Shock lung (ARDS)

General information:

• Definition - acute respiratory failure:
  
• inadequate oxygenation (PaO2 < 55 mmHg; FiO2 > 0,5).
  
• inadequate ventilation (PaCO2 > 50 mmHg).
  
• acute lung injury (ALI): PaO2/FiO2 < 200 mmHg.
  
• acute respiratory distress syndrome (ARDS): PaO2/FiO2 < 300 mmHg (acute respiratory distress, diffuse bilateral infiltrates (chest x-ray), hypoxemia, static compliance < 40 – 50 mL/cm H2O, severe form of ALI.

Classification and aetiology:

1. Hypoxemic Respiratory Failure.

• Inadequate oxygenation (low or normal PaCO2).
  
• Acute lung injury (ARDS).
  
• Systemic inflammatory reaction: SIRS
  
• Extracorporeal circulation.
  
• Shock of any causes, trauma.
  
• Systemic infection: sepsis.
  
• Pulmonary inflammation: pneumonitis, inhalation injury, aspiration
  
• Pulmonary infection: pneumonia
  
• lung contusion

2. Hypercapnic Respiratory Failure (Pump Failure):

• Hypercapnia + acute respiratory acidosis.
  
• PaCO2 = k VCO2 / VE (1 – VD/VT).
  - CO2-production (VCO2 = VA x FACO2).
  - Fractional concentration of CO2 in the alveolar gas (FACO2).
  - Alveolar volume (VA) = VCO2 / FACO2.
  - Tidal volume (VT = VD + VA).
  - Minute ventilation (VE = VA + VD).
  - Dead Space (VD = VE x [(PaCO2 – PECO2)] / PaCO2.
• CO2 production (VCO2): fever, sepsis, pain.
  
• VD/VT (increased VD): ARDS, bronchoconstriction.
  
• Minute ventilation.
  - Ventilatory pump dysfunction.
  - central respiratory drive (TBI, sedatives, anesthetics.....)
  - Abnormal respiratory efferents (spinal cord injury.....)
  - Abnormal chest / abdominal wall: pleural fluid, ascites, scoliosis....
  - Upper air way obstruction.

Pathophysiology

• diffuse alveolar injury.
  
• heterogeneous alveolar injury (different time constants).
  
• alveolar consolidation (atelectasis, dependent lung regions, functional residual capacity FRC).
  
• alveolar overstretching (non dependent regions).
  
• shear trauma (between consolidated and overstretched lung areas).
  
• pulmonary hypertension: vasoconstriction, microthrombi, Leuco - plugging, interstitial oedema.
  
• hypoxic pulmonary vasoconstriction: VA/Q- mismatch (ventilation/perfusion mismatch).
  
• intrapulmonary shunting (no significant improvement with FiO2: 1,0)
  
• endothelial dysfunction: mediator imbalance, inflammation, procoagulatory state.
  
• epithelial injury: surfactant deficiency, fluid and ion flux across the membrane.
  
• alveolar-capillary barrier lesion.
  
• bronchial obstruction (oedema, secretions, terminal airway instability, spasm).
  
• extra vascular lung water (EVLW): permeability, decreased lymphatic flow.
  
• pulmonary compliance (surfactant dysfunction, oedema, hyaline membranes) C = V / P.
  
• chest wall compliance (oedema, injury).
  
• abdominal wall compliance (abdominal compartment syndrome).
  
• ventilator induced lung injury: shear trauma, overstretching.
  
• air leak (shear trauma, over distension).
  
• fibroproliferative alveolitis.

Symptoms:

1. cardio-respiratory

• tachypnoea, dyspnoea, laboured breathing.
  
• pallor, cyanosis, stridor, retractions.
  
• coarse lung crackles.
  
• tachycardia, hemodynamic instability, poor skin perfusion.

2. distant organ dysfunction (systemic inflammatory response).

• Disseminated intravascular coagulation (DIC).
  
• Encephalopathy (agitation, altered mental status).
  
• Acute renal failure.
  
• Acute liver failure.
  
• Sepsis (gut: bacterial translocation, lung: ventilator induced lung injury).
  
• Hyperglycemia.

Diagnosis, investigations:

• ABGs: hypoxemia, hypocapnia, acute respiratory alkalosis or acute metabolic acidosis.
  
• Plain lung x-ray (bilateral infiltrates, pleural effusions).
  
• CT scan – thorax, lung, abdomen.
  
• Cultures (sputum, blood ...).
  
• Respiratory mechanics.

Treatment:

• non invasive correction of hypoxemia, hypercapnia, O2 supplementation, CPAP, BiPAP (nasal airway, face mask).

Mechanical Ventilation (MV)

• open the lung and keep the lung open inspiratory alveolar recruitment: plateu pressure (Ppl), tidal volume (VT) expiratory alveolar recruitment: positive endexpiratory pressure (PEEP).
  
• preventing lung injury: VT 6 mL/kg, Ppl < 35 mbar (prevents lung over inflation) VT, peak inspiratory pressure (PIP) minimizing (overstretching, shear injury) PEEP optimization (early sufficient expiratory recruitment). IRV (inverse ratio ventilation, I:E > 1:1): increasing – inspiratory time (Ti), mean airway pressure (Pmean), PIP, auto-PEEP.
  
• PCV (pressure control ventilation), PSV (pressure support ventilation) better than volume controlled MV.
  
• early spontaneous breathing (CPAP/ASB, BiPAP).
  
• FiO2-reduction before pressure reduction PIP / PEEP (alveolar stability).
  
• permissive hyperkapnia (PaCO2: 50 – 60 mmHg, pHa > 7,25).
  
• permissive hypoxemia (SpO2: 88 – 92%).

Initial respirator setting:

• Pressure controlled ventilation (PCV).
  
• Tube sizes:
  Newborn: 3,5
  8-18 months: 4
  2-4 years: 5
  4-6 years: 5,5
  6-8 years: 6
  8-10 years: 6,5
  
• Frequency: 25 bpm (neonate), 15 bpm (adult).
  
• VT: 5 – 10 mL/kg (Ppl < 35 mbar).
  
• Flow: 6 – 8 L/min (neonate), 20 – 40 L/min (adult).
  
• PEEP: 3 – 5 mbar (neonate), 5 – 10 mbar (adult).

Monitoring of mechanical ventilation:

• AaDO2: PAO2 – paO2 (: 10 – 15 mmHg).
  
• OI: Pmean x FiO2 x 100 / PaO2.
  
• PaO2 / FiO2.
  
• Pmean: mean airway pressure, ETCO2: pulmonary perfusion, hemodynamic, alveolar empty.

Other Therapeutic Modalities:

• high frequency ventilation, early indication, if FiO2 > 0,5/ 4 hours on conventional mechanical ventilation (CMV), percussive (VDR4) or osscillative (Sensormedics 3100 A,B) ventilation.
  
• kinetic therapy (prone position, Rotarest-bed).
  
• selective pulmonary vasodilatation: inhalation of NO, Prostacyclin (NO: 4 – 20 parts per millions).
  
• anti-inflammatory therapy
  
• Prostacyclin - Inhalation (Flolan 15 – 50 (till 100) ng/kg/min).
  
• Steroids (dexamethason 4 x 0,5 mg/kg).
  
• Ibuprofen 5-10 mg/kg/dose every 6–8 hours (maximum daily dose: 4 mg/kg BW)
  
• antiproliferatory therapy with steroids.
  
• Surfactant (e.g. Curosurf 100 – 200 mg/kg).
  
• reduction in EVLW: negative fluid balance.
  
• β-Agonist (bronchodilatation, interstitial and alveolar fluid-transport). Terbutalin-Inhalation: 0,01 – 0,03 mL/kg diluted with 1-2 mL NS every 4 – 6 hrs
  
• Extracorporeal support (ECMO) if shunt > 30%, FiO2 > 60%, Compl. < 0,5 mL/ cm H2O/kg.
  
• diagnosis and treatment of complications (air leak, pneumothorax)

supportive therapy

• hemodynamic optimization: Oxygen delivery (DO2) preload, cardiac output, SvO2 (mixed venous oxygen saturation)
  
• drainage of pleural fluid, aszites (intraabdominal pressure)
  
• bronchoscopy (lavage, source of bleeding)
  
• intestinal therapy:
  
• - early enteral feeding (bacterial translocation)
  
• - stimulation of bowel motility
  
• closed tracheobronchial suction system (high respiratory support)
  
• nutritional support (early enteral feeeding)

Prognosis:

• in children better than adults.
  
• early mortality: multi-organ failure rather than lung failure oxygen utilization defect not hypoxemia.
  
• long term pulmonary dysfunction, broncho - pulmonary dysplasia (neonate), higher susceptibility to bronchial obstruction and airway infections.

Lung contusion (Lungenquetschung)

General information:

• Children: often without rib fracture.
  
• Caused by severe shearing force.
  
• In cases of serial rib factures: lung contusion a frequent finding.
  
• Frequently pneumothorax, hemothorax.
  
• Additional injuries: abdomen, cervical spinal cord

Diagnostic workout:

• On the plain – chest X-ray a fluffy infiltrate that progresses in extent and density over a period of 24 to 48 hours.
  
• Early in the course CT of the thorax is recommended, abdomen (consolidation areas, injury of the lung and other organs ).
  
• Abdominal ultrasound examination (liver or spleen ruptures, free abdominal fluid?).
  
• Echocardiography: pericardial effusion, myocardial contractility, injury of great vessels.
  
• Bronchoscopy:
  Initially often without a result but in many cases helpful for guided pulmonary lavage and suctioning the blood plugs (cave: obstruction of the tube), as well as defining the source of bleeding. Topical injection of surfactant.

Therapy:

• A high percentage require temporary assited ventilation.
  
• Early intubation and mechanical ventilation in cases of obvious respiratory insufficiency: SpO2 < 85% (PaO2 < 50, PaCO2 > 50) with FiO2 0,21.
  
• Open lung approach.
  
• High frequency ventilation should be preferred (early in the course, air leak).
  
• Sufficient drainage: pneumo- / hemothorax.
  
• Continued or uncontrollable hemorrhage and/or massive air leak generally mandates an early thoracotomy.