Complete bacterial filter blockage by a plastic film.

We describe a case where the expiratory bacterial filter was completely blocked by a plastic membrane. Significantly increased airway pressures and inability to deliver adequate tidal volume to the patient were experienced. The cause of machine failure could not be identified until a complete preanesthesia machine manual check was performed. All machines and circuits should be checked manually, even with the availability of new and advanced self-tests in machines. Backup ventilation equipments and familiarity with signs of expiratory and inspiratory limb obstruction should not be underestimated.

Introduction

Anesthesia machine check is an integral part of the anesthesiologist's daily routine. In 1993, a joint effort between the American Society of Anesthesiologists and the U.S. Food and Drug Administration (FDA) produced the Anesthesia Apparatus Checkout Recommendations, which included both low and high pressure systems checks.[1] Newer anesthesia machines offer an electronic and automatic self-testing capability and led some providers to believe that these advanced testing can partially or totally replace those outdated recommendations. We recently experienced a case in which the expiratory bacterial filter was completely blocked by a plastic membrane. The cause of machine failure could not be identified until a complete preanesthesia machine manual check was performed.

Case Report

A 71-year-old woman with history of coronary artery disease, hypertension, and chronic obstructive pulmonary disease presented for a substernal hernia repair from her coronary bypass surgery. This was the second case of the day and the Drager Fabius GS anesthesia machine was reported to have been checked by another provider right before the case started. A circle breathing circuit with inspiratory, expiratory limbs bacterial filters, and a heat and moisture exchanger (Vital Signs, Totowa, NJ) was attached to the ventilator. After tracheal intubation, the patient was then placed on volume-controlled ventilation mode with tidal volume of 600 ml, rate of 10, 3 cmH2O positive-end expiratory pressure, and 100% fraction of oxygen. Almost immediately, peak airway pressure was noted to rise to 65 cmH2O with the delivered tidal volume less than 300 ml. Systolic blood pressure dropped to 40 mmHg with 100% oxygen saturation. End-tidal carbon dioxide (EtCO2) showed elevated baseline with minimal variations. Breath sounds were muffled bilaterally on auscultation.

Direct laryngoscopy was performed and endotracheal tube (ETT) position was confirmed by direct visualization. Mucus plug, severe bronchospasm, and tension pneumothorax were considered in the differential diagnosis. Suction of the ETT revealed minimal mucus and five albuterol puffs were given. When the circuit was disconnected from the ETT for suction, a high pressure exhalation was noticed with immediate improvement of blood pressure to 90 mmHg. We removed the circuit from the ETT and manually ventilated the patient with an Ambu bag and backup oxygen source. Normal airway pressure was achieved with adequate tidal volume. Normal EtCO2 tracing was noticed and blood pressure returned to baseline. The anesthesia circuit was suspected to be source of the problem and a new circuit was installed. Normal ventilation and vital signs were achieved with the new circuit. The case proceeded uneventfully thereafter. After the completion of the case, the circuit, all apparatus, and machine were examined closely and extensively to identify possible causes. No apparent defect in either limbs of the circuit or machine was seen. However, a plastic film was found in the expiratory bacterial filter on the machine end [Figure 1]. It was assumed that this plastic film caused the inability to exhale and resulted in high peak airway pressures with inability to deliver adequate tidal volume.

Figure 1

A plastic membrane completely covered the fault bacterial filter

To simulate our case scenario, we attached a breathing bag to the Y-piece of same breathing circuit used in the case and initiated volume-controlled ventilation with preset tidal volume of 600 ml and rate of 12 with a fresh gas flow of 2 l/min. The airway pressure increased continuously and reached a peak airway pressure of 50 cmH2O after 3–4 cycles. The desired tidal volume was not achievable and high airway pressure alarm triggered. Minimal airway pressure cycling during ventilation was also noticed. Increase in fresh gas flow accelerated the rise of peak airway pressure. Since the film could be easily put on the inspiratory filter, we then attached the faulty filter into the inspiratory limb and repeated the same test. The inspiratory airway pressure immediately reached 60 cmH2O with a low expiratory airway pressure and normal airway pressure cycling. The artificial lung (bag) did not inflate or deflate during the test.

Discussion

We investigated the reason why the preanesthesia check did not recognize this problem. The anesthesia provider only performed leak test since it was the second case of the day. After occluding the Y-piece of the same circuit and flushing the oxygen valve, the pressure gauge did not decrease confirming that no leak was found and the system was fine.

We then attempted to determine whether a complete preanesthesia machine check could have correctly identified this problem. Self-tests of the same anesthesia machine, as recommended by Drager with the faulty filter in either the expiratory or inspiratory limb, passed except during the leak test, which took longer than usual to perform and gave a warning Ventilator Fail afterward. Using the FDA recommendation, we performed the leak test by closing the pop-off valve, occluding the Y-piece, and pressurizing the circuit to 30 cmH2O with the oxygen flush valve. When the faulty filter was in the expiratory limb, the value on the pressure gauge continuously increased and the breathing bag on the bag mount kept increasing in size. When the faulty filter was in the inspiratory limb, a pressure of 30 cmH2O could not be achieved by flushing the oxygen flush valve and the bag remained deflated.

Three cases of bacterial filter blockage have been reported in the literature.[2-4] In all three cases, the obstruction occurred after the case had started normally. In our patient, the plastic film stayed in a fixed position immediately above the filter material during manufacture and caused complete occlusion of the expiratory limb filter.

Many new anesthesia workstations now incorporate technology that allows the machine to either automatically or manually walk the user through a series of self-tests to check for functionality of the electronic, mechanical, and pneumatic components. However, our case emphasizes the importance of manual checking the low and high pressure systems in all anesthesia workstations in addition to self-test. A leak test should be performed by closing the pop-off valve, occluding the Y-piece, and pressurizing the circuit to 30 cmH2O with the oxygen flush valve. A flow test with a breathing bag attached at the Y-piece should be evaluated by initiating the desired ventilation mode for the coming case. Backup ventilation equipments and familiarity with signs of expiratory and inspiratory limb obstruction should not be underestimated.