Some terminologies and formulas will be necessary to understand before we continue our discussion below.
VILI = Ventilator-Induced Lung Injury
Transpulmonary Pressure = Plateau Pressure – Pleural Pressure
Alveolar Pressure = Plateau Pressure
PEEP = Positive End-Expiratory Pressure
Driving Pressure = Pressure above PEEP or Inspiratory Pressure (Manually set in Pressure-Control)
Intrinsic PEEP = “Auto” PEEP
Integral to our ability to strategically implement mechanical ventilation in ways that maximize lung rest and recovery while minimizing the risk of causing lung Injury, is an understanding of the various forms of VILI and how they occur.
Pulmonary barotrauma is recognized as an adverse complication of mechanical ventilation that generally occurs when the lungs are exposed to excessively high pressures that overly distend the alveoli leading to rupture. Clinically it is manifested by the presence of extra-alveolar air in anatomical locations where it is not commonly observed (i.e. pleural space, mediastinum).
Who is at risk?
It is important to recognize that an acutely injured lung is much less equipped to handle the stresses and strains imposed by a ventilator than an otherwise healthy lung would tolerate; thus, any patient receiving mechanical ventilation, whether invasive or non-invasive is at risk. However, there are certain conditions where susceptibility is notably much higher mandating more caution to protect the patient (i.e. ARDS, COPD/Asthma).
How does it develop?
Barotrauma occurs when there is a significantly high enough pressure difference between a compartment and its surrounding environment. When we apply this concept to mechanical ventilation, we then refer to the lungs, and the pleural space that surrounds it. Making use of respiratory nomenclature, we apply the term “alveolar pressure” to define the pressure in the lungs and “pleural pressure” to define the pressure in the pleural space; the difference between these pressures is the “Transpulmonary Pressure”.
During invasive ventilation, we can determine the alveolar pressure by performing an inspiratory hold to obtain the plateau pressure, which can be used as its surrogate. Plateau pressure is a function of tidal volume along with the compliance of the lung and chest wall. It is increased by the application of larger tidal volumes and in conditions that are characterized by poor compliance, as seen in ARDS. Therefore, higher plateau pressures are associated with higher transpulmonary pressures; furthermore, as the transpulmonary pressure rises, alveoli become progressively distended and may rupture allowing air to escape. This free air may dissect along anatomical tissue planes causing pneumothorax, pneumomediastinum, pneumopericardium, and subcutaneous emphysema.
When should we suspect barotrauma?
It is difficult to observe patient-specific signs as many of these patients are critically ill and under sedation. We must then pay attention to the details and view them as potential clinical signs. Worsening hypoxemia, changes in airway pressure, or new crepitations palpated on physical exam should raise suspicion. In the case of pneumothorax, elevated airway pressures with sudden cardiovascular collapse signal the need for immediate intervention.
How can we diagnose it?
The diagnosis is primarily radiological, and incidental discoveries made on routine imaging may occasionally lead to confirmation.
Red: Subcutaneous Emphysema
We can best protect our patients against this potentially devastating complication by ensuring the following:
- Application of Tidal Volume: Low tidal volume or Arbitrarily determined?
The revolutionary ARDSnet study demonstrated a mortality benefit by targeting patients to receive a tidal volume of 6 rather than 12 ml/kg of predicted body weight. As mentioned earlier, alveolar pressures are adversely affected by larger tidal volumes and poor lung compliance. This causes the alveolar pressure to rise and, in turn, transpulmonary pressure along with it. Pressure controlled ventilation may be advantageous as it allows us to set the driving pressure to a safe value that minimizes the risk of causing lung injury. The caveat to this approach is that in conditions with poor lung compliance (ARDS), setting low driving pressures may lead to inadequate ventilation and life-threatening respiratory acidosis. Volume controlled ventilation may be advantageous as it allows us to set the ventilator to deliver a pre-determined, safe volume which would ideally minimize lung injury while adequately ventilating the patient. The caveat to this approach, however, is that regardless of lung compliance, the ventilator will attempt to deliver the programmed tidal volume even if it requires dangerously high driving pressures to accomplish this. ECMO then becomes a worthwhile consideration if the patient cannot be adequately ventilated without the application of exceedingly high driving pressures that only further lung injury and increase the risk of barotrauma.
- Application of PEEP: Strategic rather than aimlessly increasing? Intrinsic PEEP?
PEEP must always be applied strategically to minimize harm. When we apply PEEP, the goal should be to titrate PEEP towards maximal lung recruitment. Further increments in PEEP cause overdistension of the alveoli increasing the susceptibility for alveolar rupture. In conditions associated with dynamic hyperinflation such as obstructive lung disease, large tidal volumes combined with low expiratory times creates air-trapping or “Intrinsic PEEP”. With the accumulation of Intrinsic PEEP, alveolar distension progressively worsens increasing the risk of rupture. Minimize this risk by delivering lower tidal volumes at faster inspiratory flow rates, which allows you the opportunity to provide maximal expiratory time for the patient’s lungs to fully deflate.
- Plateau Pressure: Maintain =< 30 cm H2O
This can be achieved by maintaining a strategy of low tidal volume ventilation while minimizing excessive application of PEEP. It is also of importance to note that plateau pressure describes the total alveolar pressure rather than the alveolar pressure in separate regions of the lung that may be more diseased than in other regions. In those severely affected regions, the lung may be more susceptible to injury even despite maintaining plateau pressure <30. In that sense, the aim should really be to minimize plateau pressures as low as possible.
- Patient-Ventilator Synchrony: Optimal or Completely Dyssynchronous
To maximize patient safety during invasive ventilation, it is imperative to ensure patient comfort and synchronize the ventilator in such a manner that matches the needs of the patient along with their underlying pathophysiology. Maladaptive interactions between the patient and ventilator can lead to dyssynchronies such as double triggering, which exposes the lungs to much higher tidal volumes with each breath and increases the risk of lung injury. Optimizing analgesia is of crucial importance; however, maximizing sedation and even brief periods of medical paralysis may, at times, be necessary to ensure patient safety.
- Barotrauma occurs when transpulmonary pressures exceed a threshold
Transpulmonary pressure = Plateau Pressure – Pleural Pressure
- Barotrauma as a form of VILI that can occur when the lungs are exposed to excessive transpulmonary pressures. To minimize this risk, it is always imperative to employ lung protective ventilation strategies.
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