ARF part 1.pdf
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Acute Respiratory Failure Tammy Slater, DNP, MS, ACNP-BC Overview REVIEW â–º Please review the following modules from Intro to Acute Care and Acute Care I Interpretation of arterial blood gases Mechanical ventilation Ventilation, oxygenation, and perfusion Pulmonary diso...
Acute Respiratory Failure Tammy Slater, DNP, MS, ACNP-BC Overview REVIEW â–º Please review the following modules from Intro to Acute Care and Acute Care I Interpretation of arterial blood gases Mechanical ventilation Ventilation, oxygenation, and perfusion Pulmonary disorders Assessment and management of pneumonia in hospitalized adults and older adults with acute COPD exacerbation Definition of Acute Respiratory Failure (ARF) â–º Definition: inadequate oxygenation or ventilation which threatens function of vital organs â–º Diagnostic criteria Hypoxemia: PaO2 < 60 mmHg and/or Hypercapnia: PaCO2 > 50 mmHg Pathophysiology â–º Oxygen binds hemoglobin after it diffuses across a pressure and concentration gradient from alveoli to pulmonary capillaries, which is generated by mean airway pressure (PEEP) and fraction of inspired oxygen (FiO2) â–º CO2 elimination depends upon minute ventilation (expiratory tidal volume x RR) â–º Hyperventilation increases CO2 elimination but has NO impact on hypoxemia Siegal EM. Acute Respiratory Failure. In: McKean SC, Ross JJ, Dressler DD, Scheurer DB. eds. Principles and Practice of Hospital Medicine, 2e New York, NY: McGraw-Hill;. http://accessmedicine.mhmedical.com/content.aspx?bookid=1872§ionid=146980188. Accessed May 02, 223. Pathophysiology: Two Types of ARF Type I ARF (hypoxic) Type II ARF (hypoxic hypercarbic) â–º PaO2 < 60 mmHg or â–º PaO2 < 60 mmHg PA-PaO2 (A/a gradient) > 25 â–º PaCO2 > 50 mmHg mmHg or increased shunt (PaO2/FiO2 ratio < 300) â–º And acidosis â–º PaCO2 normal or < 50 â–º Causes: alveolar mmHg hypoventilation, increased â–º Causes: V/Q mismatch, airway resistance, loss of alveolar hypoventilation, lung surface area, chest diffusion defect, R to L shunt, low ambient oxygen deformity Intrapulmonary Shunt â–º Percent of total venous blood that BYPASSES gas exchange (alveoli) and returns de-oxygenated to systemic circulation â–º Normal = 2-5% ïƒ up to max of 8-10% Estimated to increase with each decade of life Intrapulmonary shunt Normal 2-5% No clinical symptoms Mild 5-15% Short of breath, particularly on exertion Moderate 15-30% Resistant hypoxemia, requires substantial oxygenation support Severe > 30% Difficulty with spontaneous breathing, may require mechanical ventilation Siegal EM. Acute Respiratory Failure. In: McKean SC, Ross JJ, Dressler DD, Scheurer DB. eds. Principles and Practice of Hospital Medicine, 2e New York, NY: McGraw-Hill;. http://accessmedicine.mhmedical.com/content.aspx?bookid=1872§ionid=146980188. Accessed May 05, 2023. Ahrens, T., & Rutherford, K. (1993). Essentials of Oxygenation. Jones and Bartlett: Boston. Types of Shunts Wagner, K. High-Acuity Nursing, 2006 8 Intrapulmonary Shunt â–º True shunt calculation needs mixed venous blood â–º Ways to estimate shunt A-a gradient- least accurate with increasing FiO2, most common a/A ratio (Arterial to alveolar ratio)- most accurate, allows for changes in PaCO2 PaO2/FiO2 ratio- most accurate, can be easiest (if PaCO2 stable) â–º Use a/A ratio (PaO2 / PAO2) or PaO2/FiO2 ratio to estimate shunt For a/A ratio: must calculate Alveolar gas using standard equation PAO2 = [FiO2 (Patm-PH2O)-(PaCO2/RQ)] Then divide PaO2 / PAO2 A= alveolar a= arterial Ahrens, T., & Rutherford, K. (1993). Essentials of Oxygenation. Jones and Bartlett: Boston. Estimating Shunt Using a/A ratio arterial –Alveolar ratio Normal > 0.8 -0.9 PaO2 PAO2 Example: FiO2=.60 PaO2= 91 PaCO2= 41 PAO2 = FiO2 (Atm press-vapor press) – PaCO2/ RQ The lower the PAO2=.60 (760 – 47=713) – 41 / 0.8 ratio, the worse PAO2 = 377 the shunt a/A ratio= 91/377= 0.24 or 24% shunt Worsening shunt: a/A ratio is < 0.60 (60%) indicates increased/worsening shunt Ahrens, T., & Rutherford, K. (1993). Essentials of Oxygenation. Jones and Bartlett: Boston. Estimating Shunt Using PaO2/FiO2 Ratio Normal PaO2 / FiO2 ratio = 550 on 100 % Oxygen:
