Fr. Rufino Sescon said the cardinal might be removed from the respirator as his condition steadily improves.
Reading a prepared statement, Sescon said Sin had suffered irregular heartbeat, not a heart attack as earlier reported.
Sin, 76, who retired in November 2003, was hospitalized briefly last year after having a seizure while at prayer.
He was rushed to the Cardinal Santos Medical Center in Greenhills, San Juan last Monday when his blood pressure dropped during dialysis treatment at his Villa San Miguel residence in Mandaluyong City for a long-standing kidney ailment.
"His vital signs are normal," Sescon said. "We cannot yet definitely say how long will he stay in the hospital."
Sescon said Sin was already conscious but could not freely talk because he was still on a respirator.
"The doctors are protecting his Eminence from developing possible infections. The possibility of his heart not working is still there," he stressed.
Sescon said Sin is doing well since he was able to recognize a brother and a nephew who visited him at the intensive care unit.
"He suffered from arrhythmia (irregular heart rate) during dialysis and subsequent hypotension and congestive heart failure," Sescon said in clarifying reports that Sin suffered a heart attack.
Hypotension is the medical term for low blood pressure and congestive heart failure means a weak heartbeat.
Sescon said the former Manila archbishop was even upbeat and in high spirits. Sin also ate a bowl of his favorite lugaw (congee), he said.
He added Sin is looking forward to inspecting the construction of the houses at the Cardinal Sin Village in Punta, Sta. Ana in Manila.
He said doctors and Sin’s kin have recommended that the prelate not receive visitors at the moment.
President Arroyo was reportedly among those who had wanted to immediately visit Sin but was discouraged because of his delicate condition.
To accommodate well-wishers and prayerful intentions, Sescon said a guest book will be opened at Villa San Miguel.
Peachy Yamsuan, a spokeswoman for the Manila Archdiocese, declined to disclose the nature of Sin’s illness, saying Church officials would make a statement later.
"He (Sin) was under sedation while doctors observed him. He was awake (yesterday morning) and I think that is a good sign," she said.
Manila Archbishop Gaudencio Rosales issued a circular asking the faithful to pray for Sin. The President sent her "concern and prayers" for Sin’s recovery.
In Asia’s largest Roman Catholic country, tens of millions of followers were on tenterhooks overnight about the cardinal’s condition. At one stage, a bishop who asked not to be named, said Sin’s chances were "50-50."
Revered for marshaling huge street protests that toppled two presidents, Sin has courted controversy by commenting frequently on politics and public policy.
"My duty is to put Christ in politics," he said at his retirement ceremony. "Politics without Christ is the greatest scourge of our nation."
After reaching the retirement age of 75 in August last year, Sin resigned last November as head of the Manila archdiocese that he had served for nearly three decades. Afflicted with kidney problems and diabetes, he has considerably reduced his public appearances in recent years but remained a steadfast guardian of democracy.
Sin’s retirement signaled the end of an unprecedented era of political activism by the Church, although it remains a potent force and a key backer of Mrs. Arroyo.
Rosales, who succeeded Sin at the Manila archdiocese, is also outspoken about social issues but has not waded into politics as heavily.
Former President Fidel Ramos, a Protestant, once called Sin "the divine commander-in-chief" for marshaling people power rallies that drove the late strongman Ferdinand Marcos and former actor Joseph Estrada from Malacañang in 1986 and 2001, respectively.
Sin led the predominant Catholic Church through one of the Philippines’ most politically turbulent periods, emerging as one of Asia’s most prominent religious leaders and advocates of democracy.
Known for his vocal stance on many issues, Sin called on Filipinos to surround the police and military headquarters in Camps Crame and Aguinaldo, respectively, in 1986 to protect Ramos, then the vice chief of staff, and Defense Minister Juan Ponce Enrile, after they broke away from Marcos.
That led to the people power revolt which ousted Marcos over alleged corruption and human rights violations. The largely peaceful revolt became a harbinger of change in authoritarian regimes worldwide. Marcos died in exile in Hawaii in 1989. — With AFP