Adjectives modify nouns. Adverbs modify adjectives, verbs, or other adverbs. How clear were the lungs is the question you should ask yourself? The interrogative "how" generally implies the use of an adverb. Besides, the word being modified is clear. What part of speech is "clear" in this sentence? If it's not a noun, then your choice should be simple. A less scientific method is to ask yourself which sounds correct...bilateral clear or bilaterally clear.
