Arterial Blood Gas Analysis • Measured Calculated • Oxygenation PO2 O2 Sat • pH • Ventilation PCO2 • Acid-Base status pH HCO3 • PCO2 Base excess
Gas Exchange Air 150 Lung and blood 100 PO2 Tissues 50 0 Atmosphere Mitochondria
Mechanisms of Hypoxemia • Pathophsyiologic hypoxemia • Hypoventilation • Ventilation-perfusion mismatch • Diffusion impairment • Shunt • Physiologic hypoxemia • Low inspired O2
Case #1 - Hypoventilation • An 18 year old male is brought to the ER after intravenously injecting heroin. He is comatose with a respiratory rate of 4 br/min. • Arterial blood gas analysis is performed: • pH = 7.26 • PaCO2 = 60 torr (normal = 40) • PaO2 = 65 torr (normal > 85) • Why is the patient hypoxemic?
Ideal Alveolar Gas Ventilation What is the Ideal Alveolar PO2? Mixed Venous Blood Alveolar Capillary
Gas Exchange LungCellular O2 uptake = O2 consumption CO2 excretion = CO2 production Respiratory quotient = R dependent on fuel type Normally 0.8 CO2 production O2 consumption
Ideal Gas Exchange O2 CO2 Room Air: PO2 = 150 [ .21 x (PB-PH2O)] PCO2 = 0 What is the Ideal Alveolar PO2? Mixed venous: PVO2 = 40 PVCO2 = 47 O2 Sat = 75%
Ideal Gas Exchange Room Air: PO2 = 150 [ .21 x (PB-PH2O)] PCO2 = 0 PACO2 PAO2 = 100 PACO2 = 40 Ideal Alveolar PO2 = 150 - R Mixed venous: PVO2 = 40 PVCO2 = 47 O2 Sat = 75% Ideal End-Capillary: PcO2 = 100 PcCO2 = 40 O2 Sat = 98% O2 CO2
Alveolar-arterial PO2 Difference Comparison of perfect lung to actual patient data: A-a PO2 difference = Ideal Alveolar PO2 - arterial PO2 < 15 torr (normal) Ideal Alveolar PO2 is calculated from Alveolar Air Equation: = FiO2 x (PB-PH2O) - = 150 - (for room air) PACO2 R PACO2 0.8
Effect of Hypoventilation on Alveolar Gas PAO2 O2 Saturation 100% 80% O2 Saturation (%) 60% 40% PACO2 20% 120 100 80 Alveolar PO2 and PCO2(torr) 60 40 Normal Value 20 2 4 6 8 10 Alveolar Ventilation (l/min) West JB. Pulmonary Pathophysiology 1998
Hypoventilation: Normal A-a PO2 Difference • Patient data: • PaCO2 =60torr • PaO2 = 65 torr • A-a PO2 difference = Alveolar PO2 - arterial PO2 • = (150 - 60/.08) - 65 • = 75 - 65 • = 10
Interpretation of A-a PO2 Difference(room air breathing) • Normal P(A-a)O2 < 15 indicates: • absence of oxygenation defect in lung • hypoxemia is due to hypercapnia (hypoventilation) • extrapulmonary disease process • Elevated P(A-a)O2 indicates: • oxygenation defect due to intrinsic pulmonary disease • concomitant hypercapnia will contribute to hypoxemia
Etiology of Hypoventilation West JB. Pulmonary Pathophysiology 1998
Case #2 - V/Q Mismatch • 25 year old male with a prior history of asthma is admitted to the ER with wheezing and dyspnea for the last 2 days. • Arterial blood gas analysis is performed: • pH = 7.45 • PaCO2 = 32 torr • PaO2 = 65 torr (A-a PO2 difference = 45) • Why is the patient hypoxemic?
Mucous plugs in airways Atelectasis Hyperinflation Pathology of Status Asthmaticus
V/Q Mismatch Normal (ideal) Low V/Q PAO2 = 100 PAO2 = 50 O2 CO2 O2 CO2 Mixed venous: PVO2 = 40 O2 Sat = 75% Mixed venous: PVO2 = 40 O2 Sat = 75% Ideal End-Capillary: PcO2 = 100 O2 Sat = 98% End-Capillary: PcO2 = 50 O2 Sat = 84%
Effect of Oxygen Therapy on Low V/Q RA (PIO2 = 150) 50% O2 (PIO2 = 357) PAO2 = 50 PAO2 = 200 O2 CO2 O2 CO2 Mixed venous: PVO2 = 40 O2 Sat = 75% Mixed venous: PVO2 = 40 O2 Sat = 75% End-Capillary: PcO2 = 50 O2 Sat = 84% End-Capillary: PcO2 = 200 O2 Sat = 99%
Case #3 - Diffusion Impairment • A 45 year old female with idiopathic pulmonary fibrosis comes to your office with increasing dyspnea for 3 years. • Arterial blood gas analysis is performed: • pH = 7.45 • PaCO2 = 32 torr • PaO2 = 65 torr (A-a PO2 difference = 45) • Why is the patient hypoxemic?
Diffusion Impairment Normal IPF
Diffusion Impairment Reduced Contact Time Thickened Alveolar Membrane Mixed Venous PO2 Exercise Alveolar PO2 100 80 60 PO2 mmHg 40 20 0 0.25 0.5 0.75 TIME IN CAPILLARY sec West JB. Pulmonary Pathophysiology 1998
Case #4 - Shunt • A 35 year old male is admitted to the ER with 2 days of fever, cough, purulent sputum and right sided chest pain. • Arterial blood gas analysis is performed: • pH = 7.45 • PaCO2 = 32 torr • PaO2 = 65 torr (A-a PO2 difference = 45) • Why is the patient hypoxemic?
Shunt Normal (ideal) Shunt PAO2 = 100 O2 CO2 Mixed venous: PVO2 = 40 O2 Sat = 75% Ideal End-Capillary: PcO2 = 100 O2 Sat = 98% Mixed venous: PVO2 = 40 O2 Sat = 75% End-Capillary: PcO2 = 40 O2 Sat = 75%
Assessment of Hypoxia • PO2 & O2 saturation • A-a PO2 difference • Calculation of total shunt on room air • Calculation of anatomic shunt on 100% O2
Total Shunt(room air) Mixed Venous Blood Ideal End-capillary Blood Arterial Blood Venous-like Admixture Diffusion V/Q Mismatch Anatomic Shunt
Anatomic Shunt(100% O2) Mixed Venous Blood Ideal End-capillary Blood Arterial Blood Venous Admixture Anatomic Shunt
Mechanisms of Hypercapnia Metabolic CO2 production PaCO2 CO2 excretion CO2 production Alveolar ventilation PaCO2 =
Mechanisms of Alveolar Hypoventilation Total Ventilation (VE) Dead Space Ventilation (VD) Alveolar Ventilation (VA): [VA = VE - VD] Alveolus O2 CO2 CO2 production VA CO2 production VE - VD PaCO2 = =
Assessment of Ventilation COMMAND (Drive) Neuromuscular Disease EFFORT Lung Disease VENTILATION
Alveolar Hypoventilation Neuromuscular Disease Ventilation Dead space Lung Disease Alveolus O2 CO2
Determinants of Ventilation Respiratory drive central chemoreceptors peripheral chemoreceptors lung/chest wall reflexes Respiratory muscles weakness fatigue VE Respiratory motorneurons Respiratory system impedence airways resistance lung stiffness chest wall stiffness