Modes of Mechanical Ventilation

Fellow’s conference

December 7, 2011

Cheryl Pirozzi, MD

Breath types Modes of ventilation Other strategies

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Positive-pressure mechanical ventilators

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

change in pressure or flow due to patient effort (patient-initiated breaths) or a set time (vent-initiated)

2) target variable: what controls gas delivery during the breath

set flow or set inspiratory pressure 3) Termination/cycle variable: what terminates the breath

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

COMPLIANCE = Volume / Pressure

If volume is set, pressure varies…..if pressure is set, volume varies

5 basic breath types

1. volume assist (VA)

2. volume control (VC)

3. pressure assist (PA)

4. pressure control (PC)

5. pressure support (PS)

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Positive end-expiratory pressure - PEEP

5 basic breath types

Breath Trigger Target Termination / cycle

VA Pt Inspir flow 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 independent responses, and the dashed lines represent dependent responses.

Modes of mechanical ventilation

1. Controlled mechanical ventilation (CMV)2. Assist-control ventilation (ACV)3. Synchronized intermittent mandatory ventilation (SIMV or

IMV)4. Pressure support (PS)5. CPAP6. BPAP7. Pressure-regulated volume control (PRVC) 8. Airway pressure release ventilation (APRV) and Biphasic9. Adaptive support ventilation (ASV) 10. Volume support / Automatic Pressure Ventilation11. High-frequency ventilation (HFV)

Volume-limited vs. Pressure-limited

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

Volume-limited 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

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.

Pressure-limited

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Volume-limited vs. Pressure-limited

Rappaport et al. Crit Care Med. 1994;22(1):22

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

Volume-limited vs. Pressure-limited

Prella et al. Chest. 2002;122(4):1382 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

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 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. Controlled mechanical ventilation (CMV)2. Assist-control ventilation (ACV)3. Synchronized intermittent mandatory ventilation (SIMV or

IMV)4. Pressure support (PS)5. CPAP6. BPAP7. Pressure-regulated volume control (PRVC) 8. Airway pressure release ventilation (APRV) and Biphasic9. Adaptive support ventilation (ASV) 10. Volume support / Automatic Pressure Ventilation11. High-frequency ventilation (HFV)

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

Assist-control ventilation (ACV)

1. volume assist-control ventilation (VACV): flow-targeted volume-cycled breaths

2. 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) 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)

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

Continuous level of positive airway pressure. Pt must initiate all breaths Functionally similar to PEEP Good for OSA, cardiogenic pulmonary edema maintain a continuous level of positive airway pressure in a spontaneously breathing

patien It is functionally similar to positive end-expiratory pressure (PEEP), except that PEEP

is an applied pressure against exhalation and CPAP is a pressure applied by a constant flow

CPAP typically is used for people who have breathing problems, such as sleep apnea.

infants with respiratory distress syndrome

Bilevel positive airway pressure (it’s called BPAP, not BiPAP)

Mode used during NPPV(non-invasive positive pressure ventilation)

Delivers set IPAP and EPAP. It delivers a preset inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP).

Vt is determined by difference between IPAP-EPAP

BPAP can be described as a continuous positive airway pressure system with a time-cycled or flow-cycled change of the applied CPAP level

effective management tools for chronic obstructive pulmonary disease and acute respiratory failure

Pressure-regulated volume control (PRVC)

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) 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 Mlez et al. Intensive Care Med (2010) 36:817–827

Airway pressure release ventilation (APRV)

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 Mlez et al. Intensive Care Med (2010) 36:817–827

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

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; 48:722-731.

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 Mlez et al. Intensive Care Med (2010) 36:817–827

Biphasic Ventilation

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)

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 (120-900 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)

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)

Stawicki et al. J Intensive Care Med 2009 24: 215-229

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

Adaptive Support Ventilation (ASV)

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)

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)

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) 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.

fisioterapiaemterapiaintensiva.blogspot.com

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

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

Strategies to optimize syncrony

Interactive breaths improve comfort and reduce sedation

Strategies 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

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

Pressure-Targeted Inspiratory Pressure Slope Adjusters 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

Pressure support cycle adjusters 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

Proportional Assist Ventilation 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

Neurally adjusted ventilatory assistance (NAVA) 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?

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” Overall no significant outcome benefits. Consider

if severe or refractory hypoxemia

References Murray and Nadel's Textbook of Respiratory Medicine. 5 th 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 Mlez 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

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