Respiratory Muscle Function and Outcomes following SCI 

The higher the level of motor complete SCI, the greater the 

• impact on respiratory function 
• risk for secondary complications  
• indication for ventilation support  

Any acute injury above the level of T12 neurological level of SCI will result in some respiratory dysfunction: there is often respiratory muscle (abdominal, intercostal and diaphragm muscles) weakness and/or paralysis, which is initially a flaccid paralysis due to spinal shock. Compliance changes to the lung and chest wall also develop. This often results in paradoxical breathing patterns and the work of breathing increases, with the additional recruitment of accessory muscles necessary.

The impact of changes to the sympathetic nervous system should also be considered in combination with respiratory muscle paralysis.

If the SCI is above T6, there is also an additional autonomic dysfunction with the loss of sympathetic nervous system control and predominance of parasympathetic nervous system activity. This further impacts lung volumes and airway clearance and results in:

• decreased pulmonary surfactant production
• increased pulmonary secretion production
• airway bronchospasm

Assessment of respiratory muscle activity can be informed using a review of breathing patterns, coughing effectiveness and spirometry.

Respiratory Muscles and Function 

Respiration is a combination of inspiration and expiration, according to physiological and functional needs. Inspiration involves primarily the diaphragm and the intercostal muscles. Other accessory muscles can be recruited to increase volume, rate or the force generated. 

Expiration is a passive process which occurs when the diaphragm relaxes inducing the elastic recoil of the lungs and chest wall. 

However, forced expiration to speak, sing, huff or cough primarily requires activation of the abdominal muscles, co-ordinated with glottal function. Again, other accessory muscles can be recruited as needed, especially to cough. 

NOTE:  

1. Respiratory muscles often work synergistically together, including antagonists for respiratory function eg.cough
2. Respiratory muscles often have other functional roles beyond respiration
3. Many respiratory muscles are small and prone to fatigue: although a larger muscle with a variety of muscle fibre types, the diaphragm is also prone to fatigue if breathing mechanics are significantly altered due to neurological dysfunction

Muscles of Respiration

Source: https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2022.838414/full

Primary Respiratory Muscles

INSPIRATION

DIAPHRAGM

Innervation: Phrenic nerve C3-5

Respiratory roles and commments:

Quiet Inspiration, Forced Inspiration

In addition to full innervation, optimal diaphragm contractile efficiency depends on the stabilising alignment provided by the: 

1. abdominal muscles (position the abdominal contents against the diaphragm to optimise its “dome” shape for contraction, especially in sitting)   

2. intercostal and other deep thoracic muscles (stabilise the rib cage/intercostal spaces during changes in intrathoracic pressures during diaphragm contraction and relaxation) 

In an acute SCI, the initial flaccid paralysis of the abdominal and intercostal muscles often results in paradoxical breathing patterns and an increase in the work of breathing, especially in sitting.

As spinal shock resolves a spastic paralysis of the intercostals +/- abdominal muscles may improve breathing patterns.

Use of abdominal binders in sitting can assist positioning of the diaphragm for enhanced contractile efficiency, improving lung volumes for ventilation, cough and speech.

Other functions & comments:

Generation of transdiaphragmatic pressure: by co-contracting with upper airway/larynx/tongue and pelvic floor muscles and relevant sphincters for expulsive or sustained expiration eg. cough, speech etc but also general postural stability when upright

Establishment of an anti-reflux mechanism: by the crural component of the diaphragm supporting the functioning of the lower esophageal sphincter

EXPIRATION

ABDOMINALS: RECTUS, EXTERNAL OBLIQUE, INTERNAL OBLIQUE, TRANSVERSUS

Innervation: Spinal nerves T7-12

Respiratory roles and commments:

Forced Expiration, Cough

Other functions & comments:

Assist diaphragm positioning to optimize inspiratory volumes, especially in sitting

Generate transabdominal pressure to support venous return, aid voice projection, assist bladder and bowel emptying, maintain postural stability when the trunk is under load/exertion 

Initiate active posterior pelvic tilt or trunk flexion for movement, as well as static and dynamic postural reactions 

STABILISING and ACCESSORY MUSCLES

INTERCOSTALS: EXTERNAL and INTERNAL

Innervation: Spinal nerves T7-12

Respiratory roles and commments:

Quiet Inspiration, Forced Inspiration, Forced Expiration, Cough

Inspiration and forced expiration: changing the volume of the thoracic cavity

Other functions & comments:

Stabilise the rib cage/intercostal spaces during changes in intrathoracic pressures, especially to counteract the descent of the diaphragm during inspiration and the relaxation of the diaphragm during expiration

Other deep thoracic muscles that support the intercostal muscle function include: innermost intercostal muscles, subcostalis, and transversus thoracis.

OTHER MUSCLES                                              

SCALENES Spinal Nerves 3-8

STERNOCLEIDOMASTOID Accessory Cranial Nerve XI  and C1,2

UPPER TRAPEZIUS Accessory Cranial Nerve XI    

SERRATUS ANTERIOR Long Thoracic Nerve of Brachial Plexus C5-7 

PECTORALIS MAJOR and MINOR Lateral/Medial Pectoral Nerves of Brachial Plexus C5-T1 

LATISSIMUS DORSI Thoracodorsal Nerve C6-8 

Respiratory roles and commments:

Forced Inspiration, Forced Expiration, Cough

Inspiration and forced expiration: changing the volume of the thoracic cavity via action of reversed origin and insertion, improved by positioning the upper limb in a closed kinetic chain

Other functions & comments:

Head, neck, scapular and upper limb primary movement and stabilisation

Level of SCI and Respiratory Outcomes 

The ventilation outcomes and respiratory needs listed below have a direct correlation to the degree of respiratory muscle paralysis. The impact of changes to the autonomic nervous system (ANS) should also be considered in combination with respiratory muscle paralysis.

Adapted from Source: Berlowitz DJ, Wadsworth B, Ross J. Respiratory problems and management in people with spinal cord injury. Breathe 2016; 12: 328–340.