Understanding Ambient Air and Ventilation Mechanics: Gas Composition, Airway Function, and Alveolar Ventilation

1. 7 Ambient Air, Airway, and Mechanics of Ventilation

2. Objectives • Understand gas composition in the air and the effects of imbalances on metabolism. • Discuss the structure and function of the airway. • Discuss determinants of alveolar ventilation.

3. Objectives (cont’d) • Discuss ventilation and cellular oxygenation. • Discuss adequate and inadequate ventilation.

4. Introduction • This chapter and the following two chapters introduce pathophysiological principles. • The components associated with perfusion must all function in unison if an organism is to survive.

5. Introduction (cont’d) • The first three components, ambient air, the airway, and ventilation are presented here.

6. Physiology • Composition of Ambient Air • What is breathed in directly impacts the available oxygen for cellular use.

7. Percentage and Partial Pressures of Gases in Ambient Air at Sea Level

8. Physiology (cont’d) • Patency of the Airway • Maintaining an airway is one of the most basic and important steps in prehospital medicine • Without an adequate airway, all other interventions are doomed to fail • Obstructions can occur at several anatomic locations • Upper and lower airway structures

9. Airway obstruction can occur at several levels of the upper and lower airway, including the nasopharynx, oropharynx, posterior pharynx, epiglottis, larynx, trachea, and bronchi.

10. Physiology (cont’d) • Mechanics of Ventilation • Inspiratory and expiratory muscles, accessory muscles • Change in intrathoracic pressure is what creates airflow into and out of the lungs • Intrathoracic pressure in relation to atmospheric pressure

11. Physiology (cont’d) • Mechanics of Ventilation • Factors affecting ventilation • Compliance issues • Airway resistance issues

12. A normal bronchiole A constricted bronchiole

13. Physiology (cont’d) • Mechanics of Ventilation • Pleural Space • Visceral and parietal pleura envelop the lungs • Negative pressure between them • Damage to either pleura: air or blood may fill space and cause the lung to collapse

14. The pleural lining of the lung

15. Physiology (cont’d) • Mechanics of Ventilation • Minute Ventilation • Refers to amount of air moved into and out of the lung in one minute • Minute ventilation = tidal volume x frequency • Changes in tidal volume or frequency can alter minute ventilation detrimentally

16. Physiology (cont’d) • Mechanics of Ventilation • Alveolar Ventilation • Refers to the amount of air moved in and out of the alveoli in one minute • Takes into account dead space • Alveoli are the last to be ventilated during inhalation, and the first to suffer from poor ventilation when the minute ventilation drops

17. Physiology (cont’d) • Mechanics of Ventilation • Alveolar ventilation = tidal volume - dead space. • In an average-size adult patient, the alveolar ventilation can be calculated: • (500 mL – 150 mL) = 350 mL alveolar ventilation • If something causes a drop in tidal volume, alveolar ventilation will change before dead space.

18. Physiology (cont’d) • Mechanics of Ventilation • Alveolar Ventilation • Although the patient may breathe faster to improve his minute ventilation, the amount of air available for gas exchange in the alveoli may be insufficient if the tidal volume is low.

19. Physiology (cont’d) • Mechanics of Ventilation • Alveolar Ventilation • The dead space will fill first, regardless of the volume of air breathed in. • This means alveolar ventilation suffers. • To improve gas exchange in the patient with an inadequate tidal volume, you must provide positive pressure ventilation to increase tidal volume and move more air into the alveoli.

20. Physiology (cont’d) • Mechanics of Ventilation • Alveolar Ventilation • By placing a patient with a low tidal volume on an oxygen mask, you will enrich the air in the dead air space with little getting to the alveoli; the patient needs ventilation.

21. Case Study • Just as you finish completing the morning equipment list on the ambulance, you get toned out for an industrial accident. Upon your arrival, you are met by a man who says his buddy “got a big hole in his chest from some scrap metal that flew outta the thrashing machine.”

22. Case Study (cont’d) • Although you are not familiar with exactly what a “thrashing machine” does, you do recognize that a hole in the chest wall can create significant problems. • When you arrive at the patient's side, there is blood on his shirt, and he looks like he is struggling to breathe.

23. Case Study (cont’d) • Scene Size-Up • 45-year-old male patient • BSI precautions are taken • MOI is a traumatic injury • There is only one patient • Ingress and egress can occur without difficulty from the site

24. Case Study (cont’d) • What organs or tissues may be injured due to this mechanism of injury? • The patient's obvious dyspnea points to an injury to what body system? • What precautions for your safety should you take?

25. Case Study (cont’d) • Primary Assessment Findings • Patient responsive to verbal stimuli, A&Ox3 • Airway patent, no foreign bodies or fluid • Labored breathing on inhalation, patient speaking in 1-2 word sentences • Respiratory rate is 28 times/minute • Peripheral pulse is present, chest injury is bleeding minimally

26. Case Study (cont’d) • Is this patient a high or low priority? Why? • What care should be provided immediately? • If the penetration injury pierced the right parietal pleura, what would you expect breath sounds on that side to be?

27. Case Study (cont’d) • Medical History • Patient shakes his head “no” when you ask about medical problems • Medications • He states “vitamins” when you ask about meds • Allergies • Patient denies any known allergies

28. Case Study (cont’d) • Pertinent Secondary Assessment Findings • Pupils reactive to light, airway patent • Penetration to 4 ICS on right anterior chest • Breath sounds absent on right side • Pulse oximeter reads 90% on room air

29. Case Study (cont’d) • Pertinent Secondary Assessment Findings • Muscle tone is noted to all extremities • Patient denies traumatic fall or other injury • Skin cool and dry, color ashen • B/P 110/78, Pulse 108, Respirations 26

30. Case Study (cont’d) • Will you change your treatment based on information you have now learned? • How can the change in tidal volume precipitate anaerobic metabolism? • Why is the patient's tachypnea not really helping his oxygenation status?

31. Case Study (cont’d) • Care provided: • Spinal precautions taken • Occlusive dressing applied to injury • PPV with high-flow oxygen provided • Paramedic intercept initiated prior to departure • Patient packaged and transported by ambulance

32. Case Study (cont’d) • Explain how the following interventions may help improve the patient's condition: • Oxygen administration • Positive pressure ventilation • Occlusive dressing placement

33. Summary • The airway is considered to be the “channel of life.” With no airway, the patient cannot survive. • Adequate oxygen levels in the inspired air and a good ventilatory effort are also integral to assuring adequate oxygen levels for cellular metabolism.