ANESTHESIA VENTILATORS

The anesthesia ventilator can substitute for the breathing (reservoir) bag of the circle system, the Bain circuit, and other breathing systems. As recently as the late 1980s, anesthesia ventilators were mere adjuncts to the anesthesia machine, but they have attained a prominent, central role in newer anesthesia workstations. In addition to the nearly ubiquitous role of the anesthesia ventilator in modern anesthesia workstations, many advanced intensive care unit-type ventilation features have been integrated into anesthesia ventilators. This discussion focuses on the classification, operating principles, and hazards of contemporary anesthesia ventilators.

Classification

Ventilators can be classified according to their power source, drive mechanism, cycling mechanism, and bellows type.^{[137] [138]} The following section reviews ventilator classification and terminology before the discussion of individual anesthesia machine ventilators.

Power Source

The power source required to operate a mechanical ventilator is provided by compressed gas or electricity, or both. Older pneumatic ventilators required only a pneumatic power source to function properly. Contemporary electronic ventilators from Dräger, Datex-Ohmeda, and others require an electrical only or electrical and pneumatic power sources.^{[15] [19] [20] [21] [22] [23] [24] [25] [26] [27] [139] [140]}

Drive Mechanism and Circuit Designation

Most anesthesia machine ventilators are classified as double-circuit, pneumatically driven ventilators. In a double-circuit system, a driving force (i.e., compressed gas) compresses a bag or bellows, which delivers gas to the patient. The driving gas in the Datex-Ohmeda 7000, 7810, 7100, and 7900 is 100% oxygen.^{[15] [52] [139] [140]} In the North American Dräger AV-E, a Venturi device mixes oxygen and air.^{[19] [21] [24] [25] [26] [27]}

With the introduction of circle breathing systems that integrate fresh gas decoupling, a resurgence has been seen in the use of mechanically driven anesthesia ventilators. These piston-type ventilators use a computer controlled stepper motor instead of compressed drive gas to actuate gas movement in the breathing system. In these systems, rather than having dual circuits with gas for the patient in one and the drive gas in another, there is a single gas circuit for the patient. They are classified as piston-driven, single-circuit ventilators. The piston operates much like the plunger of a syringe to deliver the desired tidal volume or airway pressure to the patient. Sophisticated computerized controls are able to provide advanced types of ventilatory support such as synchronized intermittent mandatory ventilation (SIMV), pressure-controlled ventilation (PCV), and pressure-support ventilation (PSV) in addition to the conventional control-mode ventilation (CMV). Because the patient's mechanical breath is delivered without the use of compressed gas to actuate the bellows, these systems consume less gas during the ventilator's operation than a traditional pneumatic ventilator. This may have clinical significance when the anesthesia workstation is used in a setting where no pipeline gas supply is available (e.g., remote locations, office-based anesthesia practices).

Cycling Mechanism

Most anesthesia machine ventilators are time cycled and provide ventilator support in the control mode. Inspiratory phase is initiated by a timing device. Older pneumatic ventilators use a fluidic timing device. Contemporary electronic ventilators use a solid-state timing device and are classified as time cycled and electronically controlled.

Bellows Classification

The direction of bellows movement during the expiratory phase determines the bellows classification. Ascending (standing) bellows ascend during the expiratory phase ( Fig. 9-24B ), whereas descending (hanging) bellows descend during the expiratory phase. Older pneumatic ventilators and some new anesthesia workstations use weighted descending bellows, but most contemporary electronic ventilators have an ascending bellows design. Of the two configurations, the ascending bellows design is safer. Ascending bellows do not fill if a total disconnection occurs. The bellows of a descending

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Figure 9-24 Inspiratory (A) and expiratory (B) phases of gas flow in a traditional circle system with an ascending bellows anesthesia ventilator. The bellows physically separates the driving-gas circuit from the patient's gas circuit. The driving-gas circuit is located outside the bellows, and the patient's gas circuit is inside the bellows. During the inspiratory phase (A), the driving gas enters the bellows chamber, causing the pressure within it to increase. This causes the ventilator's relief valve to close, preventing anesthetic gas from escaping into the scavenging system, and the bellows to compress, delivering the anesthetic gas within the bellows to the patient's lungs. During the expiratory phase (B), the driving gas exits the bellows chamber. The pressure within the bellows chamber and the pilot line declines to zero, causing the mushroom portion of the ventilator's relief valve to open. Gas exhaled by the patient fills the bellows before any scavenging occurs because a weighted ball is incorporated into the base of the ventilator's relief valve. Scavenging happens only during the expiratory phase, because the ventilator's relief valve is open only during expiration. (Adapted from Andrews JJ: The Circle System. A Collection of 30 Color Illustrations. Washington, DC, Library of Congress, 1998.)