Approaches for Asthma
Jon N. Meliones, MD, MS, FCCM
Professor of Pediatrics and Anesthesia
Duke University

Outline
• What is the question?
• Pressure Control vs Volume Control
• Decelerating Flow vs Constant Flow
• Ventilation Physiology
• Asthma Pathology
• Data on Decelerating Flow
• If you have to ventilate…the preferred approach

Modes of Ventilation
Limit
Pressure Volume
Pressure
PRVC
Control
Decelerating
Flow
Pressure
Volume
Constant Control Control

Outline
• What is the question?
• Ventilation Physiology
• Asthma Pathology
• Data on Decelerating Flow
• If you have to ventilate…the preferred
approach

Ventilation Myths
• Increasingly Complex (Marketing directors)
• Host of New “Toys”
• New Modes: Do Not Describe Functionality
• Different Ventilator Manufacturers
– Similar modes = different functions
– Cute names that mean nothing
– Don’t say what they do
– Measures are inaccurate

Key Functionality of Ventilators
• Flow Pattern
– Gas Flow Delivered & Distribution During Inspiration
– Decelerating or Constant
• Limit
– Safety: Prevents the Ventilator from Exceeding Preset limit
– Volume or Pressure
• Cycle
– When Inspiration Ends
• Trigger
– How the Breath is Initiated
• Breath Type
– Single or Mixed

Effects of Flow Pattern on Airway
Pressures
Decelerating Square
Flow
(l/sec)
MAP = Area Under Curve
PIP
PIP
Airway
Pressure
(cmH20)
MAP

Square Wave Flow v. Decel
Flow
• Randomized Cross Over Controlled
Study of VCV v PCV
• Saline Lavage
Decel Square Wave pValue
PIP 38.2 + 5.5 46.0 + 4.4 <0.0001
MAP 12.0 + 2.2 10.0 + 2.1 <0.02
Cdyn 15.7 + 3.0 14.2 + 3.0 <0.0001
E. Williams: CCM 2000

Peak Inspiratory Pressure
P<0.05
PIP
(cm H2O)
Cheifetz: CCM 1995

Dynamic Compliance
P<0.05

Benefits of Limiting Peak Plateau
Pressure
• There are no prospective randomized controlled
studies demonstrate ANY significant benefit based
on the method of ventilation except for limiting
Pplat:
• Benefits of Limiting Pplat: Amato; ARDS network
etc.
– Pplat<20 above PEEP
– VT< 6ml/kg
– Dec mortality 22%
• Who cares what the PaCO2 is:
– The heart still works, you can always buffer!
– As long as ICP not increased!
– Limiting Pplat determines outcome!

Pediatric Asthma Data
• Heliox: Pediatrics 2005 Nov Kim IK
– Reduces the risk of admission for greater than 12
hours by 60%
• Iipratropium bromide
– No Data to support
• Magnesium
– No good data
• NIBP
– Transient improvements

Limit
• Safety Check
• Prevents The Ventilator From
Exceeding a Set Variable
– Pressure Limit
• Controlled or Support Modes
– Volume Limit
• Controlled Modes
– Minute Volume - Support Modes

Limits
• Pressure Limits
– Dependent Variable = VT:(compliance/resistance)
– Theoretical Advantage: Limit PIP (barotrauma)
– Theoretical Disadvantage: Hypo/Hyper Ventilation
• Volume Limits
– Dependent Variable = Pressure
:(compliance/resistance)
– Theoretical Advantage: Stable Min Vent (PaCO2)
– Theoretical Disadvantage: PIP (barotrauma)
• Minute Volume
– Advantage: Auto weaning
– Disadvantage: Fast / slow breathing rates

Modes of Ventilation
Limit
Pressure Volume
Pressure
PRVC
Control
Decelerating
Flow
Pressure
Volume
Constant Control Control

Outline
• What is the question?
• Ventilation Physiology
• Asthma Pathopsiology
• Data on Decelerating Flow
• If you have to ventilate…the preferred
approach

Pathophysiology of Asthma
• Marked increased airways resistance
• Prolonged Time Constant
• TC = Resistance x Compliance
• Degree of obstruction non-uniform
resulting in varying TC
• Expiratory TC worse:
• Narrowing of airway during expiration.
• Upstream displacement of equal pressure point,
• Usually, minimal alveolar disease

Pathophysiology of
Asthma

Intrinsic PEEP: Dynamic
Hyperinflation
Beginning Premature initiation
of of Inspiration
Inspiration
End
of
Inspiration
Retained Gas
Results in PEEPi
Termination
Beginning
Premature
of
of
Termination of
Exhalation
Exhalation
Exhalation

Intrinsic PEEP/Dynamic Hyperinflation
• Expiratory gas flow continues at the
end of the time allotted for exhalation.
• PEEPi may lead to excessive MAP.
– Pulmonary effects:
• Barotrauma
– Cardiac effects:
• Impedance of venous return
• Decreased cardiac output

Systemic Venous Return
(RV Preload)
PSV RAP = mean systemic venous pressure
PPV increases
right atrial pressure
Right Atrial
spontaneous
Pressure
breathing
0
0 Max
Systemic Venous Return

Effect of Lung Volume on PVR
Overexpansion
DHI
Atelectasis
PVR
Total PVR
Small Vessels
Large Vessels
FRC
Lung Volume

Overdistention and C.O.
1000
950
PEEP 5 PEEP 10
900
Cardiac 850
Output 800
750
(mL/min) 700
650
600
550
500
10 15 20
Tidal Volume (mL/kg)
Cheifetz: CCM 1998

Outline
• What is the question?
• Ventilation Physiology
• Asthma Pathology
• Data on Decelerating Flow
• If you have to ventilate…the
preferred approach

Decelerating Flow in Asthma
• Pressure controlled ventilation in severe asthma.
Lopez Pediatr Pulmonol 1996;21:401
• Pressure-support ventilation in children with severe
asthma. Wetzel Crit Care Med 1996;24:1603-1605.
• Refractory asthma, part 2: airway interventions and
management. Jagoda A. Ann Emerg Med. 1997;29:275-
281
• Mechanical ventilation for children with status
asthmaticus. Sabato K, Hanson JH. Respir Care Clin
North Am. 2000;6:171-188.
• Decelerating Flow in 51 Pediatric Asthma Patients

Decelerating Flow in Asthma
• Hypothesis:
• VCV with constant flow distributes more
volume to the less obstructed airways with
shorter TC and less volume to longer TC.
• Uneven Ventilation, Hyperexpansion of “normal
lung” under-ventilation of obstructed units
• Elevated PIP and higher airways resistance
• Decreased Compliance
• High resistance, short IT = Premature
termination of breath and set VT not achieved

Decelerating Flow in Asthma
• Decelerating flow
• Flow varies;
• High at first (overcomes high resistance) to
achieve set pressure early in inspiration
• Lower later in inspiration to maintain this
pressure through the inspiratory time.

Decelerating Flow in Asthma
• Decelerating flow
• Provides a relatively constant inflation pressure:
• Large airways fill with peak flow, smaller airways
with slower flow
• Lung units with short TC attain final volume early
• Lung units with long TC continue to receive
volume later in inspiration
• Pressure equilibrium more even ventilation
• Lower Pplat or better ventilation for same PIP
• Increased Compliance

Decelerating Flow in 51
Pediatric Asthma Patients
Sarnaik, PCCM 2004
pH
VCV PCV
Mode of Ventilation

Decelerating Flow in 51
Pediatric Asthma Patients
PaCO2
VCV PCV
Mode of Ventilation

Decelerating Flow in 51
Pediatric Asthma Patients
• In Pts with PCO2>45, median time to
reversal was 5 hrs
• SaO2 >95% in all patients
• 2 pts with Pneumos pre PCV
• 1 pts developed pneumothorax, 1 pt subq
emphesema; all well tolerated and resolved
• 100% survival
• 100% neuro intact
• Median ventilation 4-107 hrs.

Adults Agree!
Decelerating flow not just for kids!
• Measurement of air trapping, PEEPI and DHI in
mechanically ventilated patients. Blanch Respir Care.
2005;50:110-124.
• Clinical Review: Severe Asthma Papiris Critical Care
2002;6:30-44.
• Lung Protective Strategies for Acute Severe Asthma.
Brown. J of Resp Care Pract. 2002;2
• Refractory asthma, part 2: airway interventions and
management. Jagoda Ann Emerg Med. 1997;29:275-281.
• Mechanical ventilation for children with status

Outline
• What is the question?
• Ventilation Physiology
• Asthma Pathology
• Data on Decelerating Flow
• If you have to ventilate…the
preferred approach

Ventilation Approach
• Get the gas out…Limit lung injury!
• Avoid DHI / Auto PEEP:
• Prolong exhalation times & Low rates
• Graphics to ensure complete exhalation
• Minimize Pplat: Assure adequate
oxygenation & ventilation but allow
hypercapnea if Pplat elevated

Ventilation Approach
• PEEP controversial; low but not zero,
usual <5
• Pick a mode you know…but…Decelerating
flow appears to be the best choice!
• A volume/minute ventilation
“measurement / guarantee” during
decelerating flow is preferred