Case 86 - ARDS

Question 1

define ARDS

Answer 1

• inflammation of lung parenchyma leading to impaired gas exchange, hypoexemia, noncardiogenic pulmonary edema

Question 2

what is the clinical criteria of ARDS?

Answer 2

• acute onset
• bilateral fluffly infiltrates on CXR
• non-cardiogenic pulm edema: PCWP < 18 mm Hg
• Acute lung injury: PaO2/FiO2 < 300
• ARDS: PaO2/Fio2 < 200

Question 3

what are causes of ARDS?

Answer 3

Direct lung injury:

• pneumonia
• aspiration
• air embolism
• fat embolism

Indirect lung injury:

• TRALI
• severe trauma
• sepsis
• post-CPB

Question 4

pathophys of ARDS: two hit hypothesis

Answer 4

Two-hit hypothesis

• first hit - pre-existing pulmonary pathology (i.e. the first-hit) leads to localization of neutrophils to the pulmonary microvasculature
• second hit - systemic inflammatory mediator releaes
  
  • cytokines, TNF-alpha, IL’s –> resulting in leaky pulmonary capillaries –> neutrophils entering lung parenchyma -> damages aleveolar tissue
  • result: protein-rich edema (exudate) into insterstitum and airspaces

Question 5

what is your ventilatory management for ARDS patient?

Answer 5

goals:

1) treat underlying cause

2) correct hypoxemia and hypercapnia

3) minimize ventilator-induced lung injury

Ventilator MGMT

1) set any ventilatory mode (PCV, VCV)

2) ventilation

• TV 4-6mL/kg
• adjust TV and RR:
  
  • to achieve pH 7.3 - 7.4
  • Pplat < 30 cm H2O
• okay for permissive hypercapnia

2) oxygenation

• maintain PaO2 55-80 mmHg or saturation of 88-95%
  
  • minimum PEEP of 5
  • minimum FiO2

3) lung protective strategy

• avoid barotrauma and volutrauma
• TV 4-6 mL/kg
• Pplat < 30 cm h2O
• PEEP -> prevents stress and strain of reopening and closing of alveoli

4) Acid-base management

• pH 7.3 - 7.4 is goal
• acidosis - increase RR (avoid PaCo2 < 25), increase TV (may need to go over 6 ml/kg or P plat> 30 cm h2o), consider sodium bicarb
• alkalosis - decrease RR

Question 6

can prone positioning help with ARDS?

Answer 6

prone position

• can improve oxygenation:
  
  • relieve atelectasis
  • enhance v/Q mismatch
  • more uniformily distribute lung stress and strain with tidal cycling
• no evidence of decreased mortality in ARDS patients
• **consider using for intractable hypoxemia

Question 7

what are recruitment maneuvers and do they improve oxygneation in ARDS?

Answer 7

recruitment maneuver:

• tranisent increase in transpulmonary pressure intended to promote reopening of collapsed alveoli –> improve oxygenation
• CPAP held at 35-50 cm H2o for 30 seconds
• no RCT to show benefit of mortality in ARDS

Question 8

what is high-frequency jet ventilation, and does it help with ARDS?

Answer 8

HFJV

• mode of mech vent that uses rapid RR (100-200 breaths/min) + low tV (2-5 cc/kg)
• smaller TV can help avoid volutrauma
• risks: hypercapina, resp acidosis –> increase ICP, increase CBF

no mortality benefit in ARDS

Question 9

what does inhaled nitric oxide do?

Answer 9

Inhaled NO

• potent vasodilator
• no systemic effects
• increases v/q matching: taken up by functional alveoli and promotes blood flow to functional alveoli due to pulm vasodilation in that particular area –> better oxygenation
• reduces PVR

Question 10

does fluid restriction help lung funciton in ARDS?

Answer 10

• ARDS increases pulmonary capillary permeability resulting in pulmonary edema
• liberal fluid use would worsen pulmonary edema
• restriction of fluids (dry lungs) has shown to improve oxygen indicies, ICU-free days, lower Pplat and PEEP
• mortaility rates do not differ though