High Technology and Embolism: Drs. Layon and Mahla reply

Abstract

We read with interest the letter from Dr. van der Linden, who raises an interesting point of interpretation about the editorial on hyperbaric oxygen treatment for cerebral air embolism, particularly about the following sentence: “Thus, no pathognomonic sign or high-technology study allows the definitive diagnosis of cerebral arterial gas embolism.” Dr. van der Linden interpreted this sentence to imply that no “real-time” method can detect cerebral emboli; however, in the context of the editorial, this sentence meant that cerebral air embolism is usually diagnosed clinically rather than by a high-technology test. The references cited by Dr. van der Linden suggest that emboli may be detected with transcranial Doppler ultrasonography. For example, Spencer and associates1Spencer MP Thomas GI Nicholls SC Sauvage LR Detection of middle cerebral artery emboli during carotid endarterectomy using transcranial Doppler ultrasonography.Stroke. 1990; 21: 415-423Crossref PubMed Scopus (478) Google Scholar showed that, when the middle cerebral artery was monitored by transcranial Doppler ultrasonography before an embolus formed, the gas or particulate material that embolized in that area was detected and that the artifact was distinguished from embolic signals. The authors also noted that emboli, as revealed by transcranial Doppler studies, do not necessarily progress to stroke. Indeed, gaseous emboli can be found by transcranial Doppler techniques during needle puncture of a vessel wall. In a study of 27 patients who underwent cardiopulmonary bypass, Padayachee and colleagues2Padayachee TS Parsons S Theobold R Linley J Gosling RG Deverall PB The detection of microemboli in the middle cerebral artery during cardiopulmonary bypass: a transcranial Doppler ultrasound investigation using membrane and bubble oxygenators.Ann Thorac Surg. 1987; 44: 298-302Abstract Full Text PDF PubMed Scopus (236) Google Scholar used noninvasive transcranial Doppler ultrasonography to monitor for irregularities in flow signals attributable to gaseous microemboli. These authors developed an index of gaseous microemboli—the microemboli index—by analyzing the first differential of the maximal frequency envelope of sonograms of the middle cerebral artery, by means of a microcomputer, during a period of 1 second. Normal fluctuation in this first differential was calculated in five patients who were anesthetized before cardiopulmonary bypass. The authors noted that the maximal deflection in the first differential occurred at systolic upstroke and was always less than 10 U; therefore, an abnormal microemboli index is defined as the number of first differential deflections that exceed 10 U. Thus, by using this method, the authors reported that membrane oxygenators resulted in less gaseous microemboli than did bubble oxygenators and therefore recommended the use of membrane rather than bubble oxygenators. Finally, van der Linden and Casimir-Ahn3Van der Linden J Casimir-Ahn H When do cerebral emboli appear during open heart operations? A transcranial Doppler study.Ann Thorac Surg. 1991; 51: 237-241Abstract Full Text PDF PubMed Scopus (192) Google Scholar monitored 10 patients who underwent open-heart surgical procedures and cardiopulmonary bypass by using membrane oxygenators without arterial filters to detect cerebral air emboli. Despite relatively rigorous attempts at deaerating, a 2-MHz pulsed transcranial Doppler ultrasound velocimeter that obtained signals of flow velocity from the middle cerebral artery detected emboli and showed that they were most numerous when the aortic cannula was inserted and when the empty, beating ventricle was filled. The investigators were also able to discern that aggressive deaerating maneuvers did decrease the amount of emboli. No neuropsychologic testing was performed on the patients involved in this study. Dr. van der Linden states that if monitoring is begun before cerebral air embolism occurs and if the embolus occurs in the monitored area, transcranial Doppler ultrasonography may indeed detect an embolus. If, however, the area where the cerebral air embolus occurs or flows to is not being monitored, the embolus will not be detected. Furthermore, background noise can hinder detection. Nonetheless, we appreciate and agree, in general, with the comments of Dr. van der Linden. Only by prospectively identifying the problem of gaseous cerebral air emboli and then randomizing animals to hyperbaric therapy (in animal models) or patients to conventional aggressive treatment (in clinical trials) can we determine the most appropriate manner to manage this potentially devastating complication of surgical intervention. High Technology and EmbolismMayo Clinic ProceedingsVol. 67Issue 4PreviewIn an editorial entitled “Hyperbaric Oxygen Treatment for Cerebral Air Embolism—Where Are the Data?” published in the June 1991 issue of the Mayo Clinic Proceedings (pages 641 to 646), Dr. Layon raises several important questions about the possibility of treating cerebral air embolism with hyperbaric oxygen. (The editorial was initiated by a report in which hyperbaric treatment of cerebral air embolism sustained during an open-heart surgical procedure seems to have been successful.1) One of the questions raised was “How is cerebral arterial gas embolism diagnosed?” and Dr. Full-Text PDF