TRACHEAL COLLAPSE

Clinical signs of tracheal collapse usually are seen in young calves e xhibiting inspiratory distress. The inspiratory dyspnea causes strider in the calves' breathing and a marked increase in the duration of the inspiratory phase of respiration. Calves breathe with their mouth ope n and head and neck extended, but body temperatures are seldom elevate d more than 1 degrees to 1.5 degrees C above normal. Auscultation of t he respiratory tract shows harsh, often bubbly, tracheal sounds that a re referred into the chest. Calves can be stressed easily to the point of collapse by even the simplest restraint procedures, so extreme car e must be taken during examination. Historically, most calves with tra cheal collapse have been cases of dystocia at birth. Although evidence of the collapse seldom is seen from birth, it is thought that the ini tiating injury to the trachea occurs at that time. Compression of the anterior chest wall with fracturing of the first few pairs of ribs may cause injury to the tracheal rings at the thoracic inlet. Because of the elastic nature of the rings, they tend to spring back to a circula r shape, but the compressed rings are left less rigid and have the ten dency to malformation in response to pressure changes in the chest and trachea. As the calf ages, either of two events may contribute to a g radual worsening of the condition-exuberant callus formation around th e rib fractures compresses the trachea at the thoracic inlet or the th oracic segment of the trachea weakens secondarily to the increased neg ative pressure exerted by the calf during inspiration (dynamic collaps e).