Respiratory Failure and Mechanical Ventilation

Abstract

Acute lung injury develops in response to a variety of pulmonary and extrapulmonary disease processes, ultimately resulting in widespread alveolar-capillary leak with extravasation of protein-rich, non-cardiogenic pulmonary edema. This acute phase leads to atelectasis, consolidation, surfactant degradation, and ultimately decreased lung compliance with progressive hypoxemia. Further progression of lung injury leads to a chronic stage, also known as the fibroproliferative stage, characterized by improvement in compliance despite continued poor lung function. Poor compliance at this stage is due to fibrosis and thickening of the lung interstitium. If the patient survives, the acute and fibroproliferative (chronic) stages, his or her lung function can vary from complete recovery to substantial long-lasting pulmonary functional deficits.

Appendices

Summary Points

The definitions for acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are similar and include: a diffuse bilateral process, non-cardiogenic pulmonary edema, and hypoxemia.

ALI is defined by a PaO₂/FiO₂ ratio between 200 and 300 and ARDS by a PaO₂/FiO₂ ratio below 200.

Factors that contribute to ventilator-associated lung injury (VALI) include: barotrauma, volutrauma, oxygen toxicity, biotrauma, and atelectrauma.

As lung injury progresses, the lungs can be divided into three regions: dependent areas (severe collapse and alveolar flooding), non-dependent areas (normal lung), and intermediate areas (atelectatic but recruitable).

The low tidal volume approach (6–8 mL/kg) has become standard of care to prevent alveolar overdistension, stretch, and volutrauma.

The advantages of PEEP include: increase in FRC, respiratory compliance, and V/Q mismatch, redistribution of lung edema, and protection from atelectrauma.

In an attempt to limit barotrauma, PaCO₂ is allowed to increase (permissive hypercapnea).

High frequency oscillatory ventilation (HFOV) allows for optimal alveolar recruitment with lower airway pressures while minimizing phasic alveolar opening and closing.

Airway pressure-release ventilation (APRV) is essentially CPAP with brief intermittent release, coupled with spontaneous ventilation.

Non-invasive positive pressure mechanical ventilation (NIPPV) relies on a patient interface (nasal prongs, nasal mask, face mask) to deliver ventilator support without placement of an endotracheal tube or tracheostomy.

Prone positioning, exogenous surfactant, inhaled nitric oxide, and permissive hypoxemia are being investigated as potentially beneficial adjuncts in children with acute lung injury.

Editor’s Comment

For most of us, mastering the art of ventilator management is one of the most gratifying rites of passage during residency. Check the PaCO₂, make a subtle adjustment in the ventilatory rate or the PIP, and the patient weans – you have the power and the skill and your patients are the better for it. Nevertheless, what we have been slow to realize is that, at least for our sickest patients, the treatment can be as detrimental as the disease. Ventilator-associated lung injury is a concept that has been around for a long time but is just now being accepted by the clinicians on the front lines of critical care medicine, including surgeons. This elegant concept posits that barotrauma, volutrauma, atelectrauma, oxygen toxicity, and biotrauma all contribute to lung injury and that minimizing the effects of these factors helps our patients recover more quickly and with fewer sequelae. It is tempting as surgical fellows and attendings, who learned about ventilator management several years before, to be openly critical of the young critical care physician who has instituted a strange new ventilator strategy in the care of our patient. Although obviously it is important to avoid the use of untested or investigative techniques outside of a clinical trial, it is also important for us to attempt to understand the basis for any novel or counterintuitive approach – there might very well be compelling scientific evidence behind it that you were not aware of.

When both conventional and advanced modes of ventilation fail, the next step for otherwise viable patients with reversible lung injury (influenza, aspiration pneumonitis) might be ECMO. There is now considerable experience with the use of ECMO in adolescents and even adults and it appears to be a viable option in select cases. Every tertiary care children’s center should have established criteria and a practical algorithm for patients with severe respiratory failure so that this potentially life-saving technology can be made available in a timely fashion. One of the most common issues related to the application of ECMO is waiting too long to consider it. The patient with multiple pneumothoraces and chest tubes has probably already had irreversible lung injury due to barotrauma for ECMO to be of any practical value. The same can be said of patients who have ventilator-associated ­pneumonia, severe pulmonary edema, or multiple areas of refractory and severe atelectasis.