Modes of Mechanical Ventilation
Fellow’s conference
December 7, 2011
Cheryl Pirozzi, MD
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Breath types
Modes of ventilation
Other strategies
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Positive-pressure mechanical ventilators
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Most use piston/bellows
systems
Tidal breaths generated by
gas flow, either controlled
entirely by the ventilator or
interactive with patient efforts
Breath types
Classified by:
1)
trigger variable: what initiates the breath
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target variable: what controls gas delivery during the
breath
2)
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set flow or set inspiratory pressure
Termination/cycle variable: what terminates the
breath
3)
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change in pressure or flow due to patient effort (patientinitiated breaths) or a set time (vent-initiated)
set volume, set inspiratory time, or a set flow
pressure is usually a “backup” cycle variable to
terminate gas delivery if circuit pressure rises above
an alarm limit
5 basic breath types
1.
2.
3.
4.
5.
volume assist (VA)
volume control (VC)
pressure assist (PA)
pressure control (PC)
pressure support (PS)
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5 basic breath types
Breath
Trigger
Target
VA
Pt
Inspir flow
Termination /
cycle
Set Vt
VC
Vent
Inspir flow
Set Vt
PA
Pt
insp P
Insp time
PC
Vent
insp P
Insp time
PS
Pt
insp P
% decrease
inspir flow
5 basic breaths
FIGURE 89-1 ▪ Circuit pressure, flow, and volume tracings over time depicting the five basic
breaths available on most modern mechanical ventilators. Breaths are classified by the
variables that determine the trigger (machine time or patient effort), target/limit (set flow or
set pressure), and cycle (set volume, set time, or set flow). The solid lines represent set or
Modes of mechanical ventilation
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Controlled mechanical ventilation (CMV)
Assist-control ventilation (ACV)
Synchronized intermittent mandatory ventilation (SIMV or
IMV)
Pressure support (PS)
CPAP
BPAP
Pressure-regulated volume control (PRVC)
Airway pressure release ventilation (APRV) and Biphasic
Adaptive support ventilation (ASV)
Volume support / Automatic Pressure Ventilation
High-frequency ventilation (HFV)
Volume-limited vs. Pressure-limited
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Controlled mechanical ventilation (CMV),
assist/control (A/C) ventilation, and
synchronized intermittent mandatory
ventilation (SIMV) all can be supplied through
either pressure-limited or volume-limited
modes
Volume-limited
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Volume-limited
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clinician sets peak flow rate, flow pattern (ramp vs square),
tidal volume, respiratory rate, PEEP, and FiO2.
Inspiration ends after delivery of the set tidal volume.
(I:E) ratio determined by the peak inspiratory flow rate. ↑
peak inspiratory flow → ↓ inspiratory time, ↑ expiratory
time, and ↓ I:E ratio
Airway pressures depend on set Vt and patient compliance
and airway resistance
Pressure-limited
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Pressure-limited
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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.
Pressure-limited
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Volume-limited vs. Pressure-limited
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Rappaport et al. Crit Care Med. 1994;22(1):22
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RCT PCV vs VCV in 27 pts with acute,
severe hypoxic respiratory failure
(PaO2/FIO2 < 150), not LTVV
Pressure-limited associated with lower peak
airway pressure, more rapid improvement in
compliance, fewer days of mech ventilation
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Volume-limited vs. Pressure-limited
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Prella et al. Chest. 2002;122(4):1382
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Prospective, observational study of 10 pts with
ALI or ARDS: gas exchange, airway pressures,
and end-expir CT for PCV vs VCV
No difference in PaO2, PaCO2, and PaO2/FiO2
Peak airway pressure significantly lower in PCV
compared with VCV (26 vs 31cmH2O; p <
0.001)
PCV more homogeneous gas distribution at the
apex on CT
not using low tidal volume ventilation
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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
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When ramp wave (decelerating flow pattern) used for VCV, no
longer higher peak pressures than PCV
volume-limited: the only mode that can guarantee a
constant tidal volume, ensuring a minimum minute
ventilation or LTVV
Modes of mechanical ventilation
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Controlled mechanical ventilation (CMV)
Assist-control ventilation (ACV)
Synchronized intermittent mandatory ventilation (SIMV or
IMV)
Pressure support (PS)
CPAP
BPAP
Pressure-regulated volume control (PRVC)
Airway pressure release ventilation (APRV) and Biphasic
Adaptive support ventilation (ASV)
Volume support / Automatic Pressure Ventilation
High-frequency ventilation (HFV)
Controlled mechanical ventilation (CMV)
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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): pressuretargeted time-cycled breaths
Assist-control ventilation (ACV)
1.
2.
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volume assist-control ventilation (VACV): flowtargeted 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).
Synchronized intermittent mandatory
ventilation (SIMV)
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Set ventilator breaths: set minimum minute
ventilation with respir rate + tidal volume (volume
SIMV) or inspiratory P (pressure SIMV)
Ventilator breaths are synchronized with patient
inspiratory effort
pts increase minute ventilation by add’l
spontaneous breaths, which can be unassisted or
PS
Pressure Support (PS)
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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
Comparison of waveforms
Marx: Rosen's Emergency Medicine, 7th ed.2009.
CPAP
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Continuous level of positive airway pressure.
Pt must initiate all breaths
Functionally similar to PEEP
Good for OSA, cardiogenic pulmonary edema
Bilevel positive airway pressure
(it’s called BPAP, not BiPAP)
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Mode used during NPPV
Delivers set IPAP and EPAP
Vt is determined by difference between IPAPEPAP
Pressure-regulated volume control (PRVC)
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A form of PACV that uses tidal volume as a
feedback control for continuously adjusting the
pressure target
clinician sets tidal volume target and the
ventilator then automatically sets the inspiratory
pressure within a clinician-set range to achieve
this goal
As a patient's respiratory drive exceeds the
clinician-set guaranteed rate, some PRVC
systems will provide additional patient-triggered
PA or PS breaths
Airway pressure release ventilation (APRV)
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Time-triggered, pressure-limited, and time-cycled
mode
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
Gonza ́lez et al. Intensive Care Med (2010) 36:817–827
Airway pressure release ventilation (APRV)
Airway pressure release ventilation (APRV)
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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
Airway pressure release ventilation
(APRV)
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Potential benefits:
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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
APRV- Is it better?
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RCT of APRV vs SIMV plus PSV (not LTVV) in 58 pts
with ARDS: no difference in outcome
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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
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Varpula.Acta Anaesth Scand 2004; 48:722-731.
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
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Gonza ́lez et al. Intensive Care Med (2010) 36:817–827
Biphasic Ventilation
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Similar to APRV, except that T low is longer during
biphasic ventilation, allowing more spontaneous
breaths to occur at P low
AKA Bi-Vent, BiLevel, BiPhasic, and DuoPAP
ventilation.
Biphasic Ventilation
High-Frequency Oscillatory Ventilation
(HFOV or HFV)
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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 (120900 breaths/min) coupled with very small tidal
volumes (<1 mL/kg) to provide gas exchange in the
lungs
supplied by either jets or oscillators.
High-Frequency Oscillatory Ventilation
(HFOV or HFV)
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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.
High-Frequency Oscillatory Ventilation(HFOV
or HFV)
High-Frequency Oscillatory Ventilation
(HFOV or HFV)
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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
Adaptive Support Ventilation (ASV)
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Based on respiratory mechanics vent automatically
adjusts respiratory rate and inspiratory pressure to
achieve a desired minute ventilation
Clinician sets desired minute ventilation and a patient
weight (for estimating anatomic dead space).
ASV calculates expiratory time constant from the flow
volume loop → determines the respiratory rate that
minimizes work of inspiration at a given minute ventilation.
Breaths are pressure-control + pressure support for
triggered breaths to achieve desired respiratory rate.
As respiratory mechanics change, the frequency–tidal
volume pattern is automatically adjusted to maintain this
“optimal” pattern.
Adaptive Support Ventilation (ASV)
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The delivered “minimal work” tidal volume with ASV
may be higher than 6 mL/kg
No outcome studies comparing ASV to conventional
lung-protective strategies
Volume Support (VS)
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AKA “Automatic Pressure Ventilation”
Pressure support mode that uses tidal volume as a
feedback control for continuously adjusting the pressure
support level.
Clinicians select a target tidal volume, Vent makes
automatic adjustments in inspiratory pressure within a
clinician-prescribed range.
Potential for automatic support reduction: could
“automatically” wean a patient by reducing PS as
patient effort and mechanics improve
No trials comparing VS or ASV weaning to aggressive
daily SBT strategies
Other strategies
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Tracheal Gas Insufflation (TGI)
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Technique to reduce dead space in high pCO2
situations, eg lung-protective ventilatory strategies
like LTVV.
Fresh gas is insufflated by a catheter placed at the
distal end of the ETT to flush the ETT tube free of
CO2 during exhalation
Studies show TGI reduces dead space but also has
the potential to increase PEEP.
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Inverse ratio ventilation
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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 endinspiratory pause until I exceeds E
Inverse ratio ventilation
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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
Strategies to optimize syncrony
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Interactive breaths improve comfort and
reduce sedation
Strategies
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Endotracheal Tube Resistance Compensation
Pressure-Targeted Inspiratory Pressure Slope
Adjusters
Pressure Support Cycle Adjusters
Proportional Assist Ventilation
Neurally adjusted ventilatory assistance (NAVA)
Strategies to optimize syncrony
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Endotracheal tube resistance compensation /
Automatic tube compensation = type of PSV that
applies sufficient positive pressure to overcome the
work of breathing imposed by the ETT, which can
vary from breath to breath
 Clinicians input characteristics of ETT. Vent
adjusts circuit pressure during both inspiration and
expiration
 Good for SBT or combined with other mode.
Strategies to optimize syncrony
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Pressure-Targeted Inspiratory Pressure Slope
Adjusters
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For pressure-targeted breaths (PS, PA/C)
Slope adjusters allow clinician to adjust pressure rate
of rise
Pt with vigorous breaths may desire rapid rate of rise,
or vice versa if less vigorous demands
Strategies to optimize syncrony
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Pressure support cycle adjusters
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In PS, flow cycling mechanism terminating flow at 25%
can sometimes terminate breaths too early (if long
inspiratory demands) or too late (if obstruction)
allow adjustments of the flow criteria to assure
synchrony with the end of patient effort
Strategies to optimize syncrony
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Proportional Assist Ventilation
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No set pressure, flow, or volume.
The sensed patient effort is boosted according to a
proportion of the measured work of breathing set by
the clinician.
The greater the patient effort, the greater the delivered
pressure, flow, and volume.
Strategies to optimize syncrony
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Neurally adjusted ventilatory assistance (NAVA)
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uses a diaphragmatic EMG signal to trigger and cycle
ventilatory assistance.
EMG sensor positioned in the esophagus at the level of the
diaphragm
Breaths triggered by phrenic nerve excitation of the
inspiratory muscles
Expensive!
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Which mode to use when?
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Pressure- and volume-limited modes have
unique advantages and disadvantages, but
do not significantly effect mortality,
oxygenation, or work of breathing
“innovative strategies” mostly proposed for
ARDS and “lung protection”
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Overall no significant outcome benefits. Consider
if severe or refractory hypoxemia
References
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Murray and Nadel's Textbook of Respiratory Medicine. 5th edition
Bozyk P, Hyzy R. Modes of mechanical ventilation. Up To Date. 2010
Rappaport SH, Shpiner R, Yoshihara G, Wright J, Chang P, Abraham E. Randomized,
prospective trial of pressure-limited versus volume-controlled ventilation in severe
respiratory failure. Crit Care Med. 1994;22(1):22
Prella M, Feihl F, Domenighetti G. Effects of short-term pressure-controlled ventilation on
gas exchange, airway pressures, and gas distribution in patients with acute lung
injury/ARDS: comparison with volume-controlled ventilation. Chest. 2002;122(4):1382
Chiumello D, Pelosi P, Calvi E, Bigatello LM, Gattinoni. Different modes of assisted
ventilation in patients with acute respiratory failure. Eur Respir J. 2002;20(4):925
Varpula T, Valta P, Niemi R, et al: Airway pressure release ventilation as a primary
ventilatory mode in acute respiratory distress syndrome. Acta Anaesth Scand 2004;
48:722-731.
Derdak S, Mehta S, Stewart TE, Smith T, Rogers M, Buchman TG, Carlin B, Lowson S,
Granton J, Multicenter Oscillatory Ventilation For Acute Respiratory Distress Syndrome
Trial (MOAT) Study Investigators. High-frequency oscillatory ventilation for acute
respiratory distress syndrome in adults: a randomized, controlled trial. Am J Respir Crit
Care Med. 2002;166(6):801
Stewart NI, Jagelman TA, Webster NR. Emerging modes of ventilation in the intensive care
unit. Br J Anaesth. 2011 Jul;107(1):74-82. Epub 2011 May 24
Gonza ́lez et al. Airway pressure release ventilation versus assist-control ventilation: a
comparative propensity score and international cohort study. Intensive Care Med (2010)
36:817–827
References
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Stawicki S.P. , Goyal M and Sarani B. High-Frequency Oscillatory Ventilation
(HFOV) and Airway Pressure Release Ventilation (APRV): A Practical Guide. J
Intensive Care Med 2009 24: 215-229
Putensen C, Zech S, Wrigge H, Zinserling J, Stüber F, Von Spiegel T, Mutz N.
Long-term effects of spontaneous breathing during ventilatory support in
patients with acute lung injury. Am J Respir Crit Care Med. 2001;164(1):43.
Maxwell et al. A Randomized Prospective Trial of Airway Pressure Release
Ventilation and Low Tidal Volume Ventilation in Adult Trauma Patients With
Acute Respiratory Failure. J Trauma. 2010;69: 501–511