2 Breath types Modes of ventilation Other strategies
3 Positive-pressure mechanical ventilators Most use piston/bellows systemsTidal breaths generated by gas flow, either controlled entirely by the ventilator or interactive with patient efforts
4 Breath types Classified by: trigger variable: what initiates the breathchange in pressure or flow due to patient effort (patient-initiated breaths) or a set time (vent-initiated)target variable: what controls gas delivery during the breathset flow or set inspiratory pressureTermination/cycle variable: what terminates the breathset volume, set inspiratory time, or a set flowpressure is usually a “backup” cycle variable to terminate gas delivery if circuit pressure rises above an alarm limitdemand valve senses a negative airway pressure deflection (generated by the patient trying to initiate a breath) greater than the trigger sensitivity. A trigger sensitivity of -1 to -3 cmH2O is typically set.Flow-by triggering: when return flow is less than delivered flow due to pt’s effort to initiate breath
5 5 basic breath types volume assist (VA) volume control (VC) pressure assist (PA)pressure control (PC)pressure support (PS)
6 5 basic breath types Breath Trigger Target Termination / cycle VA Pt Inspir flowSet VtVCVentPAinsp PInsp timePCPS% decrease inspir flowinspir flow has decreased to % of max value (usually 25%)Called volume-limited, pressure-limitedFlow targeted, pressure targetedVolume cycled, time-cycled, flow cycled?
7 5 basic breathsFIGURE 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.
8 Modes of mechanical ventilation Controlled mechanical ventilation (CMV)Assist-control ventilation (ACV)Synchronized intermittent mandatory ventilation (SIMV or IMV)Pressure support (PS)CPAPBPAPPressure-regulated volume control (PRVC)Airway pressure release ventilation (APRV) and BiphasicAdaptive support ventilation (ASV)Volume support / Automatic Pressure VentilationHigh-frequency ventilation (HFV)
9 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
10 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 ratioAirway pressures depend on set Vt and patient compliance and airway resistancePeak 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 . This decreases the peak airway pressure, physiologic dead space, and PaCO2, while leaving oxygenation unaltered .
11 Flow and pressure waveforms for the different flow patterns: 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
12 Pressure-limited Pressure-limited clinician sets inspiratory pressure level, I:E ratio, respiratory rate, applied PEEP, and FiO2Inspiration ends after delivery of the set inspiratory pressuretidal volume is variable and determined by inspiratory pressure, compliance, airway and tubing resistancepeak 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.
13 Image may be subject to copyright. Pressure-limitedtidal 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.
14 Volume-limited vs. Pressure-limited Rappaport et al. Crit Care Med. 1994;22(1):22RCT PCV vs VCV in 27 pts with acute, severe hypoxic respiratory failure (PaO2/FIO2 < 150), not LTVVPressure-limited associated with lower peak airway pressure, more rapid improvement in compliance, fewer days of mech ventilationCompared in a few small studies
15 Volume-limited vs. Pressure-limited Prella et al. Chest. 2002;122(4):1382Prospective, observational study of 10 pts with ALI or ARDS: gas exchange, airway pressures, and end-expir CT for PCV vs VCVNo difference in PaO2, PaCO2, and PaO2/FiO2Peak airway pressure significantly lower in PCV compared with VCV (26 vs 31cmH2O; p < 0.001)PCV more homogeneous gas distribution at the apex on CTnot using low tidal volume ventilationsequential applicationof the two studied ventilatory modesVt 9 ml/kg
16 Volume-limited vs. Pressure-limited Conclusions:no statistically significant differences in mortality, oxygenation, or work of breathingpressure-limited: lower peak airway pressures, more homogeneous gas distribution, improved synchrony, and earlier liberation from ventWhen ramp wave (decelerating flow pattern) used for VCV, no longer higher peak pressures than PCVvolume-limited: the only mode that can guarantee a constant tidal volume, ensuring a minimum minute ventilation or LTVVThere is currently no data showing
17 Modes of mechanical ventilation Controlled mechanical ventilation (CMV)Assist-control ventilation (ACV)Synchronized intermittent mandatory ventilation (SIMV or IMV)Pressure support (PS)CPAPBPAPPressure-regulated volume control (PRVC)Airway pressure release ventilation (APRV) and BiphasicAdaptive support ventilation (ASV)Volume support / Automatic Pressure VentilationHigh-frequency ventilation (HFV)
18 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 breathspressure control ventilation (PCV): pressure-targeted time-cycled breaths
19 Assist-control ventilation (ACV) volume assist-control ventilation (VACV): flow-targeted volume-cycled breathspressure assist-control ventilation (PACV): pressure-targeted time-cycled breathsguarantees 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).
20 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 effortpts increase minute ventilation by add’l spontaneous breaths, which can be unassisted or PS
21 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 breathComfortable mode, good for weaning, can be combined with SIMVNot good for full ventilatory support, high airway resistance, or central apneaflow-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
22 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 patientsMarx: Rosen's Emergency Medicine, 7th ed.2009.
23 CPAP Continuous level of positive airway pressure. Pt must initiate all breathsFunctionally similar to PEEPGood for OSA, cardiogenic pulmonary edema
24 Bilevel positive airway pressure (it’s called BPAP, not BiPAP) Mode used during NPPVDelivers set IPAP and EPAPVt is determined by difference between IPAP-EPAPBipap is brand name of machine from respironicsinsp positive airway pressure
25 Pressure-regulated volume control (PRVC) A form of PACV that uses tidal volume as a feedback control for continuously adjusting the pressure targetclinician sets tidal volume target and the ventilator then automatically sets the inspiratory pressure within a clinician-set range to achieve this goalAs a patient's respiratory drive exceeds the clinician-set guaranteed rate, some PRVC systems will provide additional patient-triggered PA or PS breathsan assist-control, pressure-targeted, time-cycled mode
26 Airway pressure release ventilation (APRV) Time-triggered, pressure-limited, and time-cycled modehigh 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 phasesLike two levels of CPAPGonza ́lez et al. Intensive Care Med (2010) 36:817–827
28 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 pressureDriving 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 deflationsGonza ́lez et al. Intensive Care Med (2010) 36:817–827
29 Airway pressure release ventilation (APRV) Potential benefits:improved alveolar recruitment and oxygenationSome observational studies show decreased peak airway pressure, improved alveolar recruitment, increased ventilation of the dependent lung zones and improved oxygenationNo mortality benefitPotential risks: In severe obstructive disease, could lead to hyperinflation and barotrauma
30 APRV- Is it better?RCT of APRV vs SIMV plus PSV (not LTVV) in 58 pts with ARDS: no difference in outcomeVarpula.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 LOSMaxwell et al. J Trauma. 2010;69: 501–511Secondary 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, LOSGonza ́lez et al. Intensive Care Med (2010) 36:817–827patients 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, andventilator-associated pneumonia in the APRV groupRCT of APRV vs PCV in 30 trauma pts: MV days, ICU stay, less sedation and paralysis, no mortality diffPutensen et al. Am J Respir Crit Care Med. 2001;164(1):43.
31 Biphasic VentilationSimilar to APRV, except that T low is longer during biphasic ventilation, allowing more spontaneous breaths to occur at P lowAKA Bi-Vent, BiLevel, BiPhasic, and DuoPAP ventilation.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.31
33 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 recruitmentHFV uses very high breathing frequencies ( breaths/min) coupled with very small tidal volumes (<1 mL/kg) to provide gas exchange in the lungssupplied 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 tubeless than anatomic dead space,
34 High-Frequency Oscillatory Ventilation (HFOV or HFV) Rationale:very small alveolar tidal volumes minimize cyclical overdistention and derecruitmentmaintains the alveoli open at a relatively constant airway pressure and thus may prevent atelectrauma and barotraumaimproves 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
35 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 :
36 High-Frequency Oscillatory Ventilation (HFOV or HFV) Several studies in adults have shown improved oxygenation but no mortality benefitOne RCT: HFV vs PCV (6 -10 mL/kg, mean 8) in 148 patients with ARDS on PEEP≥10HFV 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):801Jets 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 tubeearly (less than 16hours) improvement in PaO2/fraction of inspired oxygen comparedwith the conventional ventilation group (p 0.008); however,this difference did not persist beyond 24 hours
37 Adaptive Support Ventilation (ASV) Based on respiratory mechanics vent automatically adjusts respiratory rate and inspiratory pressure to achieve a desired minute ventilationClinician 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.Using controlled breaths, ASV initially calculates resistance and compliance as well as the expiratory time constant (resistance × compliance).ASV behavior when a patient's respiratory drive increases beyond the minute ventilation set by the clinician is complex and involves continually attempting to minimize ventilator work for the higher minute ventilation*Expiratory time constant can be obtained from the expiratory limb of the flow volume loop on a breath by breath basis [46,47]. Patients who have a long expiratory time constant (eg, COPD) receive a higher tidal volume and a lower respiratory rate when ventilated by ASV than patients with stiff lungs (eg, acute lung injury, ARDS) or chest wall stiffness (eg, kyphoscoliosis, morbid obesity, neuromuscular disorder) who expire quickly [48,49].
38 Adaptive Support Ventilation (ASV) The delivered “minimal work” tidal volume with ASV may be higher than 6 mL/kgNo outcome studies comparing ASV to conventional lung-protective strategiesUsing controlled breaths, ASV initially calculates resistance and compliance as well as the expiratory time constant (resistance × compliance).ASV behavior when a patient's respiratory drive increases beyond the minute ventilation set by the clinician is complex and involves continually attempting to minimize ventilator work for the higher minute ventilation38
39 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 improveNo trials comparing VS or ASV weaning to aggressive daily SBT strategiesAll breaths are patient-triggered, pressure-limited, and flow-cycled
40 Other strategies www.nurstoon.com/Images/novent.gif Other strategies for ventilation that aren’t necessarily modes but that we encounter
41 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 exhalationStudies show TGI reduces dead space but also has the potential to increase PEEP.Next delivered breath is effectively free of ETT CO2, thereby reducing dead space.TGI catheters can deliver fresh gas continuously or just during exhalation.fisioterapiaemterapiaintensiva.blogspot.com
42 Inverse ratio ventilation Strategy of inversing I:E ratio (I>E) to potentially improve oxygenationWhen pt is severely hypoxemic despite optimal PEEP and FiO2Can be used with volume-limited or pressure-limited mechanical ventilationIn pressure: increase I:E ratioIn volume: ramp wave- decrease peak inspiratory flow rate until I exceeds EIn volume square wave- add and increase end-inspiratory pause until I exceeds EThe inspiratory time exceeds the expiratory time during IRV (the I:E ratio is inversed), increasing the mean airway pressure and potentially improving oxygenation.
43 Inverse ratio ventilation In trials increases mean airway pressure, may improve oxygenation, never been shown to improve important clinical outcomesRequires increased sedation +/- paralysisRisks: increased risk of auto-PEEP, barotrauma and hypotensionThe inspiratory time exceeds the expiratory time during IRV (the I:E ratio is inversed), increasing the mean airway pressure and potentially improving oxygenation.43
44 Strategies to optimize syncrony Interactive breaths improve comfort and reduce sedationStrategiesEndotracheal Tube Resistance CompensationPressure-Targeted Inspiratory Pressure Slope AdjustersPressure Support Cycle AdjustersProportional Assist VentilationNeurally adjusted ventilatory assistance (NAVA)Therefore useful esp during recovery phase
45 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 breathClinicians input characteristics of ETT. Vent adjusts circuit pressure during both inspiration and expirationGood for SBT or combined with other mode.initially provides an inspiratory pressure higher than the set pressure target. As inspiration proceeds, this delivered pressure then tapers to the set inspiratory pressure target. This compensation mechanism also can operate in expiration with an initial expiratory airway pressure below the set PEEP that then rises to the set PEEP. A more square wave pattern of inspiratory and expiratory tracheal pressures is the result.
46 Strategies to optimize syncrony Pressure-Targeted Inspiratory Pressure Slope AdjustersFor pressure-targeted breaths (PS, PA/C)Slope adjusters allow clinician to adjust pressure rate of risePt with vigorous breaths may desire rapid rate of rise, or vice versa if less vigorous demandsOne technique is to use the circuit pressure graph and adjust the slope to create a “smooth square wave” appearance to the circuit pressure profile
47 Strategies to optimize syncrony Pressure support cycle adjustersIn 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
48 Strategies to optimize syncrony Proportional Assist VentilationNo 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.clinician sets the “gain” on patient-generated flow and volume
49 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 diaphragmBreaths triggered by phrenic nerve excitation of the inspiratory musclesExpensive!
50 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
52 ReferencesMurray and Nadel's Textbook of Respiratory Medicine. 5th editionBozyk P, Hyzy R. Modes of mechanical ventilation. Up To Date. 2010Rappaport 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):22Prella 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):1382Chiumello 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):925Varpula 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: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):801Stewart NI, Jagelman TA, Webster NR. Emerging modes of ventilation in the intensive care unit. Br J Anaesth Jul;107(1): Epub 2011 May 24Gonza ́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
53 ReferencesStawicki 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 :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
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