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The anesthesia delivery system rarely causes patient injuries or deaths. When complications do occur, it's much more likely that user error was the reason that the delivery system failed (see "What the Closed Claims Project Can Tell Us" on page 24). That may sound like bad news, but it means that we can prevent many, if not most, delivery-system-related complications. The key? Ensuring that anesthesia providers and other OR staff alike understand and properly use the many components of the anesthesia delivery system ' the anesthesia machine, vaporizers, breathing system, ventilator and waste gas scavenging system.
While you can't eliminate equipment failures, appropriately monitoring the patient circuit should lead to early detection of failures and enable prompt intervention before the patient suffers any harm.
Pressure monitoring
Because breathing system disconnects and misconnects aren't uncommon, you must monitor the breathing system's integrity. To this end, most anesthesia breathing systems have both an analog pressure gauge and an electronic pressure monitoring and alarm system.
- Low-pressure alarm. Low-pressure monitors are sometimes called disconnect alarms, but this may be a misnomer; while they monitor pressure, the user may infer circuit integrity only if the monitor is used appropriately. Because low-pressure monitors annunciate audible and visual alarms within 15 seconds of a drop below a minimum pressure threshold, the user must set the alarm threshold to be just less than the normal peak inspiratory pressure. That way, any slight decrease will trigger the alarm; otherwise, a circuit leak or disconnect may go undetected if the low-pressure alarm threshold remains satisfied. For example, let's say you've pulled a 3mm inner diameter tracheal tube connected to a circle system out of the patient's airway, leaving the lungs unventilated. Because the small diameter tube offers a high resistance to gas flow, the pressure increase in the circuit with each positive pressure inspiration could satisfy the low-pressure alarm threshold.
Contemporary delivery systems incorporate low-pressure alarms that are automatically enabled when you turn the ventilator on. On some old models of machine, the user may need to manually enable the low-pressure alarm system. Because the low-pressure alarm is critical during use of intermittent positive pressure ventilation (IPPV), you must be aware of the properties of the monitoring system on the individual machine. If there is any doubt about whether a monitor is interfaced with the ventilator's on/off power switch, you can test the alarm by deliberately creating a disconnect during the pre-use checkout. While modern breathing system monitors have a widely adjustable low-pressure alarm threshold, older models may provide a choice of only three settings (such as 6, 12 and 26cm H2O), which may limit the sensitivity to detect small decreases in breathing system pressure. Some low-pressure monitoring alarm systems offer an optional 60-second delay in the event that a slow ventilatory rate (such as fewer than four breaths per minute) is set.
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What the ASA Closed Claims Project Can Tell Us | ||
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The ASA's Closed Claims Project has investigated the role of equipment failures that lead to malpractice litigation in the United States.1 A 1997 analysis of the database found that gas delivery equipment problems accounted for 72 of 3,791 (2 percent) claims.2 Here's how these 72 cases break down:
Death or brain damage occurred in 76 percent of these cases. Initiating events were circuit misconnects, disconnects and gas delivery system errors. The claims project found user error occurred in 75 percent and equipment failure in 25 percent. Anesthesia providers were responsible in 70 percent of user error cases with ancillary staff responsible for 30 percent. Overall, 78 percent of claims were deemed preventable by the use of appropriate monitoring. The three main causes of injury:
As of April 2004, the ASA Closed Claims Project database included 6,448 claims, 95 of which were related to anesthesia gas delivery equipment.3 Claims for events that occurred in the late 1990s are still being processed and the most recent gas delivery system claims were for an event in 2000. Thus far, however, it appears that gas delivery equipment problems are decreasing as a proportion of total claims. Anesthesia gas delivery claims represented
These include four supplemental oxygen line events, seven anesthesia machine problems, three vaporizer problems, one ventilator problem and four breathing circuit problems. In addition, the outcomes in anesthesia gas delivery equipment claims from 1990 to 2000 seem to be less severe than earlier claims. From 1990 to 2000, 31 percent of anesthesia gas delivery system claims resulted in severe injury or death compared to 80 percent from 1970 to 1989. Among the 19 claims from 1990 to 2000 were five deaths, two brain damage, four pneumothorax, four awareness, one cardiac arrest with full recovery, three cancellations (no actual injury) and one claim with no apparent injury. Payments reflect the lower injury severity, with a median payment (in 1999 dollars) of $63,250 between 1990 and 2000 compared to $594,750 (adjusted to 1999) for earlier gas delivery equipment claims. Fifteen of the 19 post-1990 claims resulted in payment, but all were less than $500,000.3 ' James B. Eisenkraft, MD |
While you must enable, either automatically or manually, the breathing system low-pressure alarm in association with IPPV, the following pressure monitoring modalities operate continuously.
- Continuing-pressure alarm. This alarm annunciates when circuit pressure exceeds 10cm H2O for more than 15 seconds, alerting to more gradual increases in pressure, such as due to a ventilator pressure relief valve malfunction (when the valve is stuck closed, for example) or a scavenging system occlusion. In these situations, fresh gas continues to enter the breathing system from the machine flowmeters but is unable to leave. The rate of rising pressure therefore depends upon the fresh gas flow rate.1
- High-pressure alarm. This alarm annunciates immediately whenever the high-pressure limit is exceeded. On modern machines, this threshold is adjustable by the user, with a default usually set at 50cm H2O. Some older pressure monitors are not user-adjustable and have a default setting of 65cm H2O. This might be too high to detect an otherwise harmful high-pressure condition, such as a total obstruction of the tracheal tube in which breathing system pressure fails to exceed 65cm H2O. Ideally, the ventilator will have a high-pressure relief valve, the opening threshold pressure of which you set in conjunction with the high-pressure alarm limit.
- Subatmospheric-pressure alarm. This annunciates an immediate alarm when pressure drops below --10cm H2O. It should alert to potential negative pressure barotrauma situations due to suction being applied to the circuit. Negative pressure in the circuit may be caused by spontaneous respiratory efforts by the patient, a malfunctioning waste gas scavenging system, a sidestream sampling gas analyzer when fresh gas flow into the circuit is too low, a suction catheter passed into the airway or suction via the working channel of a fiberscope passed into the airway through a diaphragm.
Volume monitoring
You can use a spirometer placed in the vicinity of the expiratory unidirectional valve to monitor expired tidal and minute volumes as well as circuit integrity. A breathing system disconnect should cause a low-volume alarm to sound. Because the spirometer is usually located by the expiratory unidirectional valve at the absorber, it doesn't measure the patient's actual expired tidal volume, rather the volume measured includes both that exhaled by the patient and the gas volume compressed in the breathing system.
While the spirometer low-volume alarm is generally more useful in alerting to a low volume or possible disconnect situation, using a high-volume alarm feature might help detect unanticipated increases in tidal volume.2 Extra gas flow into the circuit could come from the machine flowmeters, by increasing the inspired-to-expired ratio or through a hole in the bellows (such as on Drager Narkomed machines with oxygen/air/venturi driving gas systems), and any of these has the potential to be added to the patient's inspired tidal volume. This may be particularly hazardous to the pediatric patient for whom a small tidal volume is intended.
A spirometer that senses gas flow direction can alert you to a reverse flow situation such as one due to an incompetent expiratory valve or to a breathing system leak. The spirometer on some old models of Datex-Ohmeda machines, for example, can be moved to a location by the patient's airway for use with a Bain circuit. While in this location, the reverse flow detection alarm feature of the Ohmeda spirometer must be disabled by the user. The user must remember to re-enable this alarm feature after he returns the spirometer to its usual position by the expiratory unidirectional valve of the circle breathing system.
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CO2 Absorbents' Hidden Dangers |
Since 1990, there have been several reports of patients who developed increased levels of carboxyhemoglobin in response to accumulation of carbon monoxide in the circle system. The carbon monoxide is generated when desflurane, enflurane and, to a lesser extent, isoflurane interact with dry CO2 absorbent, particularly Baralyme.1 While no case of patient harm has been reported to date, CO represents a potential hazard of which the anesthesia provider should be aware.2 To decrease this potential hazard, use absorbent that has the standard complement of water or add liquid water to the top of the absorbent.3 Turn off fresh gas at the end of each case to prevent the absorbent from drying out, and replace the absorbent more frequently than manufacturers' guidelines state, especially if the machine has been left unused for some time, such as over a weekend.4 Alternatively, you can use CO2 absorbents that don't contain strong bases such as Ba(OH)2 and KOH. Amsorb is an example; it doesn't contain strong base or form CO or compound A in vitro, and it turns purple when desiccated, an additional advantage.5 Other absorbents that lack strong bases include Carbolime, Medisorb, Sofnolime and Dragersorb. To help you detect carboxyhemoglobin in the blood, Masimo markets a pulse oximeter that uses eight wavelengths of light and displays COHb percentage.6 Desiccated Barlyme acting on sevoflurane can produce absorber temperatures that exceed 400'C, fires and explosions.7-11 Animal studies and a bench model demonstrate fires and explosions with sevoflurane, and there are now several reports of fires and explosions in clinical practice with sevoflurane (but none with desflurane or isoflurane) in one case causing patient injury.7-12 In late 2004, the manufacturer of Baralyme discontinued its distribution, which may minimize or even eliminate the problems of fire and explosion.10-12 The risk of fire and explosion is much less with soda lime, although it has been suggested that the absorber temperature be routinely monitored using a skin probe.13 ' James B. Eisenkraft, MD |
Gas composition in the circuit
By appropriately monitoring O2, CO2, N2O, N2 and anesthetic agent in the gas mixture at the patient's airway, you can be alerted to most gas delivery, composition and agent dosing problems.
- Oxygen. The oxygen analyzer is perhaps the most important monitor in the gas delivery system because it's both quantitative and qualitative. Many anesthesia delivery systems incorporate a galvanic fuel cell oxygen sensor located at the inspiratory unidirectional valve; this actually senses PO2 (although the display is in volumes percent). It is calibrated to 21% using room air and, unlike other technologies, isn't fooled by other gases. On contemporary machines, the O2 analyzer is automatically enabled whenever the machine is capable of delivering an anesthetic gas mixture. There are several possible causes of inadequate O2 concentration in the circuit: a hypoxic gas being delivered via the pipeline or tanks, a disconnected fresh gas hose during use of a hanging bellows ventilator, the O2 flow control valve's being turned off, failure of the fail-safe system, failure of the proportioning system, an O2 leak in the low pressure system of the machine, and a closed circuit with inadequate O2 inflow rate.
Properly setting the low-concentration alarm on the O2 analyzer is essential. A high O2 concentration alarm may also be important in certain situations. For example, during administration of an O2/helium anesthetic for laser surgery of the airway, if the helium tank were to become depleted or the O2 flush were used, a high O2 concentration would be delivered that may lead to a fire. A high-concentration oxygen alarm could therefore help to prevent an airway fire.
While most would assume that O2 flows from O2 wall outlets, this is not always the case ' and because there is no qualitative O2 analyzer between the wall outlet and the machine, you can't be sure. The oxygen monitor analyzes only the gas in the breathing system, not the presumed-to-be O2 gas flowing from an auxiliary O2 flowmeter on the machine or one connected directly to a wall outlet. In one report, two patients died when an auxiliary O2 flowmeter was connected to a wall N2O outlet; this highlights the need for the user to perform double-checks on gas connections.3
- Carbon dioxide. Capnography can provide much information about ventilation of the patient's lungs, as well as about the anesthesia delivery system itself. If failure to ventilate occurs, which might be due to a circuit disconnect or misconnect, the absence of a capnogram will annunciate an apnea alarm. CO2 rebreathing, the result of exhausted CO2 absorb-ent, incompetent inspiratory or expiratory valves, a misconfigured circuit, or a Bain circuit with inner tube disconnect, will cause an abnormal capnogram.
- Anesthetic gases and nitrogen. Monitoring concentrations of N2O and potent inhaled agent with use of appropriate high- and low-concentration alarm settings will alert you to most agent dosage problems. Low agent concentration happens if the vaporizer is turned off or empty, and could result in patient awareness. Excessive agent concentrations, on the other hand, may be caused by vaporizer malfunction, tipping or liquid agent in the circuit. Some analyzers will annunciate an alarm in the presence of mixed agents (if the vaporizer is contaminated or more than one vaporizer is on, for example).4 Agent analysis is also reassuring whenever you use a new piece of equipment, such as a Tec 6 or D-Vapor vaporizer (for desflurane) or the Aladin vaporizing system (on the Datex-Ohmeda, S5/ADU and AISYS workstations). N2 concentration monitoring may alert you to the presence of an air leak into the breathing system.
- Flows/sidestream spirometry. Many anesthesia providers use sidestream sampling gas monitors to analyze respired gases. By adding a Pitot tube flow sensing system to the sidestream sampling adapter, it becomes possible to monitor and set alarm limits for pressure, flow, volume and gas composition, all of which are being sensed at the airway.5 Such monitoring of multiple aspects of ventilation and the delivery system function by the patient's airway offers many potential advantages over the usual pressure and volume monitoring sites, including the ability to monitor the patient's inspired and expired volumes, as well as to make available flow volume and pressure volume loops.
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On the Web |
Get the FDA's generic pre-use anesthesia workstation checkout at www.fda.gov/cdrh/ humfac/anesckot.html. |
Let the technology help you
While delivery system failures, use errors and equipment failures cannot always be prevented, appropriate monitoring should facilitate detection of most such problems and permit intervention before patient harm occurs. You can do this by ensuring you not only have alarms in place, but by maintaining the alarms on your anesthesia delivery system in good working order.5
Further, you should use all indicated and required monitors correctly with alarm limits and alarm volumes set appropriately for the individual patient's situation. Don't disable alarms or monitors; they enhance patient safety by providing continuous electronic surveillance.
To this end, one study, which looked at ventilation, concluded that critical areas be double- or even triple-monitored and that monitoring equipment be self-activating. The philosophy might be equally well applied to other critical variables that are monitored during anesthesia. The current ASA standards for basic anesthetic monitoring state that when capnography or capnometry is utilized, the end tidal CO2 alarm should be audible to the anesthesiologist and the anesthesia care team. They also state that, when a pulse oximeter is being utilized, the variable pitch pulse tone and the low threshold alarm should be audible to the entire anesthesia care team.6
Have all anesthesia equipment regularly serviced by authorized personnel, and update as necessary. Always perform a recommended pre-use checkout of your delivery system and ensure you are familiar with its operation.
References
1.Eisenkraft JB. Potential for barotraumas or hypoventilation with the Draeger AV-E ventilator. J Clin Anesth 1989; 1:452-456.
2. Scheller MS et al.: Influence of fresh gas flow and I:E ratio on tidal volume and PaCO2 in ventilated patients. J Cardiothorac Anesth, 1989, 3:564.
3. "Surgery mix-up causes 2 deaths." New Haven Register. January 20, 2002.
4. Eisenkraft JB. Anesthesia Vaporizers. In: Anesthesia Equipment: Principles and Applications, Ehrenwerth J/Eisenkraft JB, eds. St. Louis, Mosby, 1993.
5. Merilainen P et al.: A novel sensor for routine continuous spirometry of intubated patients. J Clin Monit 1993; 9:374-380.
6. writeOutLink('http://www.asahq.org/publications AndServices/standards/02.pdf',0)
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