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Pathophysiology and General Management of Failure of Arterial Oxygenation. Dong Soo Kim, M.D. aruma@khu.ac.kr www.nopain365.com. Respiration ; exchange of oxygen and carbon dioxide between humans and the atmosphere Human respiration ; - Ventilation - Arterial oxygenation
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Pathophysiology and General Managementof Failure of Arterial Oxygenation Dong Soo Kim, M.D. aruma@khu.ac.kr www.nopain365.com
Respiration ; exchange of oxygen and carbon dioxide between humans and the atmosphere • Human respiration ; - Ventilation - Arterial oxygenation - Oxygen transport - Oxygen extraction and utilization • Respiratory failure ; BGA abnormalities of high PaCO2 and low PaO2 or both
Standards for Basic Anesthetic Monitoring • Standard IQualified anesthesia personnel shall be present in the room throughout the conduct of all general anesthetics, regional anesthetics and monitored anesthesia care.(Approved by the ASA House of Delegates on Oct. 15, 2003)
Standards for Basic Anesthetic Monitoring • Standard IIDuring all anesthetics, the patient’s oxygenation, ventilation, circulation, and temperature shall be continually evaluated.(Approved by the ASA House of Delegates on Oct. 15, 2003)
Definition of Hypoxemia • PaO2 = 105 – Age / 2 - PaO2 of 50 year-old man ; 80 mm Hg - PaO2 of 80 year-old man ; 65 mm Hg • Defining Hypoxemia - Harrison’s (1998) :PaO2< 80 mm Hg -Shapiro BA (1998): relative deficiency of oxygen tension in the arterial blood -Morgan GE(2006): PaO2< 60 mm Hg
Objective and Methods for Oxygenation Monitoring • Objective ;To ensure adequate O2 concentration in the inspired gas and the blood during all anesthetics • Methods ; - Inspired gas : During every administration of general anesthesia using an anesthesia machine, theconcentration of O2 in the breathing system shall be measured by an O2 analyzer with a low O2concentration limit alarm in use.(Approved by the ASA House of Delegates on Oct. 15, 2003)
Objective and Methods for Oxygenation Monitoring • Objective ;To ensure adequate O2 concentration in the inspired gas and the blood during all anesthetics • Methods ; - Blood oxygenation : During all anesthetics, a quantitative method of assessing oxygenation such as pulse oximetry shall be employed. Adequate illumination and exposure of the patient are necessary to assess color.(Approved by the ASA House of Delegates on Oct. 15, 2003)
Monitoring during Anesthesia • BP, heart rate • ECG • CVP • Pulse oximetry • Capnography • Urine output • Temperature • BIS (bispectral index) • Cardiac output • Cerebral metabolic rate of oxygen (CMRO2)
Transfer of O2 and CO2 Between Alveolar Air and Capillary Blood
Process of Oxygenation • External respiration • Blood oxygen transport • Internal respiration
External Respiration • Transfer of oxygen molecules from atmosphere to blood • Alveolar oxygen tension (PAO2) is the major limiting factor in the oxygenation of desaturated blood • Major factors involved in determining the PAO2 - Fraction of inspired oxygen (FiO2) - Alveolar gas exchange - Mixed venous oxygen content - Distribution of ventilation
Mixed Venous PO2 as an Indicator • The most reliable single physiologic indicator for monitoring the overall balance between oxygen supply and demand • PvO2 < 28 mm Hg ; - anaerobic metabolic state - blood lactate levels ↑ Hyperdynamic 45 Normal 35 Compromised PvO2 27 Metabolic disruption 20 Death
Critical level of PvO2 20 18 16 Blood lactate (mEq/L) 14 Survivors 12 Nonsurvivors 10 8 6 Upper limit of normal lactate conc. 4 2 0 20 22 24 26 28 30 32 34 36 38 40 PvO2(mm Hg) Relation between blood lactate and PvO2 JAMA 1976;236:570
Aerobic Glucose ↓ Pyruvate ↓ CO2 + H2O + ATP Anaerobic Glucose ↓ Pyruvate ↓ Lactate + ATP Cellular Metabolism
Pathophysiology of Impaired Tissue Oxygenation • Failure of arterial oxygenation • Failure of oxygen transport • Failure of tissue oxygen extraction
Mechanisms of Hypoxemia • Low alveolar O2 tension • - Low inspired O2 tension • - Alveolar hypoventilation • - Third gas effect (diffusion hypoxia) • - Increased O2 consumption • Increased alveolar - arterial O2 gradient • - Shunting • - Ventilation / perfusion mismatch • - Low mixed venous O2 tension • - Decreased cardiac output • - Increased O2 consumption • - Decreased Hb concentration
Common Factors Causing Intraoperative and/or Postoperative Hypoxemia • Low inspired O2 concentration • Hypoventilation • Ventilation / perfusion mismatch • Increased intrapulmonary Rt to Lt shunt - Atelectasis ; main contributor • Pneumothorax • Pulmonary edema • Pulmonary embolism
Low Alveolar Oxygen Tension Diffusion Hypoxia
Increased A – a Oxygen Gradient Abnormal Shunts
Increased A – a Oxygen Gradient Endobronchial Intubation
Increased A – a Oxygen Gradient V/Q Mismatch Shunt ↓V/Q ratio(shunt effect)
Increased A – a Oxygen Gradient V/Q Mismatch ↑V/Q ratio Deadspace
Effect of Anesthesia on Gas Exchange • During anesthesia, - Deadspace (normal ventilation, no perfusion) ↑ - Alveolar ventilation ↓ (rate and/or tidal volume) - intrapulmonary shunting ↑ (5-10% ↑, as a result of atelectasis, airway obstruction) • Prolonged administration of high concentration of O2(> 50%) may be associated with increases in absolute shunt(absorption atelectasis) • Inhalation anestheticsinhibithypoxic pulmonary vasoconstriction(ED50 ; 2 MAC)
Mucociliary Blanket • Normal defense mechanism of the pulmonary tree • Submucosal gland ; mucus produce (100 mL/d) • Mucus ; water (95%), glycoprotein (2%), carbohydrate (1%) • Mucus blanket moves mucus and foreign bodies toward the larynx (2 cm/min) • Inhalation anesthetics, smoke ; ciliary movement ↓
Potential Results of Retained Secretions Inflammation and partial plugging Airflow resistance ↑ Work of breathing ↑ Uneven distribution of ventilation Shunt effect Hypoxemia Total plugging Absorption atelectasis Lung compliance ↓ Stasis pneumonia Fever
Various Factors as Causes of Clinically Detectable Atelectasis following Major Surgery Bendixen HH. Respiratory care. 1974
Consequences of Hypoxemia • CNS ; cerebral vasodilation, confusion, convulsion, unconsciousness, coma • CV ; myocardial depression, coronary vasodilation, bradycardia, dysrhythmia, heart failure, shock • Resp ; hypoxic pulmonary vasoconstriction • Renal ; dec RBF and tubular function
Events Associated with Decreased PaO2 Snyder JV. Oxygen transport in the critical ill. 1995
“삼순이 없는 세상 살아서 뭐하나……”
General Management of Hypoxemia • Directed at the cause • O2 therapy • Reserve normal CV function • PEEP • Chest physical therapy
Effect of Hypoventilation on Oxygen Therapy Braun HA. Introduction to Respiratory Physiology. 1990
Effect of Ventilation / Perfusion Mismatching on Oxygen Therapy Braun HA. Introduction to Respiratory Physiology. 1990
Effect of Shunt on Oxygen Therapy Braun HA. Introduction to Respiratory Physiology. 1990
Effect of Varying Amounts of Shunt on Arterial Oxygen Tension Br J Anaesth 1973;45:711
Hypoxemia and Oxygen Therapy • Refractory hypoxemia; Secondary to true shunt mechanism • Responsive hypoxemia ; Secondary to shunt effect