When Partners HealthCare, the big Boston hospital group, announces its financial results each quarter, what often stands out are not the gains and losses for providing medical services. It’s the investment and debt portfolios.

With $7.5 billion in assets under its watch — more than at most community banks — Partners feels the swerves of the stock market in a big way. And there’s another significant factor in the quarterly swings: a complex hedging strategy on Partners’ debt. Together, these are often the tail that wags the dog.