Eric Magaña, M.D.

NM ATS

March 3, 2018

Increased WOB due to increased airway resistance

Overcome with Heliox or pressure support

High Peak pressures due to increased airway resistance

Present an addition theory as to the origins of the high peak pressures in asthma

due to the presence of auto-PEEP

Inspiratory time (Tinsp)

Patient-ventilator dyssynchrony

Use a case review to illustrate

4

Inflammatory lung disease

Primarily affects larger airways

Edema

Muscle contraction

Excessive mucus

5

Corticosteroids› About 6 hrs before clinical effect

Beta agonists› Nebulized, oral, IV

Anticholinergics› Ipratropium, Tiotropium

Leukotriene inhibitors/receptor blockers› Montelukast, Zileuton

Intravenous Magnesium

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Patients are notoriously difficult to ventilate Patients often require heavy sedation or

paralysis to alleviate dyspnea and improve ventilator synchrony

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High Peak Pressures are common

Due to high airway resistance

Volume is dumped due to high pressure

Results in inadequate ventilation and elevated pCO2

Patients are often dyssynchronous (“fighting the vent”) due to set flow pattern.

High pressure, high RR, low delivered tidal volume, low minute ventilation

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High driving pressures are needed to achieve adequate tidal volumes

If high driving pressures are used to achieve desired tidal volume, barotrauma can result

› Pneumothorax or pneumomediastinum

Synchrony is often better due to delivery of variable flow

22 yr female asthmatic presented with a several day history of dyspnea

Increased used of albuterol inhaler without controller medications

Obvious respiratory distress, poor air movement, occasional wheezing

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Patient received 5 mg albuterol via nebulizer twice in route

125 mg of solumedrol, 15 mg continuous albuterol neb, .5 mg atrovent, 2 gmsmagnesium

135 pulse, RR=38, afeb, 100/60, diaphoretic

NIPPV failed, patient was intubated and placed on mechanical ventilation

Volume A/C VT=400 ml RR=19 Peep 3 100%

Ppeak=60, Paralyzed

Pressure A/C RR=19 Tinsp =.5 (to reduce auto-PEEP), ∆P=50 achieving a Vt=350, PEEP=3

pH=6.9, pCO2=80

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pCO2 is allowed to rise as long as oxygenation is adequate

› Rather than pushing vent (risking barotrauma) to lower pCO2 to normal

Bicarb drip is sometimes used to maintain normal pH

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Bigger airways during inspiration conduct more air than smaller tubes during expiration.

More air gets in than out.

15 L/m go in but only 13 L/m come out.

The lungs become overinflated

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PEEP (extrinsic)

› Applied by ventilator in a controlled manner

Auto-PEEP Occult PEEP

Intrinsic PEEP

Dynamic hyperinflation

Air trapping

develops when not all

of the air is exhaled

prior to the inspiration

of the next breath

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Increased intrathoracic pressure

Decreased venous return

Effective hypovolemia

Can increase work to trigger a breath

Can change the position on the compliance curve

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The pressure from Auto-PEEP adds to airway pressures generated by the ventilator breath Auto-PEEP

19

Due to increased airway resistance

Due to alveolar distention

From the tidal volume

Or from PEEP

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Decreases effective driving pressure in Pressure Ventilation

Ventilator does not know Auto-PEEP is there

Increases work to take a breath

Over and beyond the increase from airway resistance

Delta P=35, PEEP=5

Ventilator does not think about increasing pressure by 35 during inspiration

During inspiration it maintains the pressure at 40

During expiration it has the pressure at 5

40

40 cm H20

5 cm H20

25 cm H20

Set PEEP

Actual PEEP

∆P=35

∆P=15

The pt requires very high Set driving pressures to achieve an adequate tidal volume

the Effective driving pressures are much lower due to the presence of auto-PEEP

The auto PEEP moves the patient up on the compliance curve to a poorly compliant region where the lung is stiff

This requires higher pressures to get the same volume

RECOGNIZE ITS PRESENCE

Flow-Time curve-baseline not return to zero

estimated end expiratory pause

Decrease tidal volume

Increase expiratory time

Increase the flow

Decrease respiratory rate

Treat cause (bronchodilators)

Tinsp was decreased to 0.5 sec in an attempt to decrease Auto-PEEP

A shorter I time gives more time to exhale

.25 sec more?

Driving pressure has to be raised to get the desired tidal volume in in a shorter period of time

Short Tinsp was too low to allow for adequate mixing of gases in the alveoli.

pCO2 rose because of this

Soda in fridge analogy

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Treat the cause

› Increased airway resistance

You can Compensate for Auto-PEEP.

Compensating for Auto-PEEP rapidly improves symptoms.

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0 cm H2O

-5 cm H2O

AIR

-5 cm H2O gradient from alveoli to

mouth is enough to get a breath

Lung

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-5 cm H2O

AIR

-5 cm H2O gradient from alveoli to

mouth is enough to get a breath, but 20

cm H2O of effort was needed to get

there (+15 to -5)

Lung

-0 cm H2O

+15 cm H2O

Takes longer to generate larger pressures

Pt starts to exhale just after a ventilator breath is triggered

Pt ends up exhaling against the breath

In volume, the breath is terminated due to this high peak pressure

Low exhaled volume and minute ventilation alarms

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CPAP and PEEP are effectively the same thing

Both expand the lung and recruit alveoli

The name changes depending of if the breath is augment by the vent or not

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+10 cm H2O

Air/vent

-5 cm H2O gradient from alveoli to

mouth is enough to get a breath, and 5

cm H2O of effort was needed to get

there (+15 to +10)

+15 cm H2O Lung

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Start at 8 cm H2O

8 vs 10

10 vs 12

12 vs 14

After patient calms down, CPAP level often can be reduced

› Due to decrease in Auto-PEEP

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Patients may not need augmentation of their breaths once the Auto-PEEP is addressed with CPAP/PEEP

Allowing the patient to breath with just CPAP may result in adequate ventilation and ventilator synchrony

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Try CPAP early on in treatment of asthma exacerbation

› Not just when trying to avoid intubation

Dramatically improves dyspnea

› Does not alter airway obstruction

Titrate level for patient comfort

Changing to BiPAP or addition of PSV is sometimes necessary

› Not needed in every patient

How much PEEP can you add?

Adding extrinsic PEEP to Auto-PEEP does NOT increase total PEEP

sometimes

dropped rate from 22 to 19 which lengthened RCL and increased Tinsp to 0.8 sec

more time for gase mixing

pCO2 dropped

• Lengthening Tinsp allowed lower driving pressures Short Tinsp required higher driving pressures to get

the same volume in a shorter period of time

Then unparalyzed the patient and used PEEP to overcome Auto-PEEP.

Kept pt in flow variable mode

Pressure mode

PSV=5 with VT≈500, PEEP=17 and comfortable

I Considered extubating the patient to decrease resistance to expiratory flow and auto-PEEP

Airway resistance

Short Tinsp required higher pressure to get a decent volume in a very short time

Auto-PEEP

Pt exhaling against the breath