Principles of Mechanical Ventilation

1. Principles of Mechanical Ventilation RET 2284 Module 1.0 Spontaneous Breathing vs. Negative / Positive Pressure Ventilation

2. Spontaneous Breathing • Ventilation and Respiration • Spontaneous Breathing or Spontaneous Ventilation • The movement of air into and out of the lungs • Main Purpose • Bring in fresh air for gas exchange into the lungs and to allow the exhalation of air that contains CO2

3. Spontaneous Breathing • Ventilation and Respiration • Respiration • The movement of gas molecules across a membrane • External Respiration • Oxygen moves from the lung into the blood stream, and CO2 moves from bloodstream into the alveoli • Internal Respiration • Carbon dioxide moves from the cells into the blood, and oxygen moves from the blood into the cells

4. Spontaneous Breathing • Ventilation and Respiration • Normal Inspiration • Accomplished by the expansion of the thorax. It occurs when the muscles of inspiration contract. • Diaphragm descends and enlarges the vertical size of the thoracic cavity • External intercostal muscles contract and raise the ribs slightly, increasing the circumference of the thorax • The activities of these muscles represent the “work” required to inspire, or inhale

5. Spontaneous Breathing • Ventilation and Respiration • Normal Exhalation • Does not require any work, it is passive • The muscles relax • The diaphragm moves upward to its resting position • The ribs return to their normal position • The volume of the thoracic cavity decreases and air is forced out of the alveoli

6. Spontaneous Breathing • Gas Flow and Pressure Gradients During Ventilation • Pressure Gradient • For air to flow through a tube or airway, pressure at one end must be higher than pressure at the other end • Air always flows from the high-pressure point to the low pressure point

7. Spontaneous Breathing • Gas Flow and Pressure Gradients During Ventilation • Pressure Gradient • Gas flows into the lungs when the pressure in the alveoli is lower than the pressure at the mouth and nose • Conversely, gas flow out to the lungs when the pressure in the alveoli is greater than the pressure at the mouth and nose • When the pressure in the mouth and alveoli are the same, as occurs at the end of inspiration or the end of expiration, no gas flow occurs

8. Spontaneous Breathing • Mechanics of Spontaneous Respiration

9. Spontaneous Breathing • Lung Characteristic • Two Primary Characteristic of the Lung • Compliance and Resistance • Two types of force oppose inflation of the lungs • Elastic force • Arise from elastic properties of lung and thorax that oppose inspiration • Frictional force • Resistance of tissues and organs as they move and become displaced during breathing and resistance to gas flow through the airways

10. Spontaneous Breathing • Lung Characteristic • Compliance • The relative ease with which a structure distends • Pulmonary physiology uses the term compliance to describe the elastic forces that oppose lung inflation (lung tissue and surrounding thoracic structures) • Described as the change in volume that corresponds to a change in pressure Compliance = V / P

11. Spontaneous Breathing • Lung Characteristic • Compliance • For spontaneous breathing patients, total compliance is about 100 mL/cm H2O • Range 50 – 170 mL/cm H2O • For intubated and mechanically ventilated patients, compliance varies • Males: 40 – 50 mL/cm H2O • Females: 35 – 45 mL/cm H2O • When compliance is measured under conditions of no gas flow, it is referred to as STATIC Compliance

12. Spontaneous Breathing • Lung Characteristic • Compliance • Monitoring changes in compliance is a valuable means of assessing changes in the patient’s condition during mechanical ventilatory support Calculate Pressure If compliance is normal at 100 mL/cm H2O, calculate the amount of pressure needed to attain a tidal volume of 500 mL • 500 ml = 5 cm H20 • 100 ml/cm H2O

13. Spontaneous Breathing • Lung Characteristic • Compliance • Static Compliance (CS ): When compliance is measured under conditions of no gas flow • Normal value is 70 – 100 mL/cm H2O • When CS is <25 cm H2O, the WOB is very difficult Exhaled tidal volume/Plateau pressure – PEEP VT / PPlat – PEEP 500 mL / 25 cm H2O – 5 cm H2O CS = 25 mL/cm H2O

14. Spontaneous Breathing • Lung Characteristic • Compliance • Changes in the condition of the lungs or chest wall (or both) affect total respiratory system compliance and the pressure required to inflate the lungs • Diseases that reduce the compliance of the lung increase the pressure required to inflate the lung, e.g., ARDS • Diseases that increase the compliance of the lung decrease the pressure required to inflate the lung, e.g. emphysema

15. Spontaneous Breathing • Lung Characteristic • Resistance • Frictional forces associated with ventilation are the result of the anatomical structure of the conductive airways and the tissue viscous resistance of the lungs and adjacent tissue and organs • During mechanical ventilation, resistance of the airways (Raw) is the factor most often evaluated

16. Spontaneous Breathing • Lung Characteristics • Airway Resistance (Raw) • Expressed in (cm H2O/L/sec) • Unintubated patients • Normal: 0.6 – 2.4 cm H2O/L/sec • Intubated patients • Approximately 6 cm H2O/L/sec or higher • Increase is caused by artificial airway – smaller the tube the greater the resistance • Diseases of the airway can also cause increases in Raw

17. Spontaneous Breathing • Lung Characteristics • Airway Resistance (Raw) • With higher airway resistance, more of the pressure for breathing goes to the airways and not the alveoli; consequently, a smaller volume of gas is available for gas exchange

18. Spontaneous Breathing • Time Constants • Compliance x Resistance • A measure of how long the respiratory system takes to passively exhale (deflate) or inhale (inflate) • The differences in C and Raw affect how rapidly the lung units fill and empty

19. Spontaneous Breathing • Time Constants • Normal lung • Lung units fill within a normal length of time and with a normal volume • Low-compliance • Lung units fill rapidly • Increased resistance • Lung units fill slowly

20. Spontaneous Breathing • Time Constants A: Normal lung unit B: Low-compliance fills quickly, but with less air C: Increased resistance fills slowly. If inspiration were to end at the same time a unit A, the volume in unit C would be lower

21. Spontaneous Breathing • Time Constants • Calculation of Time Constants • Time constant = C x R • Time constant = 0.1 L/cm H2O x 1 cm H2O/(L/sec) • Time constant = 0.1 sec In a patient with a time constant of 0.1 sec., 63% of passive exhalation or inhalation occurs in 0.1 sec., 37 % of the volume remains to be exchanged

22. Spontaneous Breathing • Time Constants • TC of <3 may result in incomplete delivery of tidal volume • Prolonging the inspiratory time allows even distribution of ventilation and adequate delivery of tidal volume

23. Spontaneous Breathing • Time Constants • TC of <3 may result in incomplete emptying of the lungs, which can increase the FRC and cause air trapping • Using TC of 3 – 4 may be more adequate for exhalation (95 – 98% volume emptying level)

24. Types of Mechanical Ventilation • Negative Pressure Ventilation • Attempts to mimic the function of the respiratory muscles to allow breathing through normal physiological mechanisms • Applies subatmospheric pressure outside of the chest to inflate the lungs • Removing the negative pressure allows passive exhalation

25. Types of Mechanical Ventilation • Negative Pressure Ventilation • A negative pressure device designed for resuscitation by Woillez in 1876

26. Types of Mechanical Ventilation • Negative Pressure Ventilation Iron Lung

27. Types of Mechanical Ventilation Iron Lung • Negative Pressure Ventilation • This is the Iron Lung ward at Rancho Los Amigos Hospital, Downey, California, in the early 1950s, filled to overflowing with polio patients being treated for respiratory muscle paralysis

28. Types of Mechanical Ventilation • Negative Pressure Ventilation Chest Cuirass Iron Lung

29. Types of Mechanical Ventilation • Negative Pressure Ventilation

30. Types of Mechanical Ventilation • Negative Pressure Ventilation • Advantages • Upper airway can be maintained without the use if an endotracheal tube or tracheotomy • Patients can talk and eat • Fewer physiological disadvantages than positive pressure ventilation

31. Types of Mechanical Ventilation • Negative Pressure Ventilation • Disadvantages • Decreased accessibility to the patient • Abdominal venous blood pooling • Decreased venous return, cardiac output, systemic blood pressure (hypotension) – tank shock • Negative pressure ventilators have primarily been replaced by positive pressure ventilators that use a mask, nasal device or tracheostomy tube

32. Types of Mechanical Ventilation • Positive Pressure Ventilation • Occurs when a mechanical ventilator moves air into the patient’s lungs by way of an endotracheal tube or mask (NPPV).

33. Types of Mechanical Ventilation • Positive Pressure Ventilation • At any point during inspiration, the inflating pressure at the upper (proximal airway) equals the sum of the pressure required to overcome the compliance of the lung and chest wall and the resistance of the airways

34. Types of Mechanical Ventilation • Positive Pressure Ventilation

35. Pressures in Positive Pressure Ventilation • Baseline Pressure • Peak Pressure • Plateau Pressure • Pressure at End of Exhalation

36. Pressures in Positive Pressure Ventilation • Baseline Pressure • Pressures are read from a zero baseline value

37. Pressures in Positive Pressure Ventilation • Baseline Pressure • Continuous Positive Airway Pressure (CPAP)

38. Pressures in Positive Pressure Ventilation • Baseline Pressure • Positive End-Expiratory Pressure (PEEP) • Prevents patients from exhaling to zero (atmospheric pressure) • Increases volume of gas left in the lungs at end of normal exhalation – increases FRC

39. Pressures in Positive Pressure Ventilation • Peak Pressure (PIP)

40. Pressures in Positive Pressure Ventilation • Peak Pressure • The highest pressure recorded at the end of inspiration (PPeak, PIP) • It is the sum of two pressures • Pressure required to force the gas through the resistance of the airways and to fill alveoli

41. Pressures in Positive Pressure Ventilation • Plateau Pressure

42. Pressures in Positive Pressure Ventilation • Plateau Pressure At baseline pressure (end of exhalation), the volume of air remaining in the lungs is the FRC. At the end of inspiration, before exhalation starts, the volume of air in the lungs is the VT plus the FRC. The pressure measured at this point with no flow of air is plateau pressure

43. Pressures in Positive Pressure Ventilation • Plateau Pressure • Measured after a breath has been delivered and before exhalation • Ventilator operator has to perform an “inflation hold” • Like breath holding at the end of inspiration

44. Pressures in Positive Pressure Ventilation • Plateau Pressure • Reflects the effect of elastic recoil on the gas volume inside the alveoli and any pressure exerted by the volume in the ventilator circuit that is acted upon by the recoil of the plastic circuit

45. Pressures in Positive Pressure Ventilation • Pressure at End of Expiration • Pressure falls back to baseline during expriration

46. Pressures in Positive Pressure Ventilation • Pressure at End of Expiration • Auto-PEEP • Air trapped in the lungs during mechanical ventilation when not enough time is allowed for exhalation • Need to monitor pressure at end of exhalation

47. Pressures in Positive Pressure Ventilation • Pressure at End of Expiration • Auto-PEEP