Lying on the back when sitting is impossible

Prayer of the Sick: Standing, Gestures, Recovery and Unconsciousness

فَإِنْ لَمْ يَسْتَطِعِ الْقُعُودَ اسْتَلْقَى عَلَى قَفَاهُ، وَجَعَلَ رِجْلَيْهِ إِلَى الْقِبْلَةِ، وَأَوْمَأَ بِالرُّكُوعِ وَالسُّجُودِ.
Translation: If unable to sit, the patient lies on the back with the feet toward the qiblah and gestures with the head for bowing and prostration.

Istilqa' is lying supine. Where possible, slight support under the head helps turn the face toward the qiblah and makes head movements easier.