Principles of Mechanical Ventilation

1. Principles of Mechanical Ventilation RET 2284 Module 2.0 Classification of Ventilators; How a Breath is Delivered

2. How a Breath is Delivered • Factors Controlled and Measured by the Ventilator During Inspiration • To deliver inspiratory volume, the operator most commonly sets either a volume or a pressure • The primary variable the ventilator adjusts to achieve inspiration is called the control variable

3. How a Breath is Delivered • CONTROL VARIABLES • The primary variable the ventilator adjusts to achieve inspiration • Mechanical ventilators can control four variables, but only one at a time • Pressure • Volume • Flow • Time

4. How a Breath is Delivered • CONTROL VARIABLES • Because only one of these variables can be directly controlled at a time, a ventilator must function as either one of the following: • Pressure controller • Volume controller • Flow controller • Time controller

5. How a Breath is Delivered • Pressure Controller • When the ventilator maintains the pressure waveform in a specific pattern, the breathing is described as pressure controlled (also pressure limited or pressure targeted) • The pressure waveform is unaffected by changes in lung characteristics

6. How a Breath is Delivered • Pressure Controller • The pressure waveform will remain constant but volume and flow will vary with changes in respiratory system mechanics (airway resistance, compliance)

7. How a Breath is Delivered • Volume Controller • The ventilator maintains the volume waveform in a specific pattern, the delivered breath is volume controlled (volume limited, volume targeted) • The volume and flow waveforms remain unchanged, but the pressure waveform varies with changes in lung characteristics (resistance and compliance)

8. How a Breath is Delivered • Volume controller • Volume and flow waveforms will remain constant, but pressure will vary with changes in respiratory mechanics (airway resistance and compliance) Pressure Volume Flow

9. How a Breath is Delivered • Flow Controller • Directly measures flow and uses the flow signal as a feedback signal to control its output • Most newer ventilators measure flow and are flow controllers; volume becomes a function of flow as follows: Volume (L) = Flow (L/sec) x Inspiratory Time (sec)

10. How a Breath is Delivered • Flow Controller • Flow and volume waveforms will remain constant, but pressure will vary with changes in respiratory mechanics (airway resistance and compliance)

11. How a Breath is Delivered • Time Controller • Measures and controls inspiratory and expiratorytime • Pressure and volume waveforms vary with changes in resistance and compliance (neither pressure or volume are used as a control)

12. How a Breath is Delivered Inspiration is commonly described as pressure controlled or volume controlled (although flow and time ventilation have been defined, they are not typically used).

13. How a Breath is Delivered • PHASE VARIABLES • Ventilator-supported breath may be divided into four distinct phases • The initiation of inspiration • Inspiration itself • The end of inspiration • Expiration • To understand a breath cycle, you must know how the ventilator starts, sustains, and stops inspiration and you must know what occurs between breaths

14. How a Breath is Delivered • PHASE VARIABLES • The phase variable is a variable that is measured and used by the ventilator to initiate some phase of the breath cycle • Trigger variable – causes a breath to begin • Limit variable – limits the magnitude of any parameter (pressure, flow, volume) during inspiration • Cycle variable – causes the end of inspiration • Baseline variable – determines what happens during expiration

15. How a Breath is Delivered • Trigger Variable • Determines the start of inspiration • Any one of the following may be sensed/measured by the ventilator and used to initiate inspiration • Pressure • Volume • Flow • Time • Either the ventilator or the patient initiates a breath. If the machine initiates the breath, the trigger variable is time

16. How a Breath is Delivered • Time Triggered • Ventilatory initiates a breath according to a predetermined time interval • Rate control on ventilator is a time-triggering device Example: Rate 10 (breaths/min) = One breath every 6 seconds • An alternative approach is to provide separate timers for inspiration and expiration. Changing either or both of these timers will alter the breathing rate

17. How a Breath is Delivered • Time Triggered • Ventilatory initiates a breath according to a predetermined time interval

18. How a Breath is Delivered • Pressure Triggered • Occurs when the patient’s inspiratory effort causes a drop in pressure within the breathing circuit

19. How a Breath is Delivered • Pressure Triggered • Typically, trigger levels (sensitivity) range between 0.5 to 1.5 cm H2O below the patient’s baseline, or end-expiratory pressure • Setting the sensitivity to a higher number, e.g., 3 cm H2O makes the ventilator less sensitive and requires the patient to work harder to initiate inspiration. Conversely, setting the trigger level lower make the ventilator more sensitive

20. How a Breath is Delivered • Pressure Triggered • The operator must set the sensitivity to meet the patient’s needs. If it is not set correctly, the ventilator may not be sensitive enough to the patient’s effort, and the patient will have to work too hard to trigger the breath. • If the ventilator is too sensitive, it can autotrigger (i.e., the machine triggers a breath without the patient making an effort)

21. How a Breath is Delivered • Quiz • A patient has a baseline pressure of 10 cm H2O during mechanical ventilation. The trigger sensitivity is set at –1 cm H2O. At what pressure will the ventilator sense a patient effort and start inspiration? Answer: 9 cm H2O

22. How a Breath is Delivered • Flow Triggered • Ventilator-supported breath is initiated when the ventilator detects a drop in flow through the patient circuit during exhalation Continuous flow through circuit: “Base Flow”

23. How a Breath is Delivered • Flow Triggered • Between breaths, the flow coming out of the main flow control valve and the flow going through the exhalation valve are equal • During the patient’s inspiratory effort, the flow through the exhalation valve falls below the flow from the output valve • The difference between these two flows is the flow trigger variable

24. How a Breath is Delivered • Flow Triggered • Ventilator graphic representing flow triggering

25. How a Breath is Delivered • Quiz • The operator decides to use flow triggering for a patient and sets the base flow at 6 L/min and the trigger flow at 2 L/min. The base flow measurement must drop to what value before the ventilator will begin the inspiratory phase? Answer: 4 L/min.

26. How a Breath is Delivered • Volume Triggered • Occurs when the ventilator detects a small drop in volume in the patient circuit during exhalation. The machine interprets this drop as a patient effort and begins inspiration • Dräger Babylog • Cardiopulmonary Venturi

27. How a Breath is Delivered • Manual Triggered • A button or touch pad on the control panel labeled “Manual” breath or “Start” breath • When this control is activated, the ventilator delivers a breath according to the set variables PB 840

28. How a Breath is Delivered • Inductive Plethysmography Triggered • Infant’s ventilator-supported breath is initiated when a when chest expands Sechrist IV-200

29. How a Breath is Delivered • NAVA (Neurally Adjusted Ventilator Assist) • A esophageal probe senses the electrical activity of the diaphragm, which triggers the ventilator Maquet’s Servo-I NAVA

30. How a Breath is Delivered • Limit Variable • A limit variable is the maximum value a variable (pressure, flow, volume) can attain. This limits the variable during inspiration but does not end the inspiratory phase. • Do not confuse this with cycle variable, which always ends inspiration

31. How a Breath is Delivered • Pressure Limiting • Allows pressure to rise to a certain value but not exceed it

32. How a Breath is Delivered • Pressure Limiting • Remember; in pressure ventilation, the pressure the ventilator delivers to the patient is limited; however, reaching the pressure limit does not end the breath • Example of pressure-limiting modes • Pressure support • Pressure control

33. How a Breath is Delivered • Volume Limiting • The volume is set by the operator; however, reaching that volume does not necessarily end inspiration

34. How a Breath is Delivered • Flow Limiting • Ventilator flow to the patient reaches but does not exceed a maximum value before the end of inspiration

35. How a Breath is Delivered • Maximum Safety Pressure: Pressure Limiting vs. Pressure Cycling • All ventilators have a maximum pressure limit control, which is used to prevent excessive pressure from reaching a patient’s lungs – reaching the maximum high pressure limit ends the inspiratory phase • AKA • High pressure limit • Upper pressure limit • Pressure limit

36. How a Breath is Delivered • Cycle Variable • The variable a ventilator measures to determine the end of inspiration is called the cycling mechanism – once cycling occurs, expiratory gas flow begins • Cycle variables • Pressure • Volume • Flow • time

37. How a Breath is Delivered • Pressure Cycled • Ventilator will deliver flow until a present pressure is reached, at which point inspiration ends and expiratory flow begins • The most common application of pressure-cycling is for alarm setting (e.g., high pressure alarm) and IPPB

38. How a Breath is Delivered • Volume Cycled • Ventilator will deliver flow until a present volume is reached, at which point inspiration ends and expiratory flow begins

39. How a Breath is Delivered • Flow Cycled • Ventilator will deliver flow until a present level is met, at which point flow stops and expiration begins • The most frequent application of flow cycling is pressure control mode ventilation (to be discussed in a future module)

40. How a Breath is Delivered • Time Cycled • Expiratory flow starts because a present time interval has elapsed

41. How a Breath is Delivered • Limit Variable A B C This figure illustrates the importance of distinguishing between the terms limit and cycle. A, Inspiration is pressure limited and time cycled. B, Inspiration is flow limited and volume cycled. C, Inspiration is both flow limited and volume limited (because flow and volume reach preset values before inspiratory time ends) and time cycled (after the preset inspiratory hold time).

42. How a Breath is Delivered • Inflation Hold (Inspiratory Pause) • Designed to maintain air in the lungs at the end of inspiration • Used to obtain PPlateau for calculating CS • Occasionally used to help increase peripheral distribution of gas and improve oxygenation

43. How a Breath is Delivered • Inflation Hold (Inspiratory Pause) • Calculation of CS requires accurate measurement of the plateau pressure – the PPlateau value is inaccurate if the patient is actively breathing when the measurement is taken

44. How a Breath is Delivered • TYPES of Breaths • Spontaneous • Started by the patient (patient triggered) and VT delivery is determined by the patient (i.e., patient cycled) • Volume and pressure delivered are based on patient demand rather than a value set by the ventilator operator

45. How a Breath is Delivered • TYPES of Breaths • Mandatory • Ventilator determines the start time (time triggered) or VT (or both) • The machine triggers and/or cycles the breath

46. How a Breath is Delivered • BASELINE VARIABLE • The variable that is controlled during the expiratory phase Note: Most commonly, pressure is controlled during the expiratory phase • CPAP • PEEP

47. How a Breath is Delivered • BASELINE VARIABLE • Continuous Positive Airway Pressure (CPAP) • The application of pressures above ambient to improve oxygenation is a spontaneouslybreathing patient • Can be applied through a freestanding CPAP system or a ventilator

48. How a Breath is Delivered • BASELINE VARIABLE • Continuous Positive Airway Pressure (CPAP) • Simplified pressure-time waveform showing continuous positive airway pressure (CPAP). Breathing is spontaneous. Inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP) are present. Pressures remain positive and do not return to a zero baseline.

49. How a Breath is Delivered • BASELINE VARIABLE • Positive End Expiratory Pressure (PEEP) • The application of pressures above ambient at the end of exhalation to improve oxygenation during mechanical ventilation

50. How a Breath is Delivered • BASELINE VARIABLE • Positive End Expiratory Pressure (PEEP) • Positive end-expiratory pressure (PEEP) with intermittent mandatory breaths (also called intermittent mandatory ventilation [IMV] with PEEP). Spontaneous breaths are taken between mandatory breaths, and the baseline is maintained above zero.