Problems and Hazards

Numerous hazards are associated with anesthesia ventilators, including problems with the breathing circuit, the bellows assembly, and the control assembly.

Traditional Circle System Problems

Disconnection of the breathing circuit is a leading cause of critical incidents in anesthesia practice.^{[2] [141]} The most common disconnection site is at the Y-piece. Disconnections can be complete or partial (i.e., leaks). A common source of leaks with older absorbers is failure to close the APL (pop-off) valve on initiation of mechanical ventilation. The bag or ventilator switch on contemporary absorbers helps minimize this problem. Preexisting, undetected leaks can occur in compressed, corrugated, disposable anesthetic circuits. To detect such a leak preoperatively, the circuit must be fully expanded before the circuit is checked for leaks.^[142] Disconnections and leaks manifest more readily with the ascending bellows because the bellows will not fill.^[1]

Several disconnection monitors exist. The most important monitor is a vigilant anesthesia care provider monitoring breath sounds, chest wall excursion, and mechanical monitors.

Pneumatic and electronic pressure monitors are helpful in diagnosing disconnections. Factors that influence the monitor's effectiveness include the disconnection site, location of the pressure sensor, the threshold-pressure alarm's limit, the inspiratory flow rate, and the resistance of the disconnected breathing circuit. ^{[143] [144]} Various anesthesia machines and ventilators have different locations for the pressure sensor and different values for the threshold-pressure alarm's limit. The limit for the threshold-pressure alarm may be preset at the factory or adjustable. An audible or visual alarm is actuated if the peak inspiratory pressure of the breathing circuit does not exceed the threshold-pressure alarm's limit. When an adjustable limit for the threshold-pressure alarm is available, such as on the Dräger Narkomed 2A, 2B, 2C, 3, 4, and GS, the operator should set the pressure alarm limit to within 5 cm H₂ O of the peak inspiratory pressure.^{[14] [15] [16] [19] [21] [24]} Figure 9-25 illustrates how a partial disconnection (i.e., leak) may be unrecognized by the low-pressure monitor if the threshold-pressure alarm's limit is set too low or if the factory preset value is relatively low.

Figure 9-25 The threshold-pressure alarm's limit (dotted line) has been set appropriately (top). An alarm is actuated when a partial disconnection occurs (arrow) because the threshold-pressure alarm's limit is not exceeded by the breathing-circuit pressure. A partial disconnection (arrow) is unrecognized by the pressure monitor because the threshold-pressure alarm's limit has been set too low (bottom). (Adapted from Baromed Breathing Pressure Monitor. Operator's Instruction Manual. Telford, PA, North American Dräger, August 1986.)

Respiratory volume monitors are useful in detecting disconnections. Volume monitors sense exhaled tidal volume, minute volume, or both. The user should bracket the high and low threshold volumes slightly above and below the exhaled volumes. For example, if the exhaled minute volume of a patient is 10 L/min, reasonable alarm limits are 8 to 12 L/min. Some Datex-Ohmeda ventilators are equipped with sensors to monitor volume that use infrared light and turbine technology. The volume sensor is usually located in the expiratory limb of the breathing circuit, except in the case of the Datex-Ohmeda S/5 ADU (see "Newer Anesthesia Workstations"). Exposure of the infrared-type sensor clip to a direct beam of overhead surgical lighting can cause erroneous volume readings because the surgical beam interferes with the infrared sensor.^[145] Other types of expiratory volume sensors can be seen in systems such as the Datex-Ohmeda S/5 ADU, the Aestiva, and other workstations that incorporate the 7100 ventilator or 7900 SmartVent.^{[35] [36] [52]} These systems generally use differential pressure transduction technology to determine inhaled and exhaled volumes. Still other systems from Dräger measure exhaled volume using ultrasonic or "hot wire" sensor technology.^[20]

Carbon dioxide monitors are probably the best devices for revealing disconnections to the patient. Carbon dioxide concentration is measured near the Y-piece directly or by aspiration of a gas sample to the instrument. A drastic change in the difference between the inspiratory and end-tidal carbon dioxide concentration or the absence of carbon dioxide indicates a disconnection, a nonventilated patient, or other problems.^[1]

Misconnections of the breathing system are not uncommon, despite efforts by standards committees to eliminate

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Occlusion (i.e., obstruction) of the breathing circuit may occur. Tracheal tubes can become kinked. Hoses throughout the breathing circuit are subject to occlusion by external mechanical forces that can impinge on flow. Blockage of a bacterial filter in the expiratory limb of the circle system has caused a bilateral tension pneumothorax.^[109] Incorrect insertion of flow direction-sensitive components can result in a no-flow state.^[1] Examples of these components include some PEEP valves and cascade humidifiers. Depending on the location of the occlusion and the pressure sensor, a high-pressure alarm may alert the anesthesiologist to the problem.

Excess inflow to the breathing circuit from the anesthesia machine during the inspiratory phase can cause barotrauma. The best example of this phenomenon is oxygen flushing. Excess volume cannot be vented from the system during inspiration because the ventilator's relief valve is closed and because the APL valve is out of circuit or closed.^[51] A high-pressure alarm, if present, may be activated when the pressure becomes excessive. With Dräger systems, audible and visual alarms are actuated when the high-pressure threshold is exceeded.^{[19] [20] [21] [22] [23] [24] [25] [26] [27]} In the Modulus II Plus system, the Datex-Ohmeda 7810 ventilator automatically switches from the inspiratory to the expiratory phase when the adjustable peak-pressure threshold is exceeded.^[15] This minimizes the possibility of barotrauma if the peak-pressure threshold is set appropriately by the anesthesiologist.

On machines equipped with adjustable inspiratory pressure limiters, such as the Datex-Ohmeda S/5 ADU; Aestiva; Dräger Narkomed 6000, 2C, and GS; and Fabius GS, maximal inspiratory pressure may be set by the user to a desired peak airway pressure. An adjustable pressure relief valve opens when the user-selected pressure is reached to prevent generation of excessive airway pressure. This feature depends on the user setting the appropriate "pop-off" pressure. If it is set too low, insufficient pressure for ventilation may be generated, resulting in inadequate minute ventilation; if set too high, barotrauma may occur. The piston-driven Fabius GS and other systems may also include a factory present inspiratory pressure safety valve that opens at a preset airway pressure (e.g., 75 cm H₂ O) to minimize the risk of barotrauma.^[20]

Bellows Assembly Problems

Leaks can occur in the bellows assembly. Improper seating of the plastic bellows housing can result in inadequate ventilation because a portion of the driving gas is vented to the atmosphere. A hole in the bellows can lead to alveolar hyperinflation and possibly barotrauma in some ventilators because high-pressure driving gas can enter the patient's circuit. The value on the oxygen analyzer may increase when the driving gas is 100% oxygen, or it may decrease if the driving gas is composed of an air-oxygen mixture.^[146]

The ventilator's relief valve can cause problems. Hypoventilation occurs if the valve is incompetent, because anesthetic gas is delivered to the scavenging system during the inspiratory phase instead of to the patient. Gas molecules preferentially exit into the scavenging system because it represents the path of least resistance, and the pressure within the scavenging system can be subatmospheric. Incompetency of the ventilator's relief valve can result from a disconnected pilot line, a ruptured valve, or a damaged flapper valve.^{[147] [148]} A relief valve stuck in the closed position can produce barotrauma. Excessive suction from the scavenging system can draw the ventilator's relief valve to its seat and close the valve during the inspiratory and expiratory phases.^[1] Pressure in the breathing circuit escalates because excess anesthetic gas cannot be vented. During the expiratory phase, some newer machines from Datex-Ohmeda (i.e., S/5 ADU, 7100, and 7900 SmartVent) scavenge excess gases from the patient and from the drive gas exhausted from the ventilator. When the ventilator's relief valve opens and waste anesthetic gases are vented from the breathing circuit, the drive gas from the bellows' housing joins with the flow to enter the scavenging system. ^{[35] [36] [52]} Under certain conditions, this could overwhelm the scavenging system, resulting in pollution of the operating room with waste anesthetic gases (see "Scavenging Systems").

Control Assembly Problems

The control assembly can be the source of electrical and mechanical problems. Electrical failure can be total or partial; the former is the more obvious. Some mechanical problems include leaks within the system, faulty regulators, and faulty valves. An occluded muffler on the North American Dräger AV-E can cause barotrauma. Obstructed outflow of the driving gas closes the ventilator's relief valve, and excess gas from the patient cannot be vented.^[149]