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					Title: Pulmonary Contusions and Critical Care Management in Thoracic Trauma
Authors: Sutyak JP, Wohltmann CD, Larson J
Journal: Thoracic Surgery Clinics 17 (2007) 11-23.

Key Points
         •     Isolated blunt pulmonary contusion is rare and is more commonly part of multisystem trauma.
         •     Current therapy for blunt pulmonary contusion focuses on support of oxygenation and ventilation.
         •     Care must be taken to avoid ventilator-associated lung injury.
         •     Prevention of infection and/or early diagnosis and treatment of infection is important to avoid prolonged hospitalization and
               increased morbidity.

Clinical Conclusions
Current critical care for post-traumatic respiratory failure focuses on maintenance of adequate, not necessarily normal, lung function, avoidance of
iatrogenic injury, treatment of infection, and patience.

Section Highlights
          •    Early efforts to treat rib fractures and flail segments by external stabilization resulted in uncoordinated ventilation with resultant
               ventilatory shunting leading to respiratory failure.
          •    The application of mechanical ventilation along with the birth of critical care units resulted in improved outcomes but complications
               continued until the concept was introduced to treat pulmonary tissue injury, not just rib instability. (Avery, Benson et al. 1956)
          •    Selective positive pressure ventilation was introduced for pulmonary support, not for chest wall instability, and became the standard
               therapy. (Reid and Baird 1965; Richardson, Adams et al. 1982)
Pathophysiology of pulmonary contusion and ventilator-associated lung injury
          •    Following blunt force trauma to the chest, lacerations occur in the lung parenchyma which release blood and plasma that flood local
          •    This results in perfusion without ventilation, increased intrapulmonary shunt fraction, reduced compliance, increased pulmonary
               vascular resistance, reduced CO2 elimination, and decreased oxygenation.(Oppenheimer, Craven et al. 1979)
          •    These pathologic changes are not confined to the local zone of injury because in cases where the inflammatory response is of
               adequate magnitude, the result is diffuse pulmonary dysfunction with patches of normal functioning lung parenchyma interspersed
               with areas of fluid-filled nonfunctioning lung.
          •    The clinical impact of this original process can be aggravated by the application of therapeutic positive pressure ventilation despite
               the best intention.
Support of pulmonary function
          •    There is currently no method to speed repair of contused and secondarily injured lung tissue so current therapy is based on support
               of oxygenation and ventilation until spontaneous healing occurs.
Noninvasive ventilation
          •    Noninvasive positive pressure ventilation (NPPV), which delivers positive pressure in the form of continuous positive airway pressure
               (CPAP) or bi-level positive airway pressure (BiPAP) is a safe and effective strategy for the treatment of acute or chronic gas-exchange
               failure.(Keenan, Kernerman et al. 1997; Nava, Ambrosino et al. 1998; Keenan, Sinuff et al. 2004)
          •    Other non-invasive treatments include nasal cannula and aerosol face mask but these support only oxygenation, not oxygenation
               and ventilation as do CPAP and BiPAP.
Mechanical ventilation modes and lung protective strategies
          •    Introduction
                     o In pressure-cycled systems the pressure delivered is constant but the volume is dependent on changes in lung mechanics.
                     o In contrast, volume-cycled systems, which are the current standard by which most positive-pressure mechanical
                          ventilation is delivered, function to deliver a constant, predetermined, alveolar volume regardless of lung
                          mechanics.(Tobin 2001; Fan, Needham et al. 2005)
                     o Initially large tidal volumes were used with these systems because of the belief that would prevent alveolar collapse but
                          the notion of ventilation-associated lung injuries has lead to changes in this thought.
                     o Low-volume and low-pressure ventilation is the current recommended strategy for patients with ARDS.
          •    Controlled mandatory ventilation
                     o Assist-controlled or controlled mandatory ventilation is a mode of positive pressure, volume-controlled ventilation which
                          provides a minimum rate of set tidal volumes regardless of patient effort.
                     o This method allows the patient to initiate breaths above the minimum rate but delivers the same set tidal volume with
                          each assisted breath.
          •    Intermittent mandatory ventilation
                     o Intermittent mandatory ventilation combines periods of assist-control ventilation with spontaneous breathing.
                     o This allows assisted breaths to be synchronized with patient efforts while maintaining a preset minute volume regardless
                          of patient efforts.
                     o A potential disadvantage of this technique is that it can cause increase work of breathing, and there is a concern with
                          reduced cardiac output and venous return especially in patients with left ventricular dysfunction.
          •    Pressure-controlled ventilation
                     o In pressure-controlled ventilation (PCV), a pressure-limited breath is delivered at a minimum rate.
                    o    Tidal volume is dependent on the peak pressure limit, inspiratory time, and compliance.
                    o    The inspiratory flow pattern generated by PVC is always decelerating which may aid in preventing ventilator-associated
                         lung injury through reduced peak pressure, increased static compliance, and improved gas distribution.(Rappaport,
                         Shpiner et al. 1994)
         •    Pressure-regulated volume control ventilation
                    o Pressure-regulated volume control ventilation is an A/C mode that combines volume ventilation with pressure limitation.
                    o The ventilator delivers guaranteed minute ventilation by adjusting inspiratory times and flow with the level and time of
                         pressure continually varied to achieve volume without exceeding the pressure limit.(MacIntyre, Gropper et al. 1994)
                    o This provides a decelerating inspiratory flow pattern that produces lower peak inspiratory pressure without compromising
                         volume.(Guldager, Nielsen et al. 1997)
         •    Inverse ratio ventilation
                    o Inverse ratio ventilation may be used in combination with PVC to enact prolonged inspiratory times.
                    o PCV-inverse ratio ventilation delivers a pressure-limited breath designed to facilitate recruitment of collapsed alveoli and
                         prevent derecruitment.
         •    High-frequency oscillatory ventilation
                    o High-frequency oscillatory (HFO) ventilators generate low-amplitude proximal airway vibrations, analogous to acoustic
                         waveforms, which result in sub-dead space tidal exchanges at varying airway pressures.
                    o HFO was originally used to treat respiratory failure in premature neonates but has been used in the setting of acute lung
                         injury and ARDS as both a primary and rescue mode.(Mehta, Lapinsky et al. 2001)
         •    Nitric oxide
                    o Nitric oxide (NO) is a normal regulatory compound present in the vascular endothelium which works to increase cyclic
                         GMP, relaxing vascular smooth muscle.
                    o Inhaled NO acts locally and has an extremely short half-life of only a few seconds but is able to reduce pulmonary vascular
                    o Dosage is started a 20 ppm and titrated down as the patient improves.
                    o NO combines with oxygen to produce nitrogen dioxide (NO2) and the NO/NO2 ratio must be constantly monitored; when
                         NO is metabolized to NO2 it also combines with water to create nitrite which in turn reacts with hemoglobin to produce
                         methemoglobin, a substance that is unable to transport oxygen.
         •    Prone positioning
                    o CT scan study of respiratory failure patients shows that most alveolar collapse occurs posteriorly so prone position simply
                         works to redirect the majority of the flow to the more functional alveoli.
                    o The prone position can be technically challenging although specialty turning and padding equipment are available.
                    o The prone position is contraindicated in patients with traumatic brain injury as it increases intracranial
                         pressure.(Staudinger, Kofler et al. 2001; Guerin, Gaillard et al. 2004)
         •    Independent lung ventilation
                    o Independent lung ventilation may be useful when a significant pathologic difference exists between lungs and parallel
                         ventilation fails.
                    o Indications include bronchopleural fistula, severe unilateral pulmonary disease, pulmonary embolus, or massive
Ventilator-associated pneumonia
         •    Ventilator-associated pneumonia (VAP) is the most frequent complication related to mechanical ventilation, occurring in 9-24% of
              patients with acute respiratory failure and accounting for 50% of ICU infections.(Fagon, Chastre et al. 1989; Torres, Aznar et al. 1990)
         •    Bronchoalveolar lavage with quantitative culture has improved the accuracy of diagnosis and specificity of antibiotic therapy in VAP.
         •    In the first week, the predominant organisms are Haemophilus and gram-positive bacteria but after that, the nosocomial pathogens
              Pseudomonas, acinetobacter, Staphylococcus aureus, and methicillin-resistant S. aureus begin to appear.
         •    Strategies for the prevention of VAP include the use of non-invasive ventilation and more rapid weaning and extubation as well as
              elevating the head of the bed and emphasizing the importance of staff/visitor hand washing.

          •    Treatment strategies for pulmonary contusion are aimed as supporting the healing tissue through oxygenation and ventilation.
          •    Appropriate ventilation strategies are dependent on the condition of the individual patient and the extent of the injury.
          •    Avoiding ventilator-associated lung injury and preventing infection are key to successful outcome in these patients.


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