Mechanical ventilation, or assisted ventilation, is the medical term for artificial ventilation where mechanical means are used to assist or replace spontaneous breathing. This may involve a machine called a ventilator, or the breathing may be assisted manually by a suitably qualified professional, such as an anesthesiologist, Registered Nurse, respiratory therapist, or paramedic, by compressing a bag valve mask device.
Mechanical ventilation can be
Noninvasive, involving various types of face masks
Invasive, involving endotracheal intubation
Selection and use of appropriate techniques require an
understanding of respiratory mechanics.
Indications
There are numerous indications for endotracheal intubation
and mechanical ventilation (see table Situations Requiring Airway Control),
but, in general, mechanical ventilation should be considered when there are
clinical or laboratory signs that the patient cannot maintain an airway or
adequate oxygenation or ventilation.
Concerning findings include
Respiratory rate > 30/minute
Inability to maintain arterial oxygen saturation > 90%
with fractional inspired oxygen (FIO2) > 0.60
pH < 7.25
PaCO2 > 50 mm Hg (unless chronic and stable)
The decision to initiate mechanical ventilation should be
based on clinical judgment that considers the entire clinical situation and not
simple numeric criteria. However, mechanical ventilation should not be delayed
until the patient is in extremis.
Respiratory Mechanics
Normal inspiration generates negative intrapleural pressure,
which creates a pressure gradient between the atmosphere and the alveoli,
resulting in air inflow. In mechanical ventilation, the pressure gradient
results from increased (positive) pressure of the air source.
Peak airway pressure is measured at the airway opening (Pao)
and is routinely displayed by mechanical ventilators. It represents the total
pressure needed to push a volume of gas into the lung and is composed of
pressures resulting from inspiratory flow resistance (resistive pressure), the
elastic recoil of the lung and chest wall (elastic pressure), and the alveolar
pressure present at the beginning of the breath (positive end-expiratory
pressure [PEEP]
Resistive pressure is the product of circuit resistance and
airflow. In the mechanically ventilated patient, resistance to airflow occurs
in the ventilator circuit, the endotracheal tube, and, most importantly, the
patient’s airways. (NOTE: Even when these factors are constant, an increase in
airflow increases resistive pressure.)
Components of airway pressure during mechanical ventilation,
illustrated by an inspiratory-hold maneuver
PEEP = positive end-expiratory pressure.
Elastic pressure is the product of the elastic recoil of the
lungs and chest wall (elastance) and the volume of gas delivered. For a given
volume, elastic pressure is increased by increased lung stiffness (as in
pulmonary fibrosis) or restricted excursion of the chest wall or diaphragm (eg,
in tense ascites or massive obesity). Because elastance is the inverse of
compliance, high elastance is the same as low compliance.
End-expiratory pressure in the alveoli is normally the same
as atmospheric pressure. However, when the alveoli fail to empty completely
because of airway obstruction, airflow limitation, or shortened expiratory
time, end-expiratory pressure may be positive relative to the atmosphere. This
pressure is called intrinsic PEEP or auto PEEP to differentiate it from
externally applied (therapeutic) PEEP, which is created by adjusting the
mechanical ventilator or by placing a tight-fitting mask that applies positive
pressure throughout the respiratory cycle.
Any elevation in peak airway pressure (eg, > 25 cm H2O)
should prompt measurement of the end-inspiratory pressure (plateau pressure) by
an end-inspiratory hold maneuver to determine the relative contributions of
resistive and elastic pressures. The maneuver keeps the exhalation valve closed
for an additional 0.3 to 0.5 second after inspiration, delaying exhalation.
During this time, airway pressure falls from its peak value as airflow ceases.
The resulting end-inspiratory pressure represents the elastic pressure once
PEEP is subtracted (assuming the patient is not making active inspiratory or
expiratory muscle contractions at the time of measurement). The difference
between peak and plateau pressure is the resistive pressure.
Elevated resistive pressure (eg, > 10 cm H2O) suggests
that the endotracheal tube has been kinked or plugged with secretions or that
an intraluminal mass or bronchospasm is present.
Increased elastic pressure (eg, > 10 cm H2O) suggests
decreased lung compliance due to
Edema, fibrosis, or lobar atelectasis
Large pleural effusions, pneumothorax, or fibrothorax
Extrapulmonary restriction as may result from
circumferential burns or other chest wall deformity, ascites, pregnancy, or
massive obesity
A tidal volume too large for the amount of lung being
ventilated (eg, a normal tidal volume being delivered to a single lung because
the endotracheal tube is malpositioned)
Intrinsic PEEP (auto PEEP) can be measured in the passive
patient through an end-expiratory hold maneuver. Immediately before a breath,
the expiratory port is closed for 2 seconds. Flow ceases, eliminating resistive
pressure; the resulting pressure reflects alveolar pressure at the end of
expiration (intrinsic PEEP). Although accurate measurement depends on the
patient being completely passive on the ventilator, it is unwarranted to use
neuromuscular blockade solely for the purpose of measuring intrinsic PEEP. A
nonquantitative method of identifying intrinsic PEEP is to inspect the
expiratory flow tracing. If expiratory flow continues until the next breath or
the patient’s chest fails to come to rest before the next breath, intrinsic
PEEP is present. The consequences of elevated intrinsic PEEP include increased
inspiratory work of breathing and decreased venous return, which may result in
decreased cardiac output and hypotension.
The demonstration of intrinsic PEEP should prompt a search
for causes of airflow obstruction (eg, airway secretions, decreased elastic
recoil, bronchospasm); however, a high minute ventilation (> 20 L/minute)
alone can result in intrinsic PEEP in a patient with no airflow obstruction. If
the cause is airflow limitation, intrinsic PEEP can be reduced by shortening
inspiratory time (ie, increasing inspiratory flow) or reducing the respiratory
rate, thereby allowing a greater fraction of the respiratory cycle to be spent
in exhalation.
Means and Modes of Mechanical Ventilation
Mechanical ventilators are
Volume cycled: Delivering a constant volume with each breath
(pressures may vary)
Pressure cycled: Delivering constant pressure during each
breath (volume delivered may vary)
A combination of volume and pressure cycled
Assist-control (A/C) modes of ventilation are modes that
maintain a minimum respiratory rate regardless of whether or not the patient
initiates a spontaneous breath. Because pressures and volumes are directly
linked by the pressure-volume curve, any given volume will correspond to a
specific pressure, and vice versa, regardless of whether the ventilator is
pressure cycled or volume cycled.
Adjustable ventilator settings differ with mode but include
Respiratory rate
Tidal volume
Trigger sensitivity
Flow rate
Waveform
Inspiratory/expiratory (I/E) ratio
Volume-cycled ventilation
Volume-cycled ventilation delivers a set tidal volume. This
mode includes
Volume-control (V/C)
Synchronized intermittent mandatory ventilation (SIMV)
The resultant airway pressure is not fixed but varies with
the resistance and elastance of the respiratory system and with the flow rate
selected.
V/C ventilation is the simplest and most effective means of
providing full mechanical ventilation. In this mode, each inspiratory effort
beyond the set sensitivity threshold triggers delivery of the fixed tidal
volume. If the patient does not trigger the ventilator frequently enough, the
ventilator initiates a breath, ensuring the desired minimum respiratory rate.
SIMV also delivers breaths at a set rate and volume that is
synchronized to the patient’s efforts. In contrast to V/C, patient efforts
above the set respiratory rate are unassisted, although the intake valve opens
to allow the breath. This mode remains popular, despite not providing full
ventilator support as does V/C, not facilitating liberation of the patient from
mechanical ventilation, and not improving patient comfort.
Pressure-cycled ventilation
Pressure-cycled ventilation delivers a set inspiratory
pressure. This mode includes
Pressure control ventilation (PCV)
Pressure support ventilation (PSV)
Noninvasive modalities applied via a tight-fitting face mask
(several types available)
Hence, tidal volume varies depending on the resistance and
elastance of the respiratory system. In this mode, changes in respiratory
system mechanics can result in unrecognized changes in minute ventilation.
Because it limits the distending pressure of the lungs, this mode can
theoretically benefit patients with acute respiratory distress syndrome (ARDS);
however, no clear clinical advantage over V/C has been shown, and, if the
volume delivered by PCV is the same as that delivered by V/C, the distending
pressures will be the same.
Pressure control ventilation is a pressure-cycled form of
A/C. Each inspiratory effort beyond the set sensitivity threshold delivers full
pressure support maintained for a fixed inspiratory time. A minimum respiratory
rate is maintained.
In pressure support ventilation, a minimum rate is not set;
all breaths are triggered by the patient. The ventilator assists the patient by
delivering a pressure that continues at a constant level until the patient's
inspiratory flow falls below a preset level determined by an algorithm. Thus, a
longer or deeper inspiratory effort by the patient results in a larger tidal
volume. This mode is commonly used to liberate patients from mechanical
ventilation by letting them assume more of the work of breathing. However, no
studies indicate that this approach is more successful than others in
discontinuing mechanical ventilation.
Noninvasive positive pressure ventilation (NIPPV)
NIPPV is the delivery of positive pressure ventilation via a
tight-fitting mask that covers the nose or both the nose and mouth. Helmets
that deliver NIPPV are being studied as an alternative for patients who cannot
tolerate the standard tight-fitting face masks. Because of its use in
spontaneously breathing patients, it is primarily applied as a form of PSV or
to deliver end-expiratory pressure, although volume control can be used.
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