Sforno on Genesis 45:26/ספורנו על בראשית מ״ה:כ״ו

1ויפג לבות, he passed out. His heart stopped beating briefly, something common when people have a fainting spell. This occurred at the moment Joseph’s name was mentioned by the brothers.

2כי לא האמין להם. so that he had no chance to believe them. Therefore, ותחי רוח יעקב, once his spirit revived he never fainted again so that he was able to believe them. It had been the sudden joy that had been the cause of his fainting. Spirit has a habit of contracting when one suffers worry, etc., whereas it expands when one experiences joy, sometimes even beyond the boundaries of one’s body so that the body faints.