A Comparison of High vs. Low Tidal Volumes in Ventilator Weaning for Individuals With Sub-Acute Cervical Spinal Cord Injuries

While respiratory failure in people with SCI is common, clinicians have not come to a consensus on the best strategy to manage the mechanical ventilation of these individuals or whether to manage people with SCI differently from other patients. The Consortium for Spinal Cord Medicine has developed Clinical Practice Guidelines using protocols established at Craig Hospital in the 1980s and 1990s, but these are based only on clinical experience and retrospective, cohort data using historical controls.

These guidelines suggest that patients should be ventilated with tidal volumes (VT)of 20-25 cc/kg of ideal body weight (IBW). This recommended VT is at least twice as large as conventional VT used for general medical and surgical patients requiring mechanical ventilation. However, there is a clinical belief that people with SCI should be ventilated at higher VT to prevent atelectasis, to decrease the risk of pneumonia, and to facilitate weaning. This belief was fostered by a retrospective, concurrent cohort comparison study of individuals with SCI, which found that the use of high VT on the ventilator (mean 25.3 cc/kg, PAP<40) was associated with more rapid resolution of atelectasis and more rapid weaning from mechanical ventilation than the use of low VT (mean 15.5). Therefore, people with SCI have been clinically managed using high VT for the past 2-3 decades without prospective data to confirm these clinical impressions.

At the same time that these ventilator strategies evolved in the care of patients with SCI, several clinical studies in general medical and surgical patients requiring mechanical ventilation suggested that high VT leading to higher airway pressures could actually promote lung injury. This occurred when higher VT increased the risk of over-distending the airways and creating volume-related trauma.16-19 In multiple studies, VT of only 10-15 cc/kg IBW was found to produce alveolar over-distention, stretch injury and barotrauma. This work led to the belief that low VT of 6-8 cc/kg IBW could be helpful in protecting mechanically ventilated individuals from ventilator-induced lung injury.

Whether these findings and recommendations apply to individuals with SCI requiring mechanical ventilation is unclear. At least one study of mechanically ventilated individuals without acute respiratory distress syndrome suggests that the use of low VT (6 cc/kg) increases the risk of atelectasis.

With the lack of prospective, evidence-based data regarding optimal VT for persons with a SCI who are ventilator-dependent, clinicians may be inclined to use lower VT to prevent lung injury, unless there is more definitive data to show that higher volumes are as safe as low VT and that higher VT facilitate more rapid weaning from mechanical ventilation.