Use of Accessory Muscles in Respiration 

Definition 

Accessory muscle use refers to recruitment of non-primary respiratory muscles to assist ventilation when the diaphragm and external intercostals are insufficient. It reflects increased work of breathing. 

Physiology 

Normal quiet breathing: diaphragm + external intercostals 
Increased ventilatory demand or decreased lung compliance: CNS recruits accessory muscles to: 
Increase thoracic volume 
Improve tidal volume 
Assist forced expiration 
 
Clinical Significance 

Accessory muscle use is a red flag for respiratory compromise. 

Common causes: 

Asthma exacerbation 
COPD exacerbation 
Acute respiratory failure 
Upper airway obstruction 
Pulmonary oedema 
Neuromuscular weakness 

Physical exam findings:

Prominent neck muscle contraction 
Suprasternal, intercostal, subcostal retractions 
Tripod posture 
Tachypnoea, nasal flaring (esp. paediatrics) 

Key Points: 

Use of accessory muscles = increased work of breathing 
In asthma/COPD: suggests severe disease 
In paediatrics: retractions correlate with severity 
Persistent use despite treatment → impending respiratory failure