Ventilator Circuit Design
Traditionally ventilators on anesthesia machines have a double-circuit system design and are pneumatically powered and electronicallycontrolled (Figure 4–26). Newer machines also incorporate microprocessor control that relies on sophisticated pressure and flow sensors. This feature allows multiple ventilatory modes, electronic PEEP, tidal volume modulation, and enhanced safety
features. Some anesthesia machines have ventilators that use a single-circuit piston design (Figure 4–24).
In a double-circuit system design, tidal volume is delivered from a bellows assembly that con-sists of a bellows in a clear rigid plastic enclosure (Figure 4–26). A standing (ascending) bellows is preferred as it readily draws attention to a circuit disconnection by collapsing. Hanging (descending) bellows are rarely used and must not be weighted;
A: Datex-Ohmeda. B: Dräger.older ventilators with weighted hanging bellows continue to fill by gravity despite a disconnection in the breathing circuit.
The bellows in a double-circuit design ventila-tor takes the place of the breathing bag in the anes-thesia circuit. Pressurized oxygen or air from the ventilator power outlet (45–50 psig) is routed to the space between the inside wall of the plastic enclosure and the outside wall of the bellows. Pressurization of the plastic enclosure compresses the pleated bel-lows inside, forcing the gas inside into the breathingcircuit and patient. In contrast, during exhalation, the bellows ascend as pressure inside the plastic enclosure drops and the bellows fill up with the exhaled gas. A ventilator flow control valve regu-lates drive gas flow into the pressurizing chamber. This valve is controlled by ventilator settings in the control box (Figure 4–26). Ventilators with micro-processors also utilize feedback from flow and pres-sure sensors. If oxygen is used for pneumatic power it will be consumed at a rate at least equal to minute ventilation. Thus, if oxygen fresh gas flow is 2 L/min and a ventilator is delivering 6 L/min to the circuit, a total of at least 8 L/min of oxygen is being con-sumed. This should be kept in mind if the hospi-tal’s medical gas system fails and cylinder oxygen is required. Some anesthesia machines reduce oxygen consumption by incorporating a Venturi device that draws in room air to provide air/oxygen pneumatic power. Newer machines may offer the option of using compressed air for pneumatic power. A leak in the ventilator bellows can transmit high gas pres-sure to the patient’s airway, potentially resulting in pulmonary barotrauma. This may be indicated by ahigher than expected rise in inspired oxygen con-centration (if oxygen is the sole pressurizing gas). Some machine ventilators have a built-in drive gas regulator that reduces the drive pressure (eg, to 25 psig) for added safety.
Double-circuit design ventilators also incor-porate a free breathing valve that allows outside air to enter the rigid drive chamber and the bellows to collapse if the patient generates negative pressure by taking spontaneous breaths during mechanical ventilation.
In a piston design, the ventilator substitutes an elec-trically driven piston for the bellows (Figure 4–24); the ventilator requires either minimal or no pneumatic (oxygen) power. The major advantage of a piston ventilator is its ability to deliver accurate tidal volumes to patients with very poor lung compliance and to very small patients. During volume-controlled ventilation the piston moves at a constant velocity whereas during pressure-controlled ventilation the piston moves with decreasing velocity. As with the bellows, the piston fills with gas from the breathing circuit. To prevent generation of significant negative pressure during the downstroke of the piston the circle system configura-tion has to be modified (Figure 4–27). The ventilator must also incorporate a negative-pressure relief valve or be capable of terminating the piston’s downstroke if negative pressure is detected. Introduction of a negative-pressure relief valve to the breathing circuit may introduce the risk of air entrainment and the potential for dilution of oxygen and volatile anes-thetic concentrations if the patient breathes during mechanical ventilation and low fresh gas flows.
Whenever a ventilator is used on an anesthesia machine, the circle system’s APL valve must be functionally removed or isolated from the circuit. A bag/ventilator switch typically accomplishes this. When the switch is turned to “bag” the ventilator is excluded and spontaneous/manual (bag) ventila-tion is possible. When it is turned to “ventilator,” the breathing bag and the APL are excluded from the breathing circuit. The APL valve may be automati-cally excluded in some newer anesthesia machines when the ventilator is turned on. The ventilator con-tains its own pressure-relief (pop-off ) valve, called the spill valve, which is pneumatically closed during inspiration so that positive pressure can be gener-ated (Figure 4–26). During exhalation, the pressur-izing gas is vented out and the ventilator spill valve is no longer closed. The ventilator bellows or piston refill during expiration; when the bellows is com-pletely filled, the increase in circle system pressure causes the excess gas to be directed to the scavenging system through the spill valve. Sticking of this valve can result in abnormally elevated airway pressure during exhalation.
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