ARDS Notes

Acute Respiratory Distress Syndrome (ARDS)

ARDS Overview

• ARDS is a condition that develops in response to injury.
• Pulmonary capillaries become engorged.
• The permeability of the alveolar-capillary membrane increases.
• Interstitial and intra-alveolar edema and hemorrhage occur.
• Scattered areas of hemorrhagic alveolar consolidation are observed.
• This results in a decrease in alveolar surfactant and in alveolar collapse, or atelectasis.

Progression of ARDS

• As the disease progresses, intra-alveolar walls become lined with a thick, rippled hyaline membrane.
• This membrane contains fibrin and cellular debris.
• The anatomic alterations that develop in ARDS create a restrictive lung disorder.

Pathological Changes Associated with ARDS

• Interstitial and intra-alveolar edema and hemorrhage
• Alveolar consolidation
• Intra-alveolar hyaline membrane
• Pulmonary surfactant deficiency or abnormality
• Atelectasis

Phases of ARDS

• Exudative Phase:
  • Characterized by edema and hemorrhage.
  • Destruction of Type I alveolar cells, engorgement of capillaries, alveolar collapse, and atelectasis.
• Proliferative Phase:
  • Occurs 7-21 days after the initial injury.
  • Marked by the beginning of lung repair and an increase in surfactant production.
• Fibrotic Phase:
  • Represents end-stage fibrosis.
  • Severe cases lead to extensive fibrosis.

Etiology and Epidemiology of ARDS

• ARDS accounts for 10%–15% of all ICU admissions.
• About 25% of patients on mechanical ventilation develop ARDS.
• Clinical manifestations associated with ARDS usually appear within 6–72 hours of an inciting event and worsen rapidly.
• Between 12% and 35% of patients die within the first 72 hours.

Common Causes of ARDS

• Sepsis
• Aspiration
• Pneumonia
• Severe trauma
• Massive blood transfusion
• Lung and hematopoietic stem cell transplantation
• Drug abuse

Other Causes of ARDS

• Central nervous system (CNS) disease
• Cardiopulmonary bypass
• Disseminated intravascular coagulation
• Inhalation of toxins and irritants
• Immunologic reactions
• Oxygen toxicity

Berlin Definition of ARDS: Diagnostic Criteria

• Respiratory symptoms associated with ARDS manifest within 1 week of a known clinical event—or new or worsening symptoms over the past 7 days.
• Bilateral opacities, similar to pulmonary edema, appear on the chest radiograph or computed tomography scan.
  • The opacities cannot be fully explained by pleural effusion, lobar or lung collapse, or pulmonary nodules.
• Respiratory failure cannot be fully explained by heart failure or fluid overload.
  • An objective assessment to rule out hydrostatic pulmonary edema is required if risk factors are not present for ARDS.
• A moderate to severe impairment of oxygenation must be present, as defined by the PaO2/FIO2 ratio.
  • The severity of the hypoxemia defines the severity of the ARDS.

Severity of ARDS Based on Berlin Definition

• Mild ARDS:
  • The PaO2/FIO2 is greater than 200, but less than or equal to 300, on ventilator settings that include PEEP or CPAP greater than or equal to 5 cm H2O.
• Moderate ARDS:
  • The PaO2/FIO2 is greater than 100, but less than or equal to 200, on ventilator settings that include PEEP greater than or equal to 5 cm H2O.
• Severe ARDS:
  • The PaO2/FIO2 is less than or equal to 100 on ventilator settings that include PEEP greater than or equal to 5 cm H2O.

PF Ratio Calculation

• PF\,ratio = \frac{PaO2}{FiO2}
• Example 1: Patient has a PaO_2 of 80 mmHg on 100% O2:
  • PF\,Ratio = \frac{80}{1} = 80
• Example 2: Patient has a PaO_2 of 100 on 50% O2:
  • PF\,ratio = \frac{100}{0.5} = 200

Clinical Manifestations of ARDS

The following clinical manifestations result from the pathophysiologic mechanisms caused (or activated) by:

• Atelectasis
• Alveolar consolidation
• Increased alveolar-capillary membrane thickness

Physical Examination Findings

• Vital Signs:
  • Increased respiratory rate (tachypnea)
  • Increased heart rate (pulse)
  • Increased blood pressure
  • Substernal or intercostal retractions
  • Cyanosis
• Chest Assessment Findings:
  • Dull percussion note
  • Bronchial breath sounds
  • Crackles

Pulmonary Function Test Findings

Forced Expiratory Volume and Flowrate Findings

• FVC: Decreased \,(\downarrow)
• FEVT: Normal or Decreased \,(N \,or\, \downarrow)
• FEV_1/FVC ratio: Normal or Increased \,(N \,or\, \uparrow)
• FEF_{25\%-75\%}: Normal or Decreased \,(N \,or\, \downarrow)
• FEF_{50\%}: Normal or Decreased \,(N \,or\, \downarrow)
• FEF_{200-1200}: Normal or Decreased \,(N \,or\, \downarrow)
• PEFR: Normal or Decreased \,(N \,or\, \downarrow)
• MVV: Normal or Decreased \,(N \,or\, \downarrow)

Lung Volume and Capacity Findings

• V_T: Normal or Decreased \,(N \,or\, \downarrow)
• IRV: Decreased \,(\downarrow)
• ERV: Decreased \,(\downarrow)
• RV: Decreased \,(\downarrow)
• VC: Decreased \,(\downarrow)\n* IC: Decreased \,(\downarrow)
• FRC: Decreased \,(\downarrow)
• TLC: Decreased \,(\downarrow)
• RV/TLC ratio: Normal \N

Arterial Blood Gases in Severe ARDS

• Acute Ventilatory Failure with Hypoxemia (Acute Respiratory Acidosis)
• pH: Decreased \,(\downarrow)
• PaCO_2: Increased \,(\uparrow)
• HCO_3^-: Increased \,(\uparrow), but normal
• PaO_2: Decreased \,(\downarrow)
• SaO2/SpO2: Decreased \,(\downarrow)

Radiologic Findings

• Chest radiograph shows increased opacity, diffusely throughout lungs.
• Ground-glass appearance.

General Management of ARDS

• Intravenous corticosteroids
• Respiratory care treatment protocols
  • Oxygen therapy protocol
  • Lung expansion therapy protocol
  • Mechanical ventilation protocol

Ventilation Strategy for ARDS

• Recommended to immediately place the patient on invasive mechanical ventilation rather than doing an initial trial of noninvasive positive pressure ventilation.
• Full support mode of mechanical ventilation is recommended rather than a partially supported mode of ventilation.

ARDSnet Protocol

• ARDSnet is a network of the National Institutes of Health (NIH), National Heart, Lung, and Blood Institute (NHLBI), and Acute Respiratory Distress Syndrome (ARDS).
• Low-tidal volumes ventilation (LTVV) and high respiratory rates are used.
  • Initial tidal volume is usually set at 8 mL/kg.
  • The ability to drop down to 6 mL/kg IBW if needed to maintain a Pplat.
    • Pplat should be maintained between 25 and 30 cm H2O.
• If the Pplat drops below 25 cm H2O, the protocol is to increase the VT.
• Permissive hypercapnia is allowed.
  • PaCO_2 should not be permitted to increase to the point of severe acidosis (pH below 7.2).
  • Emphasis is on maintaining adequate gas exchange while minimizing the risks of ventilator-associated injuries (VALI).
• Common strategies used to reduce the complications of mechanical ventilation:
  • Low V_T
  • Reduction of the V_T, RR, or both to minimize auto-PEEP in COPD patients.

ARDSnet Mechanical Ventilation Protocol Summary

• OXYGENATION GOAL: PaO2 55-80 mmHg or SpO2 88-95%
• Use a minimum PEEP of 5 cm H2O. Consider use of incremental FiO_2/PEEP combinations to achieve goal.
• Inclusion Criteria:
  • Acute onset of
    • PaO2/FiO2 \le 300 (corrected for altitude)
  • Bilateral (patchy, diffuse, or homogeneous) infiltrates consistent with pulmonary edema
  • No clinical evidence of left atrial hypertension
• Part I: Ventilator Setup and Adjustment
  • Calculate predicted body weight (PBW)
    • Males = 50+ 2.3 [height (inches) - 60]
    • Females = 45.5+ 2.3 [height (inches) -60]
  • Select any ventilator mode
  • Set ventilator settings to achieve initial V_T = 8 ml/kg PBW
  • Reduce VT by 1 ml/kg at intervals < 2 hours until VT = 6ml/kg PBW.
  • Set initial rate to approximate baseline minute ventilation (not > 35 bpm).
  • Adjust V_T and RR to achieve pH and plateau pressure goals below.
• PLATEAU PRESSURE GOAL: < 30 cm H_2O
  • Check Pplat (0.5 second inspiratory pause), at least q 4h and after each change in PEEP or V_T.
  • If Pplat > 30 cm H2O: decrease VT by 1ml/kg steps (minimum = 4 ml/kg).
  • If Pplat < 25 cm H2O and VT< 6 ml/kg, increase VT by 1 ml/kg until Pplat > 25 cm H2O or V_T = 6 ml/kg.
  • If Pplat < 30 and breath stacking or dys-synchrony occurs: may increase VT in 1ml/kg increments to 7 or 8 ml/kg if Pplat remains < 30 cm H2O.

Therapeutic Goals Summarized

• Low-tidal volume, between 6 and 8 mL/kg
• High respiratory rates, not greater than 35 bpm
• Maintain the Pplat between 25 and 30 cm H2O
• Oxygenation level between PaO2 55 and 80 mm Hg, or an SpO2 between 88% and 95%
• pH between 7.30 and 7.45

Proning

• Patient is repositioned from the supine to the prone position
• Typically, 16 hours prone/8 hours supine
• Q2 head turns/swim position

Paralytics

• Neuromuscular blocking agents (NBA)
  • Control patient WOB
• Patient self-inflicted lung injury (P-SILI).
  • When the patients own drive to breathe causes lung injury

Extra Corporeal Membrane Oxygenation

• ECMO/ECLS: Blood is removed from the body, a machine removes CO2 and adds O2, then blood is returned.
• Veno-arterial (VA)/ Veno-venous (VV)
  • Placement of the catheter
  • VA lung and heart support
  • VV lung support

COVID-19 and ARDS

• WeVent International Mechanical Ventilation Group recently provided a unified respiratory monitoring and management protocol for the patient with severe COVID-19 and ARDS.
• Incorporates the use of the Sp02/FIO2 Ratio, PaO2/FIO2 Ratio, and SpO_2 to justify the up-regulation or down regulation of respiratory therapy.
• HFNC in early stages for the patient that does not have severe hypoxemia.
• NIV
• Suggested ventilator settings are as follows:
  • V_T = 6 mL/Kg of IBW
  • PEEP 10 cm H_2O
  • Driving Pressure: < 15 cm H_2O
  • Pplat < 30 cm H_2O
  • FIO2 to achieve SP02 90-97%
  • Sedation + Neural Muscular Block
• If the patient continues to deteriorate—i.e., P/F 150-200, or S/P 175-200 (Fi0_2 0.4 – 0.5)—Express PEEP is recommended as follows:
  • Initial PEEP: 10 cm H_2O
  • Increase PEEP 2 cm H_2O, every two minutes
  • Set the highest PEEP that maintains or improves S/F ratio and allows a Pplat of <30 cm H_2O
  • If the P/F is < 150, or the S/F is < 175 (FI0_2 > 0.5) after the express PEEP titration, Prone Positioning Ventilation is recommended as the next first line of treatment