Post on 02-Jun-2020
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
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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
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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