Respiratory predictive factors

Following a spinal cord injury (SCI), multiple predictive factors determine the extent of respiratory dysfunction. Increasing prevalence or severity of these factors correlates with more significant changes in respiratory function and a higher risk of developing complications, particularly in the early post-injury period. Some factors are specific to the acute phase, whereas others persist life-long.

A comprehensive respiratory assessment assists in identifying these factors and supports early management planning for timely and appropriate intervention. Typically, it is imperative to improve ventilation and secretion clearance, while reducing the work of breathing. This is both relevant during the acute management of SCI, and when a person with chronic SCI becomes acutely unwell.

Key predictive factors are the neurological level of injury (NLI), along with the degree of motor impairment and autonomic nervous system (ANS) disruption. Other predictive factors are the presence of spinal shock, concomitant injuries, medical issues and comorbidities associated with age.

The International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI) can be used to as a starting point to determine both the NLI and degree of motor impairment. However, it does not:

• include specific motor tests to grade the strength of the trunk, neck or head muscles involved in respiration (when assessing these body areas, it infers that impaired sensation of a dermatome indicates impaired motor function of the associated myotome)
• determine the degree of ANS disruption, so this must be determined symptomatically.

Neurological level of injury

The NLI represents the lowest neurological level of intact spinal nerve root innervation.

A NLI above T12 predicts some respiratory muscle impairment, increasing as the NLI becomes higher.

For example, a NLI of C5 will likely result in greater respiratory dysfunction than a NLI of T5.

Degree of motor impairment

In addition to determining the NLI, the ISNCSCI assigns an impairment grade (AIS A–E) to describe the severity of motor and sensory loss.

A higher NLI, together with a greater degree of motor impairment, predicts more significant respiratory muscle weakness or paralysis.

For example, a C5 AIS A injury will likely result in greater respiratory dysfunction than a C5 AIS D injury.

Specifically, the extent of abdominal, intercostal and diaphragm muscle weakness or paralysis, will determine:

• the reduction of tidal and inspiratory reserve volumes, as well as the overall impairment to diaphragm functioning and efficiency
• the reduction of forced expiratory volumes and flow rates, affecting the regulation of carbon dioxide (CO₂) levels and the ability to clear secretions.

Overall, the degree of these changes exponentially diminish minute ventilation and increase the work of breathing, along with the risk of respiratory complications.

Degree of autonomic nervous system disruption

If the NLI is at or above T6, the presence of autonomic nervous system (ANS) disruption predicts additional respiratory and cardiovascular effects. For example, these may be:

• increased sputum production
• bronchospasm
• nasal congestion
• arrhythmias, especially bradycardia
• reduced cardiac output
• increased risk of pulmonary embolism and pulmonary oedema.

Overall, the presence of ANS disruption in addition to respiratory muscle impairment, may further diminish minute ventilation and increase the work of breathing, along with the risk of respiratory complications.

Spinal shock

Spinal shock occurs in the acute phase following SCI and when present, predicts more significant respiratory dysfunction.

It amplifies the impact of the NLI on respiratory muscle function, including the degree of motor impairment and ANS disruption.

In addition to this, any respiratory muscle weakness or paralysis presents with flaccid muscle tone and this further alters breathing mechanics.

For example, flaccidity of the weakened or paralysed intercostal and abdominal muscles increases chest and abdominal wall compliance. This further reduces the functioning and efficiency of the diaphragm during inspiration, especially in sitting.

Typically, a paradoxical breathing pattern develops during inspiration, evidenced by a collapse of the chest wall, with a simultaneous heightened rise of the abdomen.

Overall, the presence of spinal shock with respiratory dysfunction will further diminish minute ventilation and increase the work of breathing, along with the risk of respiratory complications. This is until the overlay of spinal shock resolves in the following weeks and months post-injury.

Concomitant injuries

Other factors which may further compound respiratory dysfunction in the acute phase are:

• high-velocity or high-force trauma resulting in chest wall or abdominal injury
• risk of aspiration, including immersion injuries or excessive saliva production with some degree of dysphagia
• traumatic brain injury
• pain
• altered level of consciousness
• altered psychological state
• post-surgical recovery (e.g. spinal discectomies, fusions, intra-abdominal repairs)
• prolonged invasive ventilation or recent extubation
• use of medications, particularly opioids
• faecal impaction due to paralytic ileus or severe constipation, limiting diaphragm excursion and increasing the work of breathing
• complications from other body systems, such as fluid or electrolyte imbalances, or hypoalbuminemia.

Medical issues

Other factors which may further compound respiratory dysfunction in the acute phase are:

• post-surgical recovery (e.g. spinal discectomies, fusions, intra-abdominal repairs)
• prolonged invasive ventilation or recent extubation (e.g. ventilator associated pneumonia (VAP) and ventilator-induced diaphragmatic dysfunction (VIDD))
• use of medications, particularly opioids
• faecal impaction due to paralytic ileus or severe constipation, limiting diaphragm excursion and increasing the work of breathing
• complications from other body systems, such as fluid or electrolyte imbalances, or hypoalbuminemia.

Comorbidities and ageing

Other factors which may further compound respiratory dysfunction in the acute phase are:

• age > 50 years
• body mass index (BMI) > 25
• history of smoking
• chronic lung diseases
• pre-existing cardiovascular, neurological or musculoskeletal conditions
• mental health conditions or intellectual disability.