Anesthesia machine and equipment related problems occur in veterinary medicine. A brief window of opportunity exists during which the anesthetist can diagnose and correct the problem. Many issues arise from breathing and delivery systems, and inappropriate use of monitoring equipment.¹ The primary cause is human error and failure to perform a normal anesthesia delivery system check prior to anesthesia.² Problems involved the unidirectional valves, ventilator, gas or electrical supplies, circuit integrity, vaporizers, absorbers, and pressure regulators.¹ Buffington et al. demonstrated that a high percentage of anesthesia providers were shown to perform poorly when identifying anesthesia machine-related problems.³ Anesthetists (technician or DVM) with less experience performed worse and therefore it is important that inexperienced members be monitored closely when performing anesthesia. To prevent and manage such problems, the anesthesia provider must have a strong understanding of the anesthesia delivery system and equipment. This lecture will review the most common and life-threatening problems that are associated with anesthesia machine and equipment during the perioperative period.

Equipment Causes of Hypoxia

Hypoxia is defined as a low partial pressure (<60 mm Hg) of oxygen in tissues. It is important to determine that the cause is indeed a machine related versus patient problem. Machine-related hypoxia can be from a hypoxic inspired gas mixture or an inadequate amount of oxygen is delivered to the patient. An oxygen analyzer can identify a machine or equipment-related cause of inadequate oxygen delivery. A pulse oximeter can indicate low hemoglobin oxygen saturation; however, it cannot differentiate between equipment and patient-related problems. Supplying the incorrect gas with nitrogen or medical grade air or inappropriate monitoring of emptied oxygen tanks can create hypoxic mixtures.

Other causes include misconnection or crossover of the piping system lines, closed pipeline valves, or disconnection of the oxygen hose. Failure to deliver oxygen to the patient can also originate within the machine or circuit itself. Inadvertently lowering of the oxygen flowmeter control knob can deliver inadequate concentrations of oxygen. Any item (drapes, hoses, wires) that comes in contact with this knob can adjust the flow without the anesthetist's knowledge. This is of particular concern when using a non-rebreathing circuit where high-flows are required in order to prevent the rebreathing of exhaled gases. Newer machines have added a guard to prevent this. Dust accumulation within the flowmeter can result in the indicator ball becoming stuck within the flowmeter indicating that oxygen is being delivered when in fact the knob is set in the off position.

Flowmeters can also become inaccurate when grease or oil accumulates on the indicator or within the tube, misalignment of the tube, static electricity, improper calibration, and the stop at the top of the flowmeter tube falling onto the indicator. Absent or faulty unidirectional valves can result in an inspired hypoxic mixture of gases. Finally, oxygen can be lost to the atmosphere from leaks in the flowmeter, anesthesia machine hoses, absorbent canister, breathing circuits, and reservoir bag. The clinical relevance is dependent on the size of the leak. If the problem cannot be quickly identified, the author recommends replacement of the anesthesia machine along with using a reserve full oxygen e-cylinder as the supply source. Placing the patient on room air and ventilating with a resuscitation bag is an alternative until the problem is fixed. Postponing of the surgical procedure may be recommended.

Monitoring equipment can falsely indicate patient hypoxia. Pulse oximeters are excellent tools for detecting hypoxia; however, they do present with some limitations. Dyes (methylene blue or indocyanine green), skin pigmentation, prominent venous pulsations, stray or flickering fluorescent or operating room lights, mixing of probes between manufacturers, electrical interference from electrocautery, motion of the probe or tissue, excessive pressure on the probe, covering or blood on the tissue, and probe malposition can falsely decrease SpO₂ readings.

Equipment Causes of Hypercapnia and Hypocapnia

Hypercapnia is defined as elevated carbon dioxide (CO₂) tensions (>45 mm Hg) in the blood. Failure to fill the ventilator bellows through inadequate tidal volumes or respiratory rates can increase blood CO₂ tensions. Leaks within the machine, breathing circuit, CO₂ absorbent, and ventilator can decrease tidal volume delivery, decreasing CO₂ elimination. Kinking or obstruction of the expiratory hoses, breathing circuit, or endotracheal tube can also increase CO₂ levels. Degraded or improperly applied gaskets can create inadvertent leaks. Exhausted granules and inappropriate distribution of granules may allow gas to be directed through channels, which may limit gas to granule contact resulting in rebreathing of CO₂. CO₂ absorbents using ethylene violet indicator dyes can be photo-deactivated by fluorescent lights resulting in false impression that the absorbent is fresh increasing inspiratory CO₂ concentrations.⁴ Increased apparatus dead space from faulty unidirectional valves or excessive distance between the endotracheal tube and the breathing circuit can increase rebreathing of CO₂. Valves may stick in an upright position with condensation build-up during prolonged anesthesia. Low fresh gas flow rates while using non-rebreathing circuits decrease elimination of exhaled gases which can result in rebreathing of carbon dioxide. Coaxial tubes can be a cause of rebreathing CO₂ when the inner inspiratory tube becomes avulsed, damaged, or kinked. CO₂ problems due to the anesthetic delivery system can be detected with capnography. Expired absorbent or faulty unidirectional valves can be corrected by replacing the absorbent or valves or by increasing the fresh gas flow rate. If unexplained hypoventilation occurs, manual positive pressure ventilation should be provided. This will help rule out mechanical ventilator as the primary concern. If during manual ventilation compliance or breathing resistance is abnormal then the fault lies in the anesthesia delivery system, breathing circuit, or patient. If an obstruction appears evident, detachment of the patient from the anesthesia machine is recommended followed by ventilation with a resuscitation bag until the obstruction is removed. In most occasions, determining the cause of the problem requires further examination and may not be feasible during the perioperative period. The author recommends using a backup anesthesia machine and breathing circuit if possible and determine the cause at a later time. If it is still difficult to ventilate with a resuscitation bag or a new anesthesia delivery system, the problem is associated with the airway device or patient. The patient at this point should be suctioned or reintubated with a new endotracheal tube. Hypocapnia or low end tidal CO₂ (<35 mm Hg) occurs less frequently than hypercapnia during general anesthesia. In brief it can be a result of inappropriate ventilator settings, a leak or obstruction in the capnography sampling line, removal of the water trap from the capnography, complete airway obstruction, leak around the endotracheal cuff, and with sample dilution using high fresh gas flow rates with non- rebreathing systems.

Excessive Airway Pressure

Buildup of airway pressure within the anesthesia circuit can result in inadequate ventilation, barotrauma, and cardiovascular collapse. There are several safety mechanisms that try to compensate for dangerously high pressures. Latex reservoir bags can only maintain a maximum pressure of 50 cm H₂O. Non-latex bags may exceed this pressure. Limiting over-inflation of an endotracheal tube cuff to 20 cm H₂O will act as a safety mechanism. When airway pressure exceeds 20 cm H₂O gas can flow around the cuff. Opening the adjustable pressure limiting (pop-off) valves can also release gas into scavenging with increased airway pressures. Causes of excessive airway pressure include sticking of the oxygen quick flush valve, use of the quick flush on a non-rebreathing system, and obstruction at the expiratory limb, ventilator, APL valve, or scavenging system.

Detection of excessive pressure can be determined via the pressure manometer, over-inflation of the reservoir bag, prolonged thoracic expansion, ventilator alarm, and an ascending limb with a prolonged rise time and no plateau on capnography. Management of these situations requires immediate disconnection of the machine to the patient.

Volatile Anesthetic Agent Delivery Issues

Accidental overdose of the anesthetic agent can result in severe cardiopulmonary depression. Tipping of the vaporizer results in anesthetic agent entering the bypass chamber and increased anesthetic delivery. Filling of the vaporizer with the incorrect anesthetic agent can result in dangerously high concentrations being delivered.

The author has witnessed the use of two vaporizers simultaneously on accident during an emergency case. During that case two individuals turned on two different vaporizers which resulted in cardiovascular collapse.

Inadequate delivery of an anesthetic agent can occur due to air entrainment which dilutes the inhaled anesthetic agents, leaks in the vaporizer, empty vaporizer, incorrect agent in the vaporizer, and accidently leaving the vaporizer in the "off" position intraoperatively.

References

1.  Eisenkraft JB. Hazards of the anesthesia delivery system. In: Ehrenwerth J, Eisenkraft JB, Berry JM, eds. Anesthesia Equipment Principles and Applications. 2nd ed. Philadelphia, PA: Elsevier Saunders; 2013:591–620.

2.  Craig J, Wilson ME. A survey of anaesthetic misadventures. Anaesthesia. 1981;36(10):933–936.

3.  Buffington CW, Ramanathan S, Turndorf H. Detection of anesthesia machine faults. Anesth Analg. 1984;63(1):79–82.

4.  Andrews JJ, Johnston RV, Jr., Bee DE, Arens JF. Photodeactivation of ethyl violet: a potential hazard of Sodasorb. Anesthesiology. 1990;72(1):59–64.