Airway Pressure Airway Pressure Release Release

VentilationVentilation Muhammad Asim RanaMuhammad Asim Rana

In patients with acute lung injury (ALI) and ARDS, conventional mechanical ventilation (CV) may cause additional lung injury from overdistention of the lung during inspiration, repeated opening and closing of small bronchioles and alveoli, or from excessive stress at the margins between aerated and atelectatic lung regions. Increasing evidence suggests that smaller tidal volumes (VTs) and higher end-expiratory lung volumes (EELVs) may be protective from these forms of ventilator-associated lung injury and may improve outcomes from ALI/ARDS.

APRV was introduced to clinical APRV was introduced to clinical practice about 2 decades ago as an practice about 2 decades ago as an alternative mode for mechanical alternative mode for mechanical ventilation; however, it had not ventilation; however, it had not gained popularity until recently as gained popularity until recently as an effective & safe alternative for an effective & safe alternative for difficult to ventilate/oxygenate difficult to ventilate/oxygenate patients of ALI/ARDS patients of ALI/ARDS

What is APRVWhat is APRV

APRV was introduced initially by Stock & APRV was introduced initially by Stock & Down in 1987 as a CPAP with an Down in 1987 as a CPAP with an intermittent release phaseintermittent release phase

APRV applies CPAP (P high) for a prolonged APRV applies CPAP (P high) for a prolonged time (T high) to maintain adequate lung time (T high) to maintain adequate lung volume & alveolar recruitment, with a time volume & alveolar recruitment, with a time cycled release phase to a lower set of cycled release phase to a lower set of pressure (P low) for a short period of time pressure (P low) for a short period of time (T low) or (release time) where most of the (T low) or (release time) where most of the ventilation & CO2 removal occurs ventilation & CO2 removal occurs

The transition from The transition from P high to P low P high to P low deflates the lungs deflates the lungs and eliminates and eliminates carbon dioxide. carbon dioxide. Conversely, the Conversely, the transition from P transition from P low to P high low to P high inflates the lungs. inflates the lungs. Alveolar recruitment Alveolar recruitment is maximized by the is maximized by the high continuous high continuous positive airway positive airway pressurepressure

The difference The difference between P high and P between P high and P low is the driving low is the driving pressure. Larger pressure. Larger differences are differences are associated with associated with greater inflation and greater inflation and deflation, while deflation, while smaller differences smaller differences are associated with are associated with smaller inflation and smaller inflation and deflation. The exact deflation. The exact size of the tidal size of the tidal volume is related to volume is related to both the driving both the driving pressure and the pressure and the compliance.compliance.

T high and T low T high and T low determine the determine the frequency of inflations frequency of inflations and deflations. As an and deflations. As an example, a patient example, a patient whose T high is set to whose T high is set to 12 seconds and whose 12 seconds and whose T low is set to 3 T low is set to 3 seconds has an seconds has an inflation-deflation inflation-deflation cycle lasting 15 cycle lasting 15 seconds. This allows 4 seconds. This allows 4 inflations and inflations and deflations to be deflations to be completed each completed each minute.minute.

Spontaneous Spontaneous breathing is possible breathing is possible at both P high and P at both P high and P low, although most low, although most spontaneous spontaneous breathing occurs at breathing occurs at P high because the P high because the time spent at P low time spent at P low is brief. This is a is brief. This is a novel feature that novel feature that distinguishes APRV distinguishes APRV from other types of from other types of IRV. IRV.

If the patient has no spontaneous If the patient has no spontaneous respiratory effort, APRV becomes respiratory effort, APRV becomes typical to “inverse ratio pressure typical to “inverse ratio pressure limited, time cycle-assisted limited, time cycle-assisted mechanical ventilation (pressure mechanical ventilation (pressure control ventilation).control ventilation).

In ARDS the functional residual In ARDS the functional residual capacity & lung compliance are capacity & lung compliance are reduced, & thus the elastic work of reduced, & thus the elastic work of breathing is elevated. By applying breathing is elevated. By applying CPAP, the FRC is restored & inspiration CPAP, the FRC is restored & inspiration starts from a more favorable pressure-starts from a more favorable pressure-volume relationship, facilitating volume relationship, facilitating spontaneous ventilation & improve spontaneous ventilation & improve oxygenation.oxygenation.

Applying ‘P high’ for a ‘T high’ (80-95% Applying ‘P high’ for a ‘T high’ (80-95% of the cycle time), the mean airway of the cycle time), the mean airway pressure is increased insuring almost pressure is increased insuring almost constant lung recruitment (open lung constant lung recruitment (open lung approach), in contrast to the repetitive approach), in contrast to the repetitive inflation & deflation of the lung using inflation & deflation of the lung using conventional ventilatory methods (which conventional ventilatory methods (which could ventilator induced lung injury), or could ventilator induced lung injury), or the recruitment maneuvers which have the recruitment maneuvers which have to be done frequently to avoid to be done frequently to avoid derecruitment. derecruitment.

Mean air way pressure on APRV is Mean air way pressure on APRV is calculated using this formula:calculated using this formula:

(P High (P High хх T High) + (P Low T High) + (P Low хх T Low) T Low)

(T High + T Low)(T High + T Low)

Minute ventilation & CO2 removal in Minute ventilation & CO2 removal in APRV depend on lung compliance, APRV depend on lung compliance, airway resistance, the magnitude & airway resistance, the magnitude & duration of pressure release and the duration of pressure release and the magnitude of patient’s spontaneous magnitude of patient’s spontaneous breathing efforts.breathing efforts.

Spontaneous breathing plays a very Spontaneous breathing plays a very important role in APRV allowing the important role in APRV allowing the patient to control his/her respiratory patient to control his/her respiratory frequency without being confined to frequency without being confined to an arbitrary preset I:E ratio, thus an arbitrary preset I:E ratio, thus improving patient comfort & patient-improving patient comfort & patient-ventilator synchrony with reduction ventilator synchrony with reduction in the amount of sedation necessary.in the amount of sedation necessary.

Additionally, spontaneous breathing Additionally, spontaneous breathing helps derive the inspired gas to the helps derive the inspired gas to the nondependent lung regions by using nondependent lung regions by using patients own respiratory muscles & patients own respiratory muscles & through pleural pressure changes through pleural pressure changes without raising the applied airway without raising the applied airway pressure to a rather dangerous level, as pressure to a rather dangerous level, as in conventional mechanical ventilation, in conventional mechanical ventilation, producing more physiological producing more physiological distribution to the non dependent lung distribution to the non dependent lung regions & improving V/Q matching regions & improving V/Q matching

Adding Pressure Support to Adding Pressure Support to APRVAPRV

The addition of PSV above P High to add The addition of PSV above P High to add spontaneous breaths is feasible, but this spontaneous breaths is feasible, but this addition contradicts limiting the airway addition contradicts limiting the airway pressure & may cause significant lung pressure & may cause significant lung distention.distention.

Furthermore, the imposition of PSV to Furthermore, the imposition of PSV to APRV reduces the benefits of spontaneous APRV reduces the benefits of spontaneous breathing by altering the normal breathing by altering the normal sinusoidal flow of spontaneous breathing sinusoidal flow of spontaneous breathing

Advantages of Advantages of APRVAPRV

   APRV has not been shown to improve APRV has not been shown to improve mortality. However, it may improve mortality. However, it may improve alternative important clinical outcomes alternative important clinical outcomes compared to other modes of ventilation. In compared to other modes of ventilation. In one trial, 30 patients being mechanically one trial, 30 patients being mechanically ventilated because of trauma were randomly ventilated because of trauma were randomly assigned to receive APRV alone or pressure-assigned to receive APRV alone or pressure-limited ventilation for 72 hours followed by limited ventilation for 72 hours followed by APRV. The APRV alone group had a shorter APRV. The APRV alone group had a shorter duration of mechanical ventilation, a shorter duration of mechanical ventilation, a shorter ICU stay, and required less sedation and ICU stay, and required less sedation and pharmacologic paralysis. Mortality did not pharmacologic paralysis. Mortality did not differ between groups.differ between groups.

Effects on OxygenationEffects on Oxygenation

The improved oxygenation parameters The improved oxygenation parameters i.e., PaO2/FiO2 & lung compliance are i.e., PaO2/FiO2 & lung compliance are attributed to the beneficial effects of attributed to the beneficial effects of spontaneous breathing through better spontaneous breathing through better gas distribution & better V/Q matching gas distribution & better V/Q matching to the poorly aerated dorsal regions of to the poorly aerated dorsal regions of the lungs, along with higher mean the lungs, along with higher mean airway pressure obtained compared to airway pressure obtained compared to conventional ventilation.conventional ventilation.

Effects on hemodynamicsEffects on hemodynamics

During spontaneous breathing the During spontaneous breathing the pleural pressure decreases leading pleural pressure decreases leading to a decrease in intra thoracic & Rt to a decrease in intra thoracic & Rt atrial pressure thus improving atrial pressure thus improving venous return & improving o\pre venous return & improving o\pre load and consequently increasing load and consequently increasing the cardiac out put. the cardiac out put.

Kaplan compared the hemodynamics Kaplan compared the hemodynamics effects in patients with ALI/ARDS on effects in patients with ALI/ARDS on patients APRV vs IRV PCV; they found patients APRV vs IRV PCV; they found significantly higher cardiac index, significantly higher cardiac index, oxygen delivery, mixed venous oxygen oxygen delivery, mixed venous oxygen saturation, urine output & significantly saturation, urine output & significantly lower vasopressors & inotropes usage, lower vasopressors & inotropes usage, lactate concentration & CVP while on lactate concentration & CVP while on APRV APRV

Putnsen found same results in a Putnsen found same results in a separate studyseparate study

Effects on regional blood Effects on regional blood flow & organ perfusionflow & organ perfusion

In a study by Hering APRV improved In a study by Hering APRV improved respiratory muscle blood flow in 12 respiratory muscle blood flow in 12 pigs with ALIpigs with ALI

In a similar study by same author In a similar study by same author APRV showed improved blood flow APRV showed improved blood flow to stomach duodenum, ileum & to stomach duodenum, ileum & coloncolon

Kaplan found significant Kaplan found significant improvement in GFR in pts on APRVimprovement in GFR in pts on APRV

Effects on sedationEffects on sedation

The level of sedation & analgesia The level of sedation & analgesia required in CMV is usually equivalent required in CMV is usually equivalent to Ramsay score of 4-5, but during to Ramsay score of 4-5, but during APRV a Ramsay score of 2-3 can be APRV a Ramsay score of 2-3 can be targetedtargeted

APRV has shown to decreased the need APRV has shown to decreased the need of neuromuscular blockade use by 70% of neuromuscular blockade use by 70% & use of sedation by about 40% & use of sedation by about 40% compared to conventional ventilation compared to conventional ventilation

Duration of ICU stayDuration of ICU stay

The decreased use of sedatives & The decreased use of sedatives & neuromuscular blockade may neuromuscular blockade may translate into decreased length of translate into decreased length of mechanical ventilation & ICU length mechanical ventilation & ICU length of stayof stay

IndicationsIndications

ARDS/ALIARDS/ALI Atelactasis after major surgeryAtelactasis after major surgery Pulmonary edemaPulmonary edema Obesity/AscitiesObesity/Ascities PIP>35 & PEEP> 10 cm of waterPIP>35 & PEEP> 10 cm of water

Contraindications Contraindications

Increased Air way resistance Increased Air way resistance

Patients of COPD & Asthma Patients of COPD & Asthma

Theoretically, using short release time Theoretically, using short release time is not beneficial for patients who is not beneficial for patients who require long expiratory time require long expiratory time

Because of lower levels of sedation Because of lower levels of sedation used to allow spontaneous breathing used to allow spontaneous breathing APRV should not be used in patients APRV should not be used in patients who require deep sedation for who require deep sedation for management of their underlying management of their underlying disease (e.g.cerebral edema with disease (e.g.cerebral edema with increased ICP or status epilepticus) increased ICP or status epilepticus)

Likewise use of APRV has not been Likewise use of APRV has not been investigated in patients with investigated in patients with neuromuscular disease & is not neuromuscular disease & is not supported by any evidencesupported by any evidence

Setting APRVSetting APRV

Mechanical ventilation with PEEP Mechanical ventilation with PEEP titrated above the lower infliction titrated above the lower infliction point of the static pressure volume point of the static pressure volume curve & a low tidal volume at 6 ml/kg curve & a low tidal volume at 6 ml/kg are thought to prevent alveolar are thought to prevent alveolar collapse at end expiration and over collapse at end expiration and over distension of lung units at end-distension of lung units at end-inspiration in patients with ARDS. inspiration in patients with ARDS. This is lung protective strategy.This is lung protective strategy.

The setup at the bed side is simple The setup at the bed side is simple and the goals are same:and the goals are same:

To maintain adequate oxygenation & To maintain adequate oxygenation & ventilation without overt lung ventilation without overt lung distention during P high & avoiding distention during P high & avoiding lung derecruitment during P lowlung derecruitment during P low

Setting PressuresSetting Pressures

P high should be below the high P high should be below the high inflection point on the static volume-inflection point on the static volume-pressure curve, while P low should pressure curve, while P low should be above the low inflection point on be above the low inflection point on the same curvethe same curve

Setting TimeSetting Time

T high should allow complete T high should allow complete inflation of the lungs, as indicated by inflation of the lungs, as indicated by end-respiratory phase of no flow end-respiratory phase of no flow when spontaneous breathing is when spontaneous breathing is absent, & T low should allow for absent, & T low should allow for complete exhalation with no flow at complete exhalation with no flow at the end to assure absence of the end to assure absence of intrinsic or auto PEEPintrinsic or auto PEEP

Initial setup & transition Initial setup & transition from conventional from conventional

ventilationventilation P high is usually set between 20 & P high is usually set between 20 &

3030 P low is set between 0 & 5 cm of P low is set between 0 & 5 cm of

H2OH2O T high is 4 to 6 secondsT high is 4 to 6 seconds T low is 0.2 to 0.8 secondsT low is 0.2 to 0.8 seconds

TROUBLESHOOTITROUBLESHOOTINGNG

Maneuvers to correct poor Maneuvers to correct poor oxygenation oxygenation

1) increase either ‘P high’, ‘T high’ 1) increase either ‘P high’, ‘T high’ or both to increase mean airway or both to increase mean airway pressure; pressure;

2) change the patient position to the 2) change the patient position to the prone position along with the APRV.prone position along with the APRV.

Maneuvers to correct poor Maneuvers to correct poor ventilation ventilation

1) increase ‘P high’ and decrease ‘T high’ 1) increase ‘P high’ and decrease ‘T high’ simultaneously to increase minute simultaneously to increase minute ventilation while keeping stable mean ventilation while keeping stable mean airway pressure (preferred method);airway pressure (preferred method);

2) increase ‘T low’ by 0.05-0.1 s 2) increase ‘T low’ by 0.05-0.1 s increments;increments;

3) decrease sedation to increase the 3) decrease sedation to increase the patient’s contribution to minute patient’s contribution to minute ventilation.ventilation.

AcknowledgementsAcknowledgements

Dr. Mostafa AdelDr. Mostafa Adel Dr. Omar Alsayed Dr. Omar Alsayed Dr. Ahmed fouadDr. Ahmed fouad Dr. Ahmed HossamDr. Ahmed Hossam

Dr. Ahmed RajabDr. Ahmed Rajab Dr. Sameer Dr. Sameer

IbrahimIbrahim Dr. Bashir AhmedDr. Bashir Ahmed Dr. Sayed AfzalDr. Sayed Afzal

Thank you Thank you

For patient listening For patient listening