ANGLETON, Tex., Aug. 8 (AP) - The head pathologist at Baylor College of Medicine told jurors in the nation's first Vioxx-related civil trial Monday that clogged arteries led to a Texas man's death from an irregular heartbeat.

The pathologist, Dr. Thomas M. Wheeler, testified for the defense of Merck & Company, the maker of Vioxx, and his statements countered those of the pathologist who performed the autopsy, Dr. Maria M. Araneta. She said a heart attack more than likely caused the arrhythmia death of the Texas patient, Robert Ernst.

"Do we know in this case what caused the fatal arrhythmia?" a Merck lawyer, Gerry Lowry, asked.

"Yes, the severe atherosclerosis," Dr. Wheeler replied, using the medical term for blocked arteries.

The cause of Mr. Ernst's death lies at the center of the case. The plaintiff, Mr. Ernst's wife, Carol, claims that his fatal arrhythmia was caused by a Vioxx-induced heart attack. Dr. Araneta testified last week that the arrhythmia was very probably set off by a blood clot that caused a heart attack, but that Mr. Ernst died too quickly for his heart to show damage.

But neither her autopsy report nor Mr. Ernst's death certificate mention a heart attack or blood clot. Merck has relied heavily on those documents to corroborate its contention that no heart attack was involved.

Merck withdrew Vioxx from the market in September when a study showed that the once-popular painkiller doubled the risk of heart attack or stroke if taken for 18 months or longer. Carol Ernst contends Merck knew years ago that Vioxx could be dangerous but minimized those concerns.

Merck contends the company acted responsibly and disclosed research. Merck also claims no studies link Vioxx to arrhythmia, so the drug could not have caused Mr. Ernst's death.

In opening statements nearly a month ago, a Merck lawyer, David C. Kiernan, praised Dr. Araneta's unbiased view, saying, "No one can argue that she was tainted by the influence of one side or the other."

That was before she testified, however, and Merck lost efforts to block her appearance.

The case is the first of more than 4,200 Vioxx-related state and federal lawsuits pending against Merck to go before a jury. Two other state cases, in South Texas and New Jersey, are scheduled for trial next month, and the first federal case will go to trial in New Orleans in late November. Merck has vowed to fight them all.