Hypoxemic Respiratory Failure

Info

🌱 來自: Huppert’s Notes

Hypoxemic Respiratory Failure🚧 施工中

Hypoxemic Respiratory Failure

•   Definition: PaO2 <60 mmHg, P/F <300, or PaO2 decrease from baseline of 10 mmHg

•   Pathophysiology: Hypoxemia has five major mechanisms:

-   Hypoventilation (↑PaCO2, Normal AaDO2): See “hypercarbic respiratory failure” in next section

-   Low inspired FiO2 (Normal PaCO2, Normal AaDO2): High altitude

-   Low DLCO/diffusion impairment (Normal PaCO2, ↑AaDO2, Response to O2)

•   In normal states, O2 is a perfusion-limited gas (i.e., O2 exchange at alveolar–capillary bed limited by blood flow since O2 in the blood and alveolus equilibrates early along the capillary). Therefore, issues with O2 diffusion requires significant diffusion impairment

•   In physiologic stress (e.g., intense exercise) or pathologic states, O2 becomes a diffusion-limited gas; examples include pulmonary fibrosis (due to alveolar membrane thickening) and emphysema (due to destruction of lung → decreased surface area for diffusion)

-   V/Q mismatch (Normal PaCO2, ↑AaDO2, Response to supplemental O2)

•   Normal V/Q = 0.8 (highest at apices, lowest at bases)

•   Occurs with focal alveolar processes (e.g., pneumonia, pulmonary edema, aspiration/mucus plugging, ILD)

-   Shunt (Normal PaCO2, ↑AaDO2, No response to supplemental O2)

•   Perfused (Q > 0) but not ventilated (V = 0; therefore, V/Q = 0). Supplemental O2 does not help.

•   Both pulmonary and cardiac etiologies exist

-   Pulmonary shunting: Physiologic (pulmonary edema, ARDS, DAH, atelectasis), AVM, hepatopulmonary syndrome

-   Intracardiac shunting (assess using TTE with bubble):

•   R→L shunt: Blood from the right heart enters circulation without going through the lungs (tetralogy of Fallot). Reduced PaO2

•   L→R shunt: More common because left pressure higher (patent ductus arteriosus, trauma). Does NOT reduce PaO2. Over time, can get reversal of shunt to R→L causing hypoxemia (Eisenmenger’s syndrome)

•   Physical Exam: See physical exam under dyspnea above

•   Workup:

-   ABG: PaO2 measures gas exchange; PaCO2 measures ventilation. A-a gradient: PaO2-PaO2, normal (age+4)/4

-   EKG: Evaluate for ischemia, evidence of right heart strain to support PE (S1Q3T3, RAD, RBBB)

-   Labs: CBC, BMP, troponin, BNP

-   CXR: Evaluate for infiltrate or lobar collapse, pneumothorax, pulmonary edema, hyperinflation, or for no evidence of abnormality (Pearl: CXR often looks normal for PE, COPD, early aspiration)

•   Treatment: Supplemental O2, airway clearance (i.e., suction secretions), consider furosemide, albuterol, antibiotics, NIPPV, intubation and/or escalation of care (i.e., transfer to ICU)