEEP).
In its most basic application (Figure 3–4), air is drawn through a low-resistance vaporizer as the patient inspires. Patients spontaneously breathing room air and a potent halogenated agent often man-ifest an oxygen saturation (SpO2) <90%, a situation treated with IPPV, supplemental oxygen, or both. The fraction of inspired oxygen (Fio2) can be sup-plemented using an open-ended reservoir tube of about 400 mL, attached to a t-piece at the upstream side of the vaporizer. Across the clinical range of tidal volume and respiratory rate, an oxygen flow rate of 1 L/min gives an Fio2 of 30% to 40%, or with 4 L/min, an Fio2 of 60% to 80%. There are several commercial draw-over systems available that share common properties (Table 3–1).
The greatest advantage of draw-over systems is their simplicity and portability, making them useful
in locations where compressed gases or ventilators are not available. The presence of the nonrebreathing valve, PEEP valve, and circuit filter close to the patient’s head makes the technique awkward for head and neck surgery and pediatric cases. If the head is draped, the nonrebreathing valve is often covered as well.
The original design of a draw-over system has recently been modified to include a self-inflating bag, a ventilator, and/or a heat and moisture exchanger. The Ohmeda Universal Portable Anes-thesia Complete (U-PAC) is one example of a draw-over anesthesia system.
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