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950 CORRESPONDENCE Ann Thorac Surg comparing incidences between two series with a similar range of follow-up. We submit that such figures can be compared directly as incidences with confidence intervals without recourse to repeated processing of the raw data and the application of sophisticated statistical analysis (such as the Mantel-Haenzel statistic for actuarial curves). We believe the reporting of linear- ized incidence rates should continue for all valve-related compli- cations. Edmunds and colleagues also recommend that if linearized rates are to be reported, they should be “reported with appro- priate confidence limits (eg, standard error).” Standard error is not an appropriate confidence limit for valve-related complica- tions. We consider these events to be Poisson parameters for which the appropriate measurement of confidence intervals has been described by Mulder in 1983 [5] and used by ourselves (61 in 1987. Edmunds and colleagues define nonstructural dysfunction as ”valve dysfunction exclusive of thromboembolism and infection diagnosed by reoperation, autopsy, or clinical investigation.” It is not clear whether the clause “diagnosed by. . .I’ refers to the defined dysfunction, its exclusions, or both. We would appreci- ate clarification of this point. They also recommend “valve- related mortality, including operative deaths and permanent disability” as a useful reporting combination. Disability is not mortality. We presume that what is meant by this grouping is the sum of the following events: operative mortality, late valve- related mortality, and permanent disability, but this is ambigu- ous in the above expression. These would be minor points in most scientific articles, but in an article that aims to promote standardized definitions and reporting methods, correct syntax and clarity are of crucial importance. With the exception of the above points, we are delighted that the guidelines appear to be in total agreement with our own guidelines published a year ago 171. Samer A. M . Nashef, FRCS W. H. Bain, MD, FRCS Department of Cardiac Surgey Western Infirma ry, Level 9 Glasgow GI1 6NT Scotland References 1. Edmunds LH Jr, Clark RE, Cohn LH, Miller DC, Weisel RD. Guidelines for reporting morbidity and mortality after cardiac valvular operations. Ann Thorac Surg 1988;46:257-9. 2. Edmunds LH Jr, Clark RE, Cohn LH, Miller DC, Weisel RD. Guidelines for reporting morbidity and mortality after cardiac valvular operations. J Thorac Cardiovasc Surg 1988;96351-3. 3. Clark RE, Edmunds LH Jr, Cohn LH, Miller DC, Weisel RD. Guidelines for reporting morbidity and mortality after cardiac valvular operations. Eur J Cardiothorac Surg 1988;2:293-5. 4. Guidelines for data reporting and nomenclature for the An- nals of Thoracic Surgery. Ann Thorac Surg 1988;46:260-1. 5. Mulder PGH. An exact method of calculating a confidence interval of a Poisson parameter. Am J Epidemiol1983;117377. 6. Nashef SAM, Sethia B, Turner MA, Davidson KG, Lewis S, Bain WH. Bjork-Shiley and Carpentier-Edwards valves: a comparative analysis. J Thorac Cardiovasc Surg 1987;93:394- 404. 7. Nashef SAM, Bain WH. Valve-related events: a system of definitions. Thorac Cardiovasc Surg 1987;35:232-4. Reply To the Editor: The Committee appreciates Dr Nashef and Dr Bain‘s interest in the guidelines and offers these comments to the points raised in their letter. Available information indicates that valve thrombosis and thromboembolism are different manifestations of the same pathological process. In one instance the thrombus migrates and in the other it does not. We agree the process is influenced by valve design and materials, but it is also influenced by other factors that are independent of the prosthesis (eg, cardiac output, infection, and anticoagulant drugs). For this reason we included valve thrombosis as a subheading under “thromboembolism.” In all likelihood a new means to prevent one manifestation will also prevent the other. However, if the guidelines are followed, reported incidences of valve thrombosis may be added to the complications reported under the heading ”structural deteriora- tion” to provide the data that Dr Nashef and Dr Bain desire. Unfortunately, calculations of linearized rates for various valve-related events are not applicable and frankly misleading if the events occur randomly or at nonconstant rates. Most valve- related events are not constant (with the possible exception of anticoagulant-related bleeding). Event history analysis of retro- spective data and calculation of hazard functions indicate the mathematical risk that an event will occur at various time intervals after operation or any other starting point. Hazard diagrams are more difficult to calculate and to compare, but provide more information and more reliable information than do calculations of linearized rates. The Committee feels that it is better to learn and use the more applicable statistics than to persist with the more familiar but misleading linearized rates. Autopsy, reoperation, and clinical investigation are the only methods available to determine valve dysfunction, thromboem- bolism, and infection and, therefore, the Committee fails to perceive the ambiguity. From a patient’s viewpoint reoperation (without death) and permanent disability are adverse outcomes of prosthetic valve operations. The suggested combinations list different adverse outcomes. The suggested list is not exhaustive, but is designed to emphasize unfavorable outcomes. A prosthetic valve that requires 50% of patients to have reoperation within 1 year or one that causes 50% of patients to suffer permanent strokes would not be popular even if the valve-related mortality rate was low. The Committee again thanks the large number of individuals who contributed to the development of the published guidelines. We regret any deficiencies, but appreciate the favorable com- ments that we have received and the widespread acceptance of the document. L. Henry Edmunds, Jr, MD, Chairman Richard E. Clark, M D Lawrence H. Cohn, M D D. Craig Miller, M D Richard D. Weisel, M D Ad Hoc Liaison committee for Standardizing Definitions of Prosthetic Heart Valve Morbidity The Society of Thoracic Surgeons and the American Association of Thoracic Surgery
