1. Mechanics of Ventilation

2. Objectives Review basic pulmonary mechanics
Describe scalars: pressure, flow & volume
Describe the concept of compliance
Discuss and review pressure–volume, flow- volume loops
Review work of breathing
Application in various clinical scenarios

3. Normal Mechanics Spontaneous ventilation Inspiration Exhalation
Diaphragm descends and enlarges vertical diameter of thorax
External intercostal contraction raises ribs
Exhalation
Passive

4. Pressures and Gradients in Lungs
Transpulmonary
Transairway
- During positive-pressure ventilation, the Pbs remains atmospheric, but the pressures at the upper airways (Pawo) and in the conductive airways (airway pressure, or Paw) become positive. Alveolar pressure (PA) then becomes positive, and transpulmonary pressure (PL) increases.
- Transrespiratory pressure (Ptr ) is the pressure difference between the airway opening and the body surface: Ptr
= Pawo − Pbs . Transrespiratory pressure is used to describe the pressure required to inflate the lungs and airways during positive-pressure ventilation. In this situation, the body surface pressure (Pbs ) is atmospheric and usually is given the value zero; thus Pawo becomes the pressure reading on a ventilator gauge (Paw).
Alveolar distention pressure
Plateau

5. Mechanics of Spontaneous Ventilation

6. Mechanics of Positive Pressure Ventilation

7. Elementary Law of Mechanics
Simple model of respiratory system:
Resistive element connected to an elastic element
Interaction between pressure, volume and flow follow Newtonian physics
Simple but useful model during
assisted breathing

8. Elementary Law of Mechanics
Newton’s third law of motion
Pappl(t) = Pel (t) + Pres (t)

9. Equation of Motion P = ΔV X E + Flow x R P = ΔV/ C + Flow x R
Ventilation
Pressure
( to deliver
tidal volume)
Elastic
Pressure
( to inflate lungs and chest wall)
Resistive
Pressure
( to make air flow through airways) = +
P = ΔV X E + Flow x R
Compliance = 1/ E
P = ΔV/ C + Flow x R

10. Relevance
Pressure, airflow and volume measurements quantify basic mechanics of the respiratory system
These are resistance, compliance and work of breathing
Monitoring and analysis of these parameters & graphic display of curves and loops during mechanical ventilation is a useful way to determine not only how patient are being ventilated but also a way to assess problems occurring during ventilation

11. Mechanics of Ventilation
Dynamic mechanics
Pertain to properties of system during variable flow
Respiratory system resistance, compliance can be mathematically derived with sample flow, volume and airway pressures by multiple linear regression analysis ( or linear least square fitting models)
Static mechanics
Absence of flow
Obtained with airway occlusion on modern ventilators

12. Basic waveforms Scalars Loops Pressure Flow Volume Pressure-Volume

13. Uses of Flow, Volume, and Pressure Graphic Display
Confirm mode functions
Detect auto-PEEP
Measure the work of breathing
Adjust tidal volume and minimize over distension
Assess the effect of bronchodilator administration
Detect equipment malfunctions
Determine appropriate PEEP level

14. Pressure Scalar
A pressure - time graphs shows gradual changes in pressure over time during the breath cycle
Achieved with a manometer/ pressure gauge at the airway opening or inside the ventilator
These pressure points are used in the monitoring of patients, to describe modes of ventilation, and to calculate a variety of parameters in patients receiving mechanical ventilation

15. Pressure Scalar
During a ventilator driven breath, the airway pressure rises to a peak
This is PIP
PIP is influenced by airway resistance and compliance
Plateau Pressure
- Inspiratory pause before exhalation ( no flow)
- Reflects lung and chest wall resistance & pressure in small airways and alveoli

16. Pressure Waveform for Volume- Controlled Ventilation
Resistance = airway resistance
Compliance = compliance of the entire system (lungs, vent circuit, etc)
Δp/Δt = Flow/Compliance
Δp = R * Exp Flow
Δp = R ∗ Flow
At the beginning of inspiration the pressure between points A and B increases from the resistances in the system.
The level of the pressure at B is equivalent to the product of resistance R and flow (V); (Valid if no intrinsic PEEP exists).
The higher the selected Flow or overall Resistance, the greater the pressure rise up to point B.
Reduced inspiratory Flow and low Resistance lead to a low pressure at point B.
There may be a slight decrease in pressure (points D to E) from lung recruitment and leaks in the system.
The level of the plateau pressure is determined by the compliance and the tidal volume
During the plateau time no volume is supplied to the lung, and inspiratory flow is zero
The difference between plateau pressure (E) and end-expiratory pressure F (PEEP) is obtained by dividing the delivered volume tidal volume (VT) by compliance (C)
After point B the pressure increases in a straight line, until the peak pressure at point C is reached.
The gradient of the pressure curve is dependent on the inspiratory flow and the overall compliance.
At point C the ventilator applied the set tidal volume and no further flow is delivered (Flow = 0).
Expiration begins at point E and is passive; the elastic recoil forces of the thorax force the air against atmospheric pressure out of the lung
The change in pressure is obtained by multiplying exhalation resistance of the ventilator by expiratory flow
Once expiration is completely finished, pressure once again reaches the end-expiratory level F (PEEP)
Pressure quickly falls to plateau pressure.
This drop in pressure is equivalent to the rise in pressure caused by the resistance at the beginning of inspiration. The base line between points A and D runs parallel to the line B - C.

17. Pressure Waveform for Pressure- Controlled Ventilation
Pressure increases rapidly from the lower pressure level (ambient pressure or PEEP) until it reaches the upper pressure value (PInsp)
Pressure then remains constant for the inspiration time (Tinsp) set on the ventilator.
The drop in pressure during the expiratory phase follows the same curve as in volume-oriented ventilation, as expiration is a passive process.
Until the next breath, pressure remains at the lower pressure level PEEP.

18. Pressure Waveform for Pressure- Controlled Ventilation
As pressure is preset in pressure controlled ventilation, Pressure- time diagrams show no changes or changes which are difficult to detect as a consequence of changes in resistance and compliance of the entire system

19. PIP Pplateau Paw (cm H20) Pao Resistive Pressure Time (sec)
Transairway Pressure
Pplateau
Paw (cm H20)
Pao
Alveolar Pressure
Transairway Pressure
Resistive Pressure
Time (sec)
Elastic resistance

20. Pressure Waveform: Changes in Compliance
When compliance changes, the plateau and peak pressures change by the same amount and the pressure difference (ΔP) remains unchanged
Decreasing compliance → plateau and peak pressures rise
Increasing compliance → plateau and peak pressures fall

21. Pressure Waveform: Changes in Airway Resistance
Increasing Resistance → Peak Pressure Rises
Decreasing Resistance → Peak Pressure Falls
When the inspiratory airway resistance changes, the peak pressure changes and the plateau pressure remains the same

22. PIP vs. Pplat Bronchospasm Secretions Foreign Bodies Tube Kinks
Pulm Edema
Atelectasis
Pneumonia
Pneumothorax
ARDS
Pulmonary Fibrosis
Pplat

23. Mean Airway Pressure (Paw)
Reflects level of airway pressure and duration of elevation
Area under Pressure-Time curve
Pressure Targeted Ventilation
Mean Paw = (PIP – PEEP) X (Ti/Tt) + PEEP
Volume Control Ventilation
Mean Paw = 0.5 X (PIP – PEEP) X (Ti/Tt) + PEEP
Ti = Inspiratory time and Tt = Total cycle time

24. Increasing Mean Airway Pressure

25. Pressure – Time Scalar - Information
Mode
Volume or Pressure targeted
Triggering
Negative deflection preceding inspiration
I:E Ratio
Calculated from lengths of insp to exp
Peak Airway Pressure
Highest point in pressure tracing
Plateau Pressure
Inspiratory pause
Mean Airway Pressure
Area under inspiratory curve
Set PEEP
Start of inspiratory tracing above baseline
Auto PEEP
Expiratory tracing ending above set PEEP
Airway obstruction
Disproportionate rise in PIP
Response to therapy
Decrease in PIP

26. Flow- Time Scalar
Reveals gradual change in Inspiratory and expiratory flow
The transferred volume (Tidal Volume) is the integration of flow over time and is equivalent to area under the curve
Inspiratory flow is influenced by set ventilator mode
Respiratory compliance and resistance can be assessed only in expiratory phase

27. Flow – Time Scalar
Only volume targeted ventilation offers a choice in flow wave pattern
In pressure targeted ventilation, to maintain constancy of pressure, decelerating waveform is necessary
With each flow pattern the maximum flow rate is the same while inspiratory time

28. Flow – Time Scalar
Slow rise to peak flows - thought to improve oxygenation by allowing time for gas distribution but may result in ‘flow starvation’
Constant flow and decelerating flow are the standard forms for ventilator control. No evidence exists to suggest that using other flow forms improves clinical outcomes

29. Volume Targeted Ventilation
Tplateau

30. Pressure Targeted Ventilation
If at the end of inspiration and at the end of expiration flow = 0
C = VT/ ΔP
ΔP = PIP - PEEP
Flow in the expiratory phase permits conclusions to be drawn about the overall resistance and compliance of the lung and the system
Decelerating flow is typical of pressure- controlled ventilation
The flow falls constantly after having reached an initially high value
Under normal conditions the flow returns to zero during the course of inspiration

31. Expiratory Flow Rate and Changes in Expiratory Resistance
120 .
Time in sec V 1 2 3 4 5 6
Bronchospasm COPD
Secretions
Water in the tubing
-120
Low expiratory flow rate
Extended exhalation phase
Curved contour

32. A Higher Expiratory Flow Rate and a Decreased Expiratory Time Denote a Lower Expiratory Resistance
120 .
Time in sec V 1 2 3 4 5 6
120

33. Flow- Time Scalar: Low compliance
120 .
Time in sec V 1 2 3 4 5 6
-120
Higher peak expiratory flow Shortened Te due to greater elastic recoil

34. Flow –Time Scalar: Auto PEEP
Expiratory Flow:
If expiratory time is insufficient to allow flow to reach 0, air trapping occurs (auto-PEEP or intrinsic PEEP)
Auto-PEEP results in an increase in lung pressure in volume-controlled ventilation
Auto-PEEP can have considerable effects on gas exchange and hemodynamics
High respiratory rate
Inadequate expiratory time
Too long of an inspiratory time
Prolonged exhalation due to bronchoconstriction

35. Flow – Time Scalar - Information
Volume target
Square wave-form
Pressure target mode
Decelerating flow patter on inspiration
Auto-Peep
Failure of exp flow to return to baseline
Airway obstruction
PEF is low, Prolonged expiratory flow
Bronchodilator response
Reversal or improvement of airflow pattern
Air Leak
Decreased PEF

36. Volume –Time Scalar
Shows the gradual changes in the volume transferred during inspiration and expiration

37. Volume - Time Scalar
Inspiration
SEC
800 ml 2 3 4 5 6 1 VT

38. Volume -Time Scalar Expiration VT 800 ml 1 2 3 4 5 6 SEC
During expiration, seen in red here, the transferred volume decreases, again due to the passive recoil of the lung. Generally, what goes in comes out, unless you have a leak in the patient circuit or the patient, or gas is trapped in the lung. 1 2 3 4 5 6

39. Typical Volume Curve I-Time E-Time 1.2 A B VT Liters 1 2 3 4 5 6 -0.4
SEC
The volume-time curve shows the gradual changes in the volume that is delivered during inspiration and expiration.
During the inspiratory flow phase the volume increases continuously until the set tidal volume is achieved or the high pressure alarm limit has been reached, or I-Time has expired. 1 2 3 4 5 6
-0.4
A = inspiratory volume
B = expiratory volume

40. Volume -Time Scalar : PCV
800 ml
Expiration VT
SEC 1 2 3 4 5 6
Angle of volume rise drops as the flow decelerate

41. Lung Compliance
Can be described as the relative ease with which the structure distends
Two types of forces oppose inflation of the lungs: elastic forces and frictional forces
Elastic forces arise from the elastic properties of the lungs and chest wall
Frictional forces are the result of two factors:
The resistance of the tissues and organs
The resistance to gas flow through the airways

42. Compliance
In the clinical setting, compliance measurements are used to describe the elastic forces that oppose lung inflation
C = Δ V / Δ P
Compliance has two components
Static compliance
Dynamic compliance

43. Static Compliance ( Cstat)
Static compliance measurements are made during static or no-flow conditions
Static compliance monitors elastic resistance only
Includes recoil of lung and thorax
Therefore, the plateau pressure is used for the calculation
Cstat = VT/ Pplat-PEEP
Normal Cstat in a ventilated patient: mL/cm H2O

44. Static Compliance Decreased Increased Emphysema Consolidation Collapse
Pulmonary edema
ARDS
Pneumothorax
Abdominal distention
Obesity/ Scoliosis
Decreased
Emphysema
Increased

45. Dynamic compliance
Dynamic compliance is the total impedance to inflation and represents the sum of all forces opposing movement of gas into the lung
Indicative of the “lungs and airway resistance”
The PIP indicates the energy needed to overcome the elastic and airway resistance
Cdyn = VT/ PIP-PEEP

46. Static and dynamic compliance
Cdyn
Cstat
Decrease
Unchanged
Increase PIP, unchanged Pplat: Increase Raw
Increase
Improved Raw e.g., cleared secretions, bronchodilators
Increased PIP and Pplat :Dec lung compliance and Raw
Improved PIP and Pplat: Improved lung compliance and airway resistance

47. Compliance
Dynamic compliance of obtained through least square fit method
The inspiratory curve of the dynamic P-V loop closely approximates the static curve
Slope = C = Δ V / Δ P
Shift in lung compliance curve yield different VT

48. Loops Pressure- Volume Flow- Volume
P-V and F-V loops provide provide dynamic trends in respiratory system compliance and resistance

49. Loops Pressure-Volume Loop
Pressure X - axis & Volume Y- axis
Important for understanding optimal alveolar recruitment (volume at which compliance is maximized) and to measure patient compliance
Static P-V: super syringe method
Time consuming, error prone
Dynamic P-V loops generated during mechanical ventilation with slow steady flow and corrected for airway resistance

50. Pressure-Volume Loop Paw VT20 40 60
-60
0.2
LITERS
0.4
0.6
Paw
cmH2O VT
Pressure and Volume changes plotted against each other
Elliptical or Football shaped

51. Pressure-Volume Loop Paw Inspiration VT20 40 60
-60
0.2
LITERS
0.4
0.6
Paw
cmH2O VT
On a ventilator-initiated mandatory breath, the loop starts in left hand corner
Progresses counter clockwise

52. Pressure-Volume Loop Counterclockwise Paw Expiration Inspiration VT
When preset VT is reached expiration begins and returns to FRC
LITERS
0.6
Expiration
0.4
Hysteresis
Inspiration
0.2
Paw
cmH2O
-60 40 20 20 40 60
Hysteresis refers to unrecoverable energy, or delayed recovery of energy due to alveolar recruitment/ de recruitment; surfactant; stress relaxation; and gas absorption during the measurement of P-V curves
When the forward path is different from the reverse path, then this is referred to as hysteresis

53. Spontaneous Breath Paw Inspiration VT Clockwise 0.6 0.4 0.2 cmH2O -60
LITERS
0.6
0.4
Inspiration
0.2
Paw
cmH2O
-60 40 20 20 40 60

54. Spontaneous Breath Paw Inspiration Expiration VT Clockwise 0.6 0.4 0.2
LITERS
0.6
0.4
Inspiration
Expiration
0.2
Paw
cmH2O
-60 40 20 20 40 60

55. Assisted Breath Paw Assisted Breath VT 0.6 0.4 0.2 cmH2O -60 40 20 20
LITERS
0.6
0.4
Assisted Breath
0.2
Paw
cmH2O
-60 40 20 20 40 60

56. Assisted Breath Paw Assisted Breath Inspiration VT 0.6 0.4 0.2 cmH2O
LITERS
0.6
0.4
Assisted Breath
0.2
Inspiration
Paw
cmH2O
-60 40 20 20 40 60

57. Assisted Breath Paw Expiration Assisted Breath Inspiration VT
Clockwise to Counterclockwise
LITERS
0.6
Expiration
0.4
Assisted Breath
0.2
Inspiration
Paw
cmH2O
-60 40 20 20 40 60

58. Components of Pressure-Volume Loop
Tidal Dynamic compliance
Δ V / Δ P VT
PIP
Normal compliance is 50 – 80mL/cm H20
Slope set at 45 degree
Volume (mL)
PEEP
FRC: Balance between lung recoil and chest wall expansion
Paw (cm H2O)
FRC

59. Lung Compliance Changes and the P-V Loop
Volume Targeted Ventilation
Preset VT
Change in slope
COMPLIANCE
Increased
Normal
Decreased
Volume (mL)
Paw (cm H2O)
Decreased compliance: more pressure to deliver volume
PIP levels

60. Lung Compliance Changes and the P-V Loop
Increased
Normal
Decreased
VT levels
Pressure Targeted Ventilation
Volume (mL)
Paw (cm H2O)
Preset PIP

61. Change in resistance Normal Hysteresis Abnormal Hysteresis Volume (ml)
If resistance changes during constant flow ventilation the steepness of the right branch of the loop remains unchanged, but changes position
Normal Hysteresis
Abnormal Hysteresis
Pressure (cm H2O)

62. Inflection Points Point of change in line of a slope
Lower Inflection Point:
Represents minimal pressure for adequate alveolar recruitment (alveoli begin to fill rapidly and alveolar recruitment begins)
Upper Inflection Point:
Represents pressure resulting in regional over distension
(the lung’s maximum volume is reached in the face of continued inspiratory flow)
Upper Inflection Point
Volume (mL)
Lower Inflection Point
Pressure (cm H2O)

63. Inflection Points Initially, the volume per unit pressure rise is slow
At the lower inflection point, the lung-opening pressure is reached and the rise shows a more rapid increase in volume per unit pressure
Point at which alveolar recruitment begins
Lung recruitment may continue until the upper inflection point
At the upper inflection point, the compliance limit is reached the slope decreases again

64. Inflection Points
Ventilation should take place within the linear compliance area as dangerous shear forces may occur outside of this area
Some advocate setting PEEP at the LIP of expiratory curve. This prevents cyclical derecruitment injury
The ventilation volume (in CMV, SIMV) or inspiratory pressures (in BIPAP, PCV) must then be selected such that the upper inflection point not be exceeded

65. Inflection Point Alveolar over distention
Occur when the volume capacity of lung has been exceeded and addition pressure causes very little change in volume
May result in barotrauma, decreased venous return, etc
Correction involves decreasing the tidal volume or pressure target

66. With little or no change in VT
Over distension
Beaking
With little or no change in VT
Normal
Abnormal
Volume (ml)
Pressure (cm H2O)
Paw rises

67. Work of Breathing B A: Resistive Work B: Elastic Work A Volume (ml)
Pressure (cm H2O)

68. Work of Breathing
WOB equals area under the changing pressure curve as volume moves from zero to its peak at end inspiration

69. P-V Loops and WOB
The greater the area comprised by A & B, the greater the work

70. Flow- Volume Loops Flow-Volume Loop
Flow X axis and Volume Y- axis
Used to gain information about airway resistance and response to bronchodilators
In PFT’s inspiratory curve is below horizontal axis and expiratory curve above X- axis
Depending on brand of ventilator, orientation may vary

71. Flow -Volume Loop Volume Control
Tidal Volume
Peak Inspiratory Flow
Peak Expiratory Flow
Inspiration
Flow
Volume
FRC
Expiration
PEFR

72. F-V Loop - Pressure Control
Volume target has constant flow pattern, in pressure control, due to decelerating flow pattern the F-V appears as two opposing expiratory curves.

73. F-V Loop: Air Leak Normal Abnormal Inspiration Expiration Volume (ml)
Flow
(L/min)
Volume (ml)
Air Leak in mL
Normal
Abnormal
Expiration

74. F-V Loop: Increased Airway Resistance
BEFORE
AFTER
Worse
Better 3 3 2 1 3 V .
INSP 2 2 1 1 . . V V VT 1 1 2 2
“Scooped out”
pattern & decreased PEFR 3 3
EXP

75. F-V Loop: Airway Secretions
Inspiration
Flow
(L/min)
Volume (ml)
Normal
Abnormal
Expiration

76. F-V Loop – Auto PEEP
Does not return
to baseline

77. Summary
Ventilator waveforms provides much information on airway and lung mechanics
Assist in monitoring clinical course and response to therapy

78. References
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