The hospital's run a series of tests on him and we'll hopefully know a lot more tomorrow, but at the moment they seem to be most worried about his cardiac arrhythmia. If he's had a heart attack, it's likely to have been a minor one. Phew!
Dad has had a bit of a history of heart problems over the last 15 years or so. He suffered a major heart attack in 1990, and has had several minor incidents over the last few years as well (plus he's also had 2 strokes, a major one in 1996 and a less severe one in 2000), so when the call came this afternoon, it didn't come as a complete surprise to learn that he'd been taken into hospital again.
Thank you all who have left messages of comfort and best wishes, and I hope to have more good news for you soon!