Air Embolism and Other Accidents Using Pump Oxygenators

Transcription

1 Air Embolism and Other Accidents Using Pump Oxygenators William S. Stoney, M.D., William C. Alford, Jr., M.D., George R. Burrus, M.D., David M. Glassford, Jr., M.D., and Clarence S. Thomas, Jr., M.D. ABSTRACT A survey of 9 cardiac surgeons showed that during a six-year period, a pump oxygenator accident serious enough to cause patient injury or death occurred once per,000 procedures. A total of 26 deaths occurred as a direct result of an accident. Air embolism and disseminated intravascular coagulation were the two most common problems. Low-level alarm systems were reported to be used by 2% of the respondents and activated clotting times were used by 6%. Rigorous use of alarm systems and heparin monitoring could reduce the incidence of pump-related accidents. Accidents that are directly related to the misuse or mechanical failure of pump oxygenators have rarely been reported in the past [ll. On the other hand, the surgical literature contains a number of reports describing cerebral air embolism secondary to residual intracavitary air after valve replacement or repair of congenital defects [,6. A few recent reports advocate the use of hyperbaric treatment for air embolism but fail to discuss prevention of the event [7,8. The actual frequency of accidents related to the use of pump oxygenators is not known. Recently, we were involved in the medical and medicolegal aspects of this problem when a patient in our hospital was injured by accidental arterial line air embolism. To estimate the frequency of this problem, we sent a questionnaire concerning pump oxygenator accidents to other surgical groups. We found that the incidence of an accident serious enough to produce patient injury or death occurs about once in,000 procedures. From the Division of Cardiac Surgery, St. Thomas Hospital, and the Department of Thoracic and Cardiovascular Surgery, Vanderbilt University Medical School, Nashville, TN. Presented at the Twenty-sixth Annual Meeting of the Southern Thoracic Surgical Association, Nov -,979, San Antonio, TX. Address reprint requests to Dr. Stoney, 20 Harding Rd, Suite 50, Nashville, TN Material and Methods A questionnaire concerning techniques, safety precautions, and accidents related to pump oxygenators used in cardiopulmonary bypass operations was mailed to,700 cardiac surgeons primarily in the United States and Canada. The questionnaire asked about accidents during the six-year period from 972 through 977. A stamped return envelope was included as well as a cover letter explaining the purpose of the study and a provision for an anonymous reply. We received back of the questionnaires. Of these, 92 could not be used principally because the responding surgeon was not currently engaged in cardiac operations or the questionnaire was incomplete for other reasons. Therefore, 9 completed responses form the basis for this report. Results During the six-year period from 972 through 977, our respondents performed 7,89 operations using a pump oxygenator. In 977 alone, they did 98,602 procedures. This number includes congenital, valve replacement, and coronary artery operations. The average number of operations for the 9 respondents in 977 was 28, with the highest being,07 and the lowest, 20. Perfusionists The total number of perfusionists used by the respondents was 965, and of these, 62 (66.5%) were certified. Seventy-six percent of the respondents stated that the perfusionists were employed by a hospital, 5% stated that the perfusionists were employed by a surgeon or group of surgeons, and 6.% replied that the perfusionists were self-employed. The other % reported various combinations of employment. Fifty-seven percent of the respondents used one perfusionist per operation, % used by William S. Stoney

2 7 Stoney et al: Air Embolism and Pump Oxygenators two perfusionists, and 0% reported one or two per operation. Alarms and Activated Clotting Time A total of 2 respondents answered the question on low-level alarm systems. Of these, 6 (2%) used an alarm system and 85 (58%) did not. Of the 6 using an alarm, 65 (8%) used an audible alarm plus an automatic shutoff for the pump oxygenator. Those using an alarm reported 96 episodes of air embolism, and those not using an alarm reported 229 episodes. The question concerning the use of activated clotting times (ACT) during bypass had 25 responses. Two hundred four respondents (6%) used ACTs and 2 respondents (7%) did not. Those using ACTs reported episodes of disseminated intravascular coagulation (DIC) and those who reported no use of ACTs reported only 57 episodes. Accidents The questionnaire included several questions concerning accidents related to the pump oxygenator. These inquired about the number of episodes of arterial line air embolism, mechanical failure of the pump, electrical failure of the pump, DIC during or after bypass, and oxygenator failure. The incidence of permanent injury or death resulting from an accident was explored. The results of this part of the questionnaire are summarized in Table. There were a total of,9 accidents resulting in 00 permanent injuries and 26 deaths. Air embolism from the arterial line caused 92 deaths and DIC, 6. Mechanical, electrical, or oxygenator failure was the cause of 9 deaths. Table 2. Miscellaneous Accidents Accident ~ Ruptured arterial line or connector Air pumped into left ventricle through vent (reversed tubing) Oxygenator Leaked Clotted Pressurized Burst Contaminated Chemical injury Overheated blood Unable to rewarm Rupture of pulsatile assist device Reversed arterial and venous lines Total Incidence The final question asked if there were any other accidents related to the pump oxygenator not covered in the questionnaire. A multitude of such problems were reported, one of the most common accidents being reversal of the tubing connected to the ventricular sump. This causes air to be pumped into the left ventricle. This accident was reported eighteen times. Various problems related to the oxygenator were mentioned as well as rupture of the arterial line or arterial line connectors. Rupture of a pulsatile assist device resulted in air embolism on three occasions. The miscellaneous accidents are summarized in Table 2. The following case reports illustrate three accidents associated with a pump oxygenator. All occurred at St. Thomas Hospital. Table. Survey of Accidents during 972 through 977 among 75,000 Patients Accident Arterial line air embolism 29 Disseminated intravascular coagulation 72 Electrical failure 25 Mechanical failure Oxygenator failure 2 Total,9 Permanent Incidence Iniuries Deaths

3 8 The Annals of Thoracic Surgery Vol 29 No April 980 Case Reports Patient A 62-year-old man with unstable angina of three weeks' duration was admitted to our hospital. Arteriography showed severe stenosis of the circumflex, anterior descending, and right coronary arteries. Using cardiopulmonary bypass, vein bypass grafts were sutured to the circumflex and right coronary arteries. While the left mammary artery was being anastomosed to the anterior descending coronary artery, the arterial line suddenly was filled with air. The pump was stopped, and all possible air was evacuated from the aorta. The lines were refilled and the operation completed. The perfusionist had failed to notice that the oxygenator reservoir was empty and had not used an available low-level alarm. The patient was decerebrate for ten days but ultimately regained speech and motor function. A negligence claim was brought against the hospital and surgeon and was settled out of court. Five years later, the patient was well but still had memory impairment. COMMENT. It is probable that the perfusionist failed to notice a decrease in the volume of blood in the reservoir during rewarming and allowed the oxygenator to be pumped dry. An alarm would have prevented the accident. Patient 2 A 6-year-old woman with angina and transient episodes of cerebral ischemia was hospitalized. Arteriograms showed stenosis of the left internal carotid artery, the left main coronary artery, and the right coronary artery. A combined operation was done. Prior to occlusion of the carotid artery, 5,000 U of heparin was administered and after the carotid endarterectomy, 50 mg of protamine. The heart was then exposed by a sternotomy and after the patient was heparinized, three bypass grafts were constructed using cardiopulmonary bypass. At the completion of the procedure, the blood did not coagulate after the administration of protamine. Hematological studies showed thrombocytopenia, elevated partial thromboplastin time, decreased fibrinogen, and elevated fibrin degradation products, all indicating severe DIC. In spite of administration of heparin and multiple transfusions of platelets, fresh plasma, and blood, the patient continued to bleed and died forty-eight hours later. ACTS were not used during the operative procedure. COMMENT. It is probable that the protamine neutralized some of the heparin given at the onset of bypass, and the operation was done with inadequate heparinization, which caused DIC. An ACT would have identified the problem. Patient A 68-year-old woman with mitral insufficiency and congestive heart failure was hospitalized. She underwent mitral valve replacement without incident. At the completion of the operation, the final 200 ml of blood in the reservoir was being given slowly. The reservoir level was below the low-level alarm sensor, which had been switched off. As the perfusionist reached across the console to adjust an intravenous line, he inadvertently advanced the arterial pump control knob, emptying the reservoir and pumping air into the aorta. The head of the patient was lowered, the aorta was opened, and all air was evacuated. After a brief period of hypotension, the blood pressure returned to normal and the operation was completed. The patient never regained consciousness and died one week later. The family was given full information about the death. COMMENT. This accident occurred after the low-level alarm had been switched off. An airactivated valve would have prevented the accident. Comment The data for this study were obtained by questionnaire, and the respondents were asked to supply information based on memory rather than a review of past experience. Therefore, this information must be, at best, an approximation of the number of accidents that have occurred. It is probable that the results reported represent an underestimation since the most dramatic and worst events are the ones most frequently remembered and since many minor events were not included. Another source of error could be double reporting of the same accidents since individuals were polled rather

4 9 Stoney et al: Air Embolism and Pump Oxygenators than hospitals. Such double reports were eliminated when recognized. For the year 977, the respondents from the United States reported a total of 90,000 openheart operations. The Commission on Professional and Hospital Activities in Ann Arbor, MI, estimates that in the same year, 95,000 open-heart operations were done in the United States. Our study, therefore, probably included the great majority of open-heart procedures performed during the years considered. The most sudden and distressing accident that can occur during cardiopulmonary bypass is arterial line air embolism. Disc oxygenators, commonly used in the 960s, required a generous reserve of blood to achieve oxygenation. This reserve gave some assurance that the oxygenator would not be totally emptied. Plastic bag oxygenators also had some safeguard against air since they collapsed if the reserve was pumped out. However, the rigid shell bubble oxygenators now in use do not have a separate visible oxygenating chamber and can be pumped dry in about thirty seconds if all inflow is interrupted. The use of an alarm system is of primary importance with these oxygenators. Three types of alarms are available. A photoelectric cell can be positioned on the reservoir to detect when the level drops below the preselected reserve and then give an audible signal. This cell can also be connected to the circuit of the console to shut off the arterial pump at the same time. One manufacturer uses a variation of the reservoir alarm. In this system, the reservoir is weighed continuously. If it becomes too light, the pump is slowed automatically to allow the reservoir to regain a safe level. A second device available is an air-activated ball valve that can be placed at the outlet of the oxygenator.* The valve remains open as long as it is filled with fluid but will close if air enters the chamber of the valve. Its two big advantages are that it is inexpensive and entirely mechanical, not dependent on any electrical circuitry. The third alarm system available is an infrared light sensor that is placed on the arterial line and can detect or 2 cm of air. This device has an au- *Delta Medical Industries, Costa Mesa, CA. tomatic shutoff to the arterial pump. After a review of our own experience and the experience of others, it is our strong recommendation that each pump oxygenator be equipped with at least two of these systems. A photosensor on the reservoir, which can set off an alarm and shut off the pump, plus an air-activated mechanical valve provide reasonable safety. When massive air embolism in the ascending aorta occurs, several maneuvers should be initiated immediately. The pump should be stopped and the aorta opened by either a stab wound or removal of the aortic cannula. The head of the patient should be lowered, and the pericardium filled with saline solution until the aortotomy is flooded. Massage of the heart and great vessels will help dislodge trapped air. The patient should receive 00% oxygen to decrease the nitrogen content of the lungs and increase the nitrogen gradient between blood and lungs. It has been suggested that gentle retrograde perfusion into the superior vena cava may help flush air from smaller cerebral vessels [. As much time as is available should be used in displacing the intravascular air before resuming bypass. Subsequent measures that have been useful include intravenous steroids, mild postoperative hypothermia, and hyperbaric treatment. Of these, hyperbaric treatment appears to be the most effective. Only 2% of the respondents used a low-level alarm system, and only 20% used an alarm system equipped with an automatic shutoff. It is curious that the incidence of arterial line air embolism is about the same among alarm users and those who use no alarms. In part, this may be due to the fact that episodes of air embolism were reported for a six-year period, but the lapse in time during that period before alarms were used was not documented. For example, in our practice, a rigid requirement that perfusionists use an alarm was not instituted until after an embolic accident had occurred. Two of the major manufacturers of pump oxygenators now include low-level alarm and shutoff systems as mandatory equipment for each machine purchased. It is probable that this has been brought about by product liability lawsuits against the manufacturers after air embolism accidents. The remaining manufac-

5 0 The Annals of Thoracic Surgery Vol 29 No April 980 turers of pump oxygenators recently surveyed do not require alarm systems with each pump console but offer them as options. In this survey, two other sources of air embolism accidents were reported. The accidental reversal of the ventricular suction tubing in the pump head produced air embolism in 8 patients. Therefore, it is recommended that large red or orange markers be added to each pump head to clearly designate flow direction. The arrows indicating flow direction on most consoles are small and neat but are not large enough to be seen at a glance. The second unexpected source of air embolism was a ruptured pulsatile assist device in patients. Since this is not a widely used technique, these episodes of failure would suggest that this device is not yet entirely safe. Disseminated intravascular coagulation is not as clearly defined an accident of cardiopulmonary bypass as air embolism. However, it is likely that the majority of episodes of DIC that occur in the operating room are secondary to inadequate heparinization during bypass [9. Other factors such as sepsis, transfusion reactions, exposure of blood to plastic surfaces, and low cardiac output are all known to produce DIC. In 979, it was demonstrated that the individual response to a calculated dose of heparin shows wide variations and that a small percentage of patients will receive inadequate heparin for bypass when a calculated dose is used [2. In addition, there is considerable individual variation in heparin consumption during bypass [5]. The use of ACT will identify those patients who have inadequate heparinization following a calculated dose and will identify the need for additional heparin as bypass proceeds. In contrast to low level alarms, 6% of the respondents use ACTs, although the method has been advocated only quite recently. Again, it is our opinion that all patients undergoing cardiopulmonary bypass should be carefully monitored by ACTs or some other technique of heparin titration. A particularly dangerous situation is one in which bypass is reinstituted after protamine has been given. ACT or heparin titration is absolutely mandatory in these circumstances. We conclude that most accidents caused by air embolism from the arterial line are due to human error and could be virtually eliminated by using a combination of both electrical and mechanical shutoff devices. It is probable that many deaths and injuries secondary to DIC during cardiopulmonary bypass could be eliminated by consistent use of ACTs or heparin titration. An awareness of the multitude of other problems that can arise is a step toward prevention. This survey shows that an accident related to the pump oxygenator system occurred once per 00 procedures. An accident resulting in permanent injury or death of a patient occurred once per,000 procedures. Appropriate use of alarms and some method of heparin titration could greatly reduce the incidence of this problem. References. Kurusz M, Shaffer CW, Christman EW, et al: Runaway pump head: new cause of gas embolism during cardiopulmonary bypass. J Thorac Cardiovasc Surg 77:792, Mabry CD, Read RC, et al: Identification of heparin resistance during cardiac and vascular surgery. Arch Surg :29, 979. Mills NL: Massive air embolism during cardiopulmonary bypass: causes and management. To be presented at the Annual Meeting of the American Association for Thoracic Surgery, San Francisco, CA, April 28, 980. Nicks R: Arterial air embolism. Thorax 22:20, Roth JA, Cukingnan RA, Scott CR: Use of activated coagulation time to monitor heparin during cardiac surgery. Ann Thorac Surg 28:69, Silverstein A, Krieger HP: Neurologic complications of cardiac surgery. Arch Neurol 5:5, Steward D, Williams WG, Freedom R: Hypothermia in conjunction with hyperbaric oxygenation in the treatment of massive air embolism during cardiopulmonary bypass. Ann Thorac Surg 2: 59, Winter PM, Alvis HJ, et al: Hyperbaric treatment of cerebral air embolism during cardiopulmonary bypass. JAMA 25:786, Young JA, Kisker CT, Doty DB: Adequate anticoagulation during cardiopulmonary bypass determined by activated clotting time and the appearance of fibrin monomer. Ann Thorac Surg 26:2, 978