Discussion
Inspiratory and expiratory unidirectional valves permit gas flow in one direction only in the semiclosed circle breathing system. Unidirectional valve malfunction can occur in case of a broken disc, replacement with a different size, or a disc that is sticky due to the moisture or improper orientation. These can cause failure to open or close, leading to hypercapnia, ventilation failure, or barotrauma as the degree of malfunction [
2,
3,
6,
7].
The present two cases represented a failure to close the unidirectional valve. If the expiratory unidirectional valve fails to close completely, then previously exhaled gas, which is high in CO
2 in the expiratory limb, will be rebreathed with each inspiration, leading to hypercapnia, markedly elevating both inspiratory and expiratory segments of the CO
2 waveform [
4]. It can cause hypertension, tachycardia, and increased intracerebral pressure. Because we discovered the problems within a few minutes, there were no vital sign changes. In case 1, we did not suspect unidirectional expiratory valve malfunction at first because there was no abnormal CO
2 waveform at the prior general anesthesia and it was normal at the first 10 minutes of this anesthesia. We first thought that the error was in the capnometer, and calibrated again. To rule out increasing metabolic rate of patient by malignant hyperthermia, we checked the body temperature, but it was within normal range of temperature. Therefore, we suspected the cause of abnormal CO
2 waveform to be rebreathing of the breathing circuit. At that time, we found the broken expiratory valve disc through the transparent dome. When a disc breaks suddenly in the midst of anesthesia like our case 1, capnography can be used to identify unidirectional valve incompetence. Expiratory valve malfunction is more frequent than the inspiratory one due to its exposure to a humid alveolar gas [
8].
When an expiratory unidirectional valve is sticky, it can cause obstruction of the breathing system, which can cause exhalation failure, leading to pulmonary barotraumas and pneumothorax [
3,
7]. Yang et al. [
3] reported the case involving a misunderstood situation of failed ventilation caused by sticky expiratory unidirectional valve. If this event occurs during anesthesia, endotracheal tube obstruction, bronchial spasm, or malfunction of the anesthesia delivery system need to be considered. Especially, when the sound of air leakage caused by disconnection of endotracheal tube from the anesthetic circuit is heard and a positive pressure ventilation is easily conducted using the other resuscitation bags, an anesthesia delivery system obstruction, including the unidirectional valve, bacterial filter, and scavenging pathway should be suspected [
3,
9,
10]. This situation can arise suddenly in the middle of anesthesia. Therefore, CO
2 wave and airway pressure should be continuously monitored.
If the inspiratory valve fails to close, CO
2 gas is usually detected only at the beginning of inspiration because it would permit the exhaled CO
2 to enter the CO
2 free inspiratory limb of the breathing circuit, and to be breathed in by the patient during the next inspiratory period [
11]. As in case 2, the capnogram will display a prolonged plateau, gradually sloping descending limb without approaching the zero baseline [
12,
13]. When the incompetence of inspiratory valve is mild, it is not a severe problem. But if it is severe or the operation time is long, profound hypercapnia can occur. After case 2, we dissembled all unidirectional valve disc locking apparatuses in our hospital and examined them closely. Only case 2's disc locking apparatus of inspiratory unidirectional valve was loosely-fitting, so it could be easily mispositioned while reassembling. In case 2, the surgery was the first one done on Monday. It was presumed that the duty nurse of the prior day assembled the valve improperly after cleaning. A centrally and horizontally-located disc which at first covers the whole orifice of inspiratory valve could to move peripherally enough to make a gap as the induction of anesthesia goes on. Loose disc locking apparatus should be repaired to maintain the correct alignment.
Currently, unidirectional valve function can be tested using either the breathing method [
11], the anesthesia machine valve tester [
14], the pressure decline method and modified pressure decline method [
15]. Some new anesthesia workstations allow the machine to automatically check for functionality of electronic, mechanical, and pneumatic components including unidirectional valves. But, automatic check of ventilator does not confirm valve integrity [
6]. Therefore, it is still important for the operator to visually inspect the valves to ensure they move appropriately not only before induction but also in the middle of anesthesia especially unidirectional valve malfunction is suspected like our cases.
Over the years, the anesthesia delivery system has evolved to a complex multisystem workstation. To prevent mishaps, anesthesia care providers must be aware of the functional anatomy of the anesthesia workstation and current guidelines for developing institution-specific checkout procedures.
In conclusion, two cases of abnormal CO2 wave form caused by unidirectional valve malfunction and early detection without any complications are presented. These cases are good examples of the significance of ETCO2 monitor to verify the malfunction of unidirectional valve during general anesthesia.