فَمَن يُرِدِ ٱللَّهُ أَن يَهْدِيَهُۥ يَشْرَحْ صَدْرَهُۥ لِلْإِسْلَٰمِ وَمَن يُرِدْ أَن يُضِلَّهُۥ يَجْعَلْ صَدْرَهُۥ ضَيِّقًا حَرَجًا كَأَنَّمَا يَصَّعَّدُ فِى ٱلسَّمَآءِ
"Whomever God wills to guide, He opens his breast to Islam. And whomever He wills to let go astray, He makes his breast narrow and tight — as though he were climbing up into the sky."
The root ش-ر-ح was treated in al-Inshirāḥ in connection with the name of the sūra, and at al-Zumar 39/22. I do not repeat it here.
And the comparison is worth recording: ka-annamā yaṣṣaʿʿadu fi'l-samāʾ — "as though he were climbing up into the sky."
I record this as my own reading: the comparison describes shortness of breath — the drawing of breath growing harder as one climbs. And the verb is in Form V (yaṣṣaʿʿadu): a climbing with effort, step by step.
A note, per the method (Method): I draw from this comparison no claim about modern physiology. What the verse gives is an experience that the people of that day knew as well: the tightening of the chest as one goes up high. And this is what the limit kept in the index requires.