In the case of an intact central nervous system, respiration occurs via two systems [3, 5]:

The diaphragm is the main inspiratory muscle and is innervated by the phrenic nerve. It receives primary innervation at the C4 level, often with contribution from C3 and C5. The diaphragm provides 65 % of the tidal volume during normal breathing. Other inspiratory muscles are the external intercostal muscles and the accessory muscles (scalene, sternocleidomastoid, trapezius, and pectoralis). The intercostal muscles are innervated by segmental spinal nerves (T1–11) and assist inspiration by elevating the ribs, which expands the chest wall. The scalene muscles receive innervation by segmental nerves C4–8, the trapezius and sternocleidomastoid by C1–4 and the accessory nerves.

Expiratory muscles are the abdominal muscles and the lateral internal intercostals (T1–12) and are important for the expulsive force needed for an effective cough and to clear secretions.

The development of respiratory complications is directly correlated with the level of injury as diagnosed by the American Spinal Injury Association Impairment Scale [6]. Cervical SCI above the C3 level results in complete paralysis of all muscles involved in respiration and therefore requires immediate mechanical ventilation to sustain life [5]. Respiratory complications occurred in 84 % of patients with C1 to C4 injuries and in 60 % of patients with C5 to C8 injuries [6]. In patients with T1 to T12 lesions, respiratory complications occurred in 65 % of patients and were often related to direct chest trauma. The main respiratory complications are atelectasis, pneumonia, and ventilatory failure, but the incidence varies with the level of injury: atelectasis 40 % (C1–4), 34 % (C5–8), and 65 % (T1–12); ventilatory failure 40 % (C1–4), 23 % (C5–8), and 10 % (T1–12); and pneumonia 63 % (C1–4) and 28 % (C5–8) [3].

Other factors associated with pulmonary complications are age, preexisting (pulmonary) illness, and associated major traumatic injuries. Patients with these premorbid and comorbid conditions are at the highest risk to deteriorate rapidly over the first 5 days.

Pulmonary complications after acute tetraplegia progressively increase over the first 5 days following injury [2, 3, 6]. Several respiratory dysfunctions can lead to respiratory failure:

Although pulmonary complications are a common problem in patients with SCI, there is little evidence about their management. Current practice is mainly based on clinical experience and expert opinion [7].

In patients with acute SCI, it is important to determine the level of injury as accurately and as soon as possible because it will assist in the assessment and judgment of the patient’s likely ventilatory dysfunction. Examination must include an evaluation of the breathing pattern, effectiveness of cough, vital capacity, and respiratory rate. The relevant medical history must include review of prior lung disease, medication, substance abuse, and smoking history [3–5].

Because cervical SCI above the C3 level results in complete paralysis of all muscles involved in respiration, immediate and 24-h mechanical ventilation is required to sustain life. Usually, this will lead to tracheostomal ventilation.

In case of cervical injury at C3–5, respiratory insufficiency results as a consequence of significant respiratory muscular dysfunction. At the first sign of respiratory distress, semi-elective intubation in these patients is recommended [8]. Criteria for intubation are vital capacity deteriorating toward less than 10 ml/kg ideal body weight, increasing oxygen requirements, and increasing respiratory rate with shallow breathing or hypoventilation (PCO₂ > 45 mmHg). As the diaphragm receives primary innervation at the C4 level with contribution commonly seen from C3, there is a high degree of variability of the ability to wean in the C3 and C4 population [9]. In patients with SCI C5–6 or below, there is a probability to recover sufficient spontaneous ventilation.