ARDS Lecture

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ACUTE RESPIRATORY DISTRESS SYNDROMEMichael L. Fiore, MD Fellow in Critical Care Medicine Mary W. Lieh-Lai, MD, Director, ICU and Fellowship Program Division of Critical Care Medicine Childrens Hospital of Michigan/Wayne State UniversityChildrens Hospital of Michigan

A.K.A.Adult Respiratory Distress Syndrome Da Nang Lung Transfusion Lung Post Perfusion Lung

Childrens Hospital of Michigan

HISTORICAL PERSPECTIVESDescribed by William Osler in the 1800s Ashbaugh, Bigelow and Petty, Lancet 1967 12 patients pathology similar to hyaline membrane disease in neonates ARDS is also observed in children New criteria and definition

Childrens Hospital of Michigan

ORIGINAL DEFINITIONAcute respiratory distress Cyanosis refractory to oxygen therapy Decreased lung compliance Diffuse infiltrates on chest radiograph Difficulties: lacks specific criteria controversy over incidence and mortality

Childrens Hospital of Michigan

REVISION OF DEFINITIONS1988: four-point lung injury score Level of PEEP PaO / FiO ratio 2 2 Static lung compliance Degree of chest infiltrates 1994: consensus conference simplified the definition

Childrens Hospital of Michigan

1994 CONSENSUS Acute onsetmay follow catastrophic event Bilateral infiltrates on chest radiograph PAWP < 18 mm Hg Two categories: Acute Lung Injury - PaO /FiO ratio 2 2 < 300 ARDS - PaO /FiO ratio < 200 2 2Childrens Hospital of Michigan

EPIDEMIOLOGYEarlier numbers inadequate (vague definition) Using 1994 criteria: 17.9/100,000 for acute lung injury 13.5/100,000 for ARDS Current epidemiologic study underway In children: approximately 1% of all PICU admissions

Childrens Hospital of Michigan

INCITING FACTORSShock Aspiration of gastric contents Trauma Infections Inhalation of toxic gases and fumes Drugs and poisons MiscellaneousChildrens Hospital of Michigan

STAGESAcute, exudative phase rapid onset of respiratory failure after trigger diffuse alveolar damage with inflammatory cell infiltration hyaline membrane formation capillary injury protein-rich edema fluid in alveoli disruption of alveolar epitheliumChildrens Hospital of Michigan

STAGESSubacute, Proliferative phase: persistent hypoxemia development of hypercarbia fibrosing alveolitis further decrease in pulmonary compliance pulmonary hypertension

Childrens Hospital of Michigan

STAGESChronic phase obliteration of alveolar and bronchiolar spaces and pulmonary capillaries Recovery phase gradual resolution of hypoxemia improved lung compliance resolution of radiographic abnormalitiesChildrens Hospital of Michigan

MORTALITY40-60% Deaths due to: multi-organ failure sepsis Mortality may be decreasing in recent years better ventilatory strategies earlier diagnosis and treatmentChildrens Hospital of Michigan

PATHOGENESISInciting event Inflammatory mediators Damage to microvascular endothelium Damage to alveolar epithelium Increased alveolar permeability results in alveolar edema fluid accumulation

Childrens Hospital of Michigan

NORMAL ALVEOLUSType I cell Alveolar macrophage Endothelial Cell RBCs Type II cell Capillary

Childrens Hospital of Michigan

ACUTE PHASE OF ARDSType I cell Alveolar macrophage Endothelial Cell RBCs Type II cell Capillary Neutrophils

Childrens Hospital of Michigan

PATHOGENESIS Target organ injury from hosts inflammatory response and uncontrolled liberation of inflammatory mediators Localized manifestation of SIRS Neutrophils and macrophages play major roles Complement activation Cytokines: TNF- , IL-1 , IL-6 Platelet activation factor Eicosanoids: prostacyclin, leukotrienes, thromboxane Free radicals Nitric oxideChildrens Hospital of Michigan

PATHOPHYSIOLOGYAbnormalities of gas exchange Oxygen delivery and consumption Cardiopulmonary interactions Multiple organ involvement

Childrens Hospital of Michigan

ABNORMALITIES OF GAS EXCHANGEHypoxemia: HALLMARK of ARDS Increased capillary permeability Interstitial and alveolar exudate Surfactant damage Decreased FRC Diffusion defect and right to left shunt

Childrens Hospital of Michigan

OXYGEN EXTRACTIONCell

O2Arterial Inflow (Q)O2 O2capillary

O2 O2 O2

O2 O2

Venous Outflow (Q)

VO2 = Q x Hb X 13.4 X (SaO2 - SvO2)(Adapted from the ICU Book by P. Marino)Childrens Hospital of Michigan

OXYGEN DELIVERYDO2 = Q X CaO2 DO2 = Q X (1.34 X Hb X SaO2) X 10 Q = cardiac output CaO2 = arterial oxygen content Normal DO2: 520-570 ml/min/m2 Oxygen extraction ratio = (SaO2-SvO2/SaO2) X 100 Normal O2ER = 20-30%Childrens Hospital of Michigan

HEMODYNAMIC SUPPORTMax O2 extraction Max O2 extraction

VO2Critical DO2

VO2Critical DO2

DO2

DO2

NormalVO2 = DO2 X O2ERChildrens Hospital of Michigan

Septic Shock/ARDSAbnormal Flow Dependency

OXYGEN DELIVERY & CONSUMPTIONPathologic flow dependency Uncoupling of oxidative dependency Oxygen utilization by non-ATP producing oxidase systems Increased diffusion distance for O2 between capillary and alveolusChildrens Hospital of Michigan

CARDIOPULMONARY INTERACTIONSA = Pulmonary hypertension resulting in increased RV afterload B = Application of high PEEP resulting in decreased preload A+B = Decreased cardiac output

Childrens Hospital of Michigan

RESPIRATORY SUPPORTConventional mechanical ventilation Newer modalities: High frequency ventilation ECMO Innovative strategies Nitric oxide Liquid ventilation Childrens Hospital of MichiganExogenous surfactant

MANAGEMENTMonitoring:

Respiratory Hemodynamic Metabolic Infections Fluids/electroly tes

Childrens Hospital of Michigan

MANAGEMENTOptimize VO2/DO2 relationship DO2 hemoglobin mechanical ventilation oxygen/PEEP VO2 preload afterload contractility Childrens Hospital of Michigan

CONVENTIONAL VENTILATIONOxygen PEEP Inverse I:E ratio Lower tidal volume Ventilation in prone positionChildrens Hospital of Michigan

RESPIRATORY SUPPORTGoal: maintain sufficient oxygenation and ventilation, minimize complications of ventilatory management Improve oxygenation: PEEP, MAP, Ti, O2 Improve ventilation: change in pressureChildrens Hospital of Michigan

Mechanical Ventilation GuidelinesAmerican College of Chest Physicians Consensus Conference 1993 Guidelines for Mechanical Ventilation in ARDS When possible, plateau pressures < 35 cm H2O

Tidal volume should be decreased if necessary to achieve this, permitting increased pCO2

Childrens Hospital of Michigan

PEEP - BenefitsIncreases transpulmonary distending pressure Displaces edema fluid into interstitium Decreases atelectasis Decrease in right to left shunt Improved compliance Improved oxygenationChildrens Hospital of Michigan

No Benefit to Early Application of PEEPPepe PE et al. NEJM 1984;311:281-6. Prospective randomization of intubated patients at risk for ARDS Ventilated with no PEEP vs. PEEP 8+ for 72 hours No differences in development of ARDS, complications, duration of ventilation, time in hospital, duration of ICU stay, morbidity or mortality Childrens Hospital of Michigan

Everything hinges on the matter of evidenceCarl Sagan

Childrens Hospital of Michigan

Pressure-controlled Ventilation (PCV)Time-cycled mode Approximate square waves of a preset pressure are applied and released by means of a decelerating flow More laminar flow at the end of inspiration More even distribution of ventilation in patients with marked different Childrensresistance values from one region of Hospital of Michigan

Conventional inspiratory-expiratory ratio is reversed (I:E 2:1 to 3:1) Longer time constant Breath starts before expiratory flow from prior breath reaches baseline auto-PEEP with recruitment of alveoli Lower inflating pressures Potential for decrease in cardiac output Childrensdue of Michigan Hospital to increase in MAP

Pressure-controlled Inverse-ratio Ventilation

Extracorporeal Membrane Oxygenation (ECMO)Zapol WM et al. JAMA 1979;242(20):2193-6 Prospectively randomized 90 adult patients Multicenter trial Conventional mechanical ventilation vs. mechanical ventilation supplemented with partial venoarterial bypass Childrens Hospital of Michigan No benefit

Partial Liquid Ventilation (PLV)Ventilating the lung with conventional ventilation after filling with perfluorocarbon Perflubron 20 times O and 3 times the CO 2 2 solubility Heavier than water Higher spreading coefficient Studies in animal models suggest improved compliance and gas Childrens Hospital of Michigan exchange

Partial Liquid Ventilation (PLV)CL Leach, et al. NEJM 1996;335:761-7. The LiquiVent Study Group 13 premature infants with severe RDS refractory to conventional treatment No adverse events Increased oxygenation and improved pulmonary compliance 8 of 10 survivorsChildrens Hospital of Michigan

Hirschl et al JAMA 1996;275:383-389 10 adult patients on ECMO with ARDS Ann Surg 1998;228(5):692-700 9 adult patients with ARDS on conventional mechanical ventilation Improvements in gas exchange with few complications No randomized or case controlled Childrens Hospital trials of Michigan

Partial Liquid Ventilation (PLV)

High-Frequency Jet VentilationCarlon GC et al. Chest 1983;84:551-59 Prospective randomization of 309 adult patients with ARDS to receive HFJV vs. Volume Cycled Ventilation VCV provided a higher PaO 2 HFJV had slightly improved alveolar ventilation No difference in survival, ICU stay, or complicationsChildrens Hospital of Michigan

High Frequency Oscillating Ventilator (HFOV)Raise MAP Recruit lung volume Small changes in tidal volume Impedes venous return necessitating intrava