Carbon Dioxide Flooding the Operative Field to Minimize or Prevent Air Embolism During Open Heart Operations.
Bennet A. Mitchell
Bennet A. Mitchell
Background
Cerebral arterial air embolism during open heart surgery is not an uncommon occurrence and may be responsible for neuropsychological deficit and /or death. Early experiments with bubble oxygenators suggested that gas embolism was the apparent cause of unexplained damage, after extracorporeal circulation. The danger of air embolism depends on the nature of the gas (carbon dioxide, oxygen, or air), the site of entry and size of bubbles [1].
The problem of air embolism and its fatal consequences has long been a concern to surgeons and physycyans even before the advent of cardiopulmonary bypass. In 1885, Dr. Nocholas Senn, wrote a series of articles [2, 2a], concerning the fatal consequences of air entering the vascular system.
He wrote:
"Sudden and unexpected death during an operatin is a calamity which never fails to strike terror in the heart of the boldest surgeon. I may add, one of the most uncontrollable causes of sudden death - I allude to the air embolism".
Prior to Nicholas Senn, Dr. John Baptist Morgagni, an 18th century physician and anatomist, in his treatise, "The seats and causes of diseases" [3], ascribes a quotation to Hippocrates in the 4th century BC, on air embolism. Hippocrates description of seeing patients die of apoplexy. In part he states,
"If much flatus or air, is dispersed through the whole body, the whole man is planet struck; if through a part that is blasted... Wen a great quantity is mixt with the blood...many obstructions happen in many places through-out the veins and its progress is checked for considerable time; the course of blood is interrupted thereby; for air stagnates in some places, and penetrates into some parts slowly, and others more quickly"
The purpose of this paper is to describe the technique of flooding the operative field with carbon dioxide, when it is combined with present de-airing techniques. The results of some significant studies will be compared with nationally reported results, as indicated by neurologic events and mortality of open heart surgery. For readers that are interested in all phases of air embolism, there are several excellent reviews written by Butler, et al [4], Peirce [5], Utley et al [6], Mills et al [7], and many other investigators.
Introduction
The first reported accident of air embolism apparently occurred in 1951, when Dennis and colleagues [8], attempted to close an atrial septal defect in a child. The perfusionist failed to turn on the level sensing circuit to the arterial reservoir, and massive amounts of oxygen entered the aorta. The led one pionner in open heart surgery to say "as long as open heart surgery is been performed there will always be the problem of air embolism".
Several methods were then suggested to remove air trapped in the cardiac chambers and great vessel. Gibbon et al [9], in 1954, suggested left ventricular venting via the apex.. Senning, et al [10], suggested elective ventricular fibrillation. Roe, et al [11], suggested clamping the aorta and pulmonary artery. Needle vents were inserted in the proximal aorta allowing blood and air to escape; also for aspirating residual air with low suction [12,13]. Another method used was placing the patient in Trendelenburg position, and rocking the operating table from side-to-side [14]. Kirklin and Barratt-Boyes [15], suggested raising filling pressures and compressing the left atrial appendage while vigorously shaking and massaging the left ventricle. Anesthesia simultaneously inflated the lungs and kept positive pressure on them to assist in expelling trapped air bubbles [16].
Swan, et al [17], proposed flooding the thoracic cavity with a crystalloid solution prior to closing the heart chambers, (it was jokingly referred to as closing the heart under water). Gallo, et al [18], proposed placing a shunt from aorta-to-right atrium in the cardiopulmonary bypass tubing and keeping it open for 3 to 5 minutes after termination of bypass to evacuate residual air after valvular operations. Many of these procedures are still performed today albeit, with some modifications.
Carbon Dioxide Flooding the Operative Field
Flooding the chest with carbon dioxide was suggested as a means of dispelling air trapped in the heart and cardiac chambers. The rationale for this, was based on the fact that the solubility of carbon dioxide in blood is 25 times greater in weight than air, and any gas bubbles entering the circulation will be dsiplaced by gravity, diminished or completely absorbed; thus, minimizing the adverse consequences of neurologic complications [19, 21, 24].
However, various investigators including, Ng, et al [22], Selman, et al [19], Moore, et al [23], and Nichols, et al [20], reported on various modes of using carbon dioxide. Roe, et al [24], injected 100 percent carbon dioxide directly into the distal aorta after circulatory arrest to remove trapped air. Olinger, et al [25], also used 100 percent carbon dioxide injected directly into the left ventricle during reoperative cases when adhesions made it impossible to use conventional de-airing protocols.
Burbank, et al [26], observed that as the pump suckers removed shed blood from the chest cavity, it contained high levels of carbon dioxide, which was returned to the extracorporeal circuit leading to a hypercapnoeic acidosis. Connor, et al [27], reported on the severe hypercarbia they observed during cardiopulmonary bypass when flooding the field with carbon dioxide.
Flooding the field with carbon dioxide went into disuse since there was no instrumentation available in the early days of CPB to measure and establish its impact on outcome. This gave rise to the present de-airing techniques adapted and modified by some surgeons. Rhe old carbon dioxide technique combined with the present de-airing protocols has been rekindled by several investigators, and its impact on outcome can now be assessed with modern instrumentation [28,29,30].
Technique for Flooding the Operative Field
Flooding the field with carbon dioxide is a simple technique, easily handled by the perfusionist. It does not obstruct or impede the surgeon's surgical progress. A carbon dioxide cylinder with a flow valve managed by the perfusionist, is connected with a short piece of three sixteen tubing to a sterile 0.3 micron gas filter (Texas Medical Products, Woodlands, TX.). A sterile intravenous tubing is connected to the filter, and its free end placed on the operative field. The tip of the tubing is placed at the level of the skin incision and is secure behind a pericardial tacking suture. The carbon dioxide is allowed to flow at the rate of 1 liter per minute during the time interval when the cardiac chambers or the ascending aorta are open to air.
Confirmation of Air Displacement
Air displacement can now be assessed by various means using intraoperative echocardiography, epicardial or transoesophageal; (TEE); also infrared capnography and oxygen analyzer devices.
When infrared capnography is used, the oxygen analyzer device is installed in the anesthesia ventilators, which allow for the relative measurement of carbon dioxide and oxygen. A sterile tubing from the operative field to the device is used to sample air from the operative field. The analyzer consistently confirms the presence of a high partial pressure of carbon dioxide, in excess of 100 mm of mercury, and low oxygen content, in the order of 1% to 2%, at the level of the heart and throughout the pericardial well. At the level of the skin incision, there is a transition zone, leading to a reading that corresponds to 100% room air 1 inch above the incision up to the level of the surgeon's eyes.
Deployment of Carbon Dioxide
After initiating cardiopulmonary bypass according to the operation to be performed, and prior to the cardiotomy or aortotomy, flooding the operative field begins by initiating the flow of carbon dioxide as previously described. Once the cardiac chambers or aorta are opened, suctioning of shed blood from the field is done only when it is necessary to remove excess blood from the surgeon's field of vision. After the cardiac chambers are closed and the conventional de-airing is performed, the carbon dioxide flow is stopped , and the tubing is removed from the field.
Results
Webb, and colleagues [31], reported their results on the management of 22 patients in which TEE was used to monitor the presence of air bubles after cardiac valvular procedures, with a carefully managed conventional de-airing protocol. They found persistent residual air bubbles which remained for 30 to 45 minutes. In 56 patients in which carbon dioxide flooding was used, air bubbles disappeared in less than 1 minute in 48, and in the remaining 8, air bubbles disappeared within 1 to 24 minutes. The efficacy or flooding the operative field with carbon dioxide was clearly demonstrated in this study.
Sarabu, and colleagues [32], performed 219 cardiac procedures in which the left heart or the ascending aorta were open to air. They used carbon dioxide to dispel residual air, and retrospectively examined its impact on the total neurologic complication and mortality rates for open heart operations, compared to the results reported by The Committee for the National Database for Thoracic Surgeons, U.S. [33].
There were three patients ( 1.37% ) that suffered neurologic complications, and there were two mortalities ( 0.91% ) in this series of 219 patients. Both occurred among the patients with neurologic complications. Both had severe artherosclerotic disase of the ascending aorta, which was attributed as the cause of the neurologic complications.
Their results were significantly lower when compared to rates of neurologic complications and mortality from open heart procedures reported in the Society of Thoracic Surgeons National Cardiac Surgery Database [33].
Comments
Shortly after the inception of open heart surgery, carbon dioxide was used as an adjuunct in one form or another to minimize or, completely reverse the adverse effects of air embolims. It went into disuse over the years for the reasons previously stated. However, several investigators are now reporting on its effectiveness, when it is used in conjunction with present de-airing protocols.
Flooding the field with carbon dioxide requires further detailed study, given the inadequacy of conventional de-airing techniques, that require several naneuvers [11], including some of the various procedures previously mentioned. The combination of carbon dioxide and some of the present techniques, may reduce the complications incurred with some of the present techniques. Carbon dioxide flooding the operative field may be also very useful in minimally-invasive surgery, when incisions are so small that may preclude any of the conventional de-airing techniques from being executed. Moreover, it can be very useful in redo cases, and most importantly, it prevents the associated neurologic sequelae.
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