How to initiate invasive mechanical ventilation in adults in intensive care unit
INTRODUCTION
There are several indications for the initiation of invasive mechanical ventilation in the intensive care unit (ICU) .
Examples of conditions often requiring mechanical ventilation
ARDS: adult respiratory distress
syndrome.
DEFINITION
Invasive
mechanical ventilation is defined as the delivery of positive pressure to the
lungs via an endotracheal or tracheostomy tube. During
mechanical ventilation, a predetermined mixture of air (ie, oxygen and other
gases) is forced into the central airways and then flows into the alveoli. As
the lungs inflate, the intra-alveolar pressure increases. A termination
signal (usually flow or pressure) eventually causes the ventilator to stop
forcing air into the central airways and the central airway pressure
decreases. Expiration follows passively, with air flowing from the higher
pressure alveoli to the lower pressure central airways.
Invasive mechanical
ventilation is most often used to fully or partially replace the functions of
spontaneous breathing by performing the work of breathing and gas exchange in
patients with respiratory failure.
Invasive mechanical
ventilation may also be useful in those who require airway protection to
reduce the risk of aspiration (eg, depressed mental status from an overdose,
patients with variceal bleeding).
SELECTING AN INITIAL
MODE
Commonly used modes
There is no universal mode of invasive mechanical ventilation
that is ideal for all patients. However, common initial modes which are
suitable for most patients include:
· Volume-limited
assist control ventilation
· Pressure-limited
assist control ventilation
· Synchronized
intermittent mandatory ventilation with pressure support ventilation
(SIMV-PSV)
· Pressure support
ventilation (PSV) alone is uncommonly used as an initial mode of ventilation
but commonly used during weaning.
The
modes of mechanical ventilation are distinguished from each other by the
types of breaths that they deliver. In brief, the delivery of breaths
are typically either volume-limited or pressure-limited:
Volume-limited – Volume-limited
breaths can
be ventilator-initiated (also known as volume-controlled or volume-cycled
[VC]) or patient initiated (also known as volume-assist [VA]). VC or VA
breaths deliver a predetermined tidal volume at a set ventilator rate such
that a minimum minute ventilation (tidal volume x respiratory rate) is guaranteed.
Each tidal volume is delivered at a set inspiratory flow rate and inspiration
is terminated once the set tidal volume has been delivered. Airway pressure
is determined by the airway resistance, lung compliance, and chest wall
compliance. Modes of mechanical ventilation used in the ICU that can deliver
VC or VA breaths include volume-limited assist control and volume-limited
SIMV. Volume-limited continuous mechanical ventilation (VC-CMV) is not
generally needed in the ICU. Pressure-regulated volume controlled ventilation
(PRVC) is being increasingly used.
Pressure limited –
Pressure-limited
Breaths can be
ventilator-initiated (also known as pressure-control or pressure-cycled [PC])
or patient initiated (also known as pressure-assist [PA]). In PC or PA
breaths, the flow of air into the lung is determined by a set pressure limit
and the rate is determined by a set ventilator rate. Inspiration is
terminated once the set inspiratory time has elapsed. The tidal volume is
variable and related to compliance, airway resistance, and tubing resistance.
Pressure support – Spontaneous
breathing can be supported to a
set pressure limit; such breaths are called “pressure support (PS) breaths.”
The ventilator provides the driving pressure for each spontaneous breath,
which determines the maximal airflow rate. Inspiration is terminated once the
inspiratory flow has decreased to a predetermined percentage of its maximal
value.
Factors
influencing the choice of initial mode;
1.Level of support needed – The level of ventilator support is the proportion of the
patient's ventilatory needs that are met by the ventilator. The level of
support is determined by the following:
• The mode – Generally speaking,
among the modes used in the ICU, volume- or pressure-limited modes that use
assist control functions tend to provide the most support (ie, resting
respiratory muscles while simultaneously minimizing atrophy). In contrast,
pressure support tends to provide the least support and is associated with a
greater work of breathing.
• The indication for mechanical
ventilation – In general, patients who are mechanically ventilated for
respiratory failure need more support than those ventilated for airway
protection while those with severe respiratory failure need more support than
those with mild respiratory failure.
2.Reason for mechanical
ventilation – The indication for mechanical
ventilation may influence the mode. For example, patients with acute
respiratory distress syndrome (ARDS) are typically placed on low tidal volume
ventilation (LTVV) delivered using volume-limited assist control ventilation
or pressure-limited assist control ventilation. In contrast, a patient who is
mechanically ventilated short-term for airway protection may be suitable for
several additional modes including SIMV-PSV as well as PSV alone.
3.Presence of airflow limitation – In patients with active airflow limitation such as
severe chronic obstructive lung disease [COPD], acute asthma, or acute COPD
exacerbation, volume-limited modes of ventilation are commonly used (eg,
volume-limited assist control ventilation, SIMV-PSV). In contrast, pressure
support or pressure-limited modes including APRV are generally avoided.
4.Presence of an air leak – In patients with a prolonged air leak (eg, from
pneumothorax or lung surgery) pressure-limited ventilation or SIMV-PSV, or
even PSV alone, are preferred in order to limit additional barotrauma and
worsening of the air leak.
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● Concern for elevated
intracranial pressure (ICP) While volume-limited modes are frequently
used in patients with an elevated ICP (eg, traumatic brain injury or stroke),
pressure-limited modes are popular due to the theoretical concern that elevated
intrathoracic pressures hamper venous return from the brain and therefore
worsen ICP. However, there are no data in this population to suggest that one
mode is superior to the other and practice varies widely.
● Paralysis
For patients who are paralyzed or
heavily sedated (ie, those in whom the initiation of a spontaneous breath is
limited), PSV is contraindicated and assist controlled modes (volume- or
pressure-limited) are typically used, provided the set minute
ventilation is adequate.
SETTINGS
Volume-limited
assist control ventilation — For
patients in whom volume-limited assist control ventilation is chosen as the
initial mode of ventilation, we typically use the following initial settings:
Tidal volume – Tidal volume is typically set at 6 mL per kg predicted
body weight (PBW); 4 to 8 mL/kg PBW for patients with acute respiratory
distress syndrome (ARDS) and 6 to 8 mL/kg PBW for patients who do not have
ARDS.
Ventilator rate – Ventilator rate is typically set at 12 to 16 breaths per
minute; higher rates may be necessary for patients with ARDS (eg, ≤35 breaths
per minute).
Positive end-expiratory pressure
(PEEP) – PEEP is typically set at 5 cm H 2
O with subsequent adjustments made according to the fraction of inspired oxygen
(FiO 2 ).
FiO 2 – FiO 2 is set to maintain the peripheral oxygen saturation (SpO 2 )
between 90 and 96 percent.
Inspiratory flow – Inspiratory flow is typically set at 40 to 60 L per minute with a ramp
pattern to target an inspiratory:expiratory (I:E) ratio of approximately 1:2 to
1:3. Higher rates up to 75 L per minute that decrease the I:E ratio are
appropriate in patients with airflow obstruction.
Trigger sensitivity – Typical values are 2 L/min when flow-triggering is used or
-1 to -2 cm H 2 O when pressure-triggering is used. Pressure-triggering
should not be used when auto-PEEP is suspected.
Pressure-limited
assist control ventilation — For
patients in whom pressure-limited assist controlled ventilation is chosen as
the initial mode of ventilation, we typically set the inspiratory pressure
level to target an approximate tidal volume (eg, 4 to 8 mL/kg PBW for a patient
with ARDS) and the inspiratory time is set to deliver an I:E ratio of 1:2 to
1:3 (typically one second). The FiO 2 , ventilator rate, applied PEEP, and
trigger sensitivity are similar to those of volume-limited-assist control
ventilation (VC-AC).
The
initial inspiratory pressure varies depending upon lung compliance, airway
resistance, and tubing resistance but in general, acceptable target tidal
volumes may be reached with inspiratory pressure levels between 12 and 25 cm H
2 O. However, the clinician should bear in mind that the addition of the
applied PEEP to a set inspiratory pressure increases peak airway pressure and
may further increase the risk of barotrauma (eg, patient with a set inspiratory
pressure level of 20 cm H 2 O and an applied PEEP of 10 cm H 2 O will have a
peak airway pressure of 30 cm H 2 O).
Synchronized
intermittent mandatory ventilation with pressure support ventilation (SIMV/PSV)
— For patients in whom SIMV/PSV is
chosen as the initial mode of ventilation, we typically use similar settings to
VC-AC with the addition of pressure support (eg, 5 to 10 cm H 2 O) for
spontaneous breaths taken by the patient above the set rate. The pressure
support can be subsequently increased as needed for patient comfort and reduced
when mitigation of respiratory alkalosis is required.
Typical
initial settings for common modes of mechanical ventilation
PBW: predicted body weight (ie,
ideal body weight); ARDS: acute respiratory distress syndrome; PEEP: positive
end-expiratory pressure; FiO 2 : fraction of inspired oxygen; SpO 2 :
peripheral oxygen saturation; PaO 2 : arterial oxygen tension; I:E ratio: inspiratory:expiratory
ratio; N/A: not applicable; PSV: pressure support ventilation.
* Patients are often started on an FiO 2 of 1 immediately after intubation and weaned quickly over 30 minutes to reach the minimum FiO 2 needed to achieve the target SpO 2 . ¶ Conditions where higher FiO 2 is indicated include carbon monoxide toxicity, cluster headaches, sickle cell crisis, pneumothorax, pregnancy, and air embolism. Δ Pressure-regulated volume-controlled ventilation sets an inspiratory time rather than setting the inspiratory flow to target the same I:E ratio. |
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