British Prime Minister Tony Blair was hospitalized with chest pains this week, the first time hed been sick since he took office six years ago. He was given a small electric shock to regulate his heartbeat and released after just five hours; doctors said his ailment was very minor and not linked to stress. But British newspapers said the incident showed that the 50-year-old fitness buff must be feeling the strain of the past year. Blairs insistence on allying Britain with the U.S. in the Iraq war drew massive and passionate public opposition. And even his own Labor Party members have blamed him for Britains failing health and social services. Heart trouble seems particularly ominous because Blair came to the party leadership after his predecessor, John Smith, dropped dead of a heart attack at age 55.