Hypoxia and Hyperventilation

1. Hypoxia and Hyperventilation Iraj yasaei MD Flight physician

2. physiology of hypoxia, is at the basis of high-altitude medicine, plays an important role in aviation field environment.

3. Evangelista Torricelli (1608 –1647) was the first person to realize that the atmosphere above us creates a pressure that can, for example, support a column of mercury.

4. Relationship of Altitude to Barometric Pressure PRESSURE ALTITUDE FEET mm/HG ATMOSPHERES 0 760 1 18,000 380 1/2 34,000 190 1/4 48,000 95 1/8 63,000 47 1/16

5. Composition of the Air • 78 Percent Nitrogen N2 • 21 Percent Oxygen • 1 Percent Other • .03 percent CO2 ≈ 0

6. PERCENT COMPOSITION OF THE ATMOSPHERE REMAINS CONSTANT BUT PRESSURE DECREASES WITH ALTITUDE

7. The effect of altitude on O2 tension(breathing air) O2 120 100 80 60 40 20 0 Alveolar oxygen tension CO2 0 5000 10000 15000 20000 25000 Altitude

8. The most important feature of compensatory mechanism of body for reduced alveolar oxygen pressure (Hypoxia) : • In acute exposure…is……hyperventilation • In chronic exposure…is…. Polycythemia

9. Alveolar PO2 and PCO2 of acclimatized humans at high altitude. • Sea level is at the top right of the graph, and the summit of Mount Everest is at the bottom left. • Note that after a certain altitude has been exceeded, alveolar PO2 does not decrease further, It is defended at a level of about 35 mm Hg by the process of extreme hyperventilation, which reduces the PCO2 to less than 10 mm Hg.

10. HYPOXIA State of oxygen [O2] deficiency in the blood cells and tissues sufficient to cause impairment of function

11. Stages of Hypoxia • Indifferent Stage • Compensatory Stage • Disturbance Stage • Critical Stage

12. Indifferent Stage • Altitudes: up to 10,000 ft • Oxygen saturation 90 – 98 percent • No awareness of symptoms • Symptoms: • No noticeable impairment • decrease in night vision , acuity and color perception at 4000 ft reported

13. Compensatory Stage • Altitudes: 10,000-15,000 ft • Oxygen saturation: 80 – 90 percent • Symptoms: • Nausea, dizziness, lethargy, headache, fatigue • Decreased efficiency, increased irritability, poor judgment and impaired coordination • An increase in RR, HR, and BP compensate for lack of O2

14. Disturbance Stage • Altitudes: 15,000-20,000 ft • Oxygen saturation: 70 – 80 per cent • Compensatory mechanism no longer effective • Symptoms: • short term memory, reaction time, speech and handwriting impaired • Slow mental function (calculation impaired) • Behavior: aggressive, euphoric, over confident • Impaired muscular coordination, fine movement impossible • Muscular spasm and tetany • Drowsiness, decreased level of consciousness • Hyperventilation, cyanosis

15. Critical Stage • Altitudes: above 20,000 ft • Oxygen saturation 60 – 70 percent • All features of previous stage + loss of consciousness, convulsions and death

16. Time of Oxygen 1 Minute 2 Minutes 3 Minutes 4 Minutes 5 Minutes 6 Minutes Put Back on Oxygen

17. Types of Hypoxia • Hypemic (anemic) • Stagnant (circulatory) • Histotoxic (cellular) • Hypoxic (hypobaric)

18. Hypobaric hypoxia • A deficiency inalveolar-capilary membrane • V/Q mismatch • Sever copd Reduced pO2 in the lungs (high altitude) Red blood cells Body tissue

19. Ultrastructural changes in the wall of a pulmonary capillary • The arrows at the top show a disruption in the alveolar epithelial layer; • the arrows at the bottom show a break in the capillary endothelial layer, • with a platelet apparently adhering to the exposed basement membrane. • These changes are caused by the high mechanical stress in the capillary • wall.

20. CAUTION!!!! Failure to recognize your signs and symptoms may result in an aircraft mishap

21. Hypemic Hypoxia • An oxygen deficiency due to reduction in the oxygen carrying capacity of the blood • Anaemia • Heavy smoking • Hypovolomia • Blood donation + + + + + + + + + + Carbon monoxide + + + + +

22. Stagnant Hypoxia Adequate oxygen Reduced blood flow • Cardiogenic shock • Venous pooling • Arterial spasm • ischemia Blood moves slowly and Red blood cells not reaching tissue needs fast enough G-Forces

23. Histotoxic Hypoxia Inability of the cell to accept or use oxygen Adequate oxygen Red blood cells retain oxygen Alcohol cyanide poisoning

24. Hyperventilationsignificance • Incapacitation of healthy crew member or passenger • Confusion with hypoxia Remember treat it as hypoxia except prove otherwise

25. Pressure altitude Rate of ascent Time at altitude Temperature Physical activity Individual factors Physical fitness Self-imposed stresses Factors modifying hypoxia symptoms

26. DEATH Drugs Exhaustion Alcohol Tobacco Hypoglycemia keep self imposed stresses out of the aircraft

27. Alveolar Gas Equation • PAO2 = FiO2 (PB – PH2O) – PACO2 [FiO2 + (1-FiO2)/R] The below version of equation is useful for clinical purposes; the R value is 0.8, arterial and alveolar PCO2 are same and Water vapor pressure is 47 mm Hg.PAO2 = FIO2(PB-47) - 1.2(PaCO2) PB = barometric pressure FIO2 = fraction of inspired oxygen PIO2 = pressure of inspired oxygen in the trachea PaCO2 = arterial PCO2, equal to = alveolar PACO2 PAO2 = alveolar PO2, PaO2 = arterial PO2, R = respiratory quotient (CO2 excretion over O2 uptake in the lungs)

28. The effect of altitude on O2 tension(breathing 100% O2) O2 120 100 80 60 40 20 0 Alveolar oxygen tension CO2 30000 34000 38000 42000 46000 Altitude

29. The partial pressure of oxygen in dry air is the fraction of inspired oxygen (FIO2) times the barometric pressure; at sea level this is 0.21x(760) = 160 mm Hg. With increasing altitude, the barometric pressure falls and FIO2 remains constant. • In the upper airways (nose, larynx, trachea), water vapor is added to the inspired air. Water vapor pressure is 47 mm Hg at normal body temperature; this pressure affects all dry (nonvapor) gas pressures (oxygen, nitrogen, carbon dioxide). • Thus tracheal (inspired O2) PiO2 = 0.21(760­47) = 150 mm Hg. • As air travels toward the alveoli, carbon dioxide increases; PCO2 at the alveolar level = arterial PCO2 = 40 mm Hg (normal alveolar ventilation).

30. Since the lungs are an open system in continuous contact with the atmosphere, total alveolar gas pressure (sum of partial pressure of gases in alveoli) must be equal to barometric pressure. But since inspired PCO2 is zero and alveolar PCO2 is always equal to 40 mm Hg, the partial pressure of some other gas must fall. Water vapor does not change since it is a function of body temperature. So PiO2=PAO2+PACO2 • This expression is special case of alveolar air equation which reflects only three gas tensions. • What about nitrogen? When nitrogen is present it is necessary to introduce a correction factor into the equation. the magnitude of this correction factor varies with the respiratory exchange ratio.

31. Under normal conditions, approximately 250 ml of oxygen are added to the pulmonary circulation per minute (the VO2), while 200 ml of carbon dioxide are removed (the VCO2). The ratio of VCO2/VO2 is the respiratory quotient (R or RQ), so the normal R is approximately 0.8. Thus, as air moves from the trachea to the alveoli, PiO2 will fall 1.2 mm Hg for every I mm Hg increase in PaCO2. If tracheal PiO2 is 150 mm Hg and if PACO2 is 40 mm Hg, alveolar partial pressure of oxygen (PaO2) is 102 mm Hg. • PAO2 = PiO2 – PACO2 (FiO2 + (1 – FiO2)/R) • PAO2 = FIO2(PB-47) - 1.2(PaCO2)