Blood Gases: Pathophysiology and Interpretation

1. Blood Gases: Pathophysiology and Interpretation Tintinalli Chapter 26 Sept. 1, 2005

2. Definitions • Ventilation: Is a function of the rate and depth of breathing and determines the clearance of carbon dioxide from the body • Oxygenation: Diffusion of oxygen from the lungs to the bloodstream for subsequent delivery to the tissues

3. Minute Ventilation • Total amount of new air moved in and out of the airways and lungs each minute • Equals tidal volume multiplied by the respiratory rate • Normal tidal volume is 7mL/kg, or 500mL in adult • Normal rate is 12 breaths/minute • Normal minute ventilation is 6L/min

4. Dead Space • Anatomic dead space occurs in the trachea, bronchi, and bronchioles • Alveolar dead space (high V/Q mismatch) occurs when ventilation of the alveolar-capillary is normal but perfusion is absent • The combined dead space is physiologic dead space and is about 30% of the tidal volume • ARDS and COPD can increase dead space to 60% • Dead space over 60% typically requires intubation

5. Partial Pressures • Normal atmospheric pressure is 760mm Hg • Partial pressure of H20 is 47mm Hg and is subtracted from atmospheric pressure (760-47=713) • Remaining gases are: • Nitrogen 79% (563mm Hg) • Oxygen 21% (149mm Hg) • CO2 0.04% (0.3mm Hg)

6. Alveolar Gas Equation • For each mL of O2 leaving the alveolus, 0.8 to 1.0 mL of CO2 enters it • This is the respiratory quotient (RQ)

7. A-a Gradient • Determines the degree of lung function impairment • The A-a gradient is the partial pressure of alveolar oxygen minus the partial pressure of arterial oxygen (PAO2-PaO2) • Normal is 2-10mm Hg or 10 plus one tenth the person’s age

8. A-a Gradient • PAO2=(PB-PH2O)(FIO2)-PaCO2/RQ • PAO2=(760-47)(0.21)-40/0.08 • PAO2=100mm Hg at sea level in room air • PaO2 in a normal, healthy adult in room air at sea level is 90-100mm Hg • So, the PAO2-PaO2 is 100 minus 90, or about 10mm Hg

9. A-a Gradient • PAO2-PaO2 of 20-30mm Hg on room air indicates mild pulmonary dysfunction, and greater than 50mm Hg on room air indicates severe pulmonary dysfunction • The causes of increased gradient include intrapulmonary shunt, intracardiac shunt, and diffusion abnormalities

10. PaO2 • Factors affecting the PaO2 include alveolar ventilation, FIO2, altitude, age, and the oxyhemoglobin dissociation curve • Relation between PaO2 and SaO2: PaO2 corresponds to SaO2 60mm Hg 90% 50mm Hg 80% 40mm Hg 70% 30mm Hg 60%

11. Alveolar Ventilation • During hyperventilation, the PaCO2 falls and the PaO2 rises • If the PaCO2 falls by 1mm Hg, the PaO2 rises by about 1.0-1.2mm Hg

12. Factors Affecting Oxyhemoglobin Dissociation • pH: • The more acidic the blood, the more readily hemoglobin gives up oxygen and the higher the PaO2 • With alkalosis, hemoglobin binds more tightly to oxygen • A rise or fall in pH of 0.10 causes a fall or rise in the PaO2 of about 10%, respectively

13. Factors Affecting Oxyhemoglobin Dissociation • Partial pressure of CO2 • CO2 entering blood from tissues shifts the curve to the right • Oxygen is displaced from the hemoglobin and delivers oxygen at a higher PO2 than normal

14. Factor Affecting Oxyhemoglobin Dissociation • Temperature • With a rise in blood temperature, hemoglobin releases oxygen more readily, which increases the PO2 in the plasma

15. Factors Affecting Oxyhemoglobin Dissociation • Exercise • With exercise, muscles release large amounts of CO2 and acids • Muscle temperature can rise 3-4oC • Combined, these shift the oxyhemoglobin dissociation curve to the right, which releases O2 more readily

16. Factors Affecting Oxyhemoglobin Dissociation • 2,3-Diphosphoglycerate (2,3-DPG) • With prolonged hypoxia over several hours, 2,3-DPG quantities increase, which shifts the dissociation curve to the right • If the concentration of 2,3-DPG falls, such as in banked blood or sepsis, the curve shifts left and the PaO2 falls

17. PaO2/FIO2 Ratio • To estimate the impairment of oxygenation, calculate the PaO2/FIO2 ratio • Normally, this ratio is 500-600 • Below 300 is acute lung injury* • Below 200 is ARDS* *Along with diffuse infiltrates, normal PCWP, and appropriate mechanism

18. Pulse Oximetry • Factors that affect the pulse ox effectiveness • Impaired local perfusion (hypothermia, vasopressors) • Ambient light (fluorescent) • Nail polish (particularly blue) • Abnormal hemoglobin • Very high PO2 • Carboxyhemoglobin falsely raises readings • Methemoglobin falsely lowers the readings

19. Questions 1. A SaO2 of 90% corresponds to a PaO2 of: A. 40% B. 50% C. 60% D. 70% E. 80% 2. Which of the following affects the pulse oximetry readings? A. Blue nail polish B. Ambient fluorescent lighting C. Hypothermia D. Carboxyhemoglobin E. All will adversely affect the pulse ox readings 3. Which is false regarding oxyhemoglobin dissociation? A. As temperature rises, hemoglobin binds oxygen with more affinity B. Exercise shifts the dissociation curve to the right, releasing oxygen more readily C. Hemoglobin binds oxygen with more affinity in an alkylotic state D. During sepsis, the curve shifts left, causing a decrease in the PaO2

20. Questions 4. ARDS is defined as bilateral diffuse infiltrates, normal PCWP, appropriate mechanism, and a PaO2/FIO2 ratio of what? A. Below 500 B. Below 400 C. Below 300 D. Below 200 E. Below 100 5. True or False Minute ventilation is the respiratory rate multiplied times the tidal volume

21. Answers 1. C 2. E 3. A 4. D 5. T