1. MODES OF MECHANICAL VENTILATION
DR MALAV SHAH

2. Controlled mechanical ventilation (CMV )

3. Volume-limited vs. Pressure-limited
Controlled mechanical ventilation (CMV)
assist/control (A/C) ventilation, and
synchronized intermittent mandatory ventilation (SIMV)

4. Volume-limited
clinician sets flow rate, flow pattern (ramp vs square), tidal volume, respiratory rate, PEEP, and FiO2.
Inspiration ends after delivery of the set tidal volume ( LIMIT ) .
Airway pressures depend on set Vt and patient compliance and airway resistance
Peak flow rates of 60 L per minute may be sufficient, although higher rates are frequently necessary.
Flow pattern: square wave (constant flow), a ramp wave (decelerating flow), and a sinusoidal wave (figure 3). The ramp wave may distribute ventilation more evenly than other patterns of flow, particularly when airway obstruction is present [22]. This decreases the peak airway pressure, physiologic dead space, and PaCO2, while leaving oxygenation unaltered [23].

5. Pressure-limited
clinician sets inspiratory pressure level, I:E ratio, respiratory rate, applied PEEP, and FiO2
Inspiration ends after delivery of the set inspiratory pressure
tidal volume is variable and determined by inspiratory pressure, compliance, airway and tubing resistance
peak airway pressure is constant and equal to sum of set inspiratory pressure and applied PEEP.
tidal volumes will be larger when the set inspiratory pressure level is high or there is good compliance, little airway resistance, or little resistance from the ventilator tubing. 5

6. Image may be subject to copyright.
Pressure-limited
tidal volumes will be larger when the set inspiratory pressure level is high or there is good compliance, little airway resistance, or little resistance from the ventilator tubing.
Image may be subject to copyright. 6

7. Volume-limited vs. Pressure-limited
RCT : Pressure-limited associated with lower peak airway pressure, more rapid improvement in compliance, fewer days of mechanical ventilation
Compared in a few small studies 7

8. Volume-limited vs. Pressure-limited
Conclusions:
no statistically significant differences in mortality, oxygenation, or work of breathing
pressure-limited: lower peak airway pressures, more homogeneous gas distribution, improved synchrony, and earlier liberation from vent
volume-limited: the only mode that can guarantee a constant tidal volume, ensuring a minimum minute ventilation .
There is currently no data showing 8

9. Controlled mechanical ventilation (CMV)
Minute ventilation is determined entirely by the set respiratory rate and tidal volume / pressure.
The patient does not initiate additional breaths above that set on the ventilator.
volume control ventilation (VCV): flow-targeted volume-cycled breaths
pressure control ventilation (PCV): pressure-targeted time-cycled breaths

10. ASSIST CONTROL MODE

11. Assist-control ventilation (ACV)
volume assist-control ventilation (VACV): flow-targeted volume-cycled breaths
pressure assist-control ventilation (PACV): pressure- targeted time-cycled breaths
guarantees a set number of positive-pressure breaths.
If respiratory rate exceeds this, breaths are patient- triggered breaths (VA or PA). If respiratory rate is below guarantee, ventilator delivers mandatory breaths (VC or PC breaths).

12. IMV

13. INTERMITTENT MANDATORY VENTILATION (I.M.V.)
CMV breaths delivered at a set rate and volume. (or pressure )
In between two machine delivered breaths , spontaneous breaths are possible through a demand valve.
Developed as a weaning mode
? Work of breathing is increased

14. IMV
Primary disadvantage is chance for breath stacking, therefore care should be taken to set high press. limit properly to reduce risk of barotrauma

15. I M V
Flow
(L/min)
Pressure
(cm H2O)
Volume
(ml)
Time (sec)

16. SIMV

17. Synchronized Intermittent Mandatory Ventilation (SIMV)
This is a version of IMV wherein the mandatory breaths are synchronized with the patient’s inspiratory effort.
With the development of SIMV, the IMV version is only historical and practically defunct.
The mandatory or mechanical breath in both SIMV and IMV are traditionally volume limited / preset but can be pressure limited / preset

18. “synchronized window” refers to the time just prior to time triggering in which the vent. is responsive to the pt.’s effort (0.5 sec is typical)
Advantages include maintaining resp. muscle strength, reduces V/Q mismatch, decreases mean airway press., helps wean pt

19. PS Used to lower the WOB and augment a patient’s spont. tidal volume
Typically used in the SIMV mode to help weaning by (1) increasing spont. tidal volume (2) decreasing spont. RR (3) decreasing WOB

20. SIMV
Flow
(L/min)
Pressure
(cm H2O)
Volume
(ml)
Time (sec)

21. SIMV (Volume-Targeted Ventilation)
Flow
(L/m)
Pressure
(cm H2O)
Volume
(mL)
Spontaneous Breaths

22. SIMV Mode (Pressure-Targeted Ventilation)
Flow
Volume
(L/min)
(cm H2O)
(ml)
Set PC level
Time (sec)
Spontaneous Breath

23. SIMV / PS Flow Pressure Volume Time (sec) Set PS level (L/min)
(cm H2O)
Volume
(ml)
Time (sec)

24. SIMV + PS (Volume-Targeted Ventilation)
Flow-cycled
Flow
Pressure
Volume
(L/min)
(cm H2O)
(ml)
Set PS level
PS Breath

25. SIMV + PS (Pressure-Targeted Ventilation)
Time-Cycled
Flow-Cycled
Pressure
Flow
Volume
(L/min)
(cm H2O)
(ml)
Set PC level
Set PS level
Time (sec)
PS Breath

26. Synchronized Intermittent Mandatory Ventilation
SIMV is usually associated with greater work of breathing than AC ventilation and therefore is less frequently used as the initial ventilator mode
Negative inspiratory pressure generated by spontaneous breathing leads to increased venous return, which theoretically may help cardiac output and function

27. PRESSURE SUPPORT

28. Pressure Support (PS)
Flow-limited mode of ventilation (not volume-limited or pressure-limited)
Delivers inspiratory pressure until the inspiratory flow decreases to ~25% of its peak value.
Clinician sets inspiratory pressure, applied PEEP, and FiO2.
Patient triggers each breath
Comfortable mode, good for weaning, can be combined with SIMV
Not good for full ventilatory support, high airway resistance, or central apnea
flow-limited mode of ventilation that delivers inspiratory pressure until the inspiratory flow decreases to a predetermined percentage of its peak value. This is usually 25 percent 28

29. Comparison of waveforms
Figure 2-1. Pressure, flow, and waveforms typically encountered during mechanical ventilation in the emergency department. The nature of fresh gas delivery during mechanical ventilation is in part a function of the definition of “breath” (i.e., a delivered volume or delivered pressure) and the means by which that breath is initiated (i.e., by the patient or by a timing decision made by the ventilator). In practice, only a handful of ventilator parameters commonly are managed in the emergency department (the mode, the magnitude of the delivered breath, the rate of delivery, and Fio2). However, as this figure shows, a number of additional features can be fine-tuned to optimize the effectiveness and comfort of mechanical ventilation in critically ill patients
Marx: Rosen's Emergency Medicine, 7th ed.2009. 29

30. Inverse ratio ventilation

31. Inverse ratio ventilation
Strategy of inversing I:E ratio (I>E) to potentially improve oxygenation
When pt is severely hypoxemic despite optimal PEEP and FiO2
Can be used with volume-limited or pressure-limited mechanical ventilation
In pressure: increase I:E ratio
In volume: ramp wave- decrease peak inspiratory flow rate until I exceeds E
In volume square wave- add and increase end- inspiratory pause until I exceeds E
The inspiratory time exceeds the expiratory time during IRV (the I:E ratio is inversed), increasing the mean airway pressure and potentially improving oxygenation. 31

32. Inverse ratio ventilation
In trials increases mean airway pressure,
may improve oxygenation,
never been shown to improve important clinical outcomes
Requires increased sedation +/- paralysis
Risks: increased risk of auto-PEEP, barotrauma and hypotension
The inspiratory time exceeds the expiratory time during IRV (the I:E ratio is inversed), increasing the mean airway pressure and potentially improving oxygenation. 32

33. AUTOMATIC TUBE COMPENSATION (ATC)

34. Automatic tube compensation(ATC)
Designed to overcome the resistance of the ET by means of continuous calculations.
A ventilatory method aimed at compensating for nonlinear pressure drop across the ET during spontaneous breathing.
Overcomes the imposed work of breathing due to artificial airways.
At least as successful as use of simple T-tube or low- level PS for weaning from mechanical ventilation. .

35. How?
Gas flow through an endotracheal tube during spontaneous breathing generates a pressure gradient between the 2 ends of the tube and increases the work of breathing.

37. Automatic tube compensation ∆P (P support) α (L / r4 ) α flow α WOB
High circuit pressure
Low carinal pressure
1.Pressure drop across the circuit is the cause of WOB with an endotracheal tube.
2.ATC raises the carinal pressure and hence decreases the work of breathing
∆P (P support) α (L / r4 ) α flow α WOB

38. Negative pressure at the distal end of the tube (Ptrach) during spontaneous inspiration is modified by positive-pressure ventilation.

40. Automatic tube compensation
Indications for ATC
Patients with compromised respiratory function– COPD, malnutrition
Patients with failed previous extubation attempts
May be beneficial if an SBT fails because of a particularly narrow endotracheal tube
Difficult to wean patients

42. AIRWAY PRESSURE RELEASE VENTILATION (APRV)

43. AKA BiVent – Servo APRV – Drager BiLevel – Puritan Bennett
APRV – Hamilton
Etc.

44. Introduction APRV augments alveolar ventilation.
Airway pressure is released from an elevated baseline pressure to produce an expiration.
The elevated pressure facilitates oxygenation, while the pressure release increases minute ventilation.

45. APRV (airway pressure release ventilation)

46. Combines two separate levels of CPAP and the pt. may breathe spont
Combines two separate levels of CPAP and the pt. may breathe spont. from both levels
Periodically, pressure is dropped to the lower level, reducing mean airway press.
During spont. expir. the CPAP is dropped (released) to a lower level which simulates an effective expiration

47. Airway pressure release ventilation (APRV)
high continuous positive airway pressure (P high) is delivered for a long duration (T high) and then falls to a lower pressure (P low) for a shorter duration (T low)
allows spontaneous breathing (with or without PS) during both the inflation and deflation phases
Like two levels of CPAP
Gonza ́lez et al. Intensive Care Med (2010) 36:817–827 47

48. Airway pressure release ventilation (APRV)

49. Airway pressure release ventilation (APRV)
Based on Open Lung Concept: maximize alveolar recruitment by keeping the lung inflated for extended time with high continuous positive airway pressure
Driving pressure= difference between P high and P low. Size of the tidal volume is related to both the driving pressure and the compliance.
The transition from P high to P low deflates the lungs and eliminates CO2.
T high and T low determine the frequency of inflations and deflations
Gonza ́lez et al. Intensive Care Med (2010) 36:817–827

50. Airway pressure release ventilation (APRV)
Potential benefits:
improved alveolar recruitment and oxygenation
Some observational studies show decreased peak airway pressure, improved alveolar recruitment, increased ventilation of the dependent lung zones and improved oxygenation
No mortality benefit
Potential risks: In severe obstructive disease, could lead to hyperinflation and barotrauma

51. APRV- Is it better?
RCT of APRV vs SIMV plus PSV (not LTVV) in 58 pts with ARDS: no difference in outcome
Varpula.Acta Anaesth Scand  2004; 48:
RCT of APRV vs LTVV with SIMV in 63 trauma pts (not all with ARDS): no diff in mortality, trend towards ↑ MV days and ICU LOS
Maxwell et al. J Trauma. 2010;69: 501–511
Secondary analysis of observational cohort study of 234 pts ventilated with APRV/BI-PAP vs 1,228 with A/C:
no differences in ICU or hospital mortality, days of MV, LOS
Gonza ́lez et al. Intensive Care Med (2010) 36:817–827
patients who received mechanical ventilation for more than 12 h during a 1- month period beginning 1 April 2004 in 349 intensive care units in 23 countries. propensity score method has become a common method used for confounder adjust- ment in observational studies.
Maxwell; trend for increased ventilator days, ICU length of stay, and
ventilator-associated pneumonia in the APRV group
RCT of APRV vs PCV in 30 trauma pts:  MV days,  ICU stay, less sedation and paralysis, no mortality diff
Putensen et al. Am J Respir Crit Care Med. 2001;164(1):43. 51

52. Indications
Primarily used as an alternative ventilation technique in patients with ARDS.
Used to help protect against ventilator induced lung injury.

53. Consider APRV when the Patient Has --
Bilateral Infiltrates
PaO2/FIO2 ratio < 300 and falling
Plateau pressures greater than 30 cm H2O
No evidence of left heart failure (e.g. PAOP of 18 mm Hg or greater)
In other words, persistent ARDS

54. Possible Contraindications
Unmanaged increases in intracraneal pressure.
Large bronchopleural fistulas.
Possibly obstructive lung disease.
Technically, it may be possible to ventilate nearly any disorder.

55. Terminology
Four commonly used terms include: pressure high (P High), pressure low (P Low), time high (T High), and time low (T Low).
P High – the upper CPAP level. Analogous to MAP (mean airway pressure) and thus affects oxygenation. P High is the baseline airway pressure level and is the higher of the two airway pressure levels. Other authors have described P High as the CPAP level, the inflating pressure, or the P1 pressure (P1), also called High Peep.

56. P-High
p-High is the upper CPAP or pressure setting when utilizing APRV.
p-High regulates end-inspiratory lung volume & is analogous with mean airway pressure

57. Terminology
PEEP (Also called Plow or Low Peep) is the lower pressure setting.
P Low is the airway pressure level resulting from the pressure release. Other authors may refer to P Low as the PEEP level, the release pressure, or the P2 pressure (P2).

58. P-Low Servo I: Bi-Vent Draeger Evita
The p-Low setting, sets the lower level of CPAP during the release phase.
The term "p-Low" is used in Draeger & Hamilton medical ventilators. 
PB 840 BiLEVEL

59. T-High
T High- is the inspiratory time IT(s) phase for the high CPAP level (P High).
T High corresponds with the length of time for which P High is maintained

60. T-High
Allows for sustained recruitment allowing for improved gas exchange by increasing alveolar surface area. 

61. T-Low
T Low is the length of time for which the P Low is held (i.e. for which the airway pressure is released)
T High plus T PEEP (T low) is the total time of one cycle.
I:E ratio becomes irrelevant because APRV is really best thought of as CPAP With occasional releases

62. T-Low
The t-Low sets the time interval for the low pressure/CPAP phase (p-Low).
Allows for intermittent release in airway pressure, providing paCO2 removal.
Partially unloads the patients work of breathing associated with pure CPAP breathing.
The name "t-Low" is used in both Draeger and Hamilton ventilators. 
note- t-Low should not be considered "expiratory time" as the patient may exhale throughout the entire inspiratory phase. 
When using Bi-Level on the PB 840 there is no setting to set the low pressure interval. The operator must change frequency & "TH" to set a t-Low. This can become problematic when trying to precisely set a t-Low interval. 

64. Advantages of Spontaneous Breathing
The benefits of APRV may be related to the preservation of spontaneous breathing.
Maintaining the normal cyclic decrease in pleural pressure, augmenting venous return and improving cardiac output. (Putensen, AJRCCM, 1999)
The need for sedation is decreased.

65. Spontaneous v.s. Paralyzed
Spontaneous breathing provides ventilation to dependent lung regions which get the best blood flow, as opposed to PPV with paralyzed patients. ((Frawley, AACN Clinical Froese, Anesth, 1974).
Maintaining spontaneous ventilation tends to improve ventilation- perfusion matching by preferentially providing ventilation to dependent lung regions that receive the best blood flow. 65

66. Other Advantages of Spontaneous Breathing
Reduces atrophy of the muscles of ventilation associated with the use of PPV and paralytic agents. (Neuman, ICM,2002)

67. Initial Settings
P high cm H2O, according to the following chart.
T High/T low releases
T High (s) T low (s) Freq.
P/F MAP
<
<
<
T high range 4-6 sec.
PS- as indicated with special attention given to PIP.
T low = 0.5 sec and
P low = 0

68. Disadvantages of APRV With increased Raw (e.g.COPD)
the ability to eliminate CO2 may be more difficult
Due to limited emptying of the lung and short release periods.
If spontaneous efforts are not matched during the transition from Phigh to Plow and Plow to Phigh, may lead to increased work load and discomfort for the patient.
Limited staff experience with this mode may make implementation of its use difficult.

69. HIGH FREQUENCY VENTILATION

70. High-frequency ventilation (HFV)
HFV is time-cycled positive pressure ventilation that delivers a high frequency (60– 120 breathes per min) of small tidal volumes (1.5 mL/kg) that are usually less than the anatomic dead space
3 different modes: high-frequency positive- pressure ventilation (HFPPV), high-frequency jet ventilation (HFJV), and high-frequency oscillatory ventilation (HFOV)

71. High-Frequency Oscillatory Ventilation (HFOV or HFV)
Also based on Open Lung Concept: keeping the lung inflated for extended period of time to maximize alveolar recruitment .
HFV uses very high breathing frequencies ( breaths/min) coupled with very small tidal volumes (<1 mL/kg) to provide gas exchange in the lungs.
supplied by either jets or oscillators.
Jets inject high-frequency pulses of gas into the airways. Oscillators literally vibrate a fresh bias flow of gas delivered at the tip of the endotracheal tube
less than anatomic dead space, 71

72. High-Frequency Oscillatory Ventilation (HFOV or HFV)
Rationale:
very small alveolar tidal volumes minimize cyclical overdistention and derecruitment
maintains the alveoli open at a relatively constant airway pressure and thus may prevent atelectrauma and barotrauma
improves ventilation/perfusion (V/Q) matching by ensuring uniform aeration of the lung.
Jets inject high-frequency pulses of gas into the airways. Oscillators literally vibrate a fresh bias flow of gas delivered at the tip of the endotracheal tube 72

73. High-Frequency Oscillatory Ventilation(HFOV or HFV)
Figure 3.Waveforms depicting the key variables that are controlled during high frequency oscillation as compared to conventional ventilation. The y-axis on the left depicts changes in airway pressure seen with high frequency oscillatory ventilation and the y-axis on the right depicts changes in peak airway pressure with conventional ventilation. Note that tracheal pressure becomes negative at peak expiration, thereby making expiration an active process. Also note that as amplitude increases, delivered minute ventilation increases. A background tracing of pressure versus time using a respiratory rate of 12 and inspiratory to expiratory ratio of 1:3 with conventional ventilation is presented for comparison.
Stawicki et al. J Intensive Care Med : 73

75. High-Frequency Oscillatory Ventilation (HFOV or HFV)
Several studies in adults have shown improved oxygenation but no mortality benefit
One RCT: HFV vs PCV (6 -10 mL/kg, mean 8) in 148 patients with ARDS on PEEP≥10
HFV had higher mean airway pressure, early improvement in oxygenation, and trend towards lower mortality rate (37 vs 52%, p = 0.10)
Derdak. Am J Respir Crit Care Med. 2002;166(6):801
Jets inject high-frequency pulses of gas into the airways. Oscillators literally vibrate a fresh bias flow of gas delivered at the tip of the endotracheal tube
early (less than 16
hours) improvement in Pa
O2/fraction of inspired oxygen compared
with the conventional ventilation group (p 0.008); however,
this difference did not persist beyond 24 hours 75

76. two recent large multicenter trials involving patients with ARDS did not show improved outcomes with HFOV.
this type of ventilation cannot be recommended as first-line therapy in such patients.