Acute Respiratory Failure
Michael Donahoe MD Division of Pulmonary, Allergy, and Critical Care Medicine 2015 Acute Respiratory Failure
A condition in which the arterial Pa02 is below or the arterial PaC02 is above the range of normal values expected for that individual
Type Pa02 PaCO2 PA02–Pa 02
Hypoxemic (Type I) ⇓ ⇓ − NL ⇑ Hypercapnic (Type II) ⇓ ⇑ NL Mixed (Type III) ⇓ ⇑ ⇑ ARF - Hypoxemic
• Abnormal V/Q or shunt physiology • Disorders of lung parenchyma • Airway • Alveoli / Interstitium • Pulmonary Vascular (A-a) Gradient
A comparison of the calculated alveolar oxygen partial pressure (Big A) to the arterial oxygen partial pressure (Little A)
(PAO2) = FIO2(PATM- PH2O) - PaCO2 / RQ
ROOM AIR
(PAO2) = 0.21 (760-47) - 40 / 0.8 = 100
Normal Gradient Estimate = (Age/4) + 4 Oxygenation Measures
PARAMETER CALCULATION COMMENT
Requires the use of the alveolar gas equation. The A-a gradient is unaffected A-a gradient PAO2 - PaO2 by changes in the PaCO2. Normal value age dependent. Varies with inspired oxygen concentration.
Requires calculation using the alveolar gas equation. Varies with FiO2. a/A ratio PaO2 / PAO2 Minimal variability in ARDS with FIO2 > 0.50 and PaO2 < 100.
Respiratory Index (PAO2 - PaO2)/ PaO2 Requires calculation using the alveolar gas equation. Varies with FiO2.
No calculation from the alveolar gas equation required. Does not account P/F Ratio PaO2 / FIO2 for variations in the blood PaCO2. Varies with FiO2. Minimal variability in ARDS with FIO2 > 0.50 and PaO2 < 100
Measure of the contribution of shunt, diffusion, and low V/Q regions to the Venous Admixture Shunt Equation measured PaO2. Varies with FIO2. Requires calculation from the alveolar gas equation and the measurement or assume mixed venous oxygen content. Oxygenation Measures ICU Limitations
• Supplemental Oxygen • Stability Over Range FiO2 Variability of indices of hypoxemia in adult respiratory distress syndrome
Gowda, Madhu S.; Klocke, Robert A. Cri cal Care Medicine. 25(1):41-45, January 1997. doi:
Figure 1. Relation between PaO2/FIO2 and FIO2 for patients with <30% true shunt (top) and patients with >30% shunt (bottom). PaO2/FIO2 ratios, characterized by FIO2 values of >or=to0.5 and PaO2 values of Copyright © 2011 Critical Care Medicine. Published by Lippincott Williams & Wilkins. 2 Closed Circuit Flow Independent Mechanical Ventilation Air (100 psi) Blender 100% Wall Oxygen (100 psi) Patient Open Circuit Low Flow Patient Flow Dependent Nasal Cannula High Flow Device Flow < Patient Flow 100% Wall Oxygen (Flow Fixed) Patient RA - entrainment (variable) Open Circuit Low Flow Flow Dependent Facemask / NIV High Flow Device Flow > Patient Flow 100% Wall Oxygen (Flow Fixed) Patient Dial O2 Flow Total Flow 36 14 74 40 15 62 50 15 41 60 15 30 RA - entrainment (fxed) 80 Flush 54 95 Flush 43 Open Circuit Low Flow Flow Dependent Facemask / NIV High Flow Device Flow < Patient Flow 100% Wall Oxygen (Flow Fixed) Patient RA - entrainment (fxed) RA - entrainment (variable) •FiO2 unknown •FiO2 comparison difficult •Flow variable > FiO2 variable •Know your therapist • Standard Error = + 4% • Pigment Dependent • Light 92% • Dark 94% • ? Bilirubin • Perfusion Dependent • False Errors • Carboxyhemoglobin • Methemoglobin Hypoxemia • V/Q mismatch - titrate O2 up • chronic obstructive pulmonary disease • asthma • obesity / hypoventilation • pulmonary embolism • Shunt - titrate O2 down • pulmonary edema • atelectasis • pneumonia PaO2 vs SaO2 • Oxygen diffusion (PaO2) •Lung (A-a) •Tissues • Oxygen transport (SaO2) •Arterial oxygen content •Venous oxygen content FiO2 = 0.70 PEEP = 5.0 cm H2O PaO2 = 352 SaO2 = 95% PaO2/FiO2 = 502 FiO2 = 0.70 PEEP = 5.0 cm H2O PaO2 = 82 SaO2 = 95% PaO2/FiO2 = 117 Acute Respiratory Failure A condition in which the arterial Pa02 is below or the arterial PaC02 is above the range of normal values expected for that individual Type Pa02 PaCO2 PA02–Pa 02 Hypoxemic (Type I) ⇓ ⇓ − NL ⇑ Hypercapnic (Type II) ⇓ ⇑ NL Mixed (Type III) ⇓ ⇑ ⇑ Minute Ventilation hypermetabolism VCO2 VE = ------pCO2 ( 1 - VD / VT) dyspnea bronchospasm hypoxemia mucous plugging anxiety V/Q maldistribution Equation of Motion Resistive pressure Ventilating VT/Ti x resistance Pressure Elastic pressure VT / compliance Equation of Motion Ventilating Pressure = Elastic Pressure + Resistive Pressure (driving force) (volume) (flow) Pmus + Pvent = Pelastic Presistive (driving force) (volume/compliance) + (flow x resistance) PRS = Pao +(-Pmus) = (VT / CR) + (VT/Ti x RR) + PEEPtotal Hypercapnic Respiratory Failure •Central Nervous System •Neuromuscular Transmission •Thorax and Pleural Space Acute Respiratory Failure A condition in which the arterial Pa02 is below or the arterial PaC02 is above the range of normal values expected for that individual Type Pa02 PaCO2 PA02–Pa 02 Hypercapnic (Type II) ⇓ ⇑ NL METABOLIC hypermetabolism VCO2 VE = ------pCO2 ( 1 - VD / VT) dyspnea bronchospasm hypoxemia mucous plugging anxiety V/Q maldistribution MECHANICAL Ventilating Pressure Elastic Pressure Resistive Pressure (driving force) (volume) + (flow) PRS = Pao +(-Pmus) = (VT / CR) + (VT/Ti x RR) + PEEPtotal Acute Respiratory Failure A condition in which the arterial Pa02 is below or the arterial PaC02 is above the range of normal values expected for that individual Type Pa02 PaCO2 PA02– Pa 02 Mixed (Type III) ⇓ ⇑ ⇑ METABOLIC hypermetabolism VCO2 VE = ------pCO2 ( 1 - VD / VT) dyspnea bronchospasm hypoxemia mucous plugging anxiety V/Q maldistribution MECHANICAL Ventilating Pressure Elastic Pressure Resistive Pressure (driving force) (volume) + (flow) PRS = Pao +(-Pmus) = (VT / CR) + (VT/Ti x RR) + PEEPtotal Mechanical Ventilation • Non-invasive Mechanical Ventilation (NIV) • Continuous Positive Airway Pressure (CPAP) • with or without inspiratory support (BIPAP) • Average Volume Assured Pressure Support (AVAPS) • Helium-oxygen (Heliox) gas mixtures • Invasive Mechanical Ventilation (IMV) Non - Invasive MV • Indications • Chronic Obstructive Pulmonary Disease • Congestive Heart Failure • Contraindications • Impending Cardiovascular Collapse • Unstable upper airway (obstruction/secretions/ protection) • Recent upper airway or GI surgery Non-invasive MV n=14 n=15 ✓Reduced intubation ✓Reduced intubation ✓Reduced complications ✓Reduced complications ✓Reduced hospital LOS ✓Reduced mortality ✓Reduced mortality Ram FSF et al. Cochran Database 2004 Masip et al. JAMA. 2005;294:3124 Mechanical Ventilation IMV NIMV Volume SET Variable Flow SET Variable Timing SET Variable Pressure VARIABLE SET