According to the Glossary of Old Medical Terms by Craig Thornber:
Rising of the Lights: croup - any obstructive condition of the larynx or trachea (windpipe), characterised by a hoarse, barking cough and difficult breathing, occurring chiefly in infants and children.
which backs up the speculations in Correspondence in the British Medical Journal, Dec 1926.
Croup is a respiratory condition:
Croup, also known as laryngotracheobronchitis, is a type of respiratory infection that is usually caused by a virus. The infection leads to swelling inside the trachea, which interferes with normal breathing and produces the classic symptoms of "barking" cough, stridor, and a hoarse voice. Fever and runny nose may also be present.