Final answer:
Negative pressure is applied during deep suctioning once the suction catheter is in place at the necessary depth within the respiratory tract, to remove secretions and maintain lung integrity.
Step-by-step explanation:
Negative pressure should be applied during deep suctioning when the suction catheter has been introduced into the trachea and has reached the point where suctioning is needed. This is done in a controlled manner to remove secretions while minimizing potential damage to the mucosa. Deep suctioning is utilized in situations such as during surgery or for trauma patients to help reestablish negative pressure in the lungs and facilitate their inflation. As the catheter is withdrawn, negative pressure is used intermittently or in a continuous fashion depending on the protocol and the patient's condition. It is essential to monitor the application of suction carefully to prevent lung tissue damage.