Problems Associated with Anesthesia Ventilators
From the previous discussion, it is important to appreciate that because the ventilator’s spillvalve is closed during inspiration, fresh gas flow from the machine’s common gas outlet normally contrib-utes to the tidal volume delivered to the patient. For example, if the fresh gas flow is 6 L/min, the I:E ratio is 1:2, and the respiratory rate is 10 breaths/min, each tidal volume will include an extra 200 mL in addition to the ventilator’s output:
Thus, increasing fresh gas flow increases tidal volume, minute ventilation, and peak inspiratory pressure. To avoid problems with ventilator–fresh gas flow coupling, airway pressure and exhaled tidal volume must be monitored closely and excessive fresh gas flows must be avoided.
Intermittent or sustained high inspiratory pressures (>30 mm Hg) during positive-pressure ventilation increase the risk of pulmonary barotrauma (eg, pneumothorax) or hemodynamic compromise, or both, during anesthesia. Excessively high pressures may arise from incorrect settings on the ventilator, ventilator malfunction, fresh gas flow coupling (above), or activation of the oxygen flush during the inspiratory phase of the ventilator. Use of the oxygen flush valve during the inspiratory cycleof a ventilator must be avoided because the ventilator spill valve will be closed and the APL valve is excluded; the surge of oxygen (600–1200 mL/s) and circuit pressure will be transferred to the patient’s lungs.
In addition to a high-pressure alarm, all venti-lators have a built-in automatic or APL valve. The mechanism of pressure limiting may be as simple as a threshold valve that opens at a certain pressure or electronic sensing that abruptly terminates the ven-tilator inspiratory phase.
Large discrepancies between the set and actual tidal volume that the patient receives are oftenobserved in the operating room during volume con-trol ventilation. Causes include breathing-circuit compliance, gas compression, ventilator–fresh gas flow coupling (above), and leaks in the anesthe-sia machine, the breathing circuit, or the patient’s airway.The compliance for standard adult breath-ing circuits is about 5 mL/cm H2O. Thus, if peak inspiratory pressure is 20 cm H2O, about 100 mL of set tidal volume is lost to expanding the circuit. For this reason breathing circuits for pediatric patients are designed to be much stiffer, with compliances as small as 1.5–2.5 mL/cm H2O.
Compression losses, normally about 3%, are due to gas compression within the ventilator bellows and may be dependent on breathing-circuit volume. Thus if tidal volume is 500 mL another 15 mL of the set tidal gas may be lost. Gas sampling for capnog-raphy and anesthetic gas measurements represent additional losses in the form of gas leaks unless the sampled gas is returned to the breathing circuit, as occurs in some machines.
Accurate detection of tidal volume discrepan-cies is dependent on where the spirometer is placed. Sophisticated ventilators measure both inspira-tory and expiratory tidal volumes. It is important to note that unless the spirometer is placed at the Y-connector in the breathing circuit, compliance and compression losses will not be apparent.
Several mechanisms have been built into newer anesthesia machines to reduce tidal volume discrep-ancies. During the initial electronic self-checkout, some machines measure total system compliance and subsequently use this measurement to adjust the excursion of the ventilator bellows or piston; leaks may also be measured but are usually not compensated. The actual method of tidal volume compensation or modulation varies according to manufacturer and model. In one design a flow sensor measures the tidal volume delivered at the inspiratory valve for the first few breaths and adjusts subsequent metered drive gas flow volumes to compensate for tidal volume losses (feedback adjustment). Another design continually measures fresh gas and vaporizer flow and subtracts this amount from the metered drive gas flow (pre-emptive adjustment). Alternately, machines that use electronic control of gas flow can decouple fresh gas flow from the tidal volume by delivery of fresh gas flow only during exhalation. Lastly, the inspiratory phase of the ventilator–fresh gas flow may be diverted through a decoupling valve into the breathing bag, which is excluded from the circle system during venti-lation. During exhalation the decoupling valve opens, allowing the fresh gas that was temporarily stored in the bag to enter the breathing circuit.
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