The diaphragm and inspiratory intercostal muscles are primary muscles of inspiration. Contraction of these muscles result in increased intrathoracic volume and increased pleural negative pressure. This results in air entering the lungs inspiration.
Expiration is passive and results from the elastic recoil of inflated lungs.
The sternomastoid and scalene group of muscles are accessory inspiratory muscles. The abdominal muscles and expiratory intercostal muscles are accessory expiratory muscles.
Note the following:
Method of Exam
Use of accessory muscles
Stand behind patient and place your hands behind the sternomastoid and feel the scalene muscles during quiet respiration. If the muscle contraction is palpable during quiet tidal breathing, the accessory muscles are in use. These muscles contract normally during an attempt at deep inspiration. It will be obvious when the sternomastoid and trapezius muscles are in action.
Expiration passive or active
Observe and feel the abdominal muscles to see whether they are contracting during expiration. If they do, the patient is using active muscle contraction to expire. Note whether he is using pursed lip breathing.
Negative pleural pressure
Increase in negative pleural pressure can be recognized by intercostal and supraclavicular space retractions and downward pull of the trachea during inspiration.
Number of breaths
Count the number of breaths when the patient is unaware that you are counting his breaths. If the patient is conscious,he will develop tachypnoea. Don't count the respiratory rate until patient has settled down and is in his or her basal state. Normal respiratory rate is 10-14 breathes per minute.
Position of comfort
Observe to see whether your patient is comfortable (to breath) in the supine and sitting position. A normal person is comfortable in any position.
Orthopnoea: Short of breath in supine position, gets some relief by sitting or standing up.
Platypnea: Short of breath in erect position, better in supine position.
Person appears uncomfortable. Breathing seems voluntary. Accessory muscles are in use, expiratory muscles are active and expiration is not passive any more. The degree of negative pleural pressure is high. The respiratory rate is increased.
Respiratory rate of 18 per minute. Patient was short of breath and position was not a factor. He was using scalene group of muscles for tidal breathing. Expiration was passive. These set of findings are consistent with pleural effusion.