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Potential risk associated with anesthesia machines with single oxygen cylinders
Letters to the Editor
Potential Risk Associated with Anesthesia with Single Oxygen Cylinders
Machines
To the Editor: The following incident brought to our attention the vulnerability of anesthesia systems with only a single oxygen cylinder. An adult patient was undergoing cerebral angiography in the X-ray department under general endotracheal anesthesia, paralyzed and ventilated. The anesthetic machine had a single oxygen cylinder yolk. During the procedure, an X-ray boom hit, disconnecting and breaking the oxygen pipeline supplying the anesthetic machine. Because the oxygen cylinder on the machine was closed, the oxygen failure alarm sounded immediately, and the oxygen supply to the machine was quickly restored by opening the cylinder. For the 13 minutes that it took to summon technical help and to restore pipeline oxygen, the cylinder supplied oxygen both to the fresh gas inlet and to drive the gas driven ventilator. After restoration of pipeline oxygen, the anesthetic proceeded uneventfully. This incident passed without consequence, because technical help was readily available and the pipeline was quickly repaired. But the potential for mishap was revealed. Most anesthetic ventilators in the United States are driven by oxygen, usually piped oxygen, using approximately 10 liters per minute. A full oxygen cylinder is capable of driving a ventilator, but once it has emptied to about the I/ mark (500 psi; 34.5 x 100 kPa) it no longer contains enough pressure to do so. [Note that although this pressure is still greater than pipeline pressure (60 psi; 4.1 x 100 kPa) , the partially full cylinder, unlike the pipeline, is unable to sustain pressure against a high demand beyond the machine’s pressure regulator, as placed on it by driving a ventilator.] It is easy to calculate that, after approximately 45 minutes, a full tank (659 liters) that has driven a ventilator (10 liters per minute) as well as provided fresh gas flow (approximately 1 liter per minute), will be :fi empty and no longer able to drive the ventilator, although it will still have a reserve of about 165 liters (over 2 hours use at 1 liter per minute). In order to verify these calculations, we conducted an anesthetic using full cylinders of oxygen and nitrous oxide (no piped gases) and an Ohmeda 7000 series ventilator with standard settings (a 3
260
J. Clin. Anesth., vol. 8, May 1996
liter fresh gas flow with a 29% FiO, and an 8.8 liter minute ventilation). After 52 minutes, at a cylinder pressure of 450 psi (31 x IO0 kPa), the low oxygen supply pressure alarm sounded and the ventilator ceased to function properly, so that we commenced hand ventilating. After another 1 hour, 57 minutes of hand ventilation with the same fresh gas flow and FiO,, the cylinder’s pressure reached zero and the machine’s oxygen failure alarm sounded. The problem with an anesthetic machine that has a single oxygen cylinder is that the machine will be disabled by its fail-safe mechanism once the oxygen cylinder is completely empty, unless there is an alternative supply of air. Thus there is a potential for losing control of the anesthetic, or worse still, losing the ability to ventilate with oxygen. Since there is no pipeline failure alarm as such, it is possible (if the oxygen cylinder is turned on) that the first audible warning of pipeline failure and low cylinder pressure may be the low oxygen pressure alarm of the ventilator, assuming a ventilator is being used. If a ventilator is not being used, the situation could be worse in that there will be no audible warning of failure until both pipeline and cylinder have failed. Ideally, the anesthesiologist will, of course, notice a change in gauge pressure if the pipeline fails, before an audible alarm is needed. Although there is a considerable reserve of oxygen in a cylinder once it no longer contains enough pressure to drive a ventilator, the anesthesiologist and patient are vulnerable if the patient is paralyzed, the ventilator is disabled, and the only oxygen cylinder is nearly empty. The anesthesiologist will be preoccupied with hand ventilation, and if the cylinder empties the machine itself will be disabled. In a remote location, especially at night, technical help may not be readily available. We suggest the following precautions when an anesthetic machine with a single oxygen cylinder is used, particularly in a remote location: Be certain that the oxygen cylinder is full before starting a case, and that spare cylinders are available nearby. Turn off the oxygen cylinder so that the oxygen failure alarm will warn of pipeline failure. In the event of pipeline failure, regard the cylinders as a reserve system only and hand ventilate whenever possible in order to conserve oxygen. Alert the engineers so that pipeline gas can be restored as soon as possible. In the meantime, prepare a free-standing oxygen cylinder and Ambu bag for use in the event of
machine failure, and prepare to maintain travenously in case of machine failure.
anesthesia
in-
situation and frequent suctioning before large droplets condensation form, should minimize this problem.
William McDade, MD Resident in Anesthesiology Jane C. Ballantyne, MB BS, FRCA Assistant in Anesthesia Department of Anesthesia Massachusetts General Hospital Boston, MA 02114
of
Yolanda R. Leal Certified Registered Nurse-Anesthetist Mark D. Johnson, MD Chairperson Department of Anesthesiology Melrose Wakefield Hospital Melrose. MA
PI1 SO952-8180(96)00023-2
References
Laryugospasm Associated with Humidifier with the Laryngeal
Use of a Passive Mask Airway
1. Koehli N: Aspiration and the laryngeal mask airway [Letter]. Ana&hem 1991;46:419. 2. Michel MZ, Stubbing JF: Laryngeal mask airway and laryngeal spasm [Letter]. Anaathaia 1991;46:71. PII SO952-8180(96)00023-2
To the Editor: Laryngospasm associated with the use of a laryngeal mask airway (LMA) has been reported.‘,’ The use of a passive humidifier with an LMA, as with an endotracheal tube (ETT), can minimize heat loss and help maintain humidification of inspired gases. However, caution should be exercised when employing this technique. Moisture can accumulate in the inner lumen of the LMA and then trickle down into the larynx causing laryngospasm. The following case illustrates this point. A healthy, 24year-old female was scheduled for arthroscopy. The anticipated length of the procedure was less than one hour. The anesthetic plan included the use of an LMA with spontaneous ventilation. A passive humidifier was placed in line immediately after insertion of the LMA. Condensation was observed in the LMA 90 minutes into the procedure. As we prepared to suction the LMA, the surface tension of the water was altered, resulting in a bolus of condensation into the larynx. The patient immediately coughed and developed profound laryngospasm. Attempts to ventilate through the LMA were unsuccessful due to high airway resistance. Positive pressure was maintained with 100% oxygen; however, the laryngospasm did not resolve. Succinylcholine 30 mg was then administered intravenously. The airway improved and the procedure was completed uneventfully. The patient experienced no complications from the laryngospasm. When a passive humidifier is used in conjunction with an ETT, laryngospasm is not a consideration, as the ETT is positioned in the trachea (through the vocal cords). When a passive humidifier is used with an LMA, condensation in the LMA can trigger laryngospasm if a large droplet comes in contact with sensitive structures. The incidence of this complication probably increases in relation to the duration of the case (as more condensation accumulates). Depending on the patient’s positioning for surgery, it is usually not practical to position the LMA such that condensation would drain proximally. Therefore, if a passive humidifier is used with an LMA, awareness of the
A Method Anesthesia
and Apparatus for Testing the Machine Circuit Unidirectional
Values
To the Editor: The safe delivery of anesthesia requires a properly functioning anesthesia machine and circuit (i.e., delivery system).* Prior to use, an anesthesia delivery system should undergo a daily check and a subsequent case check.? One of the purposes of this check is to assure the proper functioning of the inspiratory and expiratory unidirectional valves in the anesthesia circuit. Neither the Food and Drug Administration nor the American Society of Anesthesiologists has defined the proper method for verifying the proper functioning of these valves. One recommendation for the circuit
valve
test indicated
a method
of alternately
disconnecting the inspiratory and expiratory limbs of the anesthesia circuit and then breathing into them as a check for the proper function of the unidirectional valves.’ With the fear of acquired immunodeficiency syndrome (AIDS), hepatitis, and respiratory disease, many anesthetists do not perform these important valve tests, fearing disease transmission while exhaling and inhaling through the circle system. Of note, whether there is adequate documentation that disease
transmission
can or does occur
from
inhaling
*Cooper JB: Anesthesia machine-related injuries: prevention and inFoundation Newsletter. Park Ridge, vestigation. Anesthesia Patient Safety IL: American Society of Anesthesiologists, 1993;57:11:18-21. tFDA publishes final version of revised apparatus checkout. Anesthesia Patient Safety Foundation Newsletter. Park Ridge, IL: American Society of Anesthesiologists, Fall, 1994. $Podraza AG, Salem MR, Joseph NJ, Brenchley JL: Rebreathing due to incompetent unidirectional valves in the circle absorber system [Abstract]. Anesthesiology 1991;75:A422.
J. Clin. Anesth., vol. 8, May 1996
261
Potential Risk Associated with Anesthesia with Single Oxygen Cylinders
Machines
To the Editor: The following incident brought to our attention the vulnerability of anesthesia systems with only a single oxygen cylinder. An adult patient was undergoing cerebral angiography in the X-ray department under general endotracheal anesthesia, paralyzed and ventilated. The anesthetic machine had a single oxygen cylinder yolk. During the procedure, an X-ray boom hit, disconnecting and breaking the oxygen pipeline supplying the anesthetic machine. Because the oxygen cylinder on the machine was closed, the oxygen failure alarm sounded immediately, and the oxygen supply to the machine was quickly restored by opening the cylinder. For the 13 minutes that it took to summon technical help and to restore pipeline oxygen, the cylinder supplied oxygen both to the fresh gas inlet and to drive the gas driven ventilator. After restoration of pipeline oxygen, the anesthetic proceeded uneventfully. This incident passed without consequence, because technical help was readily available and the pipeline was quickly repaired. But the potential for mishap was revealed. Most anesthetic ventilators in the United States are driven by oxygen, usually piped oxygen, using approximately 10 liters per minute. A full oxygen cylinder is capable of driving a ventilator, but once it has emptied to about the I/ mark (500 psi; 34.5 x 100 kPa) it no longer contains enough pressure to do so. [Note that although this pressure is still greater than pipeline pressure (60 psi; 4.1 x 100 kPa) , the partially full cylinder, unlike the pipeline, is unable to sustain pressure against a high demand beyond the machine’s pressure regulator, as placed on it by driving a ventilator.] It is easy to calculate that, after approximately 45 minutes, a full tank (659 liters) that has driven a ventilator (10 liters per minute) as well as provided fresh gas flow (approximately 1 liter per minute), will be :fi empty and no longer able to drive the ventilator, although it will still have a reserve of about 165 liters (over 2 hours use at 1 liter per minute). In order to verify these calculations, we conducted an anesthetic using full cylinders of oxygen and nitrous oxide (no piped gases) and an Ohmeda 7000 series ventilator with standard settings (a 3
260
J. Clin. Anesth., vol. 8, May 1996
liter fresh gas flow with a 29% FiO, and an 8.8 liter minute ventilation). After 52 minutes, at a cylinder pressure of 450 psi (31 x IO0 kPa), the low oxygen supply pressure alarm sounded and the ventilator ceased to function properly, so that we commenced hand ventilating. After another 1 hour, 57 minutes of hand ventilation with the same fresh gas flow and FiO,, the cylinder’s pressure reached zero and the machine’s oxygen failure alarm sounded. The problem with an anesthetic machine that has a single oxygen cylinder is that the machine will be disabled by its fail-safe mechanism once the oxygen cylinder is completely empty, unless there is an alternative supply of air. Thus there is a potential for losing control of the anesthetic, or worse still, losing the ability to ventilate with oxygen. Since there is no pipeline failure alarm as such, it is possible (if the oxygen cylinder is turned on) that the first audible warning of pipeline failure and low cylinder pressure may be the low oxygen pressure alarm of the ventilator, assuming a ventilator is being used. If a ventilator is not being used, the situation could be worse in that there will be no audible warning of failure until both pipeline and cylinder have failed. Ideally, the anesthesiologist will, of course, notice a change in gauge pressure if the pipeline fails, before an audible alarm is needed. Although there is a considerable reserve of oxygen in a cylinder once it no longer contains enough pressure to drive a ventilator, the anesthesiologist and patient are vulnerable if the patient is paralyzed, the ventilator is disabled, and the only oxygen cylinder is nearly empty. The anesthesiologist will be preoccupied with hand ventilation, and if the cylinder empties the machine itself will be disabled. In a remote location, especially at night, technical help may not be readily available. We suggest the following precautions when an anesthetic machine with a single oxygen cylinder is used, particularly in a remote location: Be certain that the oxygen cylinder is full before starting a case, and that spare cylinders are available nearby. Turn off the oxygen cylinder so that the oxygen failure alarm will warn of pipeline failure. In the event of pipeline failure, regard the cylinders as a reserve system only and hand ventilate whenever possible in order to conserve oxygen. Alert the engineers so that pipeline gas can be restored as soon as possible. In the meantime, prepare a free-standing oxygen cylinder and Ambu bag for use in the event of
machine failure, and prepare to maintain travenously in case of machine failure.
anesthesia
in-
situation and frequent suctioning before large droplets condensation form, should minimize this problem.
William McDade, MD Resident in Anesthesiology Jane C. Ballantyne, MB BS, FRCA Assistant in Anesthesia Department of Anesthesia Massachusetts General Hospital Boston, MA 02114
of
Yolanda R. Leal Certified Registered Nurse-Anesthetist Mark D. Johnson, MD Chairperson Department of Anesthesiology Melrose Wakefield Hospital Melrose. MA
PI1 SO952-8180(96)00023-2
References
Laryugospasm Associated with Humidifier with the Laryngeal
Use of a Passive Mask Airway
1. Koehli N: Aspiration and the laryngeal mask airway [Letter]. Ana&hem 1991;46:419. 2. Michel MZ, Stubbing JF: Laryngeal mask airway and laryngeal spasm [Letter]. Anaathaia 1991;46:71. PII SO952-8180(96)00023-2
To the Editor: Laryngospasm associated with the use of a laryngeal mask airway (LMA) has been reported.‘,’ The use of a passive humidifier with an LMA, as with an endotracheal tube (ETT), can minimize heat loss and help maintain humidification of inspired gases. However, caution should be exercised when employing this technique. Moisture can accumulate in the inner lumen of the LMA and then trickle down into the larynx causing laryngospasm. The following case illustrates this point. A healthy, 24year-old female was scheduled for arthroscopy. The anticipated length of the procedure was less than one hour. The anesthetic plan included the use of an LMA with spontaneous ventilation. A passive humidifier was placed in line immediately after insertion of the LMA. Condensation was observed in the LMA 90 minutes into the procedure. As we prepared to suction the LMA, the surface tension of the water was altered, resulting in a bolus of condensation into the larynx. The patient immediately coughed and developed profound laryngospasm. Attempts to ventilate through the LMA were unsuccessful due to high airway resistance. Positive pressure was maintained with 100% oxygen; however, the laryngospasm did not resolve. Succinylcholine 30 mg was then administered intravenously. The airway improved and the procedure was completed uneventfully. The patient experienced no complications from the laryngospasm. When a passive humidifier is used in conjunction with an ETT, laryngospasm is not a consideration, as the ETT is positioned in the trachea (through the vocal cords). When a passive humidifier is used with an LMA, condensation in the LMA can trigger laryngospasm if a large droplet comes in contact with sensitive structures. The incidence of this complication probably increases in relation to the duration of the case (as more condensation accumulates). Depending on the patient’s positioning for surgery, it is usually not practical to position the LMA such that condensation would drain proximally. Therefore, if a passive humidifier is used with an LMA, awareness of the
A Method Anesthesia
and Apparatus for Testing the Machine Circuit Unidirectional
Values
To the Editor: The safe delivery of anesthesia requires a properly functioning anesthesia machine and circuit (i.e., delivery system).* Prior to use, an anesthesia delivery system should undergo a daily check and a subsequent case check.? One of the purposes of this check is to assure the proper functioning of the inspiratory and expiratory unidirectional valves in the anesthesia circuit. Neither the Food and Drug Administration nor the American Society of Anesthesiologists has defined the proper method for verifying the proper functioning of these valves. One recommendation for the circuit
valve
test indicated
a method
of alternately
disconnecting the inspiratory and expiratory limbs of the anesthesia circuit and then breathing into them as a check for the proper function of the unidirectional valves.’ With the fear of acquired immunodeficiency syndrome (AIDS), hepatitis, and respiratory disease, many anesthetists do not perform these important valve tests, fearing disease transmission while exhaling and inhaling through the circle system. Of note, whether there is adequate documentation that disease
transmission
can or does occur
from
inhaling
*Cooper JB: Anesthesia machine-related injuries: prevention and inFoundation Newsletter. Park Ridge, vestigation. Anesthesia Patient Safety IL: American Society of Anesthesiologists, 1993;57:11:18-21. tFDA publishes final version of revised apparatus checkout. Anesthesia Patient Safety Foundation Newsletter. Park Ridge, IL: American Society of Anesthesiologists, Fall, 1994. $Podraza AG, Salem MR, Joseph NJ, Brenchley JL: Rebreathing due to incompetent unidirectional valves in the circle absorber system [Abstract]. Anesthesiology 1991;75:A422.
J. Clin. Anesth., vol. 8, May 1996
261