Barotrauma

 

INTRODUCTION

Background: Barotrauma is one of the sequelae of ventilator-induced lung injury (VILI). The spectrum of injuries that may be induced during mechanical ventilation spans pneumothorax (unilateral, bilateral, tension), pneumomediastinum, pneumoperitoneum, and pneumatocele formation, as well as pulmonary interstitial emphysema and alveolar fracture. Whether barotrauma, in contradistinction to volutrauma, is the appropriate term to describe these observations is currently controversial. VILI also encompasses the cytokine-, complement-, and cell-mediated immunity pathways that result in proinflammatory states.

 

Pathophysiology: All of the VILI described above stem from a mismatch between the volume of gas delivered and the amount of lung available. For instance, if the available lung can accept only 700 cc (mL) of gas, but the volume to be delivered by the ventilator is set at 850 cc (mL) , then a number of sequelae arise.

First, the ventilator high-pressure alarm rings (ie, excessive airway pressure resulting in barotrauma). Second, the most compliant alveolar units distend because they are the units that can most readily accept delivered gas. This leads to excessive alveolar stretch, overdistension, and fracture. When the alveolar units fracture, gas may escape from the alveoli and lead to the radiographically identified injury patterns described above.

These observations have led to the newer appellation of volutrauma because the excessive volume, rather than pressure, is thought to cause the injury. The high pressure is a reflection of the volume that exceeds the capacity of the available lung. Pneumothorax may be more related to the underlying disease rather than to specific volumes or pressures. This may reflect the relationship between abnormal parenchyma and altered regional time constants, leading to inappropriately large volumes of gas being delivered to otherwise normal segments.

 

Frequency:

  • In the US: Barotrauma is estimated to affect from 10-65% of all patients who require mechanical ventilation. High-risk patients are those who have acute respiratory distress syndrome (ARDS), as opposed to acute lung injury (ALI), preexisting chronic obstructive pulmonary disease (COPD), and direct traumatic injury.
  • Internationally: No geographical differences exist.

Mortality/Morbidity: Mortality and morbidity rates for ALI and ARDS remain approximately 40-50%; however, the rate associated with barotrauma depends on the form it takes. Clearly, unrecognized tension pneumothorax approaches 100% mortality, but a pneumatocele may have no associated mortality or measurable morbidity.

Race: No race predilection is known.

Sex: No sex differences are known.

Age: The process of barotrauma is independent of age; however, because aging decreases pulmonary compliance, elderly individuals are at a slightly higher risk than very young individuals.

CLINICAL

History: Historically, this process has been termed barotrauma and was originally identified following thoracic injuries in military personnel after the advent of endotracheal intubation. Nonetheless, older descriptions describe "rupture of the lung" after forceful exhalation against a closed glottis and pulmonary injury after a deep-water dive event (ie, breath holding while pearl diving). The modern notion of barotrauma depends on chest radiography and ventilator systems that record volumes or pressures or that have integrated graphics packages for waveform analysis. Two prevalent waveforms include the flow-time trace and the airway-pressure tidal-volume curve (ie, hysteresis curve).

  • Flow-time curve
    • This waveform assesses the time it takes for a patient to completely exhale prior to initiating the next breath.
    • The portion of the trace below the horizontal axis represents exhalation, and this portion should return to the horizontal line prior to the onset of inhalation (ie, deflection above the horizontal line).
  • Pressure-tidal volume curve
    • The pressure-tidal volume curve assesses the amount of delivered tidal volume as a function of airway pressure.
    • The far upper right-hand corner of the trace is the point that represents the end of inspiration. This should not extend in a horizontal line to the right because it would indicate the delivery of airway pressure without any additional accompanying tidal volume.
    • This finding indicates overdistension of compliant alveoli rather than additional alveolar ventilation. This tracing is known as a bird's-beak deformity of the dynamic pressure-tidal volume curve. Although interobserver variation may be significant, this is likely the best tool available to clinician's at present. Alternative modes that do not rely on this method of titration, such as airway pressure release ventilation or closed-loop computer-controlled ventilation, may provide alternatives to this titration scheme.

Physical: Physical examination findings may be entirely normal or may be markedly abnormal, as in the case of tension pneumothorax. Frequently, the patient's discomfort while on ventilator is the only sign of barotrauma. His or her discomfort is indicated by agitation, an elevated respiratory rate, and either a high-pressure or high tidal-volume alarm. A low exhaled volume alarm may also be activated (usually in conjunction with an increased respiratory rate) for pressure-supported breaths because an increased rate leads to decreased inspiratory time and therefore less entrained volume.

  • Hypoxemia commonly accompanies barotrauma because alveolar ventilation is markedly disturbed.
  • The onset of hypoxemia should trigger a search for the underlying cause, and barotrauma should be high on the differential list in a patient on positive-pressure ventilation.

Causes: The principal cause of barotrauma is inappropriate gas delivery for the available lung, resulting in alveolar overdistension, fracture, and, ultimately, rupture.

  • External compression resulting from either a thoracic crush injury or a concussive wave from a closed-space explosion must be included in a list of causes of barotrauma. The mechanism behind these causes is compression of a thorax filled with a volume of gas that exceeds the newly compressed volume of the thorax. This leads to the same underlying process of alveolar overdistension and rupture.
  • Similarly, intra-abdominal hypertension and abdominal compartment syndrome may reduce available lung volume.

DIFFERENTIALS

Acute Respiratory Distress Syndrome
[Aspiration Pneumonia]
Critical Care Considerations in Trauma
Diaphragm Disorders
Flail Chest
Foreign Body Aspiration
Hemothorax
Initial Evaluation of the Trauma Patient
Pneumocystis Carinii Pneumonia
Pneumonia, Bacterial
Pneumonia, Fungal
Pneumonia, Viral
Pneumothorax
Pulmonary Edema, Cardiogenic
Pulmonary Edema, Neurogenic
Pulmonary Embolism
Pulmonary Fibrosis, Idiopathic
Pulmonary Hypertension, Secondary
Sepsis, Bacterial
Shock, Distributive
Shock, Hemorrhagic
Status Asthmaticus
Systemic Inflammatory Response Syndrome
Tension Pneumothorax

WORKUP

Lab Studies:

  • No diagnostic laboratory studies are indicated; however, an arterial blood gas determination is often helpful when evaluating the consequences of barotrauma to determine the relationship of pH and PCO2.
  • An arterial lactate level is useful in determining whether the patient is engaging in anaerobic metabolism. Realize, however, that not all lactate is associated with acidosis, and not all lactate stems from hypoperfusion. A pulmonary source of lactate that correlates with the severity of an underlying lung injury has been identified.

Imaging Studies:

  • Chest radiograph
    • The most useful imaging test is a chest radiograph (CXR), which demonstrates the vast majority of pneumothoraces, pulmonary interstitial emphysema, pneumatoceles, pneumomediastinum, and pneumoperitoneum.
    • A cross-table CXR, with the patient in the decubitus position (ie, side of interest up), demonstrates small pneumothoraces better than an upright CXR.
  • CT scan
    • The most precise test is a thoracic CT scan (performed using a helical scanner), which images all of the thoracic structures and demonstrates the discontinuous nature of the resultant lung injury.
    • A CT scan facilitates accurate CT-guided placement of drainage tubes for treatment of pneumothorax. Accurate tube placement has been demonstrated to aid in reducing the number of days trauma patients stay on mechanical ventilation and also allows for immediate reevaluation of drainage adequacy.

Other Tests:

  • Various cytokine assays that may indicate lung injury are available, most notably one for interleukin (IL)-10. These tests, however, do not influence the choice of therapy for barotrauma. Whether reliable biochemical markers exist for incipient barotrauma remains unclear.
Histologic Findings: Alveolar fracture, pneumocyte rupture, alveolar hemorrhage, edema, and a variable inflammatory cell infiltrate may be identified on lung biopsy samples or during an autopsy of the lungs of patients with barotrauma.

 

TREATMENT

Medical Care:

  • Medical care in this circumstance is adjunctive and consists of sedating patients enough to ensure comfort while on the ventilator and bronchodilators to remove any element of increased airway resistance and decreased effective pulmonary airway volume.
  • Diuretic therapy to remove extravascular lung water is appropriate in patients with pulmonary edema from congestive heart failure (CHF).

Surgical Care: Surgical care is limited to the insertion of pleural drainage devices. The procedures that may be relevant to this topic address ventilator settings, manual bagging techniques, and placement of either needle thoracostomy or standard tube thoracostomy devices.

  • Manual bagging
    • Perform manual bagging in a way that mimics the ventilator settings that will be designated once the patient is intubated. This technique reproduces the approximate volume and inspiratory/expiratory (I:E) ratio both before and after the endotracheal tube is inserted.

       

    • The practitioner must know the volume of gas delivered when using bags of different sizes and when using a one- versus two-handed technique.

       

    • The practitioner must also know how to set an I:E ratio on the ventilator. (The normal I:E ratio for a spontaneously breathing person is 1:3-4).
  • Needle thoracostomy
    • The most urgent complication of barotrauma is tension pneumothorax, which is readily treated by pleural space decompression.

       

    • A needle thoracostomy is performed by inserting a catheter-over-the-needle assembly into the pleural space in the second intercostal space in the midclavicular line and then removing the needle portion.

       

    • If a tension pneumothorax or a sizable pneumothorax is present, a volume of air will be heard rushing out of the catheter.
  • Standard tube thoracostomy
    • Alternatively, because the emergent need is to decompress the chest, the practitioner may simply wish to proceed with tube thoracostomy in stable patients or with creating the passage for the tube in unstable patients.

       

    • The hemithorax is prepared with Betadine solution and is draped. A linear but oblique skin incision is then created in the direction of the rib at the level of the fifth intercostal space (ie, below the pectoral crease). This avoids the pectoralis and serratus muscle groups, which decreases muscle bleeding and facilitates passage into the chest.

       

    • Next, blunt dissection is undertaken over the top of a convenient rib, with either a Kelly clamp or a hemostat (or a finger in thin patients). Do not dissect under the rib because this injures the vein, artery, and nerve that course in this location, which leads to hemorrhage and pain.

       

    • The pleura is carefully pierced, at which time a "pop" is felt and frequently heard.

       

    • The examining finger is inserted into the passage, between the ribs and into the thorax, and swept in a 360° motion to ensure that no adhesions will prevent passage of the tube into the pleural space. Adhesions could direct the tube to traverse the parenchyma of the lung and lead to a bronchopleural fistula.

       

    • The tube is inserted to a depth equivalent to the distance from the skin insertion site to the apex of the pleural cavity and secured with at least 1.0-sized nonabsorbable (eg, silk, nylon, polypropylene) suture.

       

    • The tube is attached to a pleural drainage device and a chest radiograph is obtained to confirm accurate placement and to assess the degree of resolution of the pneumothorax.
  • The emergent need in treating a tension pneumothorax is to decompress the chest. No rush is necessary when placing the tube into the pleural space; therefore, if a needle thoracostomy has been placed, time may be taken to deliver a liberal amount of local anesthetic to make the patient more comfortable during tube insertion.

Consultations:

  • The ideal consultation is with an intensivist who is well versed in ventilator management and complication avoidance strategies. In many centers, the ICU is run by an intensivist who oversees and directs daily management, while also keeping the primary team informed of major events.
  • Other paradigms exist, and practitioners must be aware of their own practice environment as well as the limitations of their scope of practice. Practitioners must ask for assistance from individuals with specific expertise prior to a patient having a medical emergency.

Activity: Barotrauma in the hospitalized patient is not related to activity. However, because these patients are attached to a mechanical ventilator, they are limited only by the mobility of the ventilator and their intrinsic functional status.

MEDICATION

Medical therapy is aimed at sedation, bronchodilatation, reduction of extravascular lung water, and clearance of secretions.

Drug Category: Bronchodilators -- Relieve reversible bronchospasm by relaxing smooth muscles of the bronchi.
Drug Name
 Albuterol (Proventil, Ventolin) -- Used for bronchospasm. Relaxes bronchial smooth muscle by action on beta2-receptors and may improve mucociliary clearance. PO dosing available for pediatric asthma management but not appropriate in this setting.
Adult Dose Inhalant: 4-16 puffs q2-6h and prn; adjusting the dose based on response to therapy and evidence of adverse effects such as tachycardia
Pediatric Dose <12 years: Not established
>12 years (inhalant): 1-2 puffs q4-6h prn
Contraindications Documented hypersensitivity, severe tachycardia, cardiovascular instability
Interactions Beta-adrenergic blockers antagonize effects; inhaled ipratropium may increase duration of bronchodilatation by albuterol; cardiovascular effects may increase with MAOIs, inhaled anesthetics, tricyclic antidepressants, and sympathomimetics
Pregnancy C - Safety for use during pregnancy has not been established.
Precautions Caution in hyperthyroidism, hypokalemia, diabetes mellitus, cardiovascular disorders, and epileptiform disorders
Drug Name
 Ipratropium (Atrovent) -- Quaternary ammonium compound chemically related to atropine. Has antisecretory properties, and when applied locally, inhibits secretions from serous and seromucous glands lining the nasal mucosa. Solution may be mixed with albuterol in nebulizer if used within 1 h from time of mixing.
Adult Dose Metered-dose inhaler: 2-8 puffs qid; not to exceed 32 puffs in 24 h; each puff delivers 18 mcg
Pediatric Dose Not established
Contraindications Documented hypersensitivity; sensitivity to soy lecithin, soybean, peanuts, or atropine
Interactions Drugs with anticholinergic properties (eg, dronabinol) may increase toxicity; albuterol may increase effects of ipratropium
Pregnancy B - Usually safe but benefits must outweigh the risks.
Precautions Narrow-angle glaucoma, prostatic hypertrophy, bladder-neck obstruction
Drug Category: Mucolytics -- Lyse mucoid secretions to aid in clearance of mucous from the tracheobronchial tree. Minimal data support the use of N-acetyl cysteine for this indication. At times, direct instillation during flexible fiberoptic bronchoscopy may be helpful for mucous plugging.
Drug Name
 N-acetyl cysteine (Mucomyst, Mucosil) -- Mucolytic agent that reduces the adhesiveness of mucoid secretions to facilitate clearance by direct suctioning and the mucociliary elevator mechanism.
Adult Dose 3 mL of solution directly instilled into tracheobronchial tree as a lavage solution with 5 mL of isotonic sodium chloride solution q4h
Pediatric Dose Not established
Contraindications Documented hypersensitivity
Interactions None reported
Pregnancy B - Usually safe but benefits must outweigh the risks.
Precautions GI distress may occur; marked bronchospasm reported

FOLLOW-UP

Further Inpatient Care:

  • Pneumothorax
    • After draining a pneumothorax, use serial CXRs to reevaluate the patient to ensure that the pneumothorax is resolved. The presence of an air leak in the water seal chamber suggests that the bronchopleural connection has not healed and continued chest tube drainage is needed. Complete evacuation of the pneumothorax is essential in coapting the leaves of the pleura to seal the leak. Once the air leak is resolved, the pneumothorax is evacuated on CXR, and the water column in the water seal chamber continues to rise and fall with each breath (ie, is in communication with the pleural space, a process known as tidaling), then it is time to move to the first step in removing the tube.
    • At this point, various equivalent methods are available to remove the pleural tube. In all of the methods, the first step is to change from suction to water seal. Then, after 12-24 hours of observation, obtain a CXR. If no recurrent air space collection is present, the tube may be either removed or clamped.
    • Clamping the tube with 2 Kelly clamps just before the entry site in the skin allows the practitioner to assess the patient's response to having the tube removed prior to actually removing it. Obtain a CXR 4-6 hours later. If no recurrent pneumothorax develops, the tube may be safely removed.
    • Controversy surrounds the practice of obtaining a CXR after the chest tube is removed. The authors’ current practice is to obtain an image, but no evidence-based guidelines mandate this practice.
  • Pulmonary interstitial emphysema and pneumatoceles
    • A routine daily CXR may be useful while the patient remains on mechanical ventilation but is not mandatory. Adjustment in ventilator settings may be advisable to decrease positive pressure and volume to avoid further barotrauma and progression to pneumomediastinum, pneumothorax, or pneumoperitoneum.
    • Patients with pneumatoceles and concomitant pneumonia may benefit from percutaneous drainage to rule out infection if they have an air-fluid level. Certain patients with large pneumatoceles may benefit from drainage if the pneumatocele is thought to impede lung expansion and result in increased effort in breathing, but this circumstance is rare.

Transfer:

  • Transfer issues center on avoiding a tension pneumothorax or treating one if it arises. This is a standard advanced cardiac life support (ACLS) transfer priority and is not unique to barotrauma.

Deterrence/Prevention:

  • Prevention is aimed at matching the delivered gas to the available lung by monitoring the peak airway pressures and the hysteresis curve.

Complications:

  • Complications include pneumothorax, tension pneumothorax, pulmonary interstitial emphysema, and pneumatocele formation.

Prognosis:

  • The prognosis for isolated barotrauma is excellent because the sequelae are easily treated; survival is nearly uniform unless a patient has an unrecognized tension pneumothorax. However, because barotrauma frequently arises in the setting of multiple organ failure and ALI or ARDS, prognosis is related to underlying organ dysfunction rather than the mechanical consequence of mismatched gas and available lung. Future research should reveal whether lower tidal volume ventilation will be associated with short durations of mechanical ventilation as is now suspected.

Patient Education:

  • Medical care providers, rather than patients, must be educated about this entity so that all practitioners, from nurses to respiratory therapists to physicians, may recognize the presence of barotrauma. More importantly, physicians must structure ventilator settings so that the risk of barotrauma is reduced or avoided.

MISCELLANEOUS

Medical/Legal Pitfalls:

  • Failure to recognize the presence of barotrauma when it is accompanied by a potentially life-threatening condition (eg, pneumothorax) in a patient on positive-pressure ventilation is the principal medical/legal pitfall. If a practitioner chooses not to drain a pneumothorax when a patient is off positive-pressure ventilation, then the medical documentation should address this conscious choice and outline a plan for follow-up care and therapy should observation fail.

Special Concerns:

  • Traumatic brain injury
    • These patients need unimpeded cerebral venous drainage.
    • Barotrauma creates increased intrathoracic pressure and therefore impedes cerebral venous drainage, which leads to intracranial venous hypertension. Avoiding such increases is a wise management strategy.
    • Pressure-controlled ventilation and airway pressure-release ventilation are ideal methods by which to achieve this goal.
  • Intra-abdominal hypertension or abdominal compartment syndrome
    • In the presence of elevated intra-abdominal pressure, diaphragmatic excursion is limited and the available lung volume for ventilation is reduced.
    • Thus, when a postoperative (ie, abdominal) patient develops either increased airway pressure on volume-cycled ventilation or declining volumes on pressure-controlled ventilation, consider the root cause to be from an extrapulmonary source rather than from an inappropriately constructed ventilator setting.
    • A bladder pressure-monitoring catheter or an inferior vena cava (IVC) catheter helps guide therapy in this setting.
    • Possible therapy to ameliorate the barotrauma is to open the anterior abdominal wall closure rather than change the ventilator settings.