What is the differential diagnosis between dry rales and abnormal alveolar breath sounds?

　　Dry rales are a long-lasting, “musical” respiratory sound. The dry rales have a long duration and can be heard in both inspiratory and expiratory phases, but they are more pronounced during expiration, and their intensity and nature can easily change, increasing or decreasing significantly within a few moments. Dry rales can be divided into high and low pitches according to their pitch. High-pitched dry rales, also known as whistling sounds, often have an ascending tone during forceful expiration and are mostly from smaller bronchi or fine bronchi, and are also called croup because they are more common in bronchial asthma. Low-pitched dry rales, also known as snoring sounds, mostly occur in the trachea or main bronchus. Dry rales that occur in the large airways above the main bronchi are not easily heard. What is the differential diagnosis between dry rales and abnormal alveolar breath sounds?　　Dry rales can be produced for a variety of reasons. When inflammation is present in the trachea and main bronchi, the mucosa of the walls becomes edematous, congested, with increased secretions and even spasm of the bronchial smooth muscle, so that the lumen of the trachea and main bronchi narrows. In addition, dry rales may also be produced by the presence of superfluous organisms (tumors) or foreign bodies in the trachea or main bronchi, or by the presence of more and thicker secretions, resulting in narrowing or partial obstruction of the lumen. In a few cases, the narrowing of the lumen is caused by the compression of the trachea or main bronchus by a mass or lymph node outside of it.　　Alveolar breath sounds are the result of air moving in and out of the fine bronchi and alveoli. Common causes are reduced conduction of alveolar breath sounds, restriction of thoracic or lung expansion, or decreased ventilation and reduced alveolar gas flow and flow rate due to inadequate respiratory drive and increased resistance. This may be manifested as unilateral, bilateral, or restricted alveolar breath sounds that are diminished or absent.