Understanding CPAP: How Continuous Positive Airway Pressure Works

1. CPAP an Introduction Joe Lewis, M.D.City and County of Honolulu EMS

2. What is CPAP ?  Continuous   Positive   Airway   Pressure

3. Background Information • In order to understand how  CPAP works a little background information is needed. • While some of this some anatomy and physiology of normal lung will seem like review, bear with it as it will make how CPAP works crystal clear to you. • Since the terms used can have different meanings in different situations they are defined for this situation now, so their meaning in this situation is clear, as that is essential to understanding CPAP.

4. Background concepts • All the body cells metabolically consume oxygen, and discharge carbon dioxide. To cover this need, respiration takes place in the lungs and we call it ventilation or breathing. • Ventilation involves the inhalation of atmospheric air into the lungs via the nose and mouth through branching passageways, and the exhalation of carbon dioxide.

5. Background concepts Respiration-the exchange of oxygen and carbon dioxide between the atmosphere and the body cells. This occurs at the level of the cells and in the alveoli. Diffusion - the movement of gas from an area of high concentration to an area of lower concentration.

6. Lung Anatomy & Gas Exchange • The key function of lung is to bring air and blood into intimate contact in the alveolar air sacs so that oxygen can enter the blood, and carbon dioxide can leave.

7. Lung Anatomy and Physiology • Gas exchange occurs in the lungs between the alveoli and a capillary network within the alveolar wall. • Capillaries are microscopic blood vessels that exchange material between the blood and body tissues.

8. Pulmonary Anatomy  • Alveolus-smallest anatomic part of the lung, where gas exchange takes place between the oxygen rich atmospheric air and the oxygen poor  blood in the pulmonary capillary. • Alveolar wall-the physical structure which separates the alveolus from the interstitium. • Interstitium the loose tissue separating the alveoli and the alveolus from the pulmonary capillary.

9. Pulmonary Anatomy  • Pulmonary capillary wall-the wall separating the oxygen poor, carbon dioxide rich pulmonary capillary blood from the interstitium. • Pulmonary capillary-the smallest blood vessel in the lung where the  exchange of oxygen and carbon dioxide takes place.

10. The lung is made of millions of alveoli

11. The Alveolus

12. The Alveolus and blood supply

13. Red Blood Cell in capillary in between alveoli

14. Electron Microscope picture

15. Oxygen diffusing across the capillary wall and Interstitium to the red blood cell & carbon dioxide diffusing across same structures to the alveoli

16. Pulmonary Physiology Terms  • Oxygen concentration- the percentage of oxygen in  the air presented to the alveolus. • Partial pressure of oxygen-the atmospheric pressure on the oxygen delivered to the alveolus

17. Pulmonary Physiology Terms • Ventilation- the mechanical exchange of air between the lungs and the atmosphere. • Alveolar ventilation – the exchange of air between the alveoli and the atmosphere.

18. Pulmonary Physiology Terms • Respiration-the exchange of oxygen and carbon dioxide between the atmosphere and the body cells. This occurs inside cells and in the alveoli.  • Alveolar respiration-the exchange of oxygen and carbon dioxide between the atmosphere and the alveoli.  

19. Pulmonary Physiology Alveolar Respiration  Alveolar respiration or the exchange of oxygen and carbon dioxide occurs due two things: 1. A concentration gradient or difference. 2. A pressure gradient or difference.

20. Pulmonary Physiology Concentration gradient • Oxygen moves or “diffuses” from the oxygen rich air in the alveoli to the oxygen poor blood in the alveolar capillary. • It follows a “concentration gradient” which means the oxygen moves from a high oxygen area to a low oxygen area.

21. Pulmonary Physiology • When we administer high flow oxygen, like 15 liters by mask, we increase the concentration of oxygen in the alveolus. This increases the concentration gradient and this makes more oxygen diffuse or move into the capillary and therefore the blood.

22. Pulmonary Physiology • Partial pressure of oxygen-the atmospheric pressure on the oxygen delivered to the alveolus. • The atmospheric pressure at sea level is one atmosphere or 760 mm of mercury. • The partial pressure of a gas is calculated by multiplying it’s concentration by the atmospheric pressure. • The oxygen concentration in air is 21%, so 760 x 21% gives a partial pressure of oxygen of 159.6.

23. Pulmonary Physiology Pressure gradient: The pressure of the atmosphere on oxygen, also called the “partial pressure” of oxygen is higher in the alveolus then the partial pressure of oxygen in the capillary blood. This pressure difference creates a “pressure gradient” similar to the “concentration gradient” we discussed before, which makes oxygen “diffuse” or move from the alveolus air into the blood in the capillary surrounding the alveolus.

24. Pulmonary Physiology • When we administer CPAP, or “continuous positive air pressure” we are adding pressure to the atmospheric pressure on the oxygen. • This increases the “partial pressure” of oxygen in the alveolus. • This then increases the pressure difference between the oxygen in the air and the oxygen in the capillary blood around the alveolus. • This increase in the pressure difference increases the “pressure gradient” and makes more oxygen “diffuse” or move into the capillary blood around the alveolus.

25. Pulmonary Physiology CPAP takes advantage of the two physical forces which improve alveolar capillary blood oxygenation; • Concentration and • Pressure This is why CPAP is referred to as oxygen delivery at it’s highest or most efficient form.