1. Mechanical Ventilation for
Severe Asthma
Niall D. Ferguson, MD, FRCPC, MSc
Assistant Professor
Interdepartmental Division of Critical Care
Medicine
University of Toronto
2. Asthma Death Rates
Modified from TRENDS IN ASTHMA
MORBIDITY AND MORTALITY,
ALA
• Number of Deaths in 2004 3780
• Age Adjusted Mortality in 1999 1.7 /
100,000
• Age Adjusted Mortality in 2004 1.3 /
100,000
• Reduction in Mortality
18.8% decrease compared to 1999
3. Life Threatening Asthma
• Clinical
inability to speak due to severe dyspnea
altered mental status
inter-costal retractions
worsening fatigue
absence of wheezing
bradycardia
absence of pulsus paradoxus
4. Life Threatening Asthma
• Physiologic
PCO2 of > 42 µµΗγ
PEF or FEV1 <20 % predicted or
personal best
5. Pathophysiology
Different patterns of fatal asthma Severe asthma Papiris 2002
Scenario of asthma death
Variable Type 1 Type 2
Time course
Subacute worsening (days).
'Slow onset – late arrival'
Acute deterioration (hours).
'Sudden asphyxic asthma'
Frequency 80–85% 15–20%
Airways Extensive mucous plugging More or less 'empty' bronchi
Inflammation Eosinophils Neutrophils
Response to
treatment
Slow Faster
Prevention Possible (?)
6. Cause of Death
• Type 1
hypercapnic respiratory failure
mixed acidosis
asphyxia
complications of mechanical ventilation,
such as barotrauma and ventilator-
associated pneumonia
7. Cause of Death
• Type 2
rapidly severe hypercapnic respiratory
failure with combined metabolic and
respiratory acidosis
asphyxia
8. Risk Factors for Death from Asthma
• Asthma history
Prior severe exacerbation (intubation or ICU admission)
Two or more hospitalizations for asthma in the past
year
Three or more ED visits for asthma in the past year
Hospitalization or ED visit for asthma in the past
month
Using >2 canisters of SABA per month
Difficulty perceiving asthma symptoms or severity of
exacerbations
Other risk factors:
lack of a written asthma action plan
Sensitivity to Alternaria
9. Risk Factors: Death from Asthma
• Social history
Low socioeconomic status or inner-city
residence
Illicit drug use
Major psychosocial problems
• Co-morbidities
Cardiovascular disease
Other chronic lung disease
Chronic psychiatric disease
11. Asthma Severity Assessment
Symptoms Severe Respiratory
Arrest Imminent
Breathless At rest, sits
upright
Talks in Words
Alertness Usually agitated Drowsy or
confused
12. Asthma Severity Assessment
Signs Severe Respiratory
Arrest Imminent
Respiratory rate >30/minute
Accessory
muscles
Usually Paradoxical
diaphragmatic
movement
Wheeze Usually loud Absent
Pulse >120 Bradycardia
14. Medical Management of Life
Threatening Asthma
• Oxygen
maintain a SaO2 >90 percent
• Inhaled short acting B2 agonists
(SABA)
4–8 puffs every 20 minutes up to 4
hours, then every 1–4 hours as needed
• Ipratropium bromide
8 puffs every 20 minutes as needed up
to 3 hours
15. Medical Management of Life
Threatening Asthma
• Systemic corticosteroids
40–80 mg/day in 1 or 2 divided doses
until PEF reaches 70% of predicted or
personal best
• Subcutaneous epinephrine
0.3–0.5 mg every 20 minutes for 3 doses
sq
16. Medical Management of Life
Threatening Asthma
• Magnesium Sulphate (I.V.)
2 grams in adults and 25–75 mg/kg up to 2
grams in children
• Heliox
consider when using nebulizer Rx
• Methylxanthines
increases the frequency of adverse effects
• Antibiotics
evidence of pneumonia or sinusitis
17. Monitoring during treatment
• Serial Measurements of Lung Function
Failure of treatment to improve predicts a more severe
course and the need for hospitalization
• Lab Studies
Hypercapnia, metabolic acidosis, CBC, electrolytes
• Chest Radiography
exclude complications such as lobar atelectasis,
pneumothorax, pneumomediastinum,
pneumopericardium, or alternate diagnosis such as
congestive heart failure and pneumonia
• Electrocardiogram
signs of right heart strain
19. Criteria for ICU admission
• PaCO2 > 45 mmHg
• impaired consciousness
• respiratory arrest
• significant hypoxia (transcutaneous oxygen
saturation <92%) with severe airway
obstruction
• ICU admission may also be indicated for
respiratory arrest, altered mental status,
myocardial injury, and when there is need
for frequent nebulizer treatments.
20. Pulmonary Hyperinflation
• Increase in functional residual capacity
Can be as much as 2x normal
• Critical limitation of the expiratory flow:
Reduced driving forces of the expiratory flow
Low pulmonary recoil
High outward recoil of the chest wall generated by
the persistent activation of the inspiratory muscles
Increased resistance to airflow
Severely reduced airway caliber
Expiratory narrowing of the glottic aperture
21. Pulmonary Hyperinflation
• Increase in the time constant of the
respiratory system (prolonged expiration)
• Inspiration starts before static
equilibrium is reached (PEEPi)
• PEEPi depends on:
Vt
Time constant of the respiratory system
RR (shortening of the expiratory time)
25. Non-Invasive Ventilation
• Improve gas exchange, alleviate
respiratory distress, prevent
exhaustion and improve
hemodynamics.
• In selected patients NIV can shorten
the attack, improve lung function
and prevent ICU admission.
J Crit Care 1993;8:87
Am Rev Respir Dis 1982;126:812
Chest 1996;110:767
Chest 2003;123:1018
26. Mechanical Ventilation
• Life saving intervention needed by
a small minority of patients!
• Fewer deaths with controlled
hypoventilation compared with
ventilation in which carbon dioxide
levels were normalized
historical cohorts and case series have
reported mortality rates of 7.523%
27. Mechanical Ventilation
• Mechanical ventilation is associated with
hypotension, barotrauma, infection, and
myopathy, especially when prolonged
paralysis is required with muscle
relaxants and systemic corticosteroids.
• Adverse effects reported in one
retrospective study of 88 episodes of
mechanical ventilation were hypotension
(20%), barotrauma (14%), and
arrhythmias (10%).
28. Intubation
• < 1% of asthmatics require
intubation for mechanical
ventilation.
• Rapid sequence intubation is the
method of choice
• Crucial objective is to prevent any
further increase in lung
hyperinflation
29. Intubation
• Absolute indications:
Respiratory arrest
Coma
• With maximum medical therapy:
Worsening pulmonary function tests
Decreasing PaO2
Increasing PaCO2
Progressive respiratory acidosis
Declining mental status
Increasing agitation
30. Intubation
• Post-intubation hypotension
25% to 35% of patients after
intubation
loss of vascular tone due to sedation,
hypovolemia, tension pneumothorax,
or hyperinflation
• Barotrauma was found to
complicate status asthmaticus in 14
to 27% of patients
31. Mechanical Ventilation
• Initial ventilator settings
Low RR (8-12 breaths/min)
Small Vt 6-8 cc/kg of predicted body weight
High peak inspiratory flow (70-100 L/min)
Prolonged expiratory time (I:E 1:2-1:3)
PEEP to compensate for PEEPi if patient is
breathing spontaneously (80% of auto-
PEEP)
ZEEP if patient is sedated, paralyzed and on
controlled mechanical ventilation to
maximize exhalation.
32. Permissive Hypercapnia
• PaCO2 up to 90 mm Hg
• Generally tolerated when adequate
oxygenation is achieved
• The main contraindication is
intracranial disease
• Should be avoided in patients who
have already suffered an anoxic
brain injury and cerebral edema
following cardiorespiratory arrest
33. Mechanical Ventilation
• Assessing lung inflation
The volume at end-inspiration, termed
Vei, is determined by collecting
expired gas from total lung capacity to
functional residual capacity during 40
to 60 seconds of apnea. A Vei greater
than 20 mL/kg has been correlated
with barotrauma.
• Pplat < 30
• PEEPi Am Rev Respir Dis 1992;146:607.
34. Sedation
• Propofol for short term intubation
• Midazolam for prolonged
ventilation
• Morphine or fentanyl depending on
hemodynamics
Morphine can cause histamine release
35. Sedation and paralysis
• Ketamine
intravenous anesthetic with sedative,
analgesic, and bronchodilating
properties
indirectly stimulates catecholamine
release and, in a dose of up to 2 mg/kg,
will produce bronchodilation in the
critically ill asthmatic
36. Paralysis
• Indication:
When safe and effective mechanical
ventilation cannot be achieved by
sedation alone
• Short-term muscle paralysis
37. Medical Rx with MV
• Bronchodilators
Higher drug dosages are required and
the dosage should be titrated to
achieve a fall in the peak-to-pause
airway pressure gradient
• General anesthetics
Halothane and enflurane are general
anesthetic bronchodilators that can
acutely reduce peak pressure and
PaCO2 Crit Care Med 2002; 30:477–480
Intensive Care Med 1990; 16:104-107
Chest 1994;106;1401-1406
CHEST 2003; 123:891–896
38. Heliox
• Heliox, an 80:20 mixture of helium and oxygen,
can be considered in patients with respiratory
acidosis who fail conventional therapy.
• Helium is a low-density, inert gas that lowers
airway resistance and decreases respiratory work.
• Significant improvement may be noted within 10-
20 minutes of initiating therapy in the asthmatic
with severe bronchospasm.
39. Weaning from MV
• Extubation
spontaneous breathing trial once:
PaCO2 normalizes at a minute ventilation
that is less than 12 LPM
airway resistance is less than 20 cm
H2O/l
the patient follows commands, and
neuromuscular weakness has not been
identified
41. Rescue therapies
• Novalung
pumpless low resistance gas exchange
membrane reliant on arteriovenous
shunt and a sweep flow oxygen for
membrane diffusion of oxygen and
carbon dioxide.
Arteriovenous and venovenous
Patients: rescue therapy for patients
with refractory bronchospasm, who
failed all usual interventions, have
arterial PaCO2 >110 mmHg and have
severe acidosis.
Crit Care Med 2007; 35:945–948
42. Novalung
• Reasons
Bronchospasm in acute severe asthma is
usually reversible, iLA should only be required
for short periods of time
Novalung is highly effective at CO2 clearance.
Usually Oxygenation is not a major problem
Patients with acute severe asthma have have
sufficient cardiac output and blood pressure
to maintain arteriovenous shunt across the
membrane
• Cons
Large bore arterial and venous cannulation
Systemic anticoagulation
43. Life Threatening Asthma
• Be on the alert in order to identify
high risk patients
• Be ready to initiate life saving
treatments, possibly even including
inhaled anaesthetics and NovaLung
• Try your best to prevent
complications
In one study of 21 acute asthmatics with a mean PEFR of 144 L/min, nasal CPAP of 5 or 7.5 cm H2O significantly decreased respiratory rate and dyspnea compared with placebo.
If the patient deteriorates or fails to improve despite intensive therapy, intubation and mechanical ventilation must be considered.
Since intubation and mechanical ventilation can be life saving but are associated high morbidity and significant mortality